Multi-Scale Genesis Pathway for the Formation of the Near- Equatorial Tropical Cyclone Senyar

Multi-Scale Genesis Pathway for the Formation of the Near- Equatorial Tropical Cyclone Senyar | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Multi-Scale Genesis Pathway for the Formation of the Near- Equatorial Tropical Cyclone Senyar Marzuki Marzuki, Michal Brennek, Ravidho Ramadhan, Nadya Ananda, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9075294/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Formation of a near equatorial Tropical Cyclone Senyar in Strait of Malacca on 25 November 2025 brought historical flooding across northern Sumatra and Malaysia a region where low planetary vorticity and nearly land-locked topography typically prohibit cyclogenesis. Here, we determine a complex, multi-scale genesis pathway of Senyar and elucidate critical superpostion of drivers across several spatiotemporal scales. La Nina and negative Indian Ocean Dipole conditions, and northward shift of the Intertropical Convergence Zone facilitated enhanced meridional shear between equatorial westerlies and the northeast monsoon, while interaction of several types of tropical waves facilitated an increase of vorticity and moisture with concurrent decrease in vertical winds shear in the Strait of Malacca, creating a nurturing environment for pre-Senyar disturbance to grow in, as well as drove its final spin up. All key ingredients enabling Senyar genesis were identifiable in observations and can be leveraged to improve forecasting such devastating, near-equatorial events. Earth and environmental sciences/Climate sciences/Atmospheric science/Atmospheric dynamics Earth and environmental sciences/Natural hazards Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction The Tropical Cyclone (TC) Senyar formed at 12 UTC on 25 November 2025 in the Strait of Malacca (4.7°N, 99°E) (Fig. 1a), triggering catastrophic flooding and landslides in parts of Indonesia and Malaysia during 25-27 November 2025. In Indonesia, the impacts of TC Senyar were severe, including 1,207 fatalities, the displacement of up to 1,057,450 people at its peak on 8 December 2025, the destruction of 860 bridges and damage to about 301,012 housing units across 53 districts, with preliminary recovery costs estimated at IDR 59.25 trillion (approximately USD 3.7 billion) 1 . Loss and damage was amplified by TC Senyar’s slow translation speed, which produced prolonged extreme rainfall, and lack of advance warning for an area rarely affected by tropical cyclones. Therefore, understanding of the physical processes leading up to the formation of TC Senyar is crucial for improving forecast capabilities and disaster preparedness in densely populated tropical coastal regions. TC Senyar is categorized as a near equatorial tropical cyclones (NETC) as it formed close to the equator, a region traditionally considered TC-free due to low planetary vorticity (f ≈ 1.2×10⁻⁵ s⁻¹ at 4.7°N) 2,3 . Analysis of the International Best Track Archive for Climate Stewardship (IBTrACS) 4 , reveals that since 1980, only 278 NETC (forming within 6° of the equator) have been recorded globally, less than 6% of total global TC activity, and all occurred in the Indo-Pacific domain (see Fig. S1 in Supplementary Information – SI). Seasonal and inter-basin variability in background vorticity and vertical wind shear are key conditions for sustaining tropical cyclogenesis (TCG) of NETC 5,6 , and such variability explains why NETCs are rarely observed in the Atlantic 7 . In western Pacific, NETCs are favored by a background, seasonal low-level vorticity associated with the cyclonic turning of NE trade wind near the equator, and thus occur predominantly during boreal winter 6 . In contrast, NETCs in the Indian Ocean are driven by the shedding of Sumatra vortices, a synoptic-scale regional feature that frequently develops near the tip of Sumatra island within the easterly flow 8,9 , leading to their downstream propagation over the basin. The Maritime Continent (MC), a complex archipelago between the Indian and western Pacific Oceans, presents a particularly challenging environment for cyclogenesis. Only 14 NETC formed within the MC since 1980, with most (10 events) occurring over relatively open waters such as the South China Sea and the Banda Sea. The most notable MC NETC, TC Vamei (Fig. 1a), formed within 1.5° of the equator in December 2001 11 . TC Senyar is an exceptional event in this small group, as it formed in Strait of Malacca (MS) (Fig. 1b), only the forth such event since 1980. While previous MS NETCs (unnamed systems on 21 November 1988 and 18 November 1995, and TC Mandous on 4 December 2022) followed typical westward and poleward trajectories (see Fig. S1 in SI), TC Senyar exhibited an anomalous track, slowly moving equatorward before recurving eastward to make landfall approximately 100 km north of Kuala Lumpur, Malaysia. It then moved into the South China Sea while gradually weakening (Fig. 1b).This unusual behavior underscores gaps in our understanding of NETC dynamics in topographically complex coastal environments. Tracking of an elevated vorticity ‘blob’ that constitute a TC Senyar precursor (hereafter "pre-Senyar" disturbance) reveals that a chain of processes leading to TCG began on 15 November 2025 (Fig. 1c), 10 days before its ‘offical’ formation, when an area of enhanced relative vorticity first became identifiable over the South China Sea during the onset of northeast (NE) monsoon (Fig. 2). Following the NE monsoon onset on 13 November 12 , a relatively broad area of positive vorticity developed along its southern flank. The pre-Senyar moved westward with the monsoon flow (15-17 November), stalled along the east coast of Malay Peninsula (18-20 November), and then crossed the peninsula before turning southward into the MS on 21 November (Fig. 2). Until this equatorward diversion, the pre-Senyar followed a typical westward wintertime trajectory of disturbances that can eventually develop into a TC over Indian Ocean 8 , also consistent with the tracks of TC Vamei (Fig. 1b) and the three other MS systems (Fig. S1). Once in the Strait, the pre-Senyar stalled again while steadily intensifying and was named on 25 November. Relative vorticity of pre-Senyar increased gradually from 10⁻⁴ s⁻¹ on 19 November (Malay Peninsula arrival) to 6×10⁻⁴ s⁻¹ by 23 November. After temporal weakening to 2×10⁻ 4 s⁻¹ on 23 November, explosive intensification occurred on 24 November, with the disturbance reaching maximum vorticity of ~10⁻ 3 s⁻¹ at 00UTC on 25 November, representing 10-fold inctease in vorticity since 19 November and more than 3-fold increase in 30 hours (Fig. 1d, Fig. 2). In the MS region, minimum sea-level pressure (1001 hPa) and maximum 10-m wind speed (17 m s⁻¹) were attained at 09 and 02 UTC on 25 November, respectively, prior to recognition in the IBTrACS data (Fig. 1d). As the system weakened thereafter, the system became self-susteining most likely around 00 UTC on 25 November. Regardless of the precise definition of the genesis time, heavy rainfall affected the region throughout the entire period when the disturbance was in the MS region, despite its modest intensity by conventional tropical cyclone metrics. A similar pattern was observed during TC Seroja's genesis in the Flores Sea 13 , which emphasizes the need to improve understading of TCG occurring in coastal regions of densely populated areas. This study investigates physical atmospheric processes related to the formation of TC Senyar, specifically focusing on the preTCG (19-23 November) and TCG (23-26 November) periods. The analysis considers all critical components of the TCG 2,14 within a multi-scale interactions framework. To that end, average November 2025 conditions, background (period > 20 days), sub-intraseasonal variability (5-20 days) and transient disturbances (period <5 days) variability, as well as spatio-temporal variability of relevant atmospheric fields and sea surface temperatures (SST), are invesitgated. The analysis centers on the western MC and three key focus regions: the NE monsoon region in the South China Sea (NEM region), the westerly wind burst (WWB) region in eastern equatorial Indian Ocean, and the primary genesis theater in the Strait of Malacca (MS region). Reanalysis data are analyzed with dynamical diagnostics to elucidate a sequence of multiscale interactions that enabled this rare and devastating cyclogenesis event, with implications for improving operational forecasts and reducing societal vulnerability to future near-equatorial tropical cyclones. Results Multi-Scale Wind Variability The preTCG (19–23 November) and TCG (23–26 November) periods were characterized by pronounced multi-scale variability in lower-tropospheric (850 hPa) zonal winds across the eastern Indian Ocean WWB region and the South China Sea NEM region (Fig. 3a,b). In the WWB region, 850-hPa westerlies consistently exceeded the November 2025 monthly mean (8.5 m s⁻¹) throughout the preTCG and TCG periods (Fig. 3a). The background component contributed steady westerlies but gradually weakened over time. Two westerly peaks on 18 November and 24 November, both exceeding 12 m s -1 were driven primarily by the transient variability driven by convectively coupled Kelvin waves (Fig. 3d) . The second peak was notably stronger (13 m s -1 ) due to constructive interference between transient and sub-intraseasonal components (Fig. 3a) related to equatorial Rossby (ER) wave and Madden-Julian oscialltion (MJO, Fig. 3d). Following the Malaysian meteorological office's declaration of NE monsoon onset on 13 November, easterlies in the NEM region intensified rapidly after 15 November and remained substantially stronger than the monthly average throughout the preTCG and TCG periods (Fig. 3b). Area-averaged NEM easterlies reached a maximum of 16 m s⁻¹ on 21 November. While the background component remained steady easterlies, temporal evolution was dominated by sub-intraseasonal variability (Fig. 3b) reflecting the westward progression and intensification of the monsoon flow (Fig. 2) supported by westward propagating ER event (Fig. 3e). This sub-intraseasonal signal manifested as both intensifying easterly winds and spatial expansion of the easterly flow field in both meridional and zonal dimensions. Interestingly, sub-intraseasonal easterlies in the NEM region peaked approximately 4 days before sub-intraseasonal westerlies in the WWB region, indicating westward propagation of a cyclonic circulation spanning meridionally between the two regions, consistent a meridionally ‘narrow’ ER event (Fig. 3d,e). Low-level meridional winds in the MS region were predominantly northerly, with significant periodic fluctuations between strong northerlies (6.5 ms -1 on 24 November) and marginally southerly flow or weak northerlies (21 November and 25 November, respectively; Fig. 3c). In the postTCG period, strong northerlies (~ 7 ms -1 ) returned on 27 November. Time-scale decomposition reveals that transient variability dominated the meridional wind signal, with substantial contributions from sub-intraseasonal variability, particularly during the TCG period (Fig. 3c). Sub-intraseasonal and transient variations correlate well with activity of ER and a combination of CCKW and mixed-Rossby gravity (MRG) waves, respectively (Fig. 3f). The southward deflection of the pre-Senyar disturbance into the Strait of Malacca on and after 20 November coincided with the development of northerly flow in the MS region, suggesting that it was facilitated by a MRG activity. The zonal wind patterns in the WWB and NEM regions (Fig. 3a, b) exhibited coherent zonal propagation: over time, easterlies north of 5°N extended progressively westward, while westerlies south of 5°N extended progressively eastward (Fig. 2), consistend with tropical waves activity (Fig. 3d,e). It should be noted, that zonal wind magnitudes in NEM and WWB regions during preTCG and TCG periods signiffiantly exceeded November 2025 averages (Fig. 3a, b) and climatological values for November (Fig. 4). This convergent evolution is quantified by the progressive decrease in latitudinal separation between the WWB westerly maximum and the NEM easterly maximum from 17 November through 24 November (Fig. 4b). During the preTCG period, maximum easterlies remained near the climatological position of 9°N, but shifted equatorward on 24–25 November. Concurrently, the WWB westerly axis migrated from 1°N to 3°N. As a result, meridional gradient of zonal velocity (-∂u/∂y) progressively increased during preTCG and TCG period, enhancing environmental vorticity in the MS region (Fig. 4c). The gradient reached maximum on 24 November, coinciding with the explosive intensification of vorticity associated with TC Senyar (Fig. 2). This convergent shear configuration not only generated the cyclonic environmental vorticity necessary to overcome weak planetary vorticity during the formation of NETC 5,7,10,15,16 , but also induced northerly flow through the MS region (Fig. 2), which drove preSenyar and TC Senyar equatorward (Fig. 1b) through its contribution to sub-intraseasonal variability (Fig. 3c). ITCZ position, vertical wind shear and SST conditions The poleward shift of low-level westerlies in the WWB region was facilitated by a displacement of the ITCZ, which in November 2025 laid about 2–4° north of its climatological position over the eastern Indian Ocean and Sumatra (Fig. 5a). The displaced ITCZ exerted a critical dynamical influence on cyclogenesis by facilitating the poleward migration of the WWB westerly maximum from 1°N during preTCG to 3°N during TCG (Fig. 4b, c), thereby enabling the convergent shear configuration documented in the previous section. At the same time, this poleward ITCZ shift enhanced background cyclonic vorticity in the near-equatorial region through the seasonal monsoon circulation, effectively increasing absolute vorticity and partially compensating for weak planetary vorticity. However, background absolute vorticity is only one of several key conditions necessary for TCG 2,14 . Other essential factors include low-to-moderate vertical wind shear, high SST and a moist atmosphere. Strong vertical wind shear generally inhibits tropical cyclone genesis through vortex tilting and vertical decoupling mechanisms, which displace the mid-level circulation from the low-level vortex center 17,18 . Climatological November vertical shear of horizontal winds over the MS region is moderate with an average magnitude of 10 m s -1 (Fig. 5e). In November 2025, the shear magnitude weakened throughout preTCG and TCG periods, reaching minimum value of 5 m s -1 in the critical 200-800 hPa layer on 25 November (Fig. 5e). This reduction permitted enhanced vertical vortex alignment and more efficient convective organization 17–19 , favorable conditions for the development of a NETC 7 . Furthermore, during preTCG period, wind consistently blew from the north (N) to west (W) directions up to the 400 hPa level and became predominantly northerly on 23-24 November, indicating a deep, equatorward steering flow consistent with the movement of preSenyar (Fig. 1b). On 25 November, the flow shifted to N-E flow with consistent barotropic conditions up to 200 hPa and steering towards Sumatra, in agreement with the initial movement of TC Senyar (Fig. 1b, Fig. 2). SST across the western MC region during November 2025 remained near climatological values (~27–29°C) (Fig. 4a) and substantially above the classical 26.5°C threshold for tropical cyclogenesis 2 . The broader climate context during November 2025 featured concurrent La Niña conditions (Niño 3.4 Index = −0.9) and a negative Indian Ocean Dipole (IOD; Dipole Mode Index = −0.6). The former was supportive of increased SST and moisture in the South China Sea and Karimata Strait 20,21 , the latter brings positive SST anomalies in the eastern Indian Ocean, resulting in stronger westerlies in the WWB region 22,23 . As a result, large-scale low-level convergence favored enhanced convection over the MC region. Thus, for TC Senyar, the near-climatological SST conditions provided necessary but not sufficient support for genesis; the critical enabling factors were the dynamical conditions. This pattern is consistent with the dynamical-forcing paradigm for NETC genesis 5,24 : thermodynamic conditions must meet minimum thresholds (SST ≥ 26.5°C, adequate moisture), but the probability and timing of genesis are determined primarily by the evolving dynamical environment. Moisture conditions Classical tropical cyclogenesis theory emphasizes the necessity of high SST over large areas to support upward latent heat flux from the ocean, which supplies moisture for latent heat release during cloud formation within developing tropical cyclone systems 25,26 . However, from a thermodynamics perspective, the critical requirement is energy supply. Idealized numerical experiments demonstrate that TCG can occur even in dry environemnt, driven by sensible heat only 27,28 . Fundamentally, moist atmospheric convection depends not on surface latent heat flux alone, but rather on boundary layer moisture content and column-integrated water vapor, which together fuel deep convection and sustain organized vortex development 29,30 . Such conditions were present during formation of TC Senyar (Fig. 5b). On 23 November, a sub-intraseasonal low-level cyclonic circulation was present over MS and the Malay Peninsula. It was characterized by exceptionally high boundary layer (1000-850 hPa average) specific humidity (q), with area-averaged values exceeding 16 g kg -1 over MS and South China Sea. Localized regions within the cyclonic circulation exhibited specific humidity ≥15 g kg⁻¹ extending above the 850 hPa level, indicating deep boundary layer moistening (Fig. 5b). Such regions are typically confined to areas within or very near the cyclonic circulation, where they support atmospheric moistening through both horizontal flow convergence and vertical convective transport, facilitated by embedded pockets of enhanced vorticity (Fig. 2). Low-level moisture convergence serves as a critical mechanism in the TCG as it controls latent heat release and sustains organized deep convection, which in turn drives low-level vorticity amplification and vortex consolidation 31 . As a result, the MS region underwent progressive moistening throughout preTCG and TCG periods and exhibited extremely high moisture content up to 300 hPa level (Fig. 5c). Particularly notable was the large positive anomaly (relative to climatology) in mid troposphere (600-400 hPa; Fig. 5c). Elevated mid-tropospheric moisture constitutes another fundamental prerequisite for tropical cyclone formation 32,33 . The presence of substantial mid-level moisture reduces saturation deficit, thereby mitigating suppression of convection and diabatic heating that allow moist updrafts to reach higher levels in the atmosphere 33 . The observed moisture evolution exhibits characteristics qualitatively consistent with the "marsupial pouch" conceptual framework 34 , which has been applied to understand TCG in Atlantic 35 , north Indian Ocean 36 and western Pacific 37 though not specifically for near-equatorial cases. In this framework, a parent cyclonic circulation, typically associated with a synoptic scale wave or monsoon trough, provides a protective environment that shields the developing vortex from adverse external conditions such as dry air intrusion, strong vertical shear, and vorticity hostile flow patterns 34,35 . The pouch circulation maintains elevated moisture and vorticity within its confines, allowing embedded mesoscale disturbances to intensify through repeated convective cycles without being disrupted by environmental ventilation. In the case of TC Senyar, the pre-Senyar disturbance moved within a sub-intraseasonal cyclonic circulation (Fig. 5b) that appears to have functioned as a protective environment. The vortex remained embedded within a region of elevated lower relative vorticity (Fig. 2) and substantially higher moisture (Fig. 5b), compared to the surrounding environment. Hence, the pre-Senyar exhibited intensity fluctuations while undergoing gradual overall intensification (Fig. 1d) and progressively moistening its immediate environment (Fig. 5c). This behavior, in which episodic convective pulses within a protective circulation lead to stepwise intensification, is characteristic of the marsupial pouch paradigm 34,35 . Thus, the TC Senyar case extends this framework to near-equatorial cyclogenesis, demonstrating that protective sub-intraseasonal circulations can facilitate genesis even at latitudes (4.7°N) where planetary vorticity is minimal. On the larger scale, western MC moistening during November 2025 was facilitated by moisture flux convergence (Fig. 6) associated with concurrent La Niña and negative IOD. The western MC exhibited a dipole pattern in moisture transport: westerly flow from the Indian Ocean (IOD pattern) penetrated the region south of the equator, while anomalously strong easterly moisture transport (La Niña conditions) dominated north of the equator and was driven by the northeast monsoon flow. Beginning on 18 November, the maximum moisture convergence is found near the leading eadge of the NE monsoon winds, coinciding with the location of pre-Senyar disturbance (Fig. 1c). As the easterly monsoon flow progress westward (Fig. 2), the moisture convergence maximum migrated in tandem, arriving in the MS region by 21 November (Fig. 6). Pre-Senyar stalling in the MS region was reinforced by westward-propagating cyclonic circulation centered over the southern Philippines on 24 November associated with Typhoon Koto (Fig. 1), which deflected the easterly moisture flux southward into the MS (Fig. 6). As a result, at that time there are two “sweet spots” of enhanced convective potential: (1) on the eastern side of the Philippines, associated with Typhoon Koto (Fig. 1), and (2) in the MS region, where TC Senyar was developing. Both systems were driven by the same large-scale moisture transport configuration, illustrating the regional coherence of the dynamical and thermodynamic forcing. Equatorial Wave Activity and Multi-Scale Interactions Subseasonal modes have been related to TC activity, including global impacts of MJO on TCG 38–40 , effects of CCKW on TCG in Atlanic 41–43 and other basins 44 , and response to ER in Pacific 45 . Wavenumber-frequency filtering of 850 hPa horizontal wind fields reveals significant activity of multiple equatorial wave types during during preTCG and TCG periods (Fig. 7a), collectively leading to TC Senyar formation (Fig. 1). Two distinct and robust (magnitude > 1.5σ, where σ represents the standard deviation of the filtered field averaged over 5°N-5°S equatorial channel) eastward propagating CCKW events (Fig. 7a, black lines), a strong (magnitude ~ 3 σ) westward propagating ER event and two robust (magnitude > 1. 5σ) MRG events occurred over the western MC region. The first significant vorticity amplification during the preTCG period occurred when the ER interacted with the first CCKW westerlies and the MRG northerlies (19 November) flow. The CCKW provides both increased westerlies (visible over WWB region one day earlier, on 18 November, in Fig. 3a) and positive vorticity at its northern edge through meridional shear of horizontal flow 46 . Simultaneously, the MRG activity closely matches variability in meridional flow in the MS region (Fig. 3c), suggesting that MRGs were responsible for the steering flow modulation and contributed to the episodic northerly wind pulses (Fig. 3) that deflected the pre-Senyar disturbance equatorward. Throughout the preTCG period, the strong ER event (Fig. 7a) maintained sustained cyclonic vorticity within its envelope as well as protective pouch 34,47 , isolating it from a lower relative vorticity background environment (Fig. 5b). On 24 November, anomalous northerlies associated with the MRG wave guided the vortex into the optimal genesis location. On 25 November, a second strong CCKW event (magnitude ~2σ) reached the MS region, coinciding with the final explosive intensification of TC Senyar (Fig. 1d). The arrival of this CCKW was associated with a strong cyclonic vorticity on its northward side (see Fig. S6s in SI). This complex multi-scale interaction occurred within a westerly flow of MJO intraseasonal circulation (Fig. 7), but away from the MJO convective center and maximum convergence. Hence, intraseasonal conditions (MJO) created supportive large-scale conditions, but it was not the direct driver for the formation of TC Senyar. It should be noted, that tropical waves described above exhibited non-canonical spatial structures with northward shifted waveguides and relatively small meridional extent (large meridional wavenumber) As a result, the ER northern cyclonic gyre spanned only 1 - 6°N (Fig. 5 and Fig. S5b-7b in SI), compared to the typical 4 – 10°N range for canonical ER waves 48 , while the second CCKW had its zonal axis at 3°N with maximum vorticity at 5°N, right at the latitude of TC Senyar’s genesis (Fig. S6a-7a in SI). This northward shift is consistent with the 2–4° northward displacement of ITCZ position during November 2025 (Fig. 5a). The narrow meridional structure of the ER event is particularly noteworthy. A similar ‘narrow’ ER wave was documented during the formation of TC Seroja in Flores Sea 13 . While the spatial structure of tropical waves active during TC Senyar formation differed from theoretical predictions for equatorial beta-plane modes 49,50 and composite observational structures 51 , their propagation properties remain robust and consistent with canonical equatorial wave dispersion relations. To that end, the ER event propagated westward at a speed consistent with expected ER phase speeds and was subsequently involved in the formation of TC Ditwah near Sri Lanka (Fig. 1a), as evidenced by a vorticity maximum at 80°E on 27 November (Fig. 7a). It is also worth noting that superposition of equatorial waves contributed to enhanced low level convergence and moisture flux convergence over the western MC. For example, on 24 November the ER and MRG circulations contributed anomalous easterlies over South China Sea and Malaysia, while CCKW and MJO brought anomalous westerlier over eastern Indian Ocean, resulting in enhanced convergence over the MS region (Fig. S5-7 in SI). This convergent configuration amplified the moisture supply (Fig. 6), providing the thermodynamic fuel for sustained deep convection. Decomposition of 850 hPa relative vorticity into temporal scales (Fig. 7b) reveals the differential contributions of background, intraseasonal, and transient variability to the observed vorticity evolution. While the background flow supported positive vorticity anomalies over the MS through preTCG and TCG periods, through enhanced meridional shear of horizontal winds (Fig. 4c), the sub-intraseasonal and transient variability were the critical drivers of vorticity amplification. The second CCKW event projects well onto transient time scale during TCG period, marking the final spin-up of TC Senyar, while sub-intraseasonal time scale variability is dominated by ER. Periods of MRG northerly anomalies concide with negative contribution of transient time scale to observed vorticity evolution. Hence, a simple temporal decomposition provides a useful diagnosis of large-scale processes and their contribution to observed multi-scale interactions. The dominance of sub-intraseasonal and transient variability during the critical 24–25 November period (Fig. 7b) confirms that equatorial wave activity, rather than seasonal mean conditions, was the proximate driver of TC Senyar’s explosive intensification. Discussion The formation of NETC Senyar at 4.5°N challenges conventional paradigms of tropical cyclogenesis, which emphasize the necessity of sufficient planetary vorticity to organize convection into a coherent mesoscale vortex 2,3,16 . Our analysis reveals that Senyar's genesis resulted from a remarkable convolution of multi-scale atmospheric processes. This multi-scale interaction framework offers a unifying perspective on near-equatorial cyclogenesis that extends beyond a isolated case study. In November 2025, the concurrent La Niña and negative IOD conditions led to positive SST anomalies (Fig. 4a), enhanced moisture availability (Fig. 5b,c), suppressed vertical wind shear (Fig. 5e) and intensified the Walker circulation over the eastern Indian Ocean 22,52 , all associated with favorable conditions for enhanced convection 53 . On shorter, subseasonal time scales no single process appears sufficient for Senyar's formation; rather, the precise phasing and spatial superposition of multiple equatorial wave modes created the necessary environment for tropical cyclogenesis to occur. The equatorial waves involved in Senyar's genesis exhibited non-canonical spatial structures, with northward-displaced waveguides and reduced meridional scales compared to theoretical expectations 48–50 , consistent with northward shift of ITCZ (Fig. 5a). As a result, the meridional distance between equatorial westerlies and NE monsoon easterlies was smaller than typical (Fig. 5e), resulting in a strong meridional gradient of the zonal wind (∂u/∂y) 54 , which constituted substantial background relative vorticity over the western MC region (Fig. 2), which overcame weak planetary vorticity (f ≈ 1.2×10⁻⁵ s⁻¹ at 4.7°N). The vorticity budget analysis (Fig. 8a) provides a quantitative confirmation of these multi-scale contributions to evolution of pre-Senyar disturbance in the MS region. Vorticity tendency during cyclogenesis was dominated by stretching (-ζD) and zonal advection of relative vorticity (-u∂ x ζ). While the former remains positive and increases throughout the preTCG and TCG periods, the latter compensates it throughout the preTCG phase. However, on 23 November the magnitude of the zonal advection of relative vorticity starts rapidly decreasing, which means that vortex stretching becomes unbalanced and facilitates rapid growth of relative vorticity tendency. Interestingly, the vertical advection (-ω∂ p ζ) is rapidly increasing since midnight on 25 November, indicating that TC Senyar became a self-sustained system before its official genesis according to IBTrACs (Fig. 1). Thus, the dynamical framework confirms our phenomenological analysis. Temporal decomposition reveals that the evolution of vortex stretching of relative vorticity was driven by diverse factors throughout the preTCG and TCG periods. While background and sub-intraseasonal divergence (related to LaNina/IOD/MJO and ER, respectively) acting on sub-intraseasonal (ER) vorticity provided positive contribution through these periods, the variability was driven by transient divergence of sub-intraseasonal vorticity (on 23-24 November) and a combination of sub-intraseasonal and transient divergence of transient vorticity (25 November). Hence, the low-level convergence of a compound effect of MRG and CCKW (Fig. 7c) acting on ER vorticity drove initial vorticity increase during TCG period, while the final rapid increase in the magnitude of the full vortex stretching term was caused by the divergence acting on CCKW vorticity (Fig. 7a). The zonal advection of relative vorticity (Fig. 8c) was dominated by the zonal advection (across all temporal scales) of sub-intraseasonal (ER-related) vorticity. Specifically, the sub-intraseasonal advection of sub-intraseasonal vorticity became positive on 24 November, allowing the full term to become near-neutral, leading to imbalance with vortex stretching and development of a self-sustained system. This quantitative assessment confirms that TC Senyar development was driven by multi-scale dynamical interactions, wherein multiple drivers across distinct spatio temporal scales acted synergistically to foster cyclogenesis. While the predominance of dynamics over thermodynamics in NETC formation has been well established 5,24 , the critical ingredients enabling TC Senyar genesis comprised: Large meridional shear of zonal winds between NE monsoon easterlies and equatorial westerlies A ‘narrow’ ER creating protective pouch for initial disturbance in moisture rich environment A sequence of tropical waves, each providing successive vorticity pulses that amplified the nascent disturbance A strong and narrow CCKW event, uniquely positioned to deliver the final amplification through vortex stretching mechanisms While oservational framework alone cannot definitively establish which combination of these ingredients constituted necessary and sufficient conditions for TC Senyar development, this study provides a robust foundation for targeted numerical experiments designed to address this question. Furthermore, each ingredient is independently trackable through operational monitoring systems. Consequently, the present analysis framework can be leveraged to quantify the frequency of such compound favorable conditions in the current climate and project changes in their occurrence under future warming scenarios. Additionally, these ingredients can be operationally monitored to enhance forecasting skill for NETC events similar to Senyar. This operational application is particularly critical: despite TC Senyar's modest intensity, its societal impacts including devastating flooding and landslides across Aceh and North Sumatra regions with limited tropical cyclone experience and underdeveloped early warning infrastructure, were substantial. As climate change potentially modifies equatorial cyclone characteristics and spatial distribution 55 , these vulnerable communities face escalating risks, necessitating improved forecasting capabilities. Such advances can only be achieved through deeper mechanistic understanding of NETC genesis pathways. Such advances can only be achieved through deeper mechanistic understanding of NETC genesis pathways. Comparison with previous NETC cases in the MC region shows how different process combinations can lead to cyclogenesis. While TC Seroja (2021) 13 was not technically classified as a NETC (Fig. 1), it formed from a seed disturbance satisfying NETC criteria. Critically, its final intensification occurred over the Flores Sea through an interaction of a meridionally narrow ER, which fostered a developing vortex within a protective pouch, with an intense CCKW propagating within a poleward-displaced (southward-shifted) equatorial waveguide 13 . However, TC Seroja formed under different monsoonal dynamics configurations, suggesting that the ER wave pouch mechanism may represent a common pathway, the specific multi-scale process combinations vary substantially across cases. Typhoon Vamei (2001) 10,15 formed in the MC region during anomalously strong northeasterly cold surge conditions, wherein the interaction between surge-related flow and a quasi-stationary Borneo vortex generated critical cyclonic vorticity reservoir 56 . The MJO played played a comparatively minor role, and explicit equatorial wave superposition was not documented. Nevertheless, the fundamental physical mechanism underlying Vamei's rapid intensification, namely the interaction between propagating flow characterized by strong horizontal shear and a semi-stationary sub-intraseasonal Borneo vortex, bears striking similarity to the processes demonstrated here for TC Senyar. The contrasts among Vamei, Seroja, and Senyar collectively demonstrate that NETC genesis emerges through multiple distinct pathways, each involving different combinations of independently trackable atmospheric processes, yet all requiring compensating mechanisms to overcome the suppressive effect of weak planetary vorticity near the equator 2,16 . These findings carry profound implications for understanding NETC behavior under climate change. Theoretical and modeling studies suggest equatorial wave activity, particularly MJO amplitude, may intensify under greenhouse warming 57 , potentially increasing favorable multi-scale superposition events. Projected SST warming in the MC would enhance thermodynamic potential for convective organization and cyclogenesis, while IOD and ENSO teleconnections may fundamentally alter the seasonal and spatial distribution of favorable conditions 58 . Whether these factors increase NETC frequency remains uncertain and demands rigorous investigation, but the process-based diagnostic framework established here provides a robust foundation for targeted climate modeling studies and mechanistic attribution analyses. These findings advance fundamental understanding of NETC genesis mechanisms while simultaneously providing actionable pathways for improving prediction systems and enhancing disaster preparedness in one of the world's most densely populated and climatically vulnerable regions. Methods Reanalysis and Satellite data European Centre for Medium-Range Weather Forecasts ECMWF Fifth Generation Reanalysis ERA5 data 59 , including zonal and meridional winds, vertically integrated moisture divergence, relative vorticity, specific humidity, and sea surface temperature, were obtained for the period 1 January 1995 to 31 December 2025. The data are provided on a regular 0.25° × 0.25° latitude–longitude grid at multiple pressure levels with hourly temporal resolution. In addition to ERA5 data, monthly rainfall data from GSMaP_Gauge Ver.8 (standard with gauge calibration, Version 8) 60 , for the period 1999–2025 were also used. These data were employed to examine the position of the ITCZ. Measurement of key atmospheric parameters Key atmospheric variables were analyzed to characterize the dynamical and thermodynamical environment of cyclogenesis. Low-level relative vorticity at 850 hPa was used to diagnose the intensification of cyclonic circulation in the lower troposphere. The 850 hPa level was selected as the standard for low-level vorticity analysis in tropical cyclone genesis studies because it samples the lower troposphere above the planetary boundary layer and is commonly used in genesis potential indices 61 . Mean sea level pressure (MSLP) was analyzed to track the development and evolution of the cyclonic pressure center. In addition, vertically integrated moisture flux from the surface to 200 hPa was calculated to quantify moisture transport pathways and convergence supporting convective organization. The vertically integrated moisture flux was computed as the mass-weighted vertical integral of specific humidity times horizontal wind, and moisture flux convergence was obtained from the horizontal divergence of vertically integrated moisture flux 62,63 . This diagnostic is widely used to identify moisture sources and assess convective organization during tropical cyclone genesis 62,63 . To quantify the influence of regional wind variability on cyclogenesis, low-level winds at 850 hPa were spatially averaged over three key domains: the westerly wind burst (WWB) region over the eastern Indian Ocean, the northeast monsoon (NEM) region over the South China Sea associated with winter monsoon surges, and the Strait of Malacca domain, which represents meridional momentum and moisture transport through this important maritime corridor (Fig. 1b). These diagnostics were then linked to sea surface thermodynamic conditions to evaluate the role of the background environment during Senyar’s formation. A November sea surface temperature (SST) climatology for 1995–2024 was calculated, and anomalies were defined as deviations from this 30-year November mean. To examine the thermodynamic and dynamical structure of the atmosphere, a combined field of sub-intraseasonal and background variability (periods >5 days) was constructed. Low-level specific humidity (1000–850 hPa average) was analyzed together with sub-intraseasonal 850 hPa horizontal winds to identify regions of convergence and column moistening. Areas where moist layers with specific humidity exceeding 15 g kg⁻¹ extended above the 850 hPa level were identified using contour analysis. Vertical structure was further evaluated using profiles of specific humidity and wind direction, with climatological profiles (1995–2024) compared to daily profiles during November 2025 to assess departures from typical conditions. The temporal evolution of vertical wind shear magnitude was calculated using both climatological values (1995–2025) and November 2025 data, including the 200–800 hPa shear difference and the maximum shear along the vertical profile. Together, these diagnostics were used to characterize the environmental conditions associated with the formation of TC Senyar. As a large-scale context, the climatological (1999–2024) and November 2025 positions of the Intertropical Convergence Zone (ITCZ) were determined using monthly precipitation from GSMaP_Gauge Version 8 and a precipitation centroid method 64 , defined as the precipitation-weighted mean latitude within the 20°S–20°N band. This approach provides a quantitative estimate of ITCZ displacement, which was then compared with low-level wind patterns and convective activity during the period of Senyar’s formation. Temporal decomposition of meteorological fields Relatively simple temporal decomposition of meteorological fields allows separating atmospheric variability into physically meaningful time scales when driving mechanisms differ in that respect. A chain of events that led to genesis of TC Senyar took about 10 days. Hence decomposition into three time scales has been employed. The background time scale (variability with periods of 20 days or longer) encompasses processes such as low frequency IOD/La Nina state and MJO circulation. The sub-intraseasonal time scale (periods of 5 – 20 days) includes variability associated with ER activity as well as progression of NE monsoon. The transient time scale (periods shorter than 5 days) is primarily driven by CCKW and MRG activity. Before decomposition, all data were smoothed with 24-hour, centered running average to remove high frequency variability and a mean state (defined here as a November 2025 average) has been removed from each field. Next, a 5-day centered running mean and a 20-day centered running mean were calculated. The transient component was obtained by subtracting the 5-day running mean from the smoothed anomaly timeseries, the sub-intraseasonal component was computed as the difference between the 5-day and 20-day running means, and the background variability was represented by the 20-day running mean itself. As a result, this simple decomposition provided useful way to identify key processes, while keeping residual small and suitable for vorticity tendency budget calculations. Calculation of daily anomalies and equatorial wave filtering The temporal decomposition of wind and vorticity fields, together with the spatial structure of sub-intraseasonal circulation and the evolution of moisture transport patterns, provides strong evidence that propagating equatorial waves contributed critically to the formation of TC Senyar. However, temporal filtering alone cannot definitively establish propagation characteristics. Therefore, the specific wave types present during the preTCG and TCG periods are identified, their amplitudes and propagation characteristics are quantified, and it is demonstrated how their constructive superposition generated the dynamical and thermodynamic conditions necessary for the formation of TC Senyar. Identify of convectively coupled equatorial waves and MJO activity was performed through the space-time spectral filtering technique 51,65 , applied to ERA5 horizontal winds and vorticity at 850 hPa. Filtering was performed based on one full your of data (calendar year 2025) and each variable was filtered in zonal wavenumber and frequency for all latitudes in meridional band 20°S – 20°N after meridional smoothing with 3°-wide, centered running mean that accounts for possible meridional propagation of a signal. Filtering procedure retained only part of the spectrum corresponding to a given mode. Standard filtering parameters for each mode included equivalent depths of 8-90m with wave-specific period-wavenumber combinations for CCKWs (wavenumber 1-14, period 2.5-30 days), MJO (wavenumber 1-14, period 30-96 days), ER (wavenumber −1 to −10, period 9.7-48 days) and MRG (wavenumber −1 to −10, period 3-96 days) 51 . Waves’ activity was determined if a filtered respective variable-dependent thresholds, given by a standard deviation of filtered signal averaged over the equatorial belt (0-360°E, 5°S-5°N). Thresholds for 850 hPa horizontal winds are as follows: [1.07 ms -1 , 0.47 ms -1 ] for CCKW, [0.97 ms -1 ,0.30 ms -1 ] for MJO, [0.94 ms -1 , 0.72 ms -1 ] for ER and [0.67 ms -1 , 0.72 ms -1 ] for MRG (values in square brackets indicate threshold in zonal and meridional wind respectively), reflecting spatial structures of those modes 51 . Hence, activity of CCKW, MJO and ER is diagnosed using zonal wind, while activity of MRG using meridional wind (Fig. 7). Full, filtered structures of CCKW, MJO, ER and MRG (Fig. S5-7 in SI) show that modes active during exhibited non-canonical spatial structures with northward shifted waveguides and relatively small meridional extent (large meridional wavenumber). Therefore, filtered signals have been meridionally averaged within Eq-5°N, which best depict specific modes’ activity in the region critical for TC Senyar formation. Vorticity budget calculations A vorticity budget has been analyzed based on vorticity equation in Cartesian coordinates (1). ζ represents relative vorticity, x and y coordinates in eastward and northward direction. The left hand side of (1) is relative vorticity tendency, while terms on the right side represent zonal advection, meditional advection and vertical (in pressure coordinates) of relative vorticity, meridional advection of planetary vorticity vortex stretching of relative and planetary vorticity (where is divergence) and residual (which includes a tilting/twisting term as well as unresolved and parameterized processes). Budget calculation has beed done based on smoothed (24-hour running average) hourly ERA5 data. A relative contribution of a given term to the observed tendency has been quantified using signed explained covariance per band Additionally, specific terms have been temporally decomposed. Since we account for three temporal frequencies (background -B, sub-intraseasonal -SI and transient -T) and each term is a product of two variables, a decomposition yields 9 terms, each quantified also using signed explained covariance (here covariance between a decomposed and a full term is normalized by full term’s variance). All vorticity budget terms have been spatially averaged over the MS box, while contributions were calculated based on 21-25 November, a critical period of preTCG and TCG when pre-Senyar vortex was strengthening in the MS region. Hence, the budget calculation provide a quantitative assessment of key drivers of that process. Trajectory of TC Senyar precursor Accodring to IBTrACS version 4 4 , genesis of TC Senyar occurred at 12 UTC on 25 November, when the cyclone center was located in the Malacca Strait. To identify and track the precursor of TC Senyar prior to this time, a backward-tracking approach was applied using ERA5 relative vorticity at the 850-hPa level, consistent with tracking of TC Seroja precursor 13 . The backward trajectory was initialized at the first reported position of TC Senyar. The local maximum in relative vorticity was then identified backward in time within a 1° × 1° search box centered on the previous location. The newly detected vorticity maximum was used as the updated position, and the procedure was iteratively repeated at hourly intervals. Using this approach, the trajectory of the TC Senyar precursor (preSenyar) was reconstructed (Fig. 1c). PreSenyar trajectory based on vorticity at 850 hPa has been confirmed by an ensemble of trajectories derived based on vorticity on multiple levels in the atmosphere, by ranging from 800 hPa up to 300 hPa, each treated independently (Fig. S2 in SI). Declarations Contributions M.M. designed the study. M.M and D.B.B. contributed to conceptualization. M.M., M.B., R.R., N.R.A. and D.B.B. performed formal analysis, investigation, and visualization. C.S. and D.B.B. performed spatio-temporal filtering. M.B. developed the Supplementary Information. All authors discussed the results. M.M., M.B. and D.B.B. wrote draft manuscript. All authors reviewed and edited the manuscript. Data Availability Tropical cyclone tracks were obtained from NOAA’s International Best Track Archive for Climate Stewardship (IBTrACS) via the NCEI website (https://www.ncdc.noaa.gov/ibtracs). ERA5, the fifth generation of ECMWF atmospheric reanalysis of the global climate, was obtained from the Copernicus Climate Change Service Climate Data Store (https://cds.climate.copernicus.eu). The GSMaP precipitation data were provided by the Japan Aerospace Exploration Agency (JAXA) and are available at https://hokusai.eorc.jaxa.jp after registration at https://www.eorc.jaxa.jp/ptree/index.html. Code Availability The Python codes used for data processing are available from the corresponding author, Marzuki Marzuki ( [email protected] ) or Dariusz B. Baranowski, [email protected] . Acknowledgements (optional) M.M acknowledges support from the Ministry of Higher Education, Science, and Technology of Indonesia and Universitas Andalas. D.B.B. and M.B. were supported by National Science Centre (NCN) of Poland (grant no. 2022/45/B/ST10/03836). Ethics declarations Competing interests The authors declare no competing interests. References BNPB. Emergency Response Dashboard for Floods and Landslides: Aceh, North Sumatra, and West Sumatra Provinces. https://gis.bnpb.go.id/bansorsumatera2025/ (2026). Gray, W. M. Global view of the origin of tropical disturbances and storms. Mon. Weather Rev. 96 , 669–700 (1968). Anthes, R. A. Tropical Cyclones. Their Evolution, Structure and Effects . (American Meteorological Society, Boston, 1982). Knapp, K. R., Kruk, M. C., Levinson, D. H., Diamond, H. J. & Neumann, C. J. The International Best Track Archive for Climate Stewardship (IBTrACS): Unifying Tropical Cyclone Data. Bull. Am. Meteorol. Soc. 91 , 363–376 (2010). Deng, L. & Li, T. Impact of background dynamic and thermodynamic states on distinctive annual cycle of near-equatorial tropical cyclogenesis over the western North Pacific. J. Meteorol. Res. 34 , 822–835 (2020). Li, Y., Li, T., Fu, C. & Hsu, P.-C. Near-equatorial tropical cyclone formation in western North Pacific: peak season and controlling parameter. Clim. Dyn. 52 , 2765–2773 (2019). Lu, C., Ge, X. & Peng, M. Comparison of controlling parameters for near-equatorial tropical cyclone formation between western North Pacific and North Atlantic. J. Meteorol. Res. 35 , 623–634 (2021). Fine, C. M., Johnson, R. H., Ciesielski, P. E. & Taft, R. K. The role of topographically induced vortices in tropical cyclone formation over the Indian Ocean. Mon. Weather Rev. 144 , 4827–4847 (2016). Johnson, P., Ciesielski, P., Fine, C. & Wang, C. Effects of the topography of Sumatra on tropical cyclone formation over the Indian Ocean. Mausam 74 , 389–396 (2023). Chang, C., Liu, C. & Kuo, H. Typhoon Vamei: An equatorial tropical cyclone formation. Geophys. Res. Lett. 30 , (2003). Juneng, L., Tangang, F. T., Reason, C. J. C., Moten, S. & Hassan, W. A. W. Simulation of tropical cyclone Vamei (2001) using the PSU/NCAR MM5 model. Meteorol. Atmos. Phys. 97 , 273–290 (2007). MET Malaysia. Onset of the Northeast Monsoon 2025/2026 . https://www.met.gov.my/assets/content/siaran_media/default/10 November 2025 - Kenyataan Media Permulaan Monsun Timur Laut 2025_2026.pdf (2025). Latos, B. et al. The role of tropical waves in the genesis of Tropical Cyclone Seroja in the Maritime Continent. Nat. Commun. 14 , 856 (2023). Rajasree, V. P. M. et al. Tropical cyclogenesis: Controlling factors and physical mechanisms. Trop. Cyclone Res. Rev. 12 , 165–181 (2023). Chambers, C. R. S. & Li, T. Simulation of formation of a near-equatorial typhoon Vamei (2001). Meteorol. Atmos. Phys. 98 , 67–80 (2007). Steenkamp, S. C., Kilroy, G. & Smith, R. K. Tropical cyclogenesis at and near the Equator. Q. J. R. Meteorol. Soc. 145 , 1846–1864 (2019). Sall, S. M., Sauvageot, H., Gaye, A. T., Viltard, A. & De Felice, P. A cyclogenesis index for tropical Atlantic off the African coasts. Atmos. Res. 79 , 123–147 (2006). Zhao, C. & Li, T. Basin dependence of the MJO modulating tropical cyclone genesis. Clim. Dyn. 52 , 6081–6096 (2019). Wei, M., Fang, G. & Ge, Y. Tropical cyclone genesis prediction based on support vector machine considering effects of multiple meteorological parameters. J. Wind Eng. Ind. Aerodyn. 243 , 105591 (2023). Camargo, S. J. & Sobel, A. H. Western North Pacific tropical cyclone intensity and ENSO. J. Clim. 18 , 2996–3006 (2005). Wang, B. & Chan, J. C. L. How strong ENSO events affect tropical storm activity over the western North Pacific. J. Clim. 15 , 1643–1658 (2002). Saji, N. H., Goswami, B. N., Vinayachandran, P. N. & Yamagata, T. A dipole mode in the tropical Indian Ocean. Nature 401 , 360–363 (1999). Cai, W. et al. Projected response of the Indian Ocean Dipole to greenhouse warming. Nat. Geosci. 6 , 999–1007 (2013). Kilroy, G., Smith, R. K. & Montgomery, M. T. A unified view of tropical cyclogenesis and intensification. Q. J. R. Meteorol. Soc. 143 , 450–462 (2017). Emanuel, K. A. Sensitivity of tropical cyclones to surface exchange coefficients and a revised steady-state model incorporating eye dynamics. J. Atmos. Sci. 52 , 3969–3976 (1995). Emanuel, K. A. An air-sea interaction theory for tropical cyclones. Part I: Steady-state maintenance. J. Atmos. Sci. 43 , 585–605 (1986). Mrowiec, A. A., Garner, S. T. & Pauluis, O. M. Axisymmetric hurricane in a dry atmosphere: Theoretical framework and numerical experiments. J. Atmos. Sci. 68 , 1607–1619 (2011). Cronin, T. W. & Chavas, D. R. Dry and semidry tropical cyclones. J. Atmos. Sci. 76 , 2193–2212 (2019). Nolan, D. S. What is the trigger for tropical cyclogenesis? Aust. Meteorol. Mag. 56 , 241–266 (2007). Raymond, D. J. & Sessions, S. L. Evolution of convection during tropical cyclogenesis. Geophys. Res. Lett. 34 , (2007). Gu, J.-F., Tan, Z.-M. & Qiu, X. Effects of vertical wind shear on inner-core thermodynamics of an idealized simulated tropical cyclone. J. Atmos. Sci. 72 , 511–530 (2015). Rappin, E. D., Nolan, D. S. & Emanuel, K. A. Thermodynamic control of tropical cyclogenesis in environments of radiative‐convective equilibrium with shear. Q. J. R. Meteorol. Soc. 136 , 1954–1971 (2010). Kutty, G. & Gohil, K. The role of mid-level vortex in the intensification and weakening of tropical cyclones. J. Earth Syst. Sci. 126 , 94 (2017). Dunkerton, T. J., Montgomery, M. T. & Wang, Z. Tropical cyclogenesis in a tropical wave critical layer: Easterly waves. Atmos. Chem. Phys. 9 , 5587–5646 (2009). Bell, M. M. & Montgomery, M. T. Observed structure, evolution, and potential intensity of category 5 Hurricane Isabel (2003) from 12 to 14 September. Mon. Weather Rev. 136 , 2023–2046 (2008). Rajasree, V. P. M., Routray, A., George, J. P., Kumar, S. & Kesarkar, A. P. Study of cyclogenesis of developing and non-developing tropical systems of NIO using NCUM forecasting system. Meteorol. Atmos. Phys. 133 , 379–397 (2021). Wu, S. & Fang, J. The evolution and role of midtropospheric cyclonic vortex in the formation of Super Typhoon Nepartak (2016). J. Geophys. Res. Atmos. 124 , 9277–9298 (2019). Schreck III, C. J., Molinari, J. & Aiyyer, A. A global view of equatorial waves and tropical cyclogenesis. Mon. Weather Rev. 140 , 774–788 (2012). Klotzbach, P. J. The Madden–Julian oscillation’s impacts on worldwide tropical cyclone activity. J. Clim. 27 , 2317–2330 (2014). Zhang, C. Madden–Julian oscillation: Bridging weather and climate. Bull. Am. Meteorol. Soc. 94 , 1849–1870 (2013). Ventrice, M. J., Thorncroft, C. D. & Schreck III, C. J. Impacts of convectively coupled Kelvin waves on environmental conditions for Atlantic tropical cyclogenesis. Mon. Weather Rev. 140 , 2198–2214 (2012). Lawton, Q. A. & Majumdar, S. J. Convectively coupled Kelvin waves and tropical cyclogenesis: Connections through convection and moisture. Mon. Weather Rev. 151 , 1647–1666 (2023). Lawton, Q. A., Majumdar, S. J., Dotterer, K., Thorncroft, C. & Schreck III, C. J. The influence of convectively coupled Kelvin waves on African easterly waves in a wave-following framework. Mon. Weather Rev. 150 , 2055–2072 (2022). Schreck III, C. J. Convectively coupled Kelvin waves and tropical cyclogenesis in a semi-Lagrangian framework. Mon. Weather Rev. 144 , 4131–4139 (2016). Wang, S., Fang, J., Tang, X. & Tan, Z.-M. A survey of statistical relationships between tropical cyclone genesis and convectively coupled equatorial rossby waves. Adv. Atmos. Sci. 39 , 747–762 (2022). Matthews, A. J. Dynamical propagation and growth mechanisms for convectively coupled equatorial Kelvin waves over the Indian Ocean. Q. J. R. Meteorol. Soc. 147 , 4310–4336 (2021). Tory, K. J., Dare, R. A., Davidson, N. E., McBride, J. L. & Chand, S. S. The importance of low-deformation vorticity in tropical cyclone formation. Atmos. Chem. Phys. 13 , 2115–2132 (2013). Kiladis, G. N. & Wheeler, M. Horizontal and vertical structure of observed tropospheric equatorial Rossby waves. J. Geophys. Res. Atmos. 100 , 22981–22997 (1995). Matsuno, T. Quasi-Geostrophic Motions in the Equatorial Area. J. Meteorol. Soc. Japan. Ser. II 44 , 25–43 (1966). Yang, G.-Y., Hoskins, B. & Slingo, J. Convectively coupled equatorial waves. Part III: Synthesis structures and their forcing and evolution. J. Atmos. Sci. 64 , 3438–3451 (2007). Kiladis, G. N., Wheeler, M. C., Haertel, P. T., Straub, K. H. & Roundy, P. E. Convectively coupled equatorial waves. Rev. Geophys. 47 , (2009). Roose, S., Ajayamohan, R. S., Ray, P., Mohan, P. R. & Mohanakumar, K. ENSO influence on Bay of Bengal cyclogenesis confined to low latitudes. npj Clim. Atmos. Sci. 5 , 31 (2022). Baranowski, D. B., Flatau, M. K., Flatau, P. J. & Schmidt, J. M. Multiple and spin off initiation of atmospheric convectively coupled Kelvin waves. Clim. Dyn. 49 , 2991–3009 (2017). G. TAKAHASHI, H., Fukutomi, Y. & Matsumoto, J. The impact of long-lasting northerly surges of the East Asian winter monsoon on tropical cyclogenesis and its seasonal march. 気象集誌 . 第 2 輯 89 , 181–200 (2011). Studholme, J., Fedorov, A. V, Gulev, S. K., Emanuel, K. & Hodges, K. Poleward expansion of tropical cyclone latitudes in warming climates. Nat. Geosci. 15 , 14–28 (2022). Chang, C. P., Harr, P. A. & Chen, H.-J. Synoptic disturbances over the equatorial South China Sea and western Maritime Continent during boreal winter. Mon. Weather Rev. 133 , 489–503 (2005). Christensen, J. H. et al. Climate phenomena and their relevance for future regional climate change. in Climate change 2013 the physical science basis: Working group I contribution to the fifth assessment report of the intergovernmental panel on climate change 1217–1308 (Cambridge University Press, 2013). Cai, W. et al. ENSO and greenhouse warming. Nat. Clim. Chang. 5 , 849–859 (2015). Hersbach, H. et al. The ERA5 Global Reanalysis. Q. J. R. Meteorol. Soc. 146 , 1999–2049 (2020). Ramadhan, R. et al. A preliminary assessment of the GSMaP Version 08 products over Indonesian maritime continent against gauge data. Remote Sens. 15 , 1115 (2023). Wang, B. & Moon, J.-Y. An anomalous genesis potential index for MJO modulation of tropical cyclones. J. Clim. 30 , 4021–4035 (2017). Fritz, C. & Wang, Z. Water vapor budget in a developing tropical cyclone and its implication for tropical cyclone formation. J. Atmos. Sci. 71 , 4321–4332 (2014). Sun, Z., Gao, S. & Jian, M. Comparison of the Water Vapor Budget Evolution of Developing and Non-Developing Disturbances over the Western North Pacific. Remote Sens. 16 , 2396 (2024). Donohoe, A., Marshall, J., Ferreira, D. & Mcgee, D. The relationship between ITCZ location and cross-equatorial atmospheric heat transport: From the seasonal cycle to the Last Glacial Maximum. J. Clim. 26 , 3597–3618 (2013). Wheeler, M. & Kiladis, G. N. Convectively coupled equatorial waves: Analysis of clouds and temperature in the wavenumber–frequency domain. J. Atmos. Sci. 56 , 374–399 (1999). Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryInformation0903submit.docx Supplementary Information Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9075294","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":605165184,"identity":"93a0b944-500d-47c9-ae9e-25cbf0cc4475","order_by":0,"name":"Marzuki Marzuki","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA60lEQVRIiWNgGAWjYNCCAxJyDAyMDVAOiDBgJqjFmGQtDIkNSBwQwK1Ft/3s4888ZyzS57cfbmDm3WGXx3eA+eEHhgJrnFrMzqQbGPPckMjdcCYRqOVMcrHkATZjCQaDdNxaDqQxJPN8AGqRYARqaWNO3HCAwQzol8O4tZx/xnAYqCVdfgZYSz1QC/s3/FpupDE2Ax2WwHADrOUwUAsPAVtuPGNmnHNGwhDkl4Nz244nzjzMUyyRgM8v59OYP7w5Vicv33784YO3bdWJfcfbN3748Ad3iKGAA2ASFCMJxGkYBaNgFIyCUYADAACRt1SXD3fxCgAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0003-0266-812X","institution":"Universitas Andalas","correspondingAuthor":true,"prefix":"","firstName":"Marzuki","middleName":"","lastName":"Marzuki","suffix":""},{"id":605165185,"identity":"7aec4387-6aa4-462f-b751-091afb5ebe6f","order_by":1,"name":"Michal Brennek","email":"","orcid":"https://orcid.org/0000-0002-3374-3965","institution":"Polish Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Michal","middleName":"","lastName":"Brennek","suffix":""},{"id":605165186,"identity":"ba21b4ac-3de2-4b4f-a6c9-9f0916500f10","order_by":2,"name":"Ravidho Ramadhan","email":"","orcid":"","institution":"Universitas Andalas","correspondingAuthor":false,"prefix":"","firstName":"Ravidho","middleName":"","lastName":"Ramadhan","suffix":""},{"id":605165187,"identity":"53ac9f4d-fc18-45c3-b93a-ea5a46533b2b","order_by":3,"name":"Nadya Ananda","email":"","orcid":"","institution":"Universitas Andalas","correspondingAuthor":false,"prefix":"","firstName":"Nadya","middleName":"","lastName":"Ananda","suffix":""},{"id":605165188,"identity":"05de4ea0-17aa-41e3-a568-85e4098d4dc3","order_by":4,"name":"Fredolin Tangang","email":"","orcid":"","institution":"Universiti Kebangsaan Malaysia","correspondingAuthor":false,"prefix":"","firstName":"Fredolin","middleName":"","lastName":"Tangang","suffix":""},{"id":605165189,"identity":"85dc5430-eba7-4e52-b0b9-9722988fe91a","order_by":5,"name":"Carl Schreck","email":"","orcid":"https://orcid.org/0000-0001-9331-5754","institution":"North Carolina State University","correspondingAuthor":false,"prefix":"","firstName":"Carl","middleName":"","lastName":"Schreck","suffix":""},{"id":605165190,"identity":"246aba24-492b-4036-afbd-df9110755879","order_by":6,"name":"Dariusz Baranowski","email":"","orcid":"https://orcid.org/0000-0001-8848-3884","institution":"Institude of Geophysics Polish Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Dariusz","middleName":"","lastName":"Baranowski","suffix":""}],"badges":[],"createdAt":"2026-03-09 16:22:44","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9075294/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9075294/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":107520395,"identity":"0fdf02c1-ace8-44cb-a785-8034f044943f","added_by":"auto","created_at":"2026-04-22 09:00:58","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":259616,"visible":true,"origin":"","legend":"\u003cp\u003eTropical cyclone tracks and low-level vorticity evolution leading to the formation of TC Senyar.\u003cstrong\u003e a\u003c/strong\u003eTropical cyclone tracks during November 2025, derived from the IBTrACS dataset, with historical reference tracks of Typhoon Vamei (December 2001) and Tropical Cyclone Seroja (April 2021) shown for comparison. The track of Seroja is displayed only partially, extending to 20°S, whereas its complete track reaches beyond 30°S. \u003cstrong\u003eb\u003c/strong\u003e Track of Tropical Cyclone Senyar, also derived from IBTrACS. The box at 0–5°N, 90–95°E over western Sumatra denotes the observation region for WWB, while the box at 6–10°N, 102–107°E over the South China Sea east of Sumatra is used to identify NEM. The box at 4–5.5°N, 98.5–100°E marks the region for monitoring low-level meridional winds over the Strait of Malacca. \u003cstrong\u003ec\u003c/strong\u003e Precursor of TC Senyar (prior to naming), identified from ERA5 relative vorticity at 850 hPa, with data shown at 3-hour intervals. \u003cstrong\u003ed\u003c/strong\u003eTime series of maximum 850-hPa relative vorticity sampled along the trajectories of the precursor (blue) and TC Senyar (orange). Also shown are the maximum relative vorticity within the Strait of Malacca region (black line), together with the maximum 850-hPa wind speed and minimum sea-level pressure (MSLP).\u003c/p\u003e","description":"","filename":"Picture1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9075294/v1/efe951ab8d3ca0ed9cd7abe9.jpg"},{"id":107520397,"identity":"21cfe9e0-dbbc-4531-a6a3-438db00e288d","added_by":"auto","created_at":"2026-04-22 09:00:58","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":407218,"visible":true,"origin":"","legend":"\u003cp\u003eDaily 850-hPa relative vorticity (shaded) overlaid with 850-hPa winds (vectors) for the period 16–25 November 2025, derived from the ERA5 reanalysis, together with the precursor of TC Senyar (yellow dots). The yellow line indicates the complete track of the TC Senyar precursor, smoothed using a 6-hour moving average.\u003c/p\u003e","description":"","filename":"Picture2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9075294/v1/f671673706c49b953fd08d05.jpg"},{"id":107705760,"identity":"1685f1d1-045f-44d3-a39f-41e48fb2894d","added_by":"auto","created_at":"2026-04-24 09:15:06","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":269692,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of 850 hPa zonal wind, its temporal decomposition (top row) and wave-filtered decomposition (bottom row), spatially averaged over Westerly Wind Burst region (\u003cstrong\u003ea\u003c/strong\u003e and \u003cstrong\u003ed\u003c/strong\u003e), NE Monsoon region (\u003cstrong\u003eb\u003c/strong\u003e and \u003cstrong\u003ee\u003c/strong\u003e). \u003cstrong\u003ec\u003c/strong\u003e and \u003cstrong\u003ef\u003c/strong\u003e show 850 hPa meridional winds spatially averaged over Strait of Malacca region. Respective time series are smoothed with 24h, centered running average and shown in black. Temporal decomposition into background (\u0026gt;20 days), sub-intraseasoanl (5-20 days) and transient (\u0026lt;5days) variability is shown in blue, green and red, respectively. Dash-dotted lines indicate November 2025 averaged values. Wave-filtering decomposition features convectively coupled Kelvin waves (Kelvin, black), Madden-Julian oscillations (MJO, purple), equatorial Rossby waves (ER, green) and mixed Rossby-gravity waves (MRG, pink). Shading highlights pre tropical cyclogenesis (preTCG), tropical cyclogenesis (TCG) and post cyclogenesis (postTCG) periods.\u003c/p\u003e","description":"","filename":"Picture3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9075294/v1/f76ef1237a80b2ec812b432e.jpg"},{"id":107520399,"identity":"bb6db31e-6be4-4f0c-b395-b5d5b5fb8315","added_by":"auto","created_at":"2026-04-22 09:00:58","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":159607,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of low-level meridional shear between WWB and NEM regions during November 2025. \u003cstrong\u003ea\u003c/strong\u003eMean SST anomaly for 3–23 November 2025 relative to the 30-year (1995–2024) daily mean SST climatology (shaded), with the 30-year mean 850-hPa wind climatology shown as vectors. \u003cstrong\u003eb\u003c/strong\u003e Mean 850-hPa zonal wind averaged over 19–25 November as a function of latitude, calculated within two regions: 0–5°N, 90–95°E over western Sumatra representing the WWB observation region, and 6–10°N, 102–107°E over the South China Sea east of Sumatra representing the EWB region, as shown in Fig. 1b. \u003cstrong\u003ec\u003c/strong\u003e Meridional gradient of the mean zonal wind (- ∂u/∂y) (left y-axis) and the latitudinal separation between the maxima in the WWB and EWB regions. The pre-cyclogenesis (preTCG), cyclogenesis (TCG), and post-cyclogenesis (postTCG) periods are indicated by shaded bands.\u003c/p\u003e","description":"","filename":"Picture4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9075294/v1/b18f949f1f24ee7abce73cfc.jpg"},{"id":107520400,"identity":"02fdaa2e-1262-4432-a929-49633cab37de","added_by":"auto","created_at":"2026-04-22 09:00:58","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":452987,"visible":true,"origin":"","legend":"\u003cp\u003eITCZ displacement modulates moisture, circulation, and vertical shear during November 2025. \u003cstrong\u003ea \u003c/strong\u003eClimatological (1999–2024) and November 2025 positions of the ITCZ superimposed on November 2025 precipitation from GSMaP and 850-hPa wind anomalies averaged over 16–25 November 2025. Wind anomalies are shown only where their magnitudes exceed one standard deviation. \u003cstrong\u003eb\u003c/strong\u003e A sum of sub-intraseasonal and background (periods \u0026gt;5 days) low level (1000-850 hPa average) specific humidity (shading), sub-intraseasonal horizontal 850 hPa winds (arrows) and areas for which moist layers with specific humidity exceeding 15 g/kg exceed 850 hPa level (red contour). \u003cstrong\u003ec \u003c/strong\u003eSpecific humidity profiles in the MS region: 1995-2024 climatology and days in November 2025. \u003cstrong\u003ed\u003c/strong\u003e Same as \u003cstrong\u003ec\u003c/strong\u003e but for wind speed direction. \u003cstrong\u003ee\u003c/strong\u003e temporal evolution of vertical wind shear magnitude: 1995-2024 climatology (black), 200-800 hPa difference in November 2025 (blue) and maximum difference along a profile in November 2025 (red).\u003c/p\u003e","description":"","filename":"Picture5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9075294/v1/f9f60d7453cfc39daaa356b4.jpg"},{"id":107705710,"identity":"30760a0f-2ded-4125-b184-c52238d83c41","added_by":"auto","created_at":"2026-04-24 09:14:43","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":421359,"visible":true,"origin":"","legend":"\u003cp\u003eDaily mean vertically integrated moisture divergence (shaded) with overlaid moisture-flux vectors. Vectors are shown only where the integrated vapor transport magnitude exceeds 200 kg m⁻¹ s⁻¹.\u003c/p\u003e","description":"","filename":"Picture6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9075294/v1/d958fb9097f26985241663d6.jpg"},{"id":107705750,"identity":"b40dc543-1076-4bdf-b454-dc1f8e4de213","added_by":"auto","created_at":"2026-04-24 09:14:59","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":200664,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of 850-hPa relative vorticity and equatorial wave activity associated with the pre-cyclogenesis of TC Senyar.\u003cstrong\u003e a\u003c/strong\u003e Time–longitude evolution of 850-hPa relative vorticity (shaded;\u0026nbsp; x 10\u003csup\u003e-5\u003c/sup\u003e s\u003csup\u003e-1\u003c/sup\u003e) averaged over the equatorial band (Eq.–10°N) during November–early December 2025. Superimposed are the tracks of equatorial wave signals identified from zonal wind anomalies, including convectively coupled Kelvin waves (CCKWs) (black), Madden–Julian Oscillation (MJO) (purple), equatorial Rossby (ER) waves (green), and mixed Rossby–gravity (MRG) waves (magenta). The timing of TC Senyar cyclogenesis from IBTrACS is marked by the cyan circle. \u003cstrong\u003eb\u003c/strong\u003e Decomposition of 850-hPa relative vorticity along the precursor trajectory, shown for all data, and decomposed to background (\u0026gt;20 days), sub-intraseasonal (5–20 days), and transient (\u0026lt;5 days). Shaded bands denote the pre-cyclogenesis, cyclogenesis, and post-cyclogenesis periods. \u003cstrong\u003ec\u003c/strong\u003e Amplitudes of CCKW, MJO, ER, and MRG modes at 99.5E based on panel \u003cstrong\u003ea\u003c/strong\u003e but normalized by standard deviation, with dashed vertical lines indicating detection thresholds. Shaded bands are the same as in panel \u003cstrong\u003eb\u003c/strong\u003e.\u003c/p\u003e","description":"","filename":"Picture7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9075294/v1/060c7f3c8a723a3d764064dd.jpg"},{"id":107705810,"identity":"fa111f35-c689-4969-97aa-524773153f2a","added_by":"auto","created_at":"2026-04-24 09:15:22","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":258373,"visible":true,"origin":"","legend":"\u003cp\u003eVorticity budget over MS region during TC Senyar genesis.\u003cstrong\u003e a\u003c/strong\u003e Vorticity tendency budget, including vorticity tendency, zonal advection of vorticity, meridional advection of vorticity, vertical advection of vorticity, medirional advection of planetary vorticity, vortex stretching of relative vorticity, vortex stretching of planetary vorticity and a sum of other contributions, which includes twisting/tilting term as well as all unresolved, subscale processes. \u003cstrong\u003eb\u003c/strong\u003evortives stretching of relative vorticity temporarily decomposed into the leading terms. \u003cstrong\u003ec\u003c/strong\u003e zonal advection of relative vorticity temporarily decomposed into the leading terms. Numbers in brackets indicate relative contribution of each term.\u003c/p\u003e","description":"","filename":"Picture8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9075294/v1/bf4d063edaed2c5c2c71927f.jpg"},{"id":107710211,"identity":"1225ee28-9886-479e-84db-47a0b3aa70a2","added_by":"auto","created_at":"2026-04-24 09:40:10","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2847695,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9075294/v1/c98917bd-cec3-4b85-adfa-5c04827deb56.pdf"},{"id":107705328,"identity":"12b55ab8-f10f-42a2-9a8b-0d60d9ee12b6","added_by":"auto","created_at":"2026-04-24 09:11:32","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":11166510,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"SupplementaryInformation0903submit.docx","url":"https://assets-eu.researchsquare.com/files/rs-9075294/v1/d7e2f20654d5263da95f276c.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Multi-Scale Genesis Pathway for the Formation of the Near- Equatorial Tropical Cyclone Senyar","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe Tropical Cyclone (TC) Senyar formed at 12 UTC on 25 November 2025 in the Strait of Malacca (4.7\u0026deg;N, 99\u0026deg;E) (Fig. 1a), triggering catastrophic flooding and landslides in parts of Indonesia and Malaysia during 25-27 November 2025. In Indonesia, the impacts of TC Senyar were severe, including 1,207 fatalities, the displacement of up to 1,057,450 people at its peak on 8 December 2025, the destruction \u0026nbsp;of 860 bridges and damage to about 301,012 housing units across 53 districts, with preliminary recovery costs estimated at IDR 59.25 trillion (approximately USD 3.7 billion)\u003csup\u003e1\u003c/sup\u003e. Loss and damage was amplified by TC Senyar\u0026rsquo;s slow translation speed, which produced prolonged extreme rainfall, and lack of advance warning for an area rarely affected by tropical cyclones. Therefore, understanding of the physical processes leading up to the formation of TC Senyar is crucial for improving forecast capabilities and disaster preparedness in densely populated tropical coastal regions.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTC Senyar is categorized as a near equatorial tropical cyclones (NETC) as it formed close to the equator, a region traditionally considered TC-free due to low planetary vorticity (f \u0026asymp; 1.2\u0026times;10⁻⁵ s⁻\u0026sup1; at 4.7\u0026deg;N) \u003csup\u003e2,3\u003c/sup\u003e. Analysis of the International Best Track Archive for Climate Stewardship (IBTrACS)\u003csup\u003e4\u003c/sup\u003e, \u0026nbsp; reveals that since 1980, only 278 NETC (forming within 6\u0026deg; of the equator) have been recorded globally, less than 6% of total global TC activity, and all occurred in the Indo-Pacific domain (see Fig. S1 in Supplementary Information \u0026ndash; SI). Seasonal and inter-basin variability in background vorticity and vertical wind shear are key conditions for sustaining tropical cyclogenesis (TCG) of NETC\u003csup\u003e5,6\u003c/sup\u003e, and such variability explains why NETCs are rarely observed in the Atlantic\u003csup\u003e7\u003c/sup\u003e. In western Pacific, NETCs are favored by a background, seasonal low-level vorticity associated with the cyclonic turning of NE trade wind near the equator, and thus occur predominantly during boreal winter\u003csup\u003e6\u003c/sup\u003e. In contrast, NETCs in the Indian Ocean are driven by the shedding of Sumatra vortices, a synoptic-scale regional feature that frequently develops near the tip of Sumatra island within the easterly flow\u003csup\u003e8,9\u003c/sup\u003e, leading to their downstream propagation over the basin.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe Maritime Continent (MC), a complex archipelago between the Indian and western Pacific Oceans, presents a particularly challenging environment for cyclogenesis. Only 14 NETC formed within the MC since 1980, with most (10 events) occurring over relatively open waters such as the South China Sea and the Banda Sea. The most notable MC NETC, TC Vamei (Fig. 1a), formed within 1.5\u0026deg; of the equator in December 2001\u003csup\u003e11\u003c/sup\u003e. TC Senyar is an exceptional event in this small group, as it formed in Strait of Malacca (MS) (Fig. 1b), only the forth such event since 1980. While previous MS NETCs (unnamed systems on 21 November 1988 and 18 November 1995, and TC Mandous on 4 December 2022) followed typical westward and poleward trajectories (see Fig. S1 in SI), TC Senyar exhibited an anomalous track, slowly moving equatorward before recurving eastward to make landfall approximately 100 km north of Kuala Lumpur, Malaysia. It then moved into the South China Sea while gradually weakening (Fig. 1b).This unusual behavior underscores gaps in our understanding of NETC dynamics in topographically complex coastal environments.\u003c/p\u003e\n\u003cp\u003eTracking of an elevated vorticity \u0026lsquo;blob\u0026rsquo; that constitute a TC Senyar precursor (hereafter \u0026quot;pre-Senyar\u0026quot; disturbance) reveals that a chain of processes leading to TCG began on 15 November 2025 (Fig. 1c), 10 days before its \u0026lsquo;offical\u0026rsquo; formation, when an area of enhanced relative vorticity first became identifiable over the South China Sea during the onset of northeast (NE) monsoon (Fig. 2).\u0026nbsp;Following the NE monsoon onset on 13 November\u003csup\u003e12\u003c/sup\u003e, a relatively broad area of positive vorticity developed along its southern flank. The pre-Senyar moved westward with the monsoon flow (15-17 November), stalled along the east coast of Malay Peninsula (18-20 November), and then crossed the peninsula before turning southward into the MS on 21 November (Fig. 2). Until this equatorward diversion, the pre-Senyar followed a typical westward wintertime trajectory of disturbances that can eventually develop into a TC over Indian Ocean\u003csup\u003e8\u003c/sup\u003e, also consistent with the tracks of TC Vamei (Fig. 1b) and the three other MS systems (Fig. S1). Once in the Strait, the pre-Senyar stalled again while steadily intensifying and was named on 25 November. Relative vorticity of pre-Senyar increased gradually from 10⁻⁴ s⁻\u0026sup1; on 19 November (Malay Peninsula arrival) to 6\u0026times;10⁻⁴ s⁻\u0026sup1; by 23 November. After temporal weakening to 2\u0026times;10⁻\u003csup\u003e4\u003c/sup\u003e s⁻\u0026sup1; on 23 November, explosive intensification occurred on 24 November, with the disturbance reaching maximum vorticity of ~10⁻\u003csup\u003e3\u003c/sup\u003e s⁻\u0026sup1; at 00UTC on 25 November, representing 10-fold inctease in vorticity since 19 November and more than 3-fold increase in 30 hours (Fig. 1d, Fig. 2). In the MS region, \u0026nbsp;minimum sea-level pressure (1001 hPa) and maximum 10-m wind speed (17 m s⁻\u0026sup1;) were attained at 09 and 02 UTC on 25 November, respectively, prior to recognition in the IBTrACS data (Fig. 1d). As the system weakened thereafter, the system became self-susteining most likely around 00 UTC on 25 November. Regardless of the precise definition of the genesis time, heavy rainfall affected the region throughout the entire period when the disturbance was in the MS region, despite its modest intensity by conventional tropical cyclone metrics. A similar pattern was observed during TC Seroja\u0026apos;s genesis in the Flores Sea\u003csup\u003e13\u003c/sup\u003e, which emphasizes the need to improve understading of TCG occurring in coastal regions of densely populated areas.\u003c/p\u003e\n\u003cp\u003eThis study investigates physical atmospheric processes related to the formation of TC Senyar, specifically focusing on the preTCG (19-23 November) and TCG (23-26 November) periods. The analysis considers all critical components of the TCG\u003csup\u003e2,14\u003c/sup\u003e within a multi-scale interactions framework. To that end, average November 2025 conditions, background (period \u0026gt; 20 days), sub-intraseasonal variability (5-20 days) and transient disturbances (period \u0026lt;5 days) variability, as well as spatio-temporal variability of relevant atmospheric fields and sea surface temperatures (SST), are invesitgated. The analysis centers on the western MC and three key focus regions: the NE monsoon region in the South China Sea (NEM region), the westerly wind burst (WWB) region in eastern equatorial Indian Ocean, and the primary genesis theater in the Strait of Malacca (MS region). Reanalysis data are analyzed with dynamical diagnostics to elucidate a sequence of multiscale interactions that enabled this rare and devastating cyclogenesis event, with implications for improving operational forecasts and reducing societal vulnerability to future near-equatorial tropical cyclones.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cstrong\u003eMulti-Scale Wind Variability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe preTCG (19–23 November) and TCG (23–26 November) periods were characterized by pronounced multi-scale variability in lower-tropospheric (850 hPa) zonal winds across the eastern Indian Ocean WWB region and the South China Sea NEM region (Fig. 3a,b). In the WWB region, 850-hPa westerlies consistently exceeded the November 2025 monthly mean (8.5 m s⁻¹) throughout the preTCG and TCG periods (Fig. 3a). The background component contributed steady westerlies but gradually weakened over time. Two westerly peaks on 18 November and 24 November, both exceeding 12 m s\u003csup\u003e-1\u003c/sup\u003e were driven primarily by the transient variability driven by convectively coupled Kelvin waves (Fig. 3d) . The second peak was notably stronger (13 m s\u003csup\u003e-1\u003c/sup\u003e) due to constructive interference between transient and sub-intraseasonal components (Fig. 3a) related to equatorial Rossby (ER) wave and Madden-Julian oscialltion (MJO, Fig. 3d). Following the Malaysian meteorological office's declaration of NE monsoon onset on 13 November, easterlies in the NEM region intensified rapidly after 15 November and remained substantially stronger than the monthly average throughout the preTCG and TCG periods (Fig. 3b).\u0026nbsp;Area-averaged NEM easterlies reached a maximum of 16 m s⁻¹ on 21 November. While the background component remained steady easterlies, temporal evolution was dominated by sub-intraseasonal variability (Fig. 3b) reflecting the westward progression and intensification of the monsoon flow (Fig. 2) supported by westward propagating ER event (Fig. 3e). This sub-intraseasonal signal manifested as both intensifying easterly winds and spatial expansion of the easterly flow field in both meridional and zonal dimensions. Interestingly, sub-intraseasonal easterlies in the NEM region peaked approximately 4 days before sub-intraseasonal westerlies in the WWB region, indicating westward propagation of a cyclonic circulation spanning meridionally between the two regions, consistent a meridionally ‘narrow’ ER event (Fig. 3d,e).\u003c/p\u003e\n\u003cp\u003eLow-level meridional winds in the MS region were predominantly northerly, with significant periodic fluctuations between strong northerlies (6.5 ms\u003csup\u003e-1\u003c/sup\u003e on 24 November) and marginally southerly flow or weak northerlies (21 November and 25 November, respectively; Fig. 3c). In the postTCG period, strong northerlies (~ 7 ms\u003csup\u003e-1\u003c/sup\u003e) returned on 27 November. Time-scale decomposition reveals that transient variability dominated the meridional wind signal, with substantial contributions from sub-intraseasonal variability, particularly during the TCG period (Fig. 3c). Sub-intraseasonal and transient variations correlate well with activity of ER and a combination of CCKW and mixed-Rossby gravity (MRG) waves, respectively (Fig. 3f). The southward deflection of the pre-Senyar disturbance into the Strait of Malacca on and after 20 November coincided with the development of northerly flow in the MS region, suggesting that it was facilitated by a MRG activity. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe zonal wind patterns in the WWB and NEM regions (Fig. 3a, b) exhibited coherent zonal propagation: over time, easterlies north of 5°N extended progressively westward, while westerlies south of 5°N extended progressively eastward (Fig. 2), consistend with tropical waves activity (Fig. 3d,e). It should be noted, that zonal wind magnitudes in NEM and WWB regions during preTCG and TCG periods signiffiantly exceeded November 2025 averages (Fig. 3a, b) and climatological values for November (Fig. 4).\u0026nbsp;This convergent evolution is quantified by the progressive decrease in latitudinal separation between the WWB westerly maximum and the NEM easterly maximum from 17 November through 24 November (Fig. 4b). During the preTCG period, maximum easterlies remained near the climatological position of 9°N, but shifted equatorward on 24–25 November. Concurrently, the WWB westerly axis migrated from 1°N to 3°N. As a result, meridional gradient of zonal velocity \u0026nbsp;(-∂u/∂y) progressively increased during preTCG and TCG period, enhancing environmental vorticity in the MS region (Fig. 4c). The gradient reached maximum on 24 November, coinciding with the explosive intensification of vorticity associated with TC Senyar (Fig. 2). This convergent shear configuration not only generated the cyclonic environmental vorticity necessary to overcome weak planetary vorticity during the formation of NETC\u003csup\u003e5,7,10,15,16\u003c/sup\u003e, but also induced northerly flow through the MS region (Fig. 2), which drove preSenyar and TC Senyar equatorward (Fig. 1b) through its contribution to sub-intraseasonal variability (Fig. 3c).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eITCZ position, vertical wind shear and SST conditions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe poleward shift of low-level westerlies in the WWB region was facilitated by a displacement of the ITCZ, which in November 2025 laid about 2–4° north of its climatological position over the eastern Indian Ocean and Sumatra (Fig. 5a). The displaced ITCZ exerted a critical dynamical influence on cyclogenesis by facilitating the poleward migration of the WWB westerly maximum from 1°N during preTCG to 3°N during TCG (Fig. 4b, c), thereby enabling the convergent shear configuration documented in the previous section. At the same time, this poleward ITCZ shift enhanced background cyclonic vorticity in the near-equatorial region through the seasonal monsoon circulation, effectively increasing absolute vorticity and partially compensating for weak planetary vorticity. However, background absolute vorticity is only one of several key conditions necessary for TCG\u003csup\u003e2,14\u003c/sup\u003e. Other essential factors include low-to-moderate vertical wind shear, high SST and a moist atmosphere. Strong vertical wind shear generally inhibits tropical cyclone genesis through vortex tilting and vertical decoupling mechanisms, which displace the mid-level circulation from the low-level vortex center\u003csup\u003e17,18\u003c/sup\u003e. Climatological November vertical shear of horizontal winds over the MS region is moderate with an average magnitude of 10 m s\u003csup\u003e-1\u003c/sup\u003e (Fig. 5e). In November 2025, the shear magnitude weakened throughout preTCG and TCG periods, reaching minimum value of 5 m s\u003csup\u003e-1\u003c/sup\u003e in the critical 200-800 hPa layer on 25 November (Fig. 5e). This reduction permitted enhanced vertical vortex alignment and more efficient convective organization\u003csup\u003e17–19\u003c/sup\u003e, favorable conditions for the development of a NETC\u003csup\u003e7\u003c/sup\u003e. Furthermore, during preTCG period, wind consistently blew from the north (N) to west (W) directions up to the 400 hPa level and became predominantly northerly on 23-24 November, indicating a deep, equatorward steering flow consistent with the movement of preSenyar (Fig. 1b). On 25 November, the flow shifted to N-E flow with consistent barotropic conditions up to 200 hPa and steering towards Sumatra, in agreement with the initial movement of TC Senyar (Fig. 1b, Fig. 2).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSST across the western MC region during November 2025\u0026nbsp;remained near climatological values (~27–29°C) (Fig. 4a)\u0026nbsp;and substantially above the classical 26.5°C threshold for tropical cyclogenesis\u003csup\u003e2\u003c/sup\u003e. The broader climate context during November 2025 featured concurrent La Niña conditions (Niño 3.4 Index = −0.9) and a negative Indian Ocean Dipole (IOD; Dipole Mode Index = −0.6).\u0026nbsp;The former was supportive of increased SST and moisture in the South China Sea and Karimata Strait\u003csup\u003e20,21\u003c/sup\u003e, the latter brings positive SST anomalies in the eastern Indian Ocean, resulting in stronger westerlies in the WWB region\u003csup\u003e22,23\u003c/sup\u003e. As a result, large-scale low-level convergence favored enhanced convection over the MC region. Thus, for TC Senyar, the near-climatological SST conditions provided necessary but not sufficient support for genesis; the critical enabling factors were the dynamical conditions.\u0026nbsp;This pattern is consistent with the dynamical-forcing paradigm for NETC genesis\u003csup\u003e5,24\u003c/sup\u003e: thermodynamic conditions must meet minimum thresholds (SST ≥ 26.5°C, adequate moisture), but the probability and timing of genesis are determined primarily by the evolving dynamical environment.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMoisture conditions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eClassical tropical cyclogenesis theory emphasizes the necessity of high SST over large areas to support upward latent heat flux from the ocean, which supplies moisture for latent heat release during cloud formation within developing tropical cyclone systems\u003csup\u003e25,26\u003c/sup\u003e. However, from a thermodynamics perspective, the critical requirement is energy supply. Idealized numerical experiments demonstrate that TCG can occur even in dry environemnt, driven by sensible heat only\u003csup\u003e27,28\u003c/sup\u003e. Fundamentally, moist atmospheric convection depends not on surface latent heat flux alone, but rather on boundary layer moisture content and column-integrated water vapor, which together fuel deep convection and sustain organized vortex development\u003csup\u003e29,30\u003c/sup\u003e. Such conditions were present during formation of TC Senyar (Fig. 5b). On 23 November, a sub-intraseasonal low-level cyclonic circulation was present over MS and the Malay Peninsula. It was characterized by exceptionally high boundary layer (1000-850 hPa average) specific humidity (q), with area-averaged values exceeding 16 g kg\u003csup\u003e-1\u003c/sup\u003e over MS and South China Sea. Localized regions within the cyclonic circulation exhibited specific humidity ≥15 g kg⁻¹ extending above the 850 hPa level, indicating deep boundary layer moistening (Fig. 5b). Such regions are typically confined to areas within or very near the cyclonic circulation, where they support atmospheric moistening through both horizontal flow convergence and vertical convective transport, facilitated by embedded pockets of enhanced vorticity (Fig. 2).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eLow-level moisture convergence serves as a critical mechanism in the TCG as it controls latent heat release and sustains organized deep convection, which in turn drives low-level vorticity amplification and vortex consolidation\u003csup\u003e31\u003c/sup\u003e.\u0026nbsp;As a result, the MS region underwent progressive moistening throughout preTCG and TCG periods and exhibited extremely high moisture content up to 300 hPa level (Fig. 5c). Particularly notable was the large positive anomaly (relative to climatology) in mid troposphere (600-400 hPa; Fig. 5c).\u0026nbsp;Elevated mid-tropospheric moisture constitutes another fundamental prerequisite for tropical cyclone formation\u003csup\u003e32,33\u003c/sup\u003e. The presence of substantial mid-level moisture reduces saturation deficit, thereby mitigating suppression of convection and diabatic heating that allow moist updrafts to reach higher levels in the atmosphere\u003csup\u003e33\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe observed moisture evolution exhibits characteristics qualitatively consistent with the \"marsupial pouch\" conceptual framework\u003csup\u003e34\u003c/sup\u003e, which has been applied to understand TCG in Atlantic\u003csup\u003e35\u003c/sup\u003e, north Indian Ocean\u003csup\u003e36\u003c/sup\u003e and western Pacific\u003csup\u003e37\u003c/sup\u003e though not specifically for near-equatorial cases. In this framework, a parent cyclonic circulation, typically associated with a synoptic scale wave or monsoon trough, provides a protective environment that shields the developing vortex from adverse external conditions such as dry air intrusion, strong vertical shear, and vorticity hostile flow patterns\u003csup\u003e34,35\u003c/sup\u003e. The pouch circulation maintains elevated moisture and vorticity within its confines, allowing embedded mesoscale disturbances to intensify through repeated convective cycles without being disrupted by environmental ventilation. In the case of TC Senyar, the pre-Senyar disturbance moved within a sub-intraseasonal cyclonic circulation (Fig. 5b) that appears to have functioned as a protective environment. The vortex remained embedded within a region of elevated lower relative vorticity (Fig. 2) and substantially higher moisture (Fig. 5b), compared to the surrounding environment. Hence, the pre-Senyar exhibited intensity fluctuations while undergoing gradual overall intensification (Fig. 1d) and progressively moistening its immediate environment (Fig. 5c). This behavior, in which episodic convective pulses within a protective circulation lead to stepwise intensification, is characteristic of the marsupial pouch paradigm\u003csup\u003e34,35\u003c/sup\u003e. Thus, the TC Senyar case extends this framework to near-equatorial cyclogenesis, demonstrating that protective sub-intraseasonal circulations can facilitate genesis even at latitudes (4.7°N) where planetary vorticity is minimal.\u003c/p\u003e\n\u003cp\u003eOn the larger scale, western MC moistening during November 2025 was facilitated by moisture flux convergence (Fig. 6) associated with concurrent La Niña and negative IOD. The western MC exhibited a dipole pattern in moisture transport: westerly flow from the Indian Ocean (IOD pattern) penetrated the region south of the equator, while anomalously strong easterly moisture transport (La Niña conditions) dominated north of the equator and was driven by the northeast monsoon flow. Beginning on 18 November, the maximum moisture convergence is found near the leading eadge of the NE monsoon winds, coinciding with the location of pre-Senyar disturbance (Fig. 1c). As the easterly monsoon flow progress westward (Fig. 2), the moisture convergence maximum migrated in tandem, arriving in the MS region by 21 November (Fig. 6). Pre-Senyar stalling in the MS region was reinforced by westward-propagating cyclonic circulation centered over the southern Philippines on 24 November associated with Typhoon Koto (Fig. 1), which deflected the easterly moisture flux southward into the MS (Fig. 6). As a result, at that time there are two “sweet spots” of enhanced convective potential: (1) on the eastern side of the Philippines, associated with Typhoon Koto (Fig. 1), and (2) in the MS region, where TC Senyar was developing. Both systems were driven by the same large-scale moisture transport configuration, illustrating the regional coherence of the dynamical and thermodynamic forcing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEquatorial Wave Activity and Multi-Scale Interactions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSubseasonal modes have been related to TC activity, including global impacts of MJO on TCG\u003csup\u003e38–40\u003c/sup\u003e, effects of CCKW on TCG in Atlanic\u003csup\u003e41–43\u003c/sup\u003e and other basins\u003csup\u003e44\u003c/sup\u003e, and response to ER in Pacific\u003csup\u003e45\u003c/sup\u003e. Wavenumber-frequency filtering of 850 hPa horizontal wind fields reveals significant activity of multiple equatorial wave types during during preTCG and TCG periods (Fig. 7a), collectively leading to TC Senyar formation (Fig. 1). Two distinct and robust (magnitude\u0026nbsp;\u0026gt; 1.5σ, where σ represents the standard deviation of the filtered field averaged over 5°N-5°S equatorial channel) eastward propagating CCKW events (Fig. 7a, black lines), a strong (magnitude ~ 3 σ) westward propagating ER event and two robust (magnitude \u0026gt; 1. 5σ) MRG events occurred over the western MC region. The first significant vorticity amplification during the preTCG period occurred when the ER interacted with the first CCKW westerlies and the MRG northerlies (19 November) flow. The CCKW provides both increased westerlies (visible over WWB region one day earlier, on 18 November, in Fig. 3a) and positive vorticity at its northern edge through meridional shear of horizontal flow\u003csup\u003e46\u003c/sup\u003e. Simultaneously, the MRG activity closely matches variability in meridional flow in the MS region (Fig. 3c), suggesting that MRGs were responsible for the steering flow modulation and contributed to the episodic northerly wind pulses (Fig. 3) that deflected the pre-Senyar disturbance equatorward. Throughout the preTCG period, the strong ER event (Fig. 7a) maintained sustained cyclonic vorticity within its envelope as well as protective pouch\u003csup\u003e34,47\u003c/sup\u003e, isolating it from a lower relative vorticity background environment (Fig. 5b). On 24 November, anomalous northerlies associated with the MRG wave\u0026nbsp;guided the vortex into the optimal genesis location.\u0026nbsp;On 25 November, a second strong CCKW event (magnitude ~2σ) reached the MS region, coinciding with the final explosive intensification of TC Senyar (Fig. 1d). The arrival of this CCKW was associated with a strong cyclonic vorticity on its northward side (see Fig. S6s in SI). This complex multi-scale interaction occurred within a westerly flow of MJO intraseasonal circulation (Fig. 7), but away from the MJO convective center and maximum convergence. Hence, intraseasonal conditions (MJO) created supportive large-scale conditions, but it was not the direct driver for the formation of TC Senyar.\u003c/p\u003e\n\u003cp\u003eIt should be noted, that tropical waves described above exhibited non-canonical spatial structures with northward shifted waveguides and relatively small meridional extent (large meridional wavenumber) As a result, the ER northern cyclonic gyre spanned only 1 - 6°N (Fig. 5 and Fig. S5b-7b in SI), compared to the typical 4 – 10°N range for canonical ER waves\u003csup\u003e48\u003c/sup\u003e, while the second CCKW had its zonal axis at 3°N with maximum vorticity at 5°N, right at the latitude of TC Senyar’s genesis (Fig. S6a-7a in SI). This northward shift is consistent with the 2–4° northward displacement of ITCZ position during November 2025 (Fig. 5a). The narrow meridional structure of the ER event is particularly noteworthy. A similar ‘narrow’ ER wave was documented during the formation of TC Seroja in Flores Sea\u003csup\u003e13\u003c/sup\u003e. While the spatial structure of tropical waves active during TC Senyar formation differed from theoretical predictions for equatorial beta-plane modes\u003csup\u003e49,50\u003c/sup\u003e and composite observational structures\u003csup\u003e51\u003c/sup\u003e, their propagation properties remain robust and consistent with canonical equatorial wave dispersion relations. To that end, the ER event propagated westward at a speed consistent with expected ER phase speeds and was subsequently involved in the formation of TC Ditwah near Sri Lanka (Fig. 1a), as evidenced by a vorticity maximum at 80°E on 27 November (Fig. 7a). It is also worth noting that superposition of equatorial waves contributed to enhanced low level convergence and moisture flux convergence over the western MC. For example, on 24 November the ER and MRG circulations contributed anomalous easterlies over South China Sea and Malaysia, while CCKW and MJO brought anomalous westerlier over eastern Indian Ocean, resulting in enhanced convergence over the MS region (Fig. S5-7 in SI). This convergent configuration amplified the moisture supply (Fig. 6), providing the thermodynamic fuel for sustained deep convection.\u003c/p\u003e\n\u003cp\u003eDecomposition of 850 hPa relative vorticity into temporal scales (Fig. 7b) reveals the differential contributions of background, intraseasonal, and transient variability to the observed vorticity evolution. While the background flow supported positive vorticity anomalies over the MS through preTCG and TCG periods, through enhanced meridional shear of horizontal winds (Fig. 4c), the sub-intraseasonal and transient variability were the critical drivers of vorticity amplification. The second CCKW event projects well onto transient time scale during TCG period, marking the final spin-up of TC Senyar, while sub-intraseasonal time scale variability is dominated by ER. Periods of MRG northerly anomalies concide with negative contribution of transient time scale to observed vorticity evolution. Hence, a simple temporal decomposition provides a useful diagnosis of large-scale processes and their contribution to observed multi-scale interactions. The dominance of sub-intraseasonal and transient variability during the critical 24–25 November period (Fig. 7b) confirms that equatorial wave activity, rather than seasonal mean conditions, was the proximate driver of TC Senyar’s explosive intensification.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe formation of NETC Senyar at 4.5\u0026deg;N challenges conventional paradigms of tropical cyclogenesis, which emphasize the necessity of sufficient planetary vorticity to organize convection into a coherent mesoscale vortex\u003csup\u003e2,3,16\u003c/sup\u003e. Our analysis reveals that Senyar\u0026apos;s genesis resulted from a remarkable convolution of multi-scale atmospheric processes. This multi-scale interaction framework offers a unifying perspective on near-equatorial cyclogenesis that extends beyond a isolated case study.\u003c/p\u003e\n\u003cp\u003eIn November 2025, the concurrent La Ni\u0026ntilde;a and negative IOD conditions led to positive SST anomalies (Fig. 4a), enhanced moisture availability (Fig. 5b,c), suppressed vertical wind shear (Fig. 5e) and intensified the Walker circulation over the eastern Indian Ocean\u003csup\u003e22,52\u003c/sup\u003e, all associated with favorable conditions for enhanced convection\u003csup\u003e53\u003c/sup\u003e. On shorter, subseasonal time scales no single process appears sufficient for Senyar\u0026apos;s formation; rather, the precise phasing and spatial superposition of multiple equatorial wave modes created the necessary environment for tropical cyclogenesis to occur.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe equatorial waves involved in Senyar\u0026apos;s genesis exhibited non-canonical spatial structures, with northward-displaced waveguides and reduced meridional scales compared to theoretical expectations\u003csup\u003e48\u0026ndash;50\u003c/sup\u003e, consistent with northward shift of ITCZ (Fig. 5a). As a result, the meridional distance between equatorial westerlies and NE monsoon easterlies was smaller than typical (Fig. 5e), resulting in a strong meridional gradient of the zonal wind (\u0026part;u/\u0026part;y)\u003csup\u003e54\u003c/sup\u003e, which constituted substantial background relative vorticity over the western MC region (Fig. 2), which overcame weak planetary vorticity (f \u0026asymp; 1.2\u0026times;10⁻⁵ s⁻\u0026sup1; at 4.7\u0026deg;N).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe vorticity budget analysis (Fig. 8a) provides a quantitative confirmation of these multi-scale contributions to evolution of pre-Senyar disturbance in the MS region. Vorticity tendency during cyclogenesis was dominated by stretching (-\u0026zeta;D) and zonal advection of relative vorticity (-u\u0026part;\u003csub\u003ex\u003c/sub\u003e\u0026zeta;). While the former remains positive and increases throughout the preTCG and TCG periods, the latter compensates it throughout the preTCG phase. However, on 23 November the magnitude of the zonal advection of relative vorticity starts rapidly decreasing, which means that vortex stretching becomes unbalanced and facilitates rapid growth of relative vorticity tendency. Interestingly, the vertical advection (-\u0026omega;\u0026part;\u003csub\u003ep\u003c/sub\u003e\u0026zeta;) is rapidly increasing since midnight on 25 November, indicating that TC Senyar became a self-sustained system before its official genesis according to IBTrACs (Fig. 1). Thus, the dynamical framework confirms our phenomenological analysis.\u003c/p\u003e\n\u003cp\u003eTemporal decomposition reveals that the evolution of vortex stretching of relative vorticity was driven by diverse factors throughout the preTCG and TCG periods. While background and sub-intraseasonal divergence (related to LaNina/IOD/MJO and ER, respectively) acting on sub-intraseasonal (ER) vorticity provided positive contribution through these periods, the variability was driven by transient divergence of sub-intraseasonal vorticity (on 23-24 November) and a combination of sub-intraseasonal and transient divergence of transient vorticity (25 November). Hence, the low-level convergence of a compound effect of MRG and CCKW (Fig. 7c) acting on ER vorticity drove initial vorticity increase during TCG period, while the final rapid increase in the magnitude of the full vortex stretching term was caused by the divergence acting on CCKW vorticity (Fig. 7a). The zonal advection of relative vorticity (Fig. 8c) was dominated by the zonal advection (across all temporal scales) of sub-intraseasonal (ER-related) vorticity. Specifically, the sub-intraseasonal advection of sub-intraseasonal vorticity became positive on 24 November, allowing the full term to become near-neutral, leading to imbalance with vortex stretching and development of a self-sustained system.\u003c/p\u003e\n\u003cp\u003eThis quantitative assessment confirms that TC Senyar development was driven by multi-scale dynamical interactions, wherein multiple drivers across distinct spatio temporal scales acted synergistically to foster cyclogenesis. While the predominance of dynamics over thermodynamics in NETC formation has been well established\u003csup\u003e5,24\u003c/sup\u003e, the critical ingredients enabling TC Senyar genesis comprised:\u003c/p\u003e\n\u003col\u003e\n \u003cli\u003eLarge meridional shear of zonal winds between NE monsoon easterlies and equatorial westerlies\u003c/li\u003e\n \u003cli\u003eA \u0026lsquo;narrow\u0026rsquo; ER creating protective pouch for initial disturbance in moisture rich environment\u003c/li\u003e\n \u003cli\u003eA sequence of tropical waves, each providing successive vorticity pulses that amplified the nascent disturbance\u003c/li\u003e\n \u003cli\u003eA strong and narrow CCKW event, uniquely positioned to deliver the final amplification through vortex stretching mechanisms\u003c/li\u003e\n\u003c/ol\u003e\n\u003cp\u003eWhile oservational framework alone cannot definitively establish which combination of these ingredients constituted necessary and sufficient conditions for TC Senyar development, this study provides a robust foundation for targeted numerical experiments designed to address this question. Furthermore, each ingredient is independently trackable through operational monitoring systems. Consequently, the present analysis framework can be leveraged to quantify the frequency of such compound favorable conditions in the current climate and project changes in their occurrence under future warming scenarios. Additionally, these ingredients can be operationally monitored to enhance forecasting skill for NETC events similar to Senyar. This operational application is particularly critical: despite TC Senyar\u0026apos;s modest intensity, its societal impacts including devastating flooding and landslides across Aceh and North Sumatra regions with limited tropical cyclone experience and underdeveloped early warning infrastructure, were substantial. As climate change potentially modifies equatorial cyclone characteristics and spatial distribution\u003csup\u003e55\u003c/sup\u003e, these vulnerable communities face escalating risks, necessitating improved forecasting capabilities. Such advances can only be achieved through deeper mechanistic understanding of NETC genesis pathways. Such advances can only be achieved through deeper mechanistic understanding of NETC genesis pathways.\u003c/p\u003e\n\u003cp\u003eComparison with previous NETC cases in the MC region shows how different process combinations can lead to cyclogenesis. While TC Seroja (2021)\u003csup\u003e13\u003c/sup\u003e was not technically classified as a NETC (Fig. 1), it formed from a seed disturbance satisfying NETC criteria. Critically, its final intensification occurred over the Flores Sea through an interaction of a meridionally narrow ER, which fostered a developing vortex within a protective pouch, with an intense CCKW propagating within a poleward-displaced (southward-shifted) equatorial waveguide\u003csup\u003e13\u003c/sup\u003e. However, TC Seroja formed under different monsoonal dynamics configurations, suggesting that the ER wave pouch mechanism may represent a common pathway, the specific multi-scale process combinations vary substantially across cases. Typhoon Vamei (2001)\u003csup\u003e10,15\u003c/sup\u003e formed in the MC region during anomalously strong northeasterly cold surge conditions, wherein the interaction between surge-related flow and a quasi-stationary Borneo vortex generated critical cyclonic vorticity reservoir \u003csup\u003e56\u003c/sup\u003e. The MJO played played a comparatively minor role, and explicit equatorial wave superposition was not documented. Nevertheless, the fundamental physical mechanism underlying Vamei\u0026apos;s rapid intensification, namely the interaction between propagating flow characterized by strong horizontal shear and a semi-stationary sub-intraseasonal Borneo vortex, bears striking similarity to the processes demonstrated here for TC Senyar.\u003c/p\u003e\n\u003cp\u003eThe contrasts among Vamei, Seroja, and Senyar collectively demonstrate that NETC genesis emerges through multiple distinct pathways, each involving different combinations of independently trackable atmospheric processes, yet all requiring compensating mechanisms to overcome the suppressive effect of weak planetary vorticity near the equator\u003csup\u003e2,16\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThese findings carry profound implications for understanding NETC behavior under climate change. Theoretical and modeling studies suggest equatorial wave activity, particularly MJO amplitude, may intensify under greenhouse warming\u003csup\u003e57\u003c/sup\u003e, potentially increasing favorable multi-scale superposition events. Projected SST warming in the MC would enhance thermodynamic potential for convective organization and cyclogenesis, while IOD and ENSO teleconnections may fundamentally alter the seasonal and spatial distribution of favorable conditions\u003csup\u003e58\u003c/sup\u003e. Whether these factors increase NETC frequency remains uncertain and demands rigorous investigation, but the process-based diagnostic framework established here provides a robust foundation for targeted climate modeling studies and mechanistic attribution analyses. These findings advance fundamental understanding of NETC genesis mechanisms while simultaneously providing actionable pathways for improving prediction systems and enhancing disaster preparedness in one of the world\u0026apos;s most densely populated and climatically vulnerable regions.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eReanalysis and Satellite data\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eEuropean Centre for Medium-Range Weather Forecasts ECMWF Fifth Generation Reanalysis ERA5 data\u003csup\u003e59\u003c/sup\u003e, including zonal and meridional winds, vertically integrated moisture divergence, relative vorticity, \u0026nbsp;specific humidity, and sea surface temperature, were obtained for the period 1 January 1995 to 31 December 2025. The data are provided on a regular 0.25\u0026deg; \u0026times; 0.25\u0026deg; latitude\u0026ndash;longitude grid at multiple pressure levels with hourly temporal resolution. In addition to ERA5 data, monthly rainfall data from GSMaP_Gauge Ver.8 (standard with gauge calibration, Version 8)\u003csup\u003e60\u003c/sup\u003e, for the period 1999\u0026ndash;2025 were also used. These data were employed to examine the position of the ITCZ.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMeasurement of key atmospheric parameters\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eKey atmospheric variables were analyzed to characterize the dynamical and thermodynamical environment of cyclogenesis. Low-level relative vorticity at 850 hPa was used to diagnose the intensification of cyclonic circulation in the lower troposphere. The 850 hPa level was selected as the standard for low-level vorticity analysis in tropical cyclone genesis studies because it samples the lower troposphere above the planetary boundary layer and is commonly used in genesis potential indices\u003csup\u003e61\u003c/sup\u003e. Mean sea level pressure (MSLP) was analyzed to track the development and evolution of the cyclonic pressure center. In addition, vertically integrated moisture flux from the surface to 200 hPa was calculated to quantify moisture transport pathways and convergence supporting convective organization. The vertically integrated moisture flux was computed as the mass-weighted vertical integral of specific humidity times horizontal wind, and moisture flux convergence was obtained from the horizontal divergence of vertically integrated moisture flux\u003csup\u003e62,63\u003c/sup\u003e. This diagnostic is widely used to identify moisture sources and assess convective organization during tropical cyclone genesis\u003csup\u003e62,63\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eTo quantify the influence of regional wind variability on cyclogenesis, low-level winds at 850\u0026nbsp;hPa were spatially averaged over three key domains: the westerly wind burst (WWB) region over the eastern Indian Ocean, the northeast monsoon (NEM) region over the South China Sea associated with winter monsoon surges, and the Strait of Malacca domain, which represents meridional momentum and moisture transport through this important maritime corridor (Fig. 1b). These diagnostics were then linked to sea surface thermodynamic conditions to evaluate the role of the background environment during Senyar\u0026rsquo;s formation. A November sea surface temperature (SST) climatology for 1995\u0026ndash;2024 was calculated, and anomalies were defined as deviations from this 30-year November mean.\u003c/p\u003e\n\u003cp\u003eTo examine the thermodynamic and dynamical structure of the atmosphere, a combined field of sub-intraseasonal and background variability (periods \u0026gt;5 days) was constructed. Low-level specific humidity (1000\u0026ndash;850 hPa average) was analyzed together with sub-intraseasonal 850 hPa horizontal winds to identify regions of convergence and column moistening. Areas where moist layers with specific humidity exceeding 15 g kg⁻\u0026sup1; extended above the 850 hPa level were identified using contour analysis. Vertical structure was further evaluated using profiles of specific humidity and wind direction, with climatological profiles (1995\u0026ndash;2024) compared to daily profiles during November 2025 to assess departures from typical conditions. The temporal evolution of vertical wind shear magnitude was calculated using both climatological values (1995\u0026ndash;2025) and November 2025 data, including the 200\u0026ndash;800 hPa shear difference and the maximum shear along the vertical profile. Together, these diagnostics were used to characterize the environmental conditions associated with the formation of TC Senyar.\u003c/p\u003e\n\u003cp\u003eAs a large-scale context, the climatological (1999\u0026ndash;2024) and November 2025 positions of the Intertropical Convergence Zone (ITCZ) were determined using monthly precipitation from GSMaP_Gauge Version 8 and a precipitation centroid method\u003csup\u003e64\u003c/sup\u003e, defined as the precipitation-weighted mean latitude within the 20\u0026deg;S\u0026ndash;20\u0026deg;N band. This approach provides a quantitative estimate of ITCZ displacement, which was then compared with low-level wind patterns and convective activity during the period of Senyar\u0026rsquo;s formation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTemporal decomposition of meteorological fields\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eRelatively simple temporal decomposition of meteorological fields allows separating atmospheric variability into physically meaningful time scales when driving mechanisms differ in that respect. A chain of events that led to genesis of TC Senyar took about 10 days. Hence decomposition into three time scales has been employed. The background time scale (variability with periods of 20 days or longer) encompasses processes such as low frequency IOD/La Nina state and MJO circulation. The sub-intraseasonal time scale (periods of 5 \u0026ndash; 20 days) includes variability associated with ER activity as well as progression of NE monsoon. The transient time scale (periods shorter than 5 days) is primarily driven by CCKW and MRG activity. Before decomposition, all data were smoothed with 24-hour, centered running average to remove high frequency variability and a mean state (defined here as a November 2025 average) has been removed from each field. Next, a 5-day centered running mean and a 20-day centered running mean were calculated. The transient component \u0026nbsp; was obtained by subtracting the 5-day running mean from the smoothed anomaly timeseries, the sub-intraseasonal component was computed as the difference between the 5-day and 20-day running means, and the background variability was represented by the 20-day running mean itself. As a result, this simple decomposition provided useful way to identify key processes, while keeping residual small and suitable for vorticity tendency budget calculations.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCalculation of daily anomalies and equatorial wave filtering\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe temporal decomposition of wind and vorticity fields, together with the spatial structure of sub-intraseasonal circulation and the evolution of moisture transport patterns, provides strong evidence that propagating equatorial waves contributed critically to the formation of TC Senyar. However, temporal filtering alone cannot definitively establish propagation characteristics. Therefore, the specific wave types present during the preTCG and TCG periods are identified, their amplitudes and propagation characteristics are quantified, and it is demonstrated how their constructive superposition generated the dynamical and thermodynamic conditions necessary for the formation of TC Senyar.\u003c/p\u003e\n\u003cp\u003eIdentify of convectively coupled equatorial waves and MJO activity was performed through the space-time spectral filtering technique\u003csup\u003e51,65\u003c/sup\u003e, applied to ERA5 horizontal winds and vorticity at 850 hPa. Filtering was performed based on one full your of data (calendar year 2025) and each variable was filtered in zonal wavenumber and frequency for all latitudes in meridional band 20\u0026deg;S \u0026ndash; 20\u0026deg;N after meridional smoothing with 3\u0026deg;-wide, centered running mean that accounts for possible meridional propagation of a signal. Filtering procedure retained only part of the spectrum corresponding to a given mode. Standard filtering parameters for each mode included equivalent depths of 8-90m with wave-specific period-wavenumber combinations for CCKWs (wavenumber 1-14, period 2.5-30 days), MJO (wavenumber 1-14, period 30-96 days), ER (wavenumber \u0026minus;1 to \u0026minus;10, period 9.7-48 days) and MRG (wavenumber \u0026minus;1 to \u0026minus;10, period 3-96 days)\u003csup\u003e51\u003c/sup\u003e. Waves\u0026rsquo; activity was determined if a filtered respective variable-dependent thresholds, given by a standard deviation of filtered signal averaged over the equatorial belt (0-360\u0026deg;E, 5\u0026deg;S-5\u0026deg;N). Thresholds for 850 hPa horizontal winds are as follows: [1.07 ms\u003csup\u003e-1\u003c/sup\u003e, 0.47 ms\u003csup\u003e-1\u003c/sup\u003e] for CCKW, [0.97 ms\u003csup\u003e-1\u003c/sup\u003e ,0.30 ms\u003csup\u003e-1\u003c/sup\u003e] for MJO, [0.94 ms\u003csup\u003e-1\u003c/sup\u003e, 0.72 ms\u003csup\u003e-1\u003c/sup\u003e] for ER and [0.67 ms\u003csup\u003e-1\u003c/sup\u003e, 0.72 ms\u003csup\u003e-1\u003c/sup\u003e] for MRG (values in square brackets indicate threshold in zonal and meridional wind respectively), reflecting spatial structures of those modes\u003csup\u003e51\u003c/sup\u003e. Hence, activity of CCKW, MJO and ER is diagnosed using zonal wind, while activity of MRG using meridional wind (Fig. 7).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFull, filtered structures of CCKW, MJO, ER and MRG (Fig. S5-7 in SI) show that modes active during exhibited non-canonical spatial structures with northward shifted waveguides and relatively small meridional extent (large meridional wavenumber). Therefore, filtered signals have been meridionally averaged within Eq-5\u0026deg;N, which best depict specific modes\u0026rsquo; activity in the region critical for TC Senyar formation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eVorticity budget calculations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA vorticity budget has been analyzed based on vorticity equation in Cartesian coordinates (1).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cimg 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\" width=\"481\" height=\"87\"\u003e\u003c/p\u003e\n\u003cp\u003e\u0026zeta; represents relative vorticity, x and y coordinates in eastward and northward direction. The left hand side of (1) is relative vorticity tendency, while terms on the right side represent zonal advection, meditional advection and vertical (in pressure coordinates) of relative vorticity, meridional advection of planetary vorticity \u003cv:shape id=\"_x0000_i1025\" type=\"#_x0000_t75\"\u003e\u003cimg src=\"data:image/png;base64,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\" width=\"114\" height=\"61\"\u003e\u003c/v:shape\u003e vortex stretching of relative and planetary vorticity (where \u003cimg src=\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAKgAAABACAYAAACOXDxbAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAAFiUAABYlAUlSJPAAAAeGSURBVHhe7ZzNaxNRF8afvHs/JroW6c1GXAhlWkGlYEEnFJfidOVGQRJc+sVo/4BJ923jyl1GUHQTP8FFUgWllGSlCycLBVczBP0HzrvpnXfmZtIk1eS9k5wfhDbn3rSZJ8/MnHvuaXNERGAYTfmPGmAYnWCDMlrDBmW0hg3KaA0blNEaNiijNWxQRmvYoIzWsEEZrWGDMlrDBmW0hg3KaE3mDBqGISqVChYWFpDL5ZDL5fDw4UN12swzNTpRxnBdlyzLolarRUREzWaTAJDruurUmWZadMrRFLTbra6uYmdnB9+/f1eHmBhZ1Clzt/g05ufn4fu+Gv4rKpUKCoWCGs40WdQpcwYNwxDlchn5fB65XA6rq6v4/ft3Yo7neSgUCsjn89je3k6MFYvFoQR9/Pgx7t69q4YzwzA6yTlyXOJ5HvL5PPL5PDqdTuI1KmPXSb3n60wQBGSaJpmmSc1mk4iIarUaASDDMKLnpVKJgiBIzbkMw6BSqZSIqdTrdQJAvu+rQ5lgGJ2IiGzbJt/3ybKsKF6v18l13Ug/+fo0JqFTqkHlL1YflmVRrVZTp0+MUqmUKoh8b3F83ycAtLW1FcVarRYBGHgMpVKJbNtWw5lhFJ2IiCzL6olLgwZBkIjHmYROqQYlInIcJ3EG+b5Ptm0TgIFXoHEgDaf+bhl3HCcRl1cMuYrtF1ORH0y9XleHMsGoOtGecdX58k7Uj0np1DcHPXLkCADg1KlTAIC5uTl4ngfbtrG5udmT242bL1++AACuXLmSGj9//nwi3mg0YBgGzpw5s29M5dmzZxBCYGVlRR2aOJVKBcViUQ3vy6g6hWEIADh58mQi/uTJE9y5cycRizMpnfoa9MOHDzBNE8eOHUvEr1+/DgD49OlTIj5ufvz4AQA4fPhwIv7y5UsYhtEj1Lt373D58uXoeRiG8DwPi4uLiXkq6+vrsG1bDWeGUXX6+vUrAODcuXNRrFqtYnl5GXNzc7GZSSalU1+Dvn37FgsLC2o4OvDd3V11aKzIK3qcdruNp0+f4sGDB4l4GIbwfR/z8/NRbG1tDQCwvLwcm5mk3W7D933cuHFDHcoMo+gEAH/+/Ek89zwP7XYb9+7dS8TjTFKnVIO2220AwNLSkjoUIYRQQ2Pl0qVLAIDXr18DALa3t3Hz5k2YptkjpiynyK/lchlLS0vodrvRa9O2/arVKizL2vfKoTuj6AQAP3/+jL6vVCpoNBrY2NhIzFGZqE5qUkpEtLW11XcxMcqWmWVZhJRqQNpDXUWmUa/XSQhB2CuXyHJIGnKRJ4SIEnn5fmQZKk4QBGQYxsAV/iSR25WjMopOcvEkhBjq2CetU6pBS6VSol4WR37w4169TZpardb3mCfBvz6Zx8WkdUo1qBCirwhCiIm+wUlhmua+ZZX/Bwe9go6TSevUk4PKBUbaYqJarcL3/dRkO41isRi1eg16jFpO+Ze0223s7Ozg1q1b6lBEp9NBuVxGoVBALpdDPp+H53nqtKlmGJ08z0t87sViMSplHQjVsXIXSd3iarVaZBgGmaaZiE8DjuMMPC7LsshxnGh3xnXdVJ3+JbpdQQfp1Gw2Ezm/7/v73o2HocegMseUSXWr1SLXdckwDLIsq2+ynWUOmvQPs6//N+hm0IPoJBfc6rbrsCRu8Z7nYXNzEwBw/Phx5HI5XLx4Ebu7u9jY2MCbN296CvdZR96mZXlmFBYXFwd2+0wLB9Xp9OnTAIBfv36pQ8OhOnbWsCxrqKtgq9WKehEMwyDHcXqaLIIgiBo14k0U9Xp97FfbcTOsTq7rRiUu0zSjK2g8FZJ3ZCFE4k5tmiYJIWI/LeUWP0vIGuCgPFLm37I9jWIdQ3Ejbm1tUbPZTDSlSGNnOTUaVidZnpRpgNRNaiFjrusmus2CICDLslLTgJk2qOM4PWdsGkKInnn7bVjITh+ZQ2bZnDSkTvKkVHNUWd9NwzRNsm2bSqVSX/Onv3JGEEIk+kXT6Ce8jPfbsDBNkwzD6Ct8lhhGp7STWMb7rfwdxyFjb6erHz110Fnh1atX8H0fV69eVYcSNBoNIGVxIONnz55NxCVCCOTzeVy4cEEdyhTD6NTpdOD7fk93k4zHu8rinDhxAt1ud9+fPbMGXVlZARENrErIVbo6T/bGqnHECtq+7/9dkVoDhtFJrtDVTqr3798DAK5du5aIY29D6MWLFwCAb9++qcMRM2vQYTl69KgaQrVaRbfbxe3bt9UhhGGI+/fv4/nz5wCAz58/q1OmjkOHDqkhhGGI9fV1WJaV2iC+traGR48eQQiBjx8/qsP/Q73nM0nUXFM2S6T96QTtrWRl3ilX/rL8NM3IHaMgCKI/DzIMI3VlXqvVorzTtu2o3JTWZcYGHQJZt8M+bWky4Y+PyRqgFfsPH9NKq9WKVuzYa2lUzSm30eMnd7PZjGqiaQvOqfjPIsz0wjkoozVsUEZr2KCM1rBBGa1hgzJawwZltIYNymgNG5TRGjYoozVsUEZr2KCM1rBBGa1hgzJawwZltIYNymgNG5TRGjYoozVsUEZr2KCM1rBBGa1hgzJawwZltOa/T9YOs7rGJTcAAAAASUVORK5CYII=\" width=\"168\" height=\"64\"\u003e\n \u003cv:shape id=\"_x0000_i1025\" type=\"#_x0000_t75\"\u003e\u0026nbsp;\u003c/v:shape\u003eis divergence) and residual (which includes a tilting/twisting term as well as unresolved and parameterized processes). Budget calculation has beed done based on smoothed (24-hour running average) hourly ERA5 data. A relative contribution of a given term to the observed tendency has been quantified using signed explained covariance per band \u003cv:shape id=\"_x0000_i1025\" type=\"#_x0000_t75\"\u003e\u003cimg src=\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAWUAAAA0CAYAAABSICldAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAAFiUAABYlAUlSJPAAABF+SURBVHhe7Z3PaxzlH8ff8a7tJJ5EJHQ8WGop6LSVpHgI2Fl7sohuaS9CRZktFARpZGIP0ghuVISiZXMoeLDs1vZQxA3ZPaTY3Qi2m7LBiJfO4qF42mkq/gHP9+B8nu/zPPvM7qbNj036ecGQzDwz8zzz7Mz7+fX5PM+QEEKAYRiGGQieMg8wDMMwWweLMsMwzADBoswwDDNAsCgzDMMMECzKDMMwAwSLMsMwzADBoswwDDNAsCgzDMMMECzKDMMwAwSLMsMwzADBoswwDDNAsCgzDMMMECzKDMMwA0RXUZ6amsLMzIx5mGGYR6TVamFqagpzc3NmELMNKZVKyOVyiONYHqvX68hkMmi1Wtq5fSMstNtt4XmeCILADGKYHU2xWBSu6woAAoDwfd885ZFpNpvC8zwRRZEQQoggCGQ85uZ5nnn5puD7vkxDrVYT4r+pfQWADj3YivTXajUZR7vdNoO3hGazKXzf19JTLBaF4zii2Wxq5/aDVZSz2ey6vowMsx0IgkA4jiPFqFwur+t34Pu+vLcQQnieJ4rFogwLw1AIIUQYhh0CuFm0223hOI4Wv+d5IpvNdojgVqU/n89vmOg/KoVCoeOZC4WCcBynI9960SHKpPBUmjPMk0CxWNRqhzaazabIZrMCgHAcRxQKBSESIaNaY7lcFiIRJnW/VqsJ13W1+9H1IqmN0rnNZlOKnRBCRFGk1Urz+bwQiTgBEGEYiiiKZC23XC6LcrksHMcRnudp6bPtm+TzeZnWIAg6xIZYz/RTbZP2VdQwx3GE4zgd59RqNe0cSku/cZhppEJIzVOCzlE1st1uC1hq777vr7lg10SZSknKNIZ5UnBdV2SzWfOwJIoiTQyazaYAIAWo2WwK13VFPp8XURSJcrkswjDUBMgUAoKa5OYHLYxvst1ua4VHsViUtUYSEc/zxC+//CLCMJT3zWazotlsinK5LJB0Q9RqNblvVsAozn67MNcz/WEYaiJG+R4EgWi32zLNqkjWajXhOI4sCIIgkK2SR4mjWCyKMAzl7xoEgVZ4lcvljgJWJAKsFkZCyZtal8LeRBPlQqGQmrkMs1OJoqjjQzexdemZHxsA7aPMZrMy3Pf91MoOiYaNIAhkYVGr1aS4EGEYpjaRScAoDaZAULgN3/eF4zjmYSvrmX5TMElgCVsB4LquKBQKUrTVVox4hDhMqCAhSLBN0n7jtdaWtV+kV22BYXYi9KF3G5QxBZeEnATOtq9+iGkfrDD6Y00cxxFImst0D1WQPM9LvTYMQ00sTfE0xYmgLgu1G6Ab65l+z/O0fDLzvVgsas9ALRYk3Ra+pba61jhM6LdtNpuinbRGbKT9xiTq3d4vFWkSt7y8jCiK8NZbb+nmGQzzhPD000/L/+v1Ok6cOKGFP//88/L/69evw3EcHDlyBADw999/AwCOHDmCOI6Ry+WQz+fl+d2oVCoYHx83DwMAVldXUavVIITA/Pw8Tp8+jZGRERneaDTw5ptvatcQS0tLeOedd+T+3bt3cfToUblfrVZx/PhxuQ8AuVwOAHDp0iUEQYCLFy9q4TYeN/10bRzHaDQaGBsbU+6g5/unn36KgwcPyv1///0X+K8aiwcPHmB+fh5vvPGGDMcjxKGatwHAnj17gCSuy5cv47PPPtPCe0Hp+fHHH80gK1KUK5UKAODQoUNqeCqtVgu5XA7Dw8MYGhrC0NAQMpnMtrG/zGQyePHFF7VjMzMz8lnq9boWtpHY0sJsHnv37oXjOLh+/TqQvNsfffQRXnnlFXmO4zj4448/AABzc3OYnJzEpUuXZDjRarVw6tQpnD17FgcOHJDHJyYm8Ndff2nnIrkXADzzzDNmkITirdfrOHXqlLR/pXd079692vlEpVLByy+/LPer1aomno1GA/v27cPs7Kz8nu/cuSOfa3x8HJVKBaVSSV5Tr9cxPDws99cj/YcPHwYA/Pnnn0DyPLOzs9r1VNA5joPR0VGUSiXtG11eXgYSu+FTp07J42uJg84/fPhwhzD7vo9vv/0Wd+/exbFjx7QwlRdeeME8hJGREXieh2q1agbZoSqzv4Y+JKqOZ7NZOUiQT0Y5+2nuDAKUfpMwGTXfTNLSwmwetcQ6gprCnudp/cU0uENh5ntOzWjf963NVLq/CjWLqelto5hYQ9E7ot7b7I5QMfteKX3qoJ7neXKATLU8oGtoX/0eyoqZ4HqkX80TGnBzXVeeR7pC+Urfp9odQcccZbCO6CcONY3m9YTZL23STqwvzEFTgvI37XoVmdv04L2gH9cmIt0StZ6YHe/riTowsV5sZHqZ7YMp9NuRMDG/e9LwDRtzk3w+39VShQqXbvcgIBSV73ZTgkrXfhR/I4iiSLiu21cB8ijQSO56sdHpZbYPZDb3JIradsRTbLq7DQQ2LR59JtRy6UdbIJQLbCOHKv2ep1Kr1UQ2Mbin2rjahKGmg5M4rFAmAOh4gamZYm7FYlG0FUNvatqoTU7KVDoHlpqrOspK3jjqtUJxw3UUzy/C932tqdQtvXR+WlqEMtJM55hNK7o/NWHp5UCfrR5m84miSIRh2NH9wQweSDSo229FXT/dBFmsUTvXJMoklv2W9NRsp9LBrDWqD4RETIMgEFEUyW4SM02UVjOj1D4nJzHyJ7E20+x5nrUvjgoIulYktqZI+oLM9Jppoz4tlbT0EmlpCQyXX/M3otKbbE0LhYIIw1C0221pb24WGgzDbA30/Zr6YGNNokwd5P1AtU4zEdS3okIip9aM04SPBCetYCBRNq9TcV1XGyggSAjVmrGtL4ieTW2KUCGiXiv6SK8tLVSYmffyLUboJMrqcdNpYCeitjJ6bWaeMcxW0O+72HXqTpVWq4XV1VV4nmcGWbl8+TIA4OOPPzaDOvjtt98AANPT09ImkExXTHvCmzdvwnVdeZ5JtVrF8PAwzp07ZwYByXNEUYT9+/ebQahWqzh06FCHfarJ7du3AQCvvfaaPKaa2qh0S29aWr7++mu4rtszHQCwuLgIJHalxMrKCpDYzO5U5ufnkVQqem7z8/Pm5RpkBskbb+u1PQ59izIZx6u2m92oVqtWMVpYWIDjONoxEhbV6PvXX38FLMJSrVY1A3gVMgz/4IMPzCAJCappjx3HMaIownvvvacd/+effwBDbG/dugXHcTQ7VNsx9EivLS30DLZrKpUKdu/erR2rVqvwPE/L54WFBfi+r51XKpU6Xhx1M+0yN4NMJiPjJ3tS2icnhs3AFHHeeHvc7XHoW5TXSqPR6HCIiOMYlUqlowa4tLQEz/M0T59r1651CAvV1k3hI9Jq1yoknmZhQbV1U6yvXr3akTZTaOM4RqlU6ri2V3ptaaFnGB0dVc78v5G+6XFpCjjl8cTEhHbe77//jiAIIIRAPp+H53kQQqBcLsN1Xe35NosrV67AcRwEQSALX8/zkM1mceHCBfN0DVXQe22ZTMa8nGE2FXJuMXXHRt+iTDVFm1fSiRMnNO+YNKhL4+TJk9rxSqXSUTNsNBodwkK19X379mnHCapdm10IKnfu3OkQTyi1dTXTSqUSoijS3CqpRq22GM6fPw8kXlsqvdKblhYbFy9ehOM4WmuCapeql1ZawbR//34pdAsLCzK/jx07hmw2K88jzy4SNFp5Zm5uDsPDwzh48CDiOMbU1BSGhobkKhpqGF1v21cZGRnBJ598Ij2dcrkcDh48iFKp1LOQWM/uC4bZaMgd3Kxs2XgKAJ577jlAaarbGBkZQTab1dwb6QO+evWq1r8KAEEQ4Pbt27KEmJ2dxeTkJMIw1Lok6F5qv6q6jEq9XsfU1JTch+J2OTMzoxUGCwsLPWt9jUYDe/bsQavV0mrsJAyqu2Yul0MQBJpbJeUR/c3lcnj99dexuroKrDG9trQcOXIEruvi2rVriONYilqlUsEPP/ygPRsVQi+99JI8dv/+ffn/7OysdJE9ceKEvNacq+Dzzz8HkgLH8zyMjo6i3W6jWCxicnIScRxjcXERP/30ExqNBs6fP493330XYRhi165dWtiZM2fw4Ycfolwuy3NPnjwp99XfFgBOnz6NBw8eSMG2uS4zzE4lk8lgeHhY7z6kEb9+RgbJhpjsd5HYyJoWFnSuap/sKasUqJB1g2mdQPa5qg2gaovsJnPXqvaBdH43yLaXTOgISh/ZNXdzIqF7qDaMlK61pDctLc1k2SDKO3UKSBXfsIsWihtpmgUKWdrY7CqDHtMsdrPqMMMoHtqncBv+Glz8me6wHbROv3bEG02amWo2mxWu6+o6Rv/wh/FkkO8yX4Ja2PqWaRZDYypIFTPMjCdcp2kimXSoMDcrOE86/XjcbTRUAesnDbJPeWJiAqurqx3NS2ZnofYnm/SaZtGcClLFDNuoaSK3G3Ecy/55deDbZnnyuExOTuKbb76R4yKtVqtj0FPdNmpGR3o2sxtvqzhw4ACOHz8ux362ArJG69a1KiF1TnN+YHYOaQ45BBSHmFqy5pla60IXz0QzzKz5UtOtUCiIKFkPTa1JU/fGTnz/bA5E1L2zXs9bs8xCVy6XhZfM36B2W9F7sJE1avN92Gromfupqa43FLfpJJaG1snnum7PPllm+0L95UhZBYHczGGZZrFbX7QZRgW86dpO3pJBn9NE7hQof1TyljX72sq8L/QbUJ8+dU04yRwxNDcLFWy2+9VqNfkb2rqTiHa7LfL5vPztqZvJjJPOoX0V9Xp6z9R3JS2OcpcFXlXM66mrld5Zs+sMlvEl37IqyWZAHrq2b86G9qbQQ9s+PIZhHg0qpGrKUlHm4I5Q5kGJkom5HGWh1jCZ14TEmuY9IdGmMYA0/C5LNvm+L7LJ3OiRsqyVLU7ap2cRytQGdIye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width=\"357\" height=\"52\"\u003e\u003c/v:shape\u003e Additionally, specific terms have been temporally decomposed. Since we account for three temporal frequencies (background -B, sub-intraseasonal -SI and transient -T) and each term is a product of two variables, a decomposition yields 9 terms, each quantified also using signed explained covariance (here covariance between a decomposed and a full term is normalized by full term\u0026rsquo;s variance). All vorticity budget terms have been spatially averaged over the MS box, while contributions were calculated based on 21-25 November, a critical period of preTCG and TCG when pre-Senyar vortex was strengthening in the MS region. Hence, the budget calculation provide a quantitative assessment of key drivers of that process.\u0026nbsp;\n\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTrajectory of TC Senyar precursor\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAccodring to IBTrACS version 4\u003csup\u003e4\u003c/sup\u003e, genesis of TC Senyar occurred at 12 UTC on 25 November, when the cyclone center was located in the Malacca Strait. To identify and track the precursor of TC Senyar prior to this time, a backward-tracking approach was applied using ERA5 relative vorticity at the 850-hPa level, consistent with tracking of TC Seroja precursor\u003csup\u003e13\u003c/sup\u003e. The backward trajectory was initialized at the first reported position of TC Senyar. The local maximum in relative vorticity was then identified backward in time within a 1\u0026deg; \u0026times; 1\u0026deg; search box centered on the previous location. The newly detected vorticity maximum was used as the updated position, and the procedure was iteratively repeated at hourly intervals. Using this approach, the trajectory of the TC Senyar precursor (preSenyar) was reconstructed (Fig. 1c). PreSenyar trajectory based on vorticity at 850 hPa has been confirmed by an ensemble of trajectories derived based on vorticity on multiple levels in the atmosphere, by ranging from 800 hPa up to 300 hPa, each treated independently (Fig. S2 in SI).\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eContributions\u003c/p\u003e\n\u003cp\u003eM.M. designed the study. M.M and D.B.B. contributed to conceptualization. M.M., M.B., R.R., N.R.A. and D.B.B. performed formal analysis, investigation, and visualization. C.S. and D.B.B. performed spatio-temporal filtering. M.B. developed the Supplementary Information. All authors discussed the results. M.M., M.B. and D.B.B. wrote draft manuscript. All authors reviewed and edited the manuscript.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTropical cyclone tracks were obtained from NOAA\u0026rsquo;s International Best Track Archive for Climate Stewardship (IBTrACS) via the NCEI website (https://www.ncdc.noaa.gov/ibtracs). ERA5, the fifth generation of ECMWF atmospheric reanalysis of the global climate, was obtained from the Copernicus Climate Change Service Climate Data Store (https://cds.climate.copernicus.eu). The GSMaP precipitation data were provided by the Japan Aerospace Exploration Agency (JAXA) and are available at https://hokusai.eorc.jaxa.jp \u0026nbsp;after registration at https://www.eorc.jaxa.jp/ptree/index.html.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCode Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;The Python codes used for data processing are available from the corresponding author, Marzuki Marzuki ([email protected]) or Dariusz B. Baranowski, [email protected].\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eAcknowledgements (optional)\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eM.M acknowledges support from the Ministry of Higher Education, Science, and Technology of Indonesia and Universitas Andalas. D.B.B. and M.B. were supported by National Science Centre (NCN) of Poland (grant no. 2022/45/B/ST10/03836).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics declarations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCompeting\u0026nbsp;interests\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBNPB. Emergency Response Dashboard for Floods and Landslides: Aceh, North Sumatra, and West Sumatra Provinces. https://gis.bnpb.go.id/bansorsumatera2025/ (2026).\u003c/li\u003e\n\u003cli\u003eGray, W. M. Global view of the origin of tropical disturbances and storms. \u003cem\u003eMon. Weather Rev.\u003c/em\u003e \u003cstrong\u003e96\u003c/strong\u003e, 669\u0026ndash;700 (1968).\u003c/li\u003e\n\u003cli\u003eAnthes, R. A. \u003cem\u003eTropical Cyclones. Their Evolution, Structure and Effects\u003c/em\u003e. (American Meteorological Society, Boston, 1982).\u003c/li\u003e\n\u003cli\u003eKnapp, K. R., Kruk, M. C., Levinson, D. H., Diamond, H. J. \u0026amp; Neumann, C. J. The International Best Track Archive for Climate Stewardship (IBTrACS): Unifying Tropical Cyclone Data. \u003cem\u003eBull. Am. Meteorol. Soc.\u003c/em\u003e \u003cstrong\u003e91\u003c/strong\u003e, 363\u0026ndash;376 (2010).\u003c/li\u003e\n\u003cli\u003eDeng, L. \u0026amp; Li, T. Impact of background dynamic and thermodynamic states on distinctive annual cycle of near-equatorial tropical cyclogenesis over the western North Pacific. \u003cem\u003eJ. Meteorol. Res.\u003c/em\u003e \u003cstrong\u003e34\u003c/strong\u003e, 822\u0026ndash;835 (2020).\u003c/li\u003e\n\u003cli\u003eLi, Y., Li, T., Fu, C. \u0026amp; Hsu, P.-C. Near-equatorial tropical cyclone formation in western North Pacific: peak season and controlling parameter. \u003cem\u003eClim. Dyn.\u003c/em\u003e \u003cstrong\u003e52\u003c/strong\u003e, 2765\u0026ndash;2773 (2019).\u003c/li\u003e\n\u003cli\u003eLu, C., Ge, X. \u0026amp; Peng, M. Comparison of controlling parameters for near-equatorial tropical cyclone formation between western North Pacific and North Atlantic. \u003cem\u003eJ. Meteorol. Res.\u003c/em\u003e \u003cstrong\u003e35\u003c/strong\u003e, 623\u0026ndash;634 (2021).\u003c/li\u003e\n\u003cli\u003eFine, C. M., Johnson, R. H., Ciesielski, P. E. \u0026amp; Taft, R. K. The role of topographically induced vortices in tropical cyclone formation over the Indian Ocean. \u003cem\u003eMon. Weather Rev.\u003c/em\u003e \u003cstrong\u003e144\u003c/strong\u003e, 4827\u0026ndash;4847 (2016).\u003c/li\u003e\n\u003cli\u003eJohnson, P., Ciesielski, P., Fine, C. \u0026amp; Wang, C. Effects of the topography of Sumatra on tropical cyclone formation over the Indian Ocean. \u003cem\u003eMausam\u003c/em\u003e \u003cstrong\u003e74\u003c/strong\u003e, 389\u0026ndash;396 (2023).\u003c/li\u003e\n\u003cli\u003eChang, C., Liu, C. \u0026amp; Kuo, H. Typhoon Vamei: An equatorial tropical cyclone formation. \u003cem\u003eGeophys. Res. Lett.\u003c/em\u003e \u003cstrong\u003e30\u003c/strong\u003e, (2003).\u003c/li\u003e\n\u003cli\u003eJuneng, L., Tangang, F. T., Reason, C. J. C., Moten, S. \u0026amp; Hassan, W. A. W. Simulation of tropical cyclone Vamei (2001) using the PSU/NCAR MM5 model. \u003cem\u003eMeteorol. Atmos. Phys.\u003c/em\u003e \u003cstrong\u003e97\u003c/strong\u003e, 273\u0026ndash;290 (2007).\u003c/li\u003e\n\u003cli\u003eMET Malaysia. \u003cem\u003eOnset of the Northeast Monsoon 2025/2026\u003c/em\u003e. https://www.met.gov.my/assets/content/siaran_media/default/10 November 2025 - Kenyataan Media Permulaan Monsun Timur Laut 2025_2026.pdf (2025).\u003c/li\u003e\n\u003cli\u003eLatos, B. \u003cem\u003eet al.\u003c/em\u003e The role of tropical waves in the genesis of Tropical Cyclone Seroja in the Maritime Continent. \u003cem\u003eNat. Commun.\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 856 (2023).\u003c/li\u003e\n\u003cli\u003eRajasree, V. P. M. \u003cem\u003eet al.\u003c/em\u003e Tropical cyclogenesis: Controlling factors and physical mechanisms. \u003cem\u003eTrop. Cyclone Res. Rev.\u003c/em\u003e \u003cstrong\u003e12\u003c/strong\u003e, 165\u0026ndash;181 (2023).\u003c/li\u003e\n\u003cli\u003eChambers, C. R. S. \u0026amp; Li, T. Simulation of formation of a near-equatorial typhoon Vamei (2001). \u003cem\u003eMeteorol. Atmos. Phys.\u003c/em\u003e \u003cstrong\u003e98\u003c/strong\u003e, 67\u0026ndash;80 (2007).\u003c/li\u003e\n\u003cli\u003eSteenkamp, S. C., Kilroy, G. \u0026amp; Smith, R. K. Tropical cyclogenesis at and near the Equator. \u003cem\u003eQ. J. R. Meteorol. Soc.\u003c/em\u003e \u003cstrong\u003e145\u003c/strong\u003e, 1846\u0026ndash;1864 (2019).\u003c/li\u003e\n\u003cli\u003eSall, S. M., Sauvageot, H., Gaye, A. T., Viltard, A. \u0026amp; De Felice, P. A cyclogenesis index for tropical Atlantic off the African coasts. \u003cem\u003eAtmos. Res.\u003c/em\u003e \u003cstrong\u003e79\u003c/strong\u003e, 123\u0026ndash;147 (2006).\u003c/li\u003e\n\u003cli\u003eZhao, C. \u0026amp; Li, T. Basin dependence of the MJO modulating tropical cyclone genesis. \u003cem\u003eClim. Dyn.\u003c/em\u003e \u003cstrong\u003e52\u003c/strong\u003e, 6081\u0026ndash;6096 (2019).\u003c/li\u003e\n\u003cli\u003eWei, M., Fang, G. \u0026amp; Ge, Y. Tropical cyclone genesis prediction based on support vector machine considering effects of multiple meteorological parameters. \u003cem\u003eJ. Wind Eng. Ind. Aerodyn.\u003c/em\u003e \u003cstrong\u003e243\u003c/strong\u003e, 105591 (2023).\u003c/li\u003e\n\u003cli\u003eCamargo, S. J. \u0026amp; Sobel, A. H. Western North Pacific tropical cyclone intensity and ENSO. \u003cem\u003eJ. Clim.\u003c/em\u003e \u003cstrong\u003e18\u003c/strong\u003e, 2996\u0026ndash;3006 (2005).\u003c/li\u003e\n\u003cli\u003eWang, B. \u0026amp; Chan, J. C. L. How strong ENSO events affect tropical storm activity over the western North Pacific. \u003cem\u003eJ. Clim.\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, 1643\u0026ndash;1658 (2002).\u003c/li\u003e\n\u003cli\u003eSaji, N. H., Goswami, B. N., Vinayachandran, P. N. \u0026amp; Yamagata, T. A dipole mode in the tropical Indian Ocean. \u003cem\u003eNature\u003c/em\u003e \u003cstrong\u003e401\u003c/strong\u003e, 360\u0026ndash;363 (1999).\u003c/li\u003e\n\u003cli\u003eCai, W. \u003cem\u003eet al.\u003c/em\u003e Projected response of the Indian Ocean Dipole to greenhouse warming. \u003cem\u003eNat. Geosci.\u003c/em\u003e \u003cstrong\u003e6\u003c/strong\u003e, 999\u0026ndash;1007 (2013).\u003c/li\u003e\n\u003cli\u003eKilroy, G., Smith, R. K. \u0026amp; Montgomery, M. T. A unified view of tropical cyclogenesis and intensification. \u003cem\u003eQ. J. R. Meteorol. Soc.\u003c/em\u003e \u003cstrong\u003e143\u003c/strong\u003e, 450\u0026ndash;462 (2017).\u003c/li\u003e\n\u003cli\u003eEmanuel, K. A. Sensitivity of tropical cyclones to surface exchange coefficients and a revised steady-state model incorporating eye dynamics. \u003cem\u003eJ. Atmos. Sci.\u003c/em\u003e \u003cstrong\u003e52\u003c/strong\u003e, 3969\u0026ndash;3976 (1995).\u003c/li\u003e\n\u003cli\u003eEmanuel, K. A. An air-sea interaction theory for tropical cyclones. Part I: Steady-state maintenance. \u003cem\u003eJ. Atmos. Sci.\u003c/em\u003e \u003cstrong\u003e43\u003c/strong\u003e, 585\u0026ndash;605 (1986).\u003c/li\u003e\n\u003cli\u003eMrowiec, A. A., Garner, S. T. \u0026amp; Pauluis, O. M. Axisymmetric hurricane in a dry atmosphere: Theoretical framework and numerical experiments. \u003cem\u003eJ. Atmos. Sci.\u003c/em\u003e \u003cstrong\u003e68\u003c/strong\u003e, 1607\u0026ndash;1619 (2011).\u003c/li\u003e\n\u003cli\u003eCronin, T. W. \u0026amp; Chavas, D. R. Dry and semidry tropical cyclones. \u003cem\u003eJ. Atmos. Sci.\u003c/em\u003e \u003cstrong\u003e76\u003c/strong\u003e, 2193\u0026ndash;2212 (2019).\u003c/li\u003e\n\u003cli\u003eNolan, D. S. What is the trigger for tropical cyclogenesis? \u003cem\u003eAust. Meteorol. Mag.\u003c/em\u003e \u003cstrong\u003e56\u003c/strong\u003e, 241\u0026ndash;266 (2007).\u003c/li\u003e\n\u003cli\u003eRaymond, D. J. \u0026amp; Sessions, S. L. Evolution of convection during tropical cyclogenesis. \u003cem\u003eGeophys. Res. Lett.\u003c/em\u003e \u003cstrong\u003e34\u003c/strong\u003e, (2007).\u003c/li\u003e\n\u003cli\u003eGu, J.-F., Tan, Z.-M. \u0026amp; Qiu, X. Effects of vertical wind shear on inner-core thermodynamics of an idealized simulated tropical cyclone. \u003cem\u003eJ. Atmos. Sci.\u003c/em\u003e \u003cstrong\u003e72\u003c/strong\u003e, 511\u0026ndash;530 (2015).\u003c/li\u003e\n\u003cli\u003eRappin, E. D., Nolan, D. S. \u0026amp; Emanuel, K. A. Thermodynamic control of tropical cyclogenesis in environments of radiative‐convective equilibrium with shear. \u003cem\u003eQ. J. R. Meteorol. Soc.\u003c/em\u003e \u003cstrong\u003e136\u003c/strong\u003e, 1954\u0026ndash;1971 (2010).\u003c/li\u003e\n\u003cli\u003eKutty, G. \u0026amp; Gohil, K. The role of mid-level vortex in the intensification and weakening of tropical cyclones. \u003cem\u003eJ. Earth Syst. Sci.\u003c/em\u003e \u003cstrong\u003e126\u003c/strong\u003e, 94 (2017).\u003c/li\u003e\n\u003cli\u003eDunkerton, T. J., Montgomery, M. T. \u0026amp; Wang, Z. Tropical cyclogenesis in a tropical wave critical layer: Easterly waves. \u003cem\u003eAtmos. Chem. Phys.\u003c/em\u003e \u003cstrong\u003e9\u003c/strong\u003e, 5587\u0026ndash;5646 (2009).\u003c/li\u003e\n\u003cli\u003eBell, M. M. \u0026amp; Montgomery, M. T. Observed structure, evolution, and potential intensity of category 5 Hurricane Isabel (2003) from 12 to 14 September. \u003cem\u003eMon. Weather Rev.\u003c/em\u003e \u003cstrong\u003e136\u003c/strong\u003e, 2023\u0026ndash;2046 (2008).\u003c/li\u003e\n\u003cli\u003eRajasree, V. P. M., Routray, A., George, J. P., Kumar, S. \u0026amp; Kesarkar, A. P. Study of cyclogenesis of developing and non-developing tropical systems of NIO using NCUM forecasting system. \u003cem\u003eMeteorol. Atmos. Phys.\u003c/em\u003e \u003cstrong\u003e133\u003c/strong\u003e, 379\u0026ndash;397 (2021).\u003c/li\u003e\n\u003cli\u003eWu, S. \u0026amp; Fang, J. The evolution and role of midtropospheric cyclonic vortex in the formation of Super Typhoon Nepartak (2016). \u003cem\u003eJ. Geophys. Res. Atmos.\u003c/em\u003e \u003cstrong\u003e124\u003c/strong\u003e, 9277\u0026ndash;9298 (2019).\u003c/li\u003e\n\u003cli\u003eSchreck III, C. J., Molinari, J. \u0026amp; Aiyyer, A. A global view of equatorial waves and tropical cyclogenesis. \u003cem\u003eMon. Weather Rev.\u003c/em\u003e \u003cstrong\u003e140\u003c/strong\u003e, 774\u0026ndash;788 (2012).\u003c/li\u003e\n\u003cli\u003eKlotzbach, P. J. The Madden\u0026ndash;Julian oscillation\u0026rsquo;s impacts on worldwide tropical cyclone activity. \u003cem\u003eJ. Clim.\u003c/em\u003e \u003cstrong\u003e27\u003c/strong\u003e, 2317\u0026ndash;2330 (2014).\u003c/li\u003e\n\u003cli\u003eZhang, C. Madden\u0026ndash;Julian oscillation: Bridging weather and climate. \u003cem\u003eBull. Am. Meteorol. Soc.\u003c/em\u003e \u003cstrong\u003e94\u003c/strong\u003e, 1849\u0026ndash;1870 (2013).\u003c/li\u003e\n\u003cli\u003eVentrice, M. J., Thorncroft, C. D. \u0026amp; Schreck III, C. J. Impacts of convectively coupled Kelvin waves on environmental conditions for Atlantic tropical cyclogenesis. \u003cem\u003eMon. Weather Rev.\u003c/em\u003e \u003cstrong\u003e140\u003c/strong\u003e, 2198\u0026ndash;2214 (2012).\u003c/li\u003e\n\u003cli\u003eLawton, Q. A. \u0026amp; Majumdar, S. J. Convectively coupled Kelvin waves and tropical cyclogenesis: Connections through convection and moisture. \u003cem\u003eMon. Weather Rev.\u003c/em\u003e \u003cstrong\u003e151\u003c/strong\u003e, 1647\u0026ndash;1666 (2023).\u003c/li\u003e\n\u003cli\u003eLawton, Q. A., Majumdar, S. J., Dotterer, K., Thorncroft, C. \u0026amp; Schreck III, C. J. The influence of convectively coupled Kelvin waves on African easterly waves in a wave-following framework. \u003cem\u003eMon. Weather Rev.\u003c/em\u003e \u003cstrong\u003e150\u003c/strong\u003e, 2055\u0026ndash;2072 (2022).\u003c/li\u003e\n\u003cli\u003eSchreck III, C. J. Convectively coupled Kelvin waves and tropical cyclogenesis in a semi-Lagrangian framework. \u003cem\u003eMon. Weather Rev.\u003c/em\u003e \u003cstrong\u003e144\u003c/strong\u003e, 4131\u0026ndash;4139 (2016).\u003c/li\u003e\n\u003cli\u003eWang, S., Fang, J., Tang, X. \u0026amp; Tan, Z.-M. A survey of statistical relationships between tropical cyclone genesis and convectively coupled equatorial rossby waves. \u003cem\u003eAdv. Atmos. Sci.\u003c/em\u003e \u003cstrong\u003e39\u003c/strong\u003e, 747\u0026ndash;762 (2022).\u003c/li\u003e\n\u003cli\u003eMatthews, A. J. Dynamical propagation and growth mechanisms for convectively coupled equatorial Kelvin waves over the Indian Ocean. \u003cem\u003eQ. J. R. Meteorol. Soc.\u003c/em\u003e \u003cstrong\u003e147\u003c/strong\u003e, 4310\u0026ndash;4336 (2021).\u003c/li\u003e\n\u003cli\u003eTory, K. J., Dare, R. A., Davidson, N. E., McBride, J. L. \u0026amp; Chand, S. S. The importance of low-deformation vorticity in tropical cyclone formation. \u003cem\u003eAtmos. Chem. Phys.\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 2115\u0026ndash;2132 (2013).\u003c/li\u003e\n\u003cli\u003eKiladis, G. N. \u0026amp; Wheeler, M. Horizontal and vertical structure of observed tropospheric equatorial Rossby waves. \u003cem\u003eJ. Geophys. Res. Atmos.\u003c/em\u003e \u003cstrong\u003e100\u003c/strong\u003e, 22981\u0026ndash;22997 (1995).\u003c/li\u003e\n\u003cli\u003eMatsuno, T. Quasi-Geostrophic Motions in the Equatorial Area. \u003cem\u003eJ. Meteorol. Soc. Japan. Ser. II\u003c/em\u003e \u003cstrong\u003e44\u003c/strong\u003e, 25\u0026ndash;43 (1966).\u003c/li\u003e\n\u003cli\u003eYang, G.-Y., Hoskins, B. \u0026amp; Slingo, J. Convectively coupled equatorial waves. Part III: Synthesis structures and their forcing and evolution. \u003cem\u003eJ. Atmos. Sci.\u003c/em\u003e \u003cstrong\u003e64\u003c/strong\u003e, 3438\u0026ndash;3451 (2007).\u003c/li\u003e\n\u003cli\u003eKiladis, G. N., Wheeler, M. C., Haertel, P. T., Straub, K. H. \u0026amp; Roundy, P. E. Convectively coupled equatorial waves. \u003cem\u003eRev. Geophys.\u003c/em\u003e \u003cstrong\u003e47\u003c/strong\u003e, (2009).\u003c/li\u003e\n\u003cli\u003eRoose, S., Ajayamohan, R. S., Ray, P., Mohan, P. R. \u0026amp; Mohanakumar, K. ENSO influence on Bay of Bengal cyclogenesis confined to low latitudes. \u003cem\u003enpj Clim. Atmos. Sci.\u003c/em\u003e \u003cstrong\u003e5\u003c/strong\u003e, 31 (2022).\u003c/li\u003e\n\u003cli\u003eBaranowski, D. B., Flatau, M. K., Flatau, P. J. \u0026amp; Schmidt, J. M. Multiple and spin off initiation of atmospheric convectively coupled Kelvin waves. \u003cem\u003eClim. Dyn.\u003c/em\u003e \u003cstrong\u003e49\u003c/strong\u003e, 2991\u0026ndash;3009 (2017).\u003c/li\u003e\n\u003cli\u003eG. TAKAHASHI, H., Fukutomi, Y. \u0026amp; Matsumoto, J. The impact of long-lasting northerly surges of the East Asian winter monsoon on tropical cyclogenesis and its seasonal march. \u003cem\u003e気象集誌\u003c/em\u003e\u003cem\u003e. \u003c/em\u003e\u003cem\u003e第\u003c/em\u003e\u003cem\u003e 2 \u003c/em\u003e\u003cem\u003e輯\u003c/em\u003e \u003cstrong\u003e89\u003c/strong\u003e, 181\u0026ndash;200 (2011).\u003c/li\u003e\n\u003cli\u003eStudholme, J., Fedorov, A. V, Gulev, S. K., Emanuel, K. \u0026amp; Hodges, K. Poleward expansion of tropical cyclone latitudes in warming climates. \u003cem\u003eNat. Geosci.\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, 14\u0026ndash;28 (2022).\u003c/li\u003e\n\u003cli\u003eChang, C. P., Harr, P. A. \u0026amp; Chen, H.-J. Synoptic disturbances over the equatorial South China Sea and western Maritime Continent during boreal winter. \u003cem\u003eMon. Weather Rev.\u003c/em\u003e \u003cstrong\u003e133\u003c/strong\u003e, 489\u0026ndash;503 (2005).\u003c/li\u003e\n\u003cli\u003eChristensen, J. H. \u003cem\u003eet al.\u003c/em\u003e Climate phenomena and their relevance for future regional climate change. in \u003cem\u003eClimate change 2013 the physical science basis: Working group I contribution to the fifth assessment report of the intergovernmental panel on climate change\u003c/em\u003e 1217\u0026ndash;1308 (Cambridge University Press, 2013).\u003c/li\u003e\n\u003cli\u003eCai, W. \u003cem\u003eet al.\u003c/em\u003e ENSO and greenhouse warming. \u003cem\u003eNat. Clim. Chang.\u003c/em\u003e \u003cstrong\u003e5\u003c/strong\u003e, 849\u0026ndash;859 (2015).\u003c/li\u003e\n\u003cli\u003eHersbach, H. \u003cem\u003eet al.\u003c/em\u003e The ERA5 Global Reanalysis. \u003cem\u003eQ. J. R. Meteorol. Soc.\u003c/em\u003e \u003cstrong\u003e146\u003c/strong\u003e, 1999\u0026ndash;2049 (2020).\u003c/li\u003e\n\u003cli\u003eRamadhan, R. \u003cem\u003eet al.\u003c/em\u003e A preliminary assessment of the GSMaP Version 08 products over Indonesian maritime continent against gauge data. \u003cem\u003eRemote Sens.\u003c/em\u003e \u003cstrong\u003e15\u003c/strong\u003e, 1115 (2023).\u003c/li\u003e\n\u003cli\u003eWang, B. \u0026amp; Moon, J.-Y. An anomalous genesis potential index for MJO modulation of tropical cyclones. \u003cem\u003eJ. Clim.\u003c/em\u003e \u003cstrong\u003e30\u003c/strong\u003e, 4021\u0026ndash;4035 (2017).\u003c/li\u003e\n\u003cli\u003eFritz, C. \u0026amp; Wang, Z. Water vapor budget in a developing tropical cyclone and its implication for tropical cyclone formation. \u003cem\u003eJ. Atmos. Sci.\u003c/em\u003e \u003cstrong\u003e71\u003c/strong\u003e, 4321\u0026ndash;4332 (2014).\u003c/li\u003e\n\u003cli\u003eSun, Z., Gao, S. \u0026amp; Jian, M. Comparison of the Water Vapor Budget Evolution of Developing and Non-Developing Disturbances over the Western North Pacific. \u003cem\u003eRemote Sens.\u003c/em\u003e \u003cstrong\u003e16\u003c/strong\u003e, 2396 (2024).\u003c/li\u003e\n\u003cli\u003eDonohoe, A., Marshall, J., Ferreira, D. \u0026amp; Mcgee, D. The relationship between ITCZ location and cross-equatorial atmospheric heat transport: From the seasonal cycle to the Last Glacial Maximum. \u003cem\u003eJ. Clim.\u003c/em\u003e \u003cstrong\u003e26\u003c/strong\u003e, 3597\u0026ndash;3618 (2013).\u003c/li\u003e\n\u003cli\u003eWheeler, M. \u0026amp; Kiladis, G. N. Convectively coupled equatorial waves: Analysis of clouds and temperature in the wavenumber\u0026ndash;frequency domain. \u003cem\u003eJ. Atmos. Sci.\u003c/em\u003e\u003cstrong\u003e56\u003c/strong\u003e, 374\u0026ndash;399 (1999).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-9075294/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9075294/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Formation of a near equatorial Tropical Cyclone Senyar in Strait of Malacca on 25 November 2025 brought historical flooding across northern Sumatra and Malaysia a region where low planetary vorticity and nearly land-locked topography typically prohibit cyclogenesis. Here, we determine a complex, multi-scale genesis pathway of Senyar and elucidate critical superpostion of drivers across several spatiotemporal scales. La Nina and negative Indian Ocean Dipole conditions, and northward shift of the Intertropical Convergence Zone facilitated enhanced meridional shear between equatorial westerlies and the northeast monsoon, while interaction of several types of tropical waves facilitated an increase of vorticity and moisture with concurrent decrease in vertical winds shear in the Strait of Malacca, creating a nurturing environment for pre-Senyar disturbance to grow in, as well as drove its final spin up. All key ingredients enabling Senyar genesis were identifiable in observations and can be leveraged to improve forecasting such devastating, near-equatorial events.","manuscriptTitle":"Multi-Scale Genesis Pathway for the Formation of the Near- Equatorial Tropical Cyclone Senyar","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-22 09:00:53","doi":"10.21203/rs.3.rs-9075294/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"communications-earth-and-environment","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"commsenv","sideBox":"Learn more about [Communications Earth and Environment](https://www.nature.com/commsenv/)","snPcode":"","submissionUrl":"","title":"Communications Earth \u0026 Environment","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Communications Series","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"fafd1db9-4ea9-4797-a643-8f1504f1db04","owner":[],"postedDate":"April 22nd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":64403848,"name":"Earth and environmental sciences/Climate sciences/Atmospheric science/Atmospheric dynamics"},{"id":64403849,"name":"Earth and environmental sciences/Natural hazards"}],"tags":[],"updatedAt":"2026-04-22T09:00:53+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-22 09:00:53","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9075294","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9075294","identity":"rs-9075294","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}