Changing Northeast India Summer Monsoon Onset Dynamics in a Warming World

Changing Northeast India Summer Monsoon Onset Dynamics in a Warming World | 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 Changing Northeast India Summer Monsoon Onset Dynamics in a Warming World Simanta Das, Rahul Mahanta, Bijit Kumar Banerjee, B. N. Goswami This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6739417/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The planetary-scale onset of the Indian Summer Monsoon over Northeast India (NEI) typically occurs about two weeks earlier than the monsoon the onset over Kerala, extending the length of the rainy season in NEI to approximately 150 days. The dynamics of the May onset over NEI are strongly linked to interactions with extra tropical Rossby waves, which through downward and eastward propagation of potential vorticity (PV) drive low-level relative vorticity and initiate the onset process. Additionally, the quasi-biweekly oscillation (QBWO) also plays a significant role in modulating the onset dynamics over NEI. While climate change is known to weaken the seasonal mean monsoon over NEI, its role in amplifying extra tropical influences on the onset and the trend of early onset over NEI remains unclear. Here we assess these changes by examining tropical circulation patterns and extra tropical influences, specifically focusing on the intrusion of potential vorticity, over two distinct periods: 1940–1960 and 2002–2023. Our analysis reveals a notable increase in extra tropical influence in recent decades. In the 2002–2023 period, with clear evidence of downward PV intrusion and the generation of low-level cyclonic vorticity, both of which are largely absent during the earlier period. Lead-lag composites of PV anomalies at 200hPa further support these findings, showing prominent eastward-propagating PV anomalies around the onset in recent years. In contrast, during the 1940–1960 periods, monsoon onset over NEI was predominantly governed by local meso-scale convective activity and self-organization, as indicated by the strengthening of low-level cyclonic vorticity two days after the onset. During the earlier period, heat sources were more likely driven by local convective instability, as indicated by higher convective available potential energy, which was approximately 500 J/kg greater in May compared to recent decades. Furthermore, the study highlights the role of air-sea interactions in influencing the onset dynamics. In recent decades, the Bay of Bengal Sea surface temperature show a stronger correlation with the onset process, indicating that changes in SST patterns and associated heat fluxes play a crucial role in facilitating the extra tropical influences. These findings underscore a significant shift in the mechanisms driving monsoon onset over NEI, with increasing extra tropical influences and altered air-sea interactions in recent decades. This evolving dynamic, likely influenced by climate change, has critical implications for understanding and predicting monsoon variability in NEI. Earth and environmental sciences/Climate sciences/Atmospheric science Earth and environmental sciences/Climate sciences/Climate change Northeast India (NEI) Onset Potential Vorticity QBWO SST Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction The rainfall over Northeast India (NEI) during the boreal summer holds a unique place within the Indian Summer Monsoon Rainfall (ISMR). The dominant mode of inter-annual variability in June–September rainfall reveals that NEI rainfall often exhibits an out of phase relationship with the rest of India during large-scale interannual drought and flood events 1,2 . The JJAS mean rainfall over NEI is decreasing at a rate of 3.6 percent per degree increase of global mean temperature 3 , contrasting with a 3.5 percent per degree increase over Central India (CI) 4 . The annual cycle of daily rainfall over NEI, when compared to that over CI, indicates that the rainy season in NEI begins in May 5 . The climate of Northeast India within the ISMR region has a distinct identity, shaped by the mighty Brahmaputra River valley and its surrounding horseshoe-shaped topography. These observations led Mishra et al. 6 to define LRS using a definition of local onset and demise at all locations and find that the onset over NEI begins as early as early May , extending the LRS to nearly 160 days. Supporting this, Sharma et al. 7 show that May rainfall over NEI is organized on a super-synoptic spatial scale and argue that May rainfall is part of the Indian Summer Monsoon (ISM) and demonstrate that its inclusion of May rainfall significantly changes the variability and predictability of seasonal monsoon rainfall over NEI. Recent studies provide compelling evidence that the onset of the ISM over NEI typically occurs around mid-May, with withdrawal by mid-October, resulting in a length of the rainy season (LRS) of approximately 150 days 5,8 . These analyses collectively establish that May rainfall is an integral component of the ISMR over NEI. The ISMR during June–September is a core component of a convectively coupled system, functioning as an off-equatorial heat source driven by the latent heat released by rainfall that influences low-level cross-equatorial winds, the low-level westerly jet, and the monsoon trough 9–11 while the low-level convergence of moisture modulating the rainfall itself. While the June–September (JJAS) monsoon is primarily governed by the northward seasonal migration of the Inter Tropical Convergence Zone (ITCZ) 9–11 , the prevailing thinking was that the onset over NEI cannot occur in May as the ITCZ is located still close to the equator. However, Das et al. 5 revealed that onset over NEI could occur May even in the absence of ITCZ through interactions between extra tropical Rossby waves and regional orography playing a crucial role in driving the large-scale onset over NEI in mid-May in recent decades. They also proposed an alternative mechanism, suggesting that the 10–20 day quasi-biweekly oscillation (QBWO) coinciding with the onset could facilitate cyclonic winds interacting with the local orography, thereby enhancing convection and triggering the onset. While the climate change is known to weaken the seasonal mean monsoon over NEI 3 , its role in the increasing extratropical influence on the onset and in particular the early onset over NEI compared to the monsoon onset over Kerela (MoK) remains unclear. This study aims to investigate the impact of climate change on the onset and LRS days over NEI, as well as how the dynamics of the May onset over NEI have evolved over the years by comparing the potential drivers of onset over NEI in two periods, 1940–1960 and 2002–2023. The onset dynamics over NEI are closely linked to extra tropical transient Rossby waves and the tropical quasi-biweekly oscillations (QBWO) that play a critical role in initiating the onset. Therefore, it is essential to examine how climate change or multi-decadal variations in these Rossby waves have influenced the onset day over NEI. Climate change has a profound impact on global atmospheric circulation, with recent studies highlighting significant changes in the tropics. Evidence suggests that the tropical belt has expanded over the past few decades, a shift with potentially far-reaching implications for subtropical regions and the global climate system 12 . Research indicates that stratospheric cooling, driven by climate change, causes a lifting of the tropopause, which in turn leads to the poleward movement of tropospheric jet streams 13,14 . Lau et al. 15 found that stronger tropical SST anomalies significantly influence the strength of the jet stream over South Asia. The strength and position of the jet stream play a critical role in shaping the amplitude and wavelength of extra tropical Rossby waves. Given the observed influence of stronger tropical SST anomalies on the jet stream over South Asia 15 , an important scientific question arises: under these changing conditions of Rossby waves, how do the dynamics over NEI during May evolve over time? Furthermore, it is essential to investigate whether the strengthened jet streams have any impact on the influence of extra tropical transient Rossby wave and on the onset processes over NEI and the resultant onset date. A critical aspect of monsoon dynamics is the local air-sea interaction, particularly how sea surface temperatures (SSTs) respond to the onset day and, conversely, how variations in SSTs influence the onset day. The interaction between the atmosphere and ocean plays a significant role in shaping the dynamics of the Indian Summer Monsoon Rainfall (ISMR) annual cycle, its intra-seasonal and inter-annual variability 11,16–22 . The onset vortex is influenced by boundary conditions linked to local SSTs 23 . Atmospheric convection in the tropics is also sensitive to SST variations and fluctuations in surface fluxes from the underlying ocean. SSTs above 28°C, in conjunction with other factors, are crucial for initiating organized deep convection in the tropical atmosphere 24–28 . About one week before the Indian monsoon onset over Kerala, a warm pool (SST > 30.5°C) forms over the Arabian Sea 29 . These elevated SSTs in the Arabian Sea help generate a vortex that plays a crucial role in triggering the Indian monsoon onset over Kerala in early June 23 . A similar mechanism concerning the onset day in May over NEI remains a scientific question that needs to be addressed. This study aims to investigate the local air-sea response around the onset day and analyze how climate change may have impacted these interactions by comparing the periods 1940–1960 and 2002–2023 in the month of May. The paper is organized as follows. After describing the data and methods used in the study in Section 2, the results are presented in Section 3. Having discussed the shift in large-scale stationary wave pattern associated with May (Section 3.1), the change in rainfall pattern over NEI is presented in Section 3.2. This is followed by a discussion on increase in Extra tropical Transient Rossby waves on onset process in recent decades (Section 3.3), we discuss change in Onset Dynamics process using ERA5 (Section 3.4) and finally change in air sea interaction with Bay of Bangle (BOB) (Section 3.5). The Conclusions and Discussions are summarized in Section 4. Data and Methodology 2.1 Data In this study, we have utilized data from the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation reanalysis dataset, ERA5 30 , to analyze winds, geopotential, rainfall, air temperature (from 200hPa to 600hPa), potential vorticity, and sea surface temperature for the periods 1940–1960 and 2002–2023. ERA5 reanalysis data is available at a horizontal resolution of 25 km. Moreover, ERA5 provides hourly data on a comprehensive range of atmospheric, land-surface, and oceanic parameters, offering high-resolution and detailed climate information essential for long-term climate studies. The ERA5 rainfall data has been shown to represent both the amplitude and variability of observed rainfall over the North East India (NEI) region well when compared with Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) datasets for the period 1998–2021 5 . Given its consistency and reliability across a broad set of atmospheric variables, ERA5 was selected as the primary dataset for all analyses in this study focused on the impacts of climate change on monsoon onset over the NEI region. 2.2 Methodology The Tropospheric Temperature (TT), derived from the vertically averaged air temperature between 600hPa and 200hPa, has been widely used to develop indices for identifying monsoon onset and withdrawal over NEI. In this study, the TT gradient over NEI is calculated as the difference between the vertically averaged TT over the North Box (88 0 E–100 0 E, 5 0 N–35 0 N) and the South Box (40 0 E–100 0 E, 15 0 S–5 0 N) (Figure 2b). The date on which these gradient changes sign from negative to positive is defined as the monsoon onset date over NEI, while the date on which it changes from positive to negative is considered as the monsoon withdrawal date. Again, the stationary wave over Indian Summer Monsoon Region is obtained from geopotential and wind daily anomalies from which the zonal mean is removed and time averaged for May during both periods 1940-1960 and 2002-2023. To extract the Quasi-Biweekly Oscillation (QBWO) signals from wind anomalies, a two-dimensional (2D) Lanczos bandpass filter 31 with a 10–20-day window is applied using the NCAR Command Language (NCL) 32 . To remove long-term trends, the SST data has been de-trended prior to analysis using Climate Data Operators (CDO) 33 . Results 3.1 Shift in large-scale stationary wave pattern associated with May The monsoon onset over NEI is closely linked to the region's orography, which acts as a facilitator for organizing a super-synoptic convective heat source through interaction with a quasi-stationary low-level vortex. This vortex drives orographic uplift, playing a critical role in initiating the 'onset' over NEI 5 . To understand how climate change has influenced the onset dynamics in this region, we first analyze the global stationary Rossby wave-train ‘background’ that dominates the May climate in the Northern Hemisphere during the two periods: 1940–1960 and 2002–2023 (Figure 1a, 1b, 1c, 1d). The stationary wave in May is constructed from daily anomalies of winds and geopotential from which the zonal mean is removed and time averaged over both periods at 200hPa and 500hPa. Interestingly, the wave train exhibits distinct change in stationary wave during May with the vorticity patterns at 200hPa over the South Asian monsoon region showing a pair of negative (cyclonic) and positive (anticylonic) vorticity during 1940–1960 and a pair of positive and negative vorticity in the recent period (2002–2023) (Figure 1a, 1b). Additionally, the vortex located to the east, spanning NEI and south/central China, appears to be barotropic in nature in both periods. Thus, a notable shift is observed in recent decades, as the vorticity at 200hPa and 500hPa over this region transitions from positive to negative keeping the barotropic nature same. These substantial changes in large-scale wind circulation patterns and Rossby waves between 1940–1960 and 2002–2023 have profound implications for monsoon dynamics over NEI. During the earlier period (1940–1960), the climatological geopotential and winds at 200hPa (Figure 1a) reveal a stationary Rossby wave train in the Northern Hemisphere, characterized by a zonal wave number of 2–3. In contrast, in recent decades (Figure 1b), this pattern has shifted to a zonal wave number of 3–4, accompanied by a transition from negative to positive geopotential anomalies over the Indian Summer Monsoon region during May. These notable shifts in large-scale circulation and Rossby waves raise a critical scientific question: how have these changes influenced the monsoon onset and its dynamics over NEI? This study seeks to explore whether these altered atmospheric patterns have directly impacted the onset dynamics, offering valuable insights into the evolving relationship between climate and monsoon behavior over NEI. 3.2 Change in rainfall pattern over NEI The climatological annual cycle of daily rainfall (Figure 2a), when compared across the two periods, suggests that the overall pattern of the annual rainfall cycle remains largely unchanged. However, the mean rainfall has decreased in the recent period, both annually and specifically in May. For both periods, more than 50% of the annual rainfall range exceeds in May itself, indicating that the rainy season begins in May 34 . Based on the objective definition of onset over NEI 5,8 , marked by the change in the sign of the north-south propagation of the TT gradient, the onset date in the earlier period is identified as May 14 th , while in the recent period, it occurs around May 18 th . Interestingly, the withdrawal date remains almost unchanged across the two periods (Figure 2c). The regions used for calculating the TT gradient are shown in Figure 2b. A noteworthy observation is the role of low-level vorticity at 850hPa, averaged over 88°–100°E, 15°–27°N, in the onset dynamics over NEI. In the recent period, the onset is strongly associated with an increase in low-level cyclonic vorticity (Figure 2c, cyan), which intensifies by a factor of 3–4 on the onset day. In contrast, during the earlier period, the onset over NEI is not directly linked to such an increase in low-level cyclonic vorticity (Figure 2c, blue). Instead, the cyclonic vorticity intensifies by a similar factor of 3–4 only in June. Despite the establishment of the heat source in May during the earlier period, the absence of a significant increase in low-level cyclonic vorticity raises critical questions. Did the earlier period produce synoptic-scale rain spells in the absence of such vorticity, or were the rainfall events primarily driven by more frequent localized meso-scale convection? To determine whether the rainfall patterns during May in the earlier period exhibit synoptic-scale characteristics like those observed in June in daily time scale, we analyzed daily rainfall anomalies averaged over the region 88 0 –100 0 E, 22 0 –29.5 0 N. The analysis reveals that in both periods, May rainfall occurs in distinct spells, with five-day spells being common and spells lasting up to 9–10 days also observed (Figure 3). Notably, the earlier period shows a higher frequency of spells lasting more than 9–10 days. To further explore the spatial distribution and persistence of daily wet rainfall anomalies, composite daily anomalies were constructed for all spells equal to or longer than five days. These composites were calculated for days 1 through 5 of such spells, focusing on rainfall anomalies exceeding the 0.75th quantile of their standard deviation. The composite spatial structure of rainfall and 850hPa winds associated with these wet spells is shown in Figure 4 and Figure S1, respectively. The results highlight that wet spells are organized and of synoptic scale in both periods, beginning with weak anomalies on day 1, intensifying by days 2 and 3, and maintaining their strength thereafter in both periods. In the recent period, it may be noted that the rain tends to get organized into two belts on the leeside of the two mountain ranges indicating the role of low-level wind induced uplift on the organization (Figure 4 and S2). On the other hand, during the early period the synoptic scale organization seems to arise from a simultaneous self-organization of several meso-scale convective clusters. This is consistent with composite of low-level wind anomalies at the time of onset (Figure 5b, 5d). It may be noted that, the wind anomalies, with a persistent cyclonic vortex over NEI during the spells bring in additional moisture from Bay of Bengal and organize the rain bands along the orographic ranges (Figure 5d). However, during the earlier period the low-level winds are dominated by a weak anticyclonic vortex (Figure 5b) and divergence of moisture from NEI. In addition to its inability to bring additional moisture to the region, they fail to generate wind induced uplift and contribute to the organization. As a result, in the earlier period, the spells are initiated simultaneously in both valleys, local meso-scale convective activity and self-organization as may be seen in Figure 4(e,g,i). Despite the absence of increased vorticity on the onset day in the earlier period, the region experiences persistent synoptic-scale rainfall spells in May, which cannot be classified as mere local thunderstorms. These findings suggest that the heat source necessary for generating such synoptic-scale spells is already established in May during both the periods. This raises a critical question: do the dynamics of monsoon onset remain consistent across the two periods, or have they undergone significant changes? 3.3 Increase in Extra tropical Transient Rossby waves on onset process in recent decades In both periods, the background barotropic anti-cyclonic and cyclonic vorticity over the east and NEI (Figure 1) play a crucial role in facilitating the climatological onset of the Indian monsoon over NEI, occurring on May 14 th during 1940–1960 and May 18 th during 2002–2023. The interaction of low-level cyclonic winds (Figure S1) with the region's orography, combined with boundary layer convergence driven by cyclonic vorticity, has the potential to generate meso-scale weather systems of approximately 200 km in size on the southern lee side of the mountains, as seen in Figure 4. These systems are likely intensified through a CISK-type mechanism. Such an organized heat source can further attract moisture convergence from the Bay of Bengal, allowing the system to sustain itself and effectively establish the monsoon season in the region for both periods. The composite of wind anomalies at 200hPa and 850hPa (Figure 5) during the onset day reveals distinct differences between the two time periods. At the upper level, the anti-cyclonic vorticity (Figures 5a and 5c) exhibits a southeastward intrusion in recent decades, leading to the development of a low-pressure system over the Tibetan Plateau region (Figure 5c). In contrast, in the earlier period, this low-pressure system was positioned further north at the time of onset (Figure 5a). At the lower level (850hPa), the wind anomaly composite highlights the formation of a cyclonic vorticity over NEI in recent decades (Figure 5d), which likely plays a crucial role in triggering monsoon onset by creating an unstable atmosphere over the region. However, in the earlier period, a more asymmetric quasi-oscillatory pattern formed over the Bay of Bengal at the time of onset (Figure 5b). As we know that the climate of NEI is influenced by extra tropical transient Rossby waves through downward or southward intrusion 5 , the key question now is whether these changes in wind anomalies at the time of onset day have led to an increased influence of extra tropical transient Rossby waves in recent decades. The lead-lag composite of 850hPa relative vorticity, averaged over a broader region (88°E–100°E, 15°N–29°N) from seven days before to seven days after the onset (Figure 6a), reveals distinct differences between the two periods. In recent decades, large-scale low-level vorticity, initially around 2×10 -6 S -1 prior to onset, intensifies to approximately 5×10 -6 S -1 at the time of onset. In contrast, during the earlier period, low-level vorticity starts at a much weaker 0.3×10 -6 S -1 before onset and only strengthens to 4×10 -6 S -1 two days after the onset. It is noted that the organized heat source during onset level of background vorticity prior to onset by (3.0-3.5) ×10 -6 S -1 to the level after onset. Due to significantly higher level of background vorticity (2×10 -6 S -1 ), the transient wave vorticity could organize the convective activity 2 days before the onset, continue to intensity and reach steady level on onset day. On the other hand, the background vorticity in the early period is so low that the wave vorticity cannot generate enough large-scale updraft to generate organized convection on synoptic or larger scales. On the day of onset, several mesoscale convective events organize a heat source that through CISK type feedback intensifies the vorticity and in 2 days reaches the quasi-equilibrium level. The onset process also significantly increases average rainfall over the region. In recent decades, daily rainfall rises from ~6.5 mm/day before onset to nearly 3–4 times that amount following onset. For the earlier period, a similar three- to four-fold increase occurs, but only after two days from the onset date. Given that eastward-propagating extra tropical Rossby waves can influence low-level relative vorticity in May through the eastward and downward intrusion of potential vorticity (PV) 5 , it is crucial to examine how this PV intrusion has evolved between the two periods. A lead-lag composite of PV anomalies, averaged over the same region and analyzed as a function of height from 50hPa to 1000hPa from seven days before to seven days after onset, shows that in recent decades, the downward intrusion of positive PV from the stratosphere triggers an increase in cyclonic relative vorticity at low levels (850-700hPa), intensifying the background cyclonic vorticity at the time of onset (Figure 6d). Additionally, the lead-lag composite of PV anomalies at 200hPa (Figure S3), spanning three days before to three days after onset, reveals a southeastward-propagating transient Rossby wave with a wavelength of approximately 4000 km in recent decades. The fact that this transient Rossby wave remains prominent even after compositing over 21 years highlights its crucial role in the monsoon onset process in recent decades. However, in the earlier period, no such downward PV intrusion occurs at the time of onset (Figure 6c) with only the background weak cyclonic vorticity at low levels (850-700hPa), nor is there any indication of a southeastward-propagating transient Rossby wave (Figure S4) as the travelling transient Rossby waves get dissipated at the time of onset over the NEI region at 200hPa. From this analysis, it is evident that the influence of extra tropical transient Rossby waves has significantly increased in recent decades compared to the earlier period. This raises an important question that if extra tropical transient Rossby waves did not play a significant role in the earlier period, what alternative mechanisms triggered the onset of the monsoon over NEI during that time and increase the low-level vorticity after two days of onset date? 3.4 Change in Onset Dynamics process Since the downward and southeastward intrusion of PV is not observed at the time of onset in the earlier period, we analyze the evolution of the quasi-biweekly oscillation (QBWO) over the Indian subcontinent concerning the onset day, from seven days before to seven days after. This is crucial as daily May rainfall over NEI is predominantly influenced by a 10–20 day QBWO 5 . The QBWO, recognized as a westward-propagating, convectively unstable equatorial Rossby wave 20,35 , typically originates over the Western Pacific warm pool before propagating westward over the North Bay of Bengal or NEI. To investigate this, a 10–20 day bandpass-filtered daily wind anomaly at 850hPa over the period 1940–1960 was computed using Lanczos filter weights. A lead-lag composite of the filtered rainfall anomalies with respect to the onset day (Figure 7) reveals that an asymmetric QBWO travels toward NEI, and at the time of onset, these oscillations merge, generating anti-cyclonic vorticity over the Bay of Bengal. This vorticity pattern likely helps initiate the onset process over NEI during the earlier period. Interestingly, this anti-cyclonic vorticity later transitions into cyclonic vorticity over NEI (Figure 7), coinciding with increased vorticity and rainfall over the region two days after onset (Figure 6a, 6b). Furthermore, a comparison of the annual cycle of convective available potential energy (CAPE) between the two periods (Figure S5) indicates that during the earlier period, CAPE increases by approximately 500 J/kg at the time of onset. In contrast, in recent decades, CAPE remains unchanged at the time of onset. This suggests that in the earlier period, the onset process over NEI was triggered by the traveling asymmetric QBWO, which, along with enhanced CAPE, facilitated the onset. Once initiated, the Matsuno–Gill response 36 sustained elevated northward moisture transport during May and beyond. However, in recent decades, this traveling asymmetric QBWO is absent (Figure S6). Instead, a northward propagating wave appears to generate cyclonic vorticity over the Bay of Bengal at the time of onset. Again, the spatial distribution of CAPE compared between the two periods (Figure S7) reveals that CAPE was more concentrated around the southern valley of the region in the earlier period. Since the influence of subtropical transient waves was weaker during this time, the initiation of wet rain spells was primarily observed in the southern valley (Figure 4). However, in recent decades, with an increased influence of subtropical winds, wet spells now initiate in both valleys, indicating a major shift in the dynamics of monsoon onset over NEI. The significance of the QBWO is further appreciated by the fact that, even after compositing over 21 years, its influence remains clearly visible during onset in the earlier period. These findings indicate that the monsoon onset process over NEI is evolving in a warming world, with a fundamental shift in the underlying mechanisms driving its initiation. 3.5 Change in air sea interaction with Bay of Bangle (BOB) The Indian Summer Monsoon is a coupled atmosphere-ocean system with intricate feedback mechanisms between atmospheric circulation and oceanic conditions 22 . However, the oceanic response over the Bay of Bengal (BOB) during the monsoon season in May, particularly in relation to monsoon onset over NEI, remains unclear. In this section, we explore how sea surface temperature (SST) over the BOB influences and in turn responds to the onset over NEI and examine how this relationship has evolved in a warming climate. The spatial structure of the lead-lag composite of de-trended SST anomalies across the North Indian Ocean (NIO) is shown in Figures S8 and S9 from 7-days before onset to 7-days after onset during recent and earlier decades respectively. It may be noted that the SST anomalies over the whole of NIO evolves coherently through the NEI onset till a week beyond onset. If we focus on the BOB (Figure 8b), during recent years, the positive SST anomalies persist over BOB till onset and become negative after the onset as could be seen from lead-lag composite SST anomalies averaged over BOB (Figure 8a). This is suggestive of an air-sea interaction on intra seasonal time scales where the increased moisture supply from higher than normal SST prior to the onset triggers the onset but invigoration of circulation or vorticity (Figure 6a) following the onset cools the ocean through increased evaporation. This air-sea interaction is rather weak compared to that associated with the Monsoon Intra Seasonal Oscillations (MISO) 37 but consistent that associated with the quasi-biweekly oscillation (QBWO) 17 . However, in the earlier period, the SST over BOB shows no significant evolution around the onset over NEI and remains close to zero throughout a two-week period around the NEI (Figure 8, S8) suggesting lack of air-sea interaction during NEI. These findings suggest that with the changing climate over NEI, the SST response over BOB has also evolved, aligning with shifts in the dynamics of monsoon onset over the region. Conclusions and Discussions This study provides compelling evidence that the onset dynamics of the Indian Summer Monsoon over NEI have undergone significant changes between the periods 1940–1960 and 2002–2023. During both periods, the May rainfall over NEI is distinctly monsoonal, marked by its organized and persistent nature, occurring in active and break spells that help establish a significant monsoon heat source over the region. Also, neither the length of the rainy season (LRS) nor the climatological onset date changes significantly. However, this study reveals that in a warming world, the onset process over NEI and its persistence are undergoing notable changes. Examining both atmospheric and oceanic effects is necessary to comprehend how the monsoon onset dynamics over NEI are changing in a warming environment. Our analysis of lead lag composite of PV help confirms that the influence of extra tropical transient Rossby wave have increased in a worming world. Again with the expansion of jet stream, the anti-cyclonic vorticity at 200hPa has significantly changed its position at time of onset moving more towards the Tibetan plateau region in the recent decades and the intensity of the lower level winds at 850hPa at the time of onset is also seen to be increased over northern valley of the region (Figure 5) which may affect the large scale organization of the rainfall spells in recent decades. We specifically observe notable change in the dominant processes that drive onset during the two periods. While a moving anti-symmetric QBWO together with local increase in instability like CAPE initiates the onset over NEI in the earlier period (1940–1960), in the recent decades (2002–2023) we have seen an increasing impact of extra tropical transient Rossby waves to initiate the onset over the region. The downward and southeastward intrusion of PV from the stratosphere in recent decades has been a key driver of increased low-level cyclonic vorticity, accelerating the onset process. In contrast, in the earlier period this southeastward and downward intrusion of PV from upper atmosphere gets dissipated at the time of onset resulting in weaker relative vorticity over the region which exhibited a more gradual onset. The lack of help from extra tropical Rossy waves is compensated by an asymmetric QBWO contributed to the monsoon onset through a complex interaction of westward-propagating waves over the Bay of Bengal. This study also reveals significant shifts in dynamics of organized wet spells in May over the Northeast India as the first organized wet spell leads to the NEI onset. The wet spells initiation has changed from the early to recent. In recent decades the initiation of the wet spells are more organized at two sides of the u-shaped topography, which indicates the role of low-level wind induced uplift on the organization. However, in the earlier period, rainfall spells in both valleys were initiated simultaneously, driven by local meso-scale convection and self-organization. This is evident from the fact that the large scale 850hPa low-level vorticity is increased by 3–4 times over the region in the month of June. For these large scale synoptic organization of vorticity at 850hPa in recent decades, the SST response with respect to the onset date is also seen over NIO as the SST over BOB regions gets cooler once the onset process start which suggests an intraseasonal air-sea interaction, where elevated SSTs prior to onset enhance moisture supply and trigger the monsoon onset, while strengthened circulation and vorticity after onset cool the ocean via increased evaporation. The compelling evidence presented here regarding the change in onset dynamic process over NEI suggests the need for a comprehensive reassessment of monsoon prediction models, improved climate adaptation strategies, and a deeper understanding of the evolving interactions between tropical and extra tropical systems in a warming world. Declarations Conflict of interest/Competing interest: No conflict of interest Author contributions BNG and S Das designed and conceptualized the study and wrote the initial manuscript. S Das carried out all the analyses and generated the graphics. All the authors contributed to writing the final manuscript. All the authors read the final manuscript and approved it for submission. Availability of data and material: Fifth-generation reanalysis (ERA5) data from the European Centre for Medium-Range Weather Forecasting (ECMWF) can be downloaded from https://climate.copernicus.eu/climate-reanalysis. Code availability: Used MATLAB, NCL, Python and climate data operator (CDO) Acknowledgement S. Das is grateful to Department of Science and Technology (DST), India for INSPIRE fellowship with grant No. DST/INSPIRE Fellowship/2020/IF200121 and thanks Cotton University for providing the research facilities. BNG thanks Science and Engineering Research Board (SERB), Government of India for supporting the computational facility for this work and Gauhati University for the Honorary Professor of Excellence. R.M. extends his appreciation to Cotton University for providing the essential infrastructure and resources necessary for conducting this research. References Shukla, J. Interannual variability of monsoons. Monsoons 14 , 399–464 (1987). Mishra, V., Smoliak, B. V., Lettenmaier, D. P. & Wallace, J. M. A prominent pattern of year-to-year variability in Indian Summer Monsoon Rainfall. Proc. Natl. Acad. Sci. 109 , 7213–7217 (2012). Zahan, Y., Mahanta, R., Rajesh, P. V. & Goswami, B. N. Impact of climate change on North-East India (NEI) summer monsoon rainfall. Clim. Change 164 , 2 (2021). Rajesh, P. V., Goswami, B. N., Choudhury, B. A. & Zahan, Y. Large Sensitivity of Simulated Indian Summer Monsoon Rainfall (ISMR) to Global Warming: Implications of ISMR Projections. J. Geophys. Res. Atmospheres 126 , e2020JD033511 (2021). Das, S., Goswami, D. J., Mahanta, R., Saha, P. & Goswami, B. N. Dynamics of May ‘onset’ of Indian summer monsoon over Northeast India. Q. J. R. Meteorol. Soc. 150 , 4533–4549 (2024). Misra, V., Bhardwaj, A. & Mishra, A. Local onset and demise of the Indian summer monsoon. Clim. Dyn. 51 , 1609–1622 (2018). Sharma, D., Das, S. & Goswami, B. N. Variability and predictability of the Northeast India summer monsoon rainfall. Int. J. Climatol. 43 , 5248–5268 (2023). Saha, P., Mahanta, R. & Goswami, B. N. Present and future of the South Asian summer monsoon’s rainy season over Northeast India. Npj Clim. Atmospheric Sci. 6 , 170 (2023). Chakraborty, A. & Goswami, D. Prediction of slope stability using multiple linear regression (MLR) and artificial neural network (ANN). Arab. J. Geosci. 10 , 385 (2017). Gadgil, S. The Indian Monsoon and Its Variability. Annu. Rev. Earth Planet. Sci. 31 , 429–467 (2003). Webster, P. J. et al. Monsoons: Processes, predictability, and the prospects for prediction. J. Geophys. Res. Oceans 103 , 14451–14510 (1998). Seidel, D. J., Fu, Q., Randel, W. J. & Reichler, T. J. Widening of the tropical belt in a changing climate. Nat. Geosci. 1 , 21–24 (2008). Williams, G. P. Circulation sensitivity to tropopause height. J. Atmospheric Sci. 63 , 1954–1961 (2006). Lorenz, D. J. & DeWeaver, E. T. Tropopause height and zonal wind response to global warming in the IPCC scenario integrations. J. Geophys. Res. Atmospheres 112 , 2006JD008087 (2007). Lau, N.-C., Leetmaa, A. & Nath, M. J. Attribution of atmospheric variations in the 1997–2003 period to SST anomalies in the Pacific and Indian Ocean basins. J. Clim. 19 , 3607–3628 (2006). Roxy, M. & Tanimoto, Y. Role of SST over the Indian Ocean in influencing the intraseasonal variability of the Indian summer monsoon. J. Meteorol. Soc. Jpn. Ser II 85 , 349–358 (2007). Goswami, B. N. & Saha, P. Chapter 4 - Quasi-biweekly mode of the South Asian monsoon. in Atmospheric Oscillations (ed. Guan, B.) 69–98 (Elsevier, 2025). doi:10.1016/B978-0-443-15638-0.00004-6. Sengupta, D. & Ravichandran, M. Oscillations of Bay of Bengal sea surface temperature during the 1998 Summer Monsoon. Geophys. Res. Lett. 28 , 2033–2036 (2001). Ashok, K., Guan, Z., Saji, N. H. & Yamagata, T. Individual and Combined Influences of ENSO and the Indian Ocean Dipole on the Indian Summer Monsoon. (2004). Kikuchi, K. & Wang, B. Global perspective of the quasi-biweekly oscillation. J. Clim. 22 , 1340–1359 (2009). Goswami, B. N., Rao, S. A., Sengupta, D. & Chakravorty, S. Monsoons to Mixing in the Bay of Bengal: Multiscale Air-Sea Interactions and Monsoon Predictability. Oceanography 29 , 18–27 (2016). Webster, P. J. The coupled monsoon system. in The Asian Monsoon 3–66 (Springer Berlin Heidelberg, 2006). doi:10.1007/3-540-37722-0_1. Joseph, P. V. Warm Pool over the Indian Ocean and Monsoon Onset . (1990). Gray, W. M. Tropical cyclone genesis. (1975). Gray, W. M. The formation of tropical cyclones. Meteorol. Atmospheric Phys. 67 , 37–69 (1998). Gray, W. M. Global view of the origin of tropical disturbances and storms. Mon. Weather Rev. 96 , 669–700 (1968). Harr, P. A. & Chan, J. C. L. Monsoon impacts on tropical cyclone variability. Glob. Monsoon Syst. Res. Forecast 512 , 542 (2005). Gadgil, S., Joseph, P. V. & Joshi, N. V. Ocean–atmosphere coupling over monsoon regions. Nature 312 , 141–143 (1984). Seetaramayya, P. & Master, A. Observed air-sea interface conditions and a monsoon depression during MONEX-79. Arch. Meteorol. Geophys. Bioclimatol. Ser. B Theor. Appl. Climatol. 33 , 61–67 (1984). Hersbach, H. et al. The ERA5 global reanalysis. Q. J. R. Meteorol. Soc. 146 , 1999–2049 (2020). Duchon, C. E. Lanczos filtering in one and two dimensions. J. Appl. Meteorol. 1962-1982 1016–1022 (1979). Boulder, C. The NCAR Command Language (NCL, Version 6.4. 0)[Software] . (2019). Uwe, S. CDO user guide (2.3. 0). Zenodo. (2023). Wang, B. & LinHo, x. Rainy season of the Asian–Pacific summer monsoon. J. Clim. 15 , 386–398 (2002). Chatterjee, P. & Goswami, B. N. Structure, genesis and scale selection of the tropical quasi‐biweekly mode. Q. J. R. Meteorol. Soc. 130 , 1171–1194 (2004). Gill, A. E. Some simple solutions for heat‐induced tropical circulation. Q. J. R. Meteorol. Soc. 106 , 447–462 (1980). Lau, W. K.-M. & Waliser, D. E. Intraseasonal Variability in the Atmosphere-Ocean Climate System . (Springer Science & Business Media, 2011). Additional Declarations No competing interests reported. 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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-6739417","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":465075969,"identity":"d45eea50-b4a9-4c04-bd7e-43dee5b18530","order_by":0,"name":"Simanta Das","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA6UlEQVRIiWNgGAWjYDACZgglZ9/MfABIS8gQqSWBwdiAvS0BpIWHSKsSGBI38JwxADEJa5Fv5078XPjDjnG7RM7nVzdqLHgY2A8f3YBPi8Fh3s3SMxKSmS1n5G6zzjkGdBhPWtoNvFqYeTdI8yQwszHcyN1mnMMG1CLBY4ZXi3wz7+bfPAn1PAw3cp4Z5/wjQgvDYd5tQFsOSxicOcP8OLeNCC1Av2yz5kk7biDZ3mbGnNsnwcNGyC/y/Wc33+axqa7vZ2Z+/DnnW50cP/vhY/gdhgTYJMAkscpBgPkDKapHwSgYBaNg5AAAtcZCyor+ZhAAAAAASUVORK5CYII=","orcid":"","institution":"Cotton University","correspondingAuthor":true,"prefix":"","firstName":"Simanta","middleName":"","lastName":"Das","suffix":""},{"id":465075970,"identity":"a8c5311b-eba5-46cd-861a-ff5e13800d35","order_by":1,"name":"Rahul Mahanta","email":"","orcid":"","institution":"Cotton University","correspondingAuthor":false,"prefix":"","firstName":"Rahul","middleName":"","lastName":"Mahanta","suffix":""},{"id":465075971,"identity":"a25d9671-7826-457e-a2f3-8c2219e4ce87","order_by":2,"name":"Bijit Kumar Banerjee","email":"","orcid":"","institution":"Gauhati University","correspondingAuthor":false,"prefix":"","firstName":"Bijit","middleName":"Kumar","lastName":"Banerjee","suffix":""},{"id":465075972,"identity":"e748a156-b306-432e-9d3d-800dc9ba8eeb","order_by":3,"name":"B. N. Goswami","email":"","orcid":"","institution":"Cotton University","correspondingAuthor":false,"prefix":"","firstName":"B.","middleName":"N.","lastName":"Goswami","suffix":""}],"badges":[],"createdAt":"2025-05-24 13:53:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6739417/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6739417/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":83829455,"identity":"ca7c1250-fed2-4feb-80ea-900c42958f37","added_by":"auto","created_at":"2025-06-03 11:12:33","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":3288341,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The observed May stationary wave pattern in geopotential height and winds at 200hPa during 1940-1960. The stationary wave is obtained from daily anomalies from which the zonal mean is removed and time averaged for May. Units are m and m/sec respectively. (b) Same as (a) but for the periods: 2002-2023. (c) Same as (a) but at 500hPa. (d) Same as (b) but at 500hPa.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6739417/v1/b560d260fa3d53e2bdbfbc10.png"},{"id":83829463,"identity":"cf20d124-e2fd-48fa-99e4-3f79296179c0","added_by":"auto","created_at":"2025-06-03 11:12:33","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1289791,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Daily Annual Cycle of rainfall averaged over NEI compared with the two periods: 1940 1960 and 2002-2023 (mm/day). (b) The map highlights the regions used for calculating the tropospheric temperature (TT) gradients, marked by the thick green box. (c) Climatological TT gradient for the NEI heat source and vorticity multiplied by 106 between the region 880E to 1000E and 150N to 270N (January 1-December 31) over the two periods 1940-1960 and 2002-2023. Units of vorticity are sec-1.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6739417/v1/84dbbbe493c05979dede4599.png"},{"id":83829458,"identity":"424593ac-c48b-46c9-be90-dcae606f6a53","added_by":"auto","created_at":"2025-06-03 11:12:33","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1015388,"visible":true,"origin":"","legend":"\u003cp\u003eTime series of daily rainfall anomalies normalized with its own standard deviation of May over NEI over the two periods: 1940-1960 and 2002-2023. Unit: mm/day\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6739417/v1/be16a904f4bf560db8f6fb8f.png"},{"id":83829459,"identity":"732f8cd6-a9b5-4387-bee7-17a067ea3cd1","added_by":"auto","created_at":"2025-06-03 11:12:33","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1939926,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Composite of rainfall anomalies for day 1 of the 5 day wet spells during 1940-1960. Unit: mm/day. (b) Same as (a) but for the period 2002-2023. (c) Same as (a) but for day 2. (d) Same as (b) but for day 2. (e) Same as (a) but for day 3. (f) Same as (b) but for day 3. (g) Same as (a) but for day 4. (h) Same as (b) but for day 4. (i) Same as (a) but for day 5. (j) Same as (b) but for day 5.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6739417/v1/a9de876a7e5bdeee743f3ba7.png"},{"id":83829615,"identity":"120c10e4-312d-40f2-8144-76c900d2d651","added_by":"auto","created_at":"2025-06-03 11:20:33","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1501919,"visible":true,"origin":"","legend":"\u003cp\u003ea) Composite of wind anomaly at 200hPa at the onset day during 1940-1960. (b) Same as (a) but for the periods: 2002-2023. (c) Same as (a) but at 850hPa. (d) Same as (b) but at 850hPa.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6739417/v1/ca3fca22f101287ad9cac0d6.png"},{"id":83829457,"identity":"7e634359-062f-41c1-97c7-b7937230cf4a","added_by":"auto","created_at":"2025-06-03 11:12:33","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":2180758,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Lead–lag composite of vorticity at 850hPa (10-6 S-1) from 7-days before to 7-days after the onset averaged over the box 880 E to 1000 E and 150 N to 290 N over the two periods: 1940-1960 and 2002-2023. (b) Same as (a) but for rainfall (mm/day) over the box. (c) Lead–lag composites of potential vorticity (PV) anomalies at pressure level from 1000 to 50hPa with respect to onset dates between 1940 to 1960 averaged over the box between 880 E and 1000 E and 150 N and 290 N from seven days before onset to seven days after onset. (d) Same as (c) but for the period 2002 to 2023.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6739417/v1/f791e67c9417bd2b766c39d4.png"},{"id":83829461,"identity":"289fc7ba-ab00-48e3-9872-280d87c70c54","added_by":"auto","created_at":"2025-06-03 11:12:33","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":4275666,"visible":true,"origin":"","legend":"\u003cp\u003eLead–lag composite of 10-20 day filtered wind anomalies at 850hPa from 7-days before to 7 days after the onset over the periods: 1940-1960.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6739417/v1/831dbcd89eea505247770464.png"},{"id":83829614,"identity":"c176cba4-cc67-40cf-9428-10de32d523f4","added_by":"auto","created_at":"2025-06-03 11:20:33","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":426304,"visible":true,"origin":"","legend":"\u003cp\u003ea) Lead lag composite of SST over BOB with respect to onset day from 7 days before onset to 7 days after onset over the period 1940-1960 (red line) and 2002-2023 (blue line). b) The map highlights the regions used for calculating the SST anomaly over the BOB region, marked by the thick green box.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-6739417/v1/4b362373449f91fc2ef7cfce.png"},{"id":83830239,"identity":"184870dd-f821-41f6-88a7-edbeea269726","added_by":"auto","created_at":"2025-06-03 11:28:41","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":16764600,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6739417/v1/206a5dd1-2ad5-4ff9-9750-87254b17fcff.pdf"},{"id":83829464,"identity":"5b0dcabd-e8be-4606-b8e1-ba2dc2e40f0d","added_by":"auto","created_at":"2025-06-03 11:12:33","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":8540789,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterial.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6739417/v1/c66ee41b0925bcda258d1242.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Changing Northeast India Summer Monsoon Onset Dynamics in a Warming World","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe rainfall over Northeast India (NEI) during the boreal summer holds a unique place within the Indian Summer Monsoon Rainfall (ISMR). The dominant mode of inter-annual variability in June\u0026ndash;September rainfall reveals that NEI rainfall often exhibits an out of phase relationship with the rest of India during large-scale interannual drought and flood events\u003csup\u003e1,2\u003c/sup\u003e. The JJAS mean rainfall over NEI is decreasing at a rate of 3.6 percent per degree increase of global mean temperature\u003csup\u003e3\u003c/sup\u003e, contrasting with a 3.5 percent per degree increase over Central India (CI)\u003csup\u003e4\u003c/sup\u003e. The annual cycle of daily rainfall over NEI, when compared to that over CI, indicates that the rainy season in NEI begins in May\u003csup\u003e5\u003c/sup\u003e. The climate of Northeast India within the ISMR region has a distinct identity, shaped by the \u003cstrong\u003emighty Brahmaputra River valley\u003c/strong\u003e and its surrounding horseshoe-shaped topography. These observations led Mishra et al.\u003csup\u003e6\u003c/sup\u003e to define LRS using a definition of local onset and demise at all locations and find that the onset over NEI begins as early as \u003cstrong\u003eearly May\u003c/strong\u003e\u003cstrong\u003e,\u003c/strong\u003e extending the LRS to nearly 160 days. Supporting this, Sharma et al.\u003csup\u003e7\u003c/sup\u003e show that May rainfall over NEI is organized on a super-synoptic spatial scale and argue that \u003cstrong\u003eMay rainfall\u003c/strong\u003e is part of the Indian Summer Monsoon (ISM) and demonstrate that its inclusion of May rainfall significantly changes the variability and predictability of seasonal monsoon rainfall over NEI. Recent studies provide compelling evidence that the onset of the ISM over NEI typically occurs around mid-May, with withdrawal by mid-October, resulting in a \u003cstrong\u003elength of the rainy season (LRS)\u003c/strong\u003e of approximately 150 days\u003csup\u003e5,8\u003c/sup\u003e. These analyses collectively establish that May rainfall is an integral component of the ISMR over NEI.\u003c/p\u003e\n\u003cp\u003eThe ISMR during June\u0026ndash;September is a core component of a convectively coupled system, functioning as an off-equatorial heat source driven by the latent heat released by rainfall that influences low-level cross-equatorial winds, the low-level westerly jet, and the monsoon trough\u003csup\u003e9\u0026ndash;11\u003c/sup\u003e while the low-level convergence of moisture modulating the rainfall itself. While the June\u0026ndash;September (JJAS) monsoon is primarily governed by the northward seasonal migration of the Inter Tropical Convergence Zone (ITCZ)\u003csup\u003e9\u0026ndash;11\u003c/sup\u003e, the prevailing thinking was that the onset over NEI cannot occur in May\u0026nbsp;as the ITCZ is located still close to the equator. However, Das et al.\u003csup\u003e5\u003c/sup\u003e revealed that onset over NEI could occur May even in the absence of ITCZ through interactions between extra tropical Rossby waves and regional orography\u0026nbsp;playing a crucial role in driving the large-scale onset over NEI in mid-May in recent decades. They also proposed an alternative mechanism, suggesting that the 10\u0026ndash;20 day quasi-biweekly oscillation (QBWO) coinciding with the onset could facilitate cyclonic winds interacting with the local orography, thereby enhancing convection and triggering the onset. While the climate change is known to weaken the seasonal mean monsoon over NEI\u003csup\u003e3\u003c/sup\u003e, its role in the increasing extratropical influence on the onset and in particular the early onset over NEI compared to the monsoon onset over Kerela (MoK) remains unclear. This study aims to investigate the impact of climate change on the onset and LRS days over NEI, as well as how the dynamics of the May onset over NEI have evolved over the years by comparing the potential drivers of onset over NEI in two periods, 1940\u0026ndash;1960 and 2002\u0026ndash;2023.\u003c/p\u003e\n\u003cp\u003eThe onset dynamics over NEI are closely linked to extra tropical transient Rossby waves and the tropical quasi-biweekly oscillations (QBWO) that play a critical role in initiating the onset. Therefore, it is essential to examine how climate change or multi-decadal variations in these Rossby waves have influenced the onset day over NEI. Climate change has a profound impact on global atmospheric circulation, with recent studies highlighting significant changes in the tropics. Evidence suggests that the tropical belt has expanded over the past few decades, a shift with potentially far-reaching implications for subtropical regions and the global climate system\u003csup\u003e12\u003c/sup\u003e. Research indicates that stratospheric cooling, driven by climate change, causes a lifting of the tropopause, which in turn leads to the poleward movement of tropospheric jet streams\u003csup\u003e13,14\u003c/sup\u003e. Lau et al.\u003csup\u003e15\u003c/sup\u003e found that stronger tropical SST anomalies significantly influence the strength of the jet stream over South Asia. The strength and position of the jet stream play a critical role in shaping the amplitude and wavelength of extra tropical Rossby waves. Given the observed influence of stronger tropical SST anomalies on the jet stream over South Asia\u003csup\u003e15\u003c/sup\u003e, an important scientific question arises: under these changing conditions of Rossby waves, how do the dynamics over NEI during May evolve over time? Furthermore, it is essential to investigate whether the strengthened jet streams have any impact on the influence of extra tropical transient Rossby wave and on the onset processes over NEI and the resultant onset date.\u003c/p\u003e\n\u003cp\u003eA critical aspect of monsoon dynamics is the local air-sea interaction, particularly how sea surface temperatures (SSTs) respond to the onset day and, conversely, how variations in SSTs influence the onset day. The interaction between the atmosphere and ocean plays a significant role in shaping the dynamics of the Indian Summer Monsoon Rainfall (ISMR) annual cycle, its intra-seasonal and inter-annual variability\u003csup\u003e11,16\u0026ndash;22\u003c/sup\u003e. The onset vortex is influenced by boundary conditions linked to local SSTs\u003csup\u003e23\u003c/sup\u003e. Atmospheric convection in the tropics is also sensitive to SST variations and fluctuations in surface fluxes from the underlying ocean. SSTs above 28\u0026deg;C, in conjunction with other factors, are crucial for initiating organized deep convection in the tropical atmosphere\u003csup\u003e24\u0026ndash;28\u003c/sup\u003e. About one week before the Indian monsoon onset over Kerala, a warm pool (SST \u0026gt; 30.5\u0026deg;C) forms over the Arabian Sea\u003csup\u003e29\u003c/sup\u003e. These elevated SSTs in the Arabian Sea help generate a vortex that plays a crucial role in triggering the Indian monsoon onset over Kerala in early June\u003csup\u003e23\u003c/sup\u003e. A similar mechanism concerning the onset day in May over NEI remains a scientific question that needs to be addressed. This study aims to investigate the local air-sea response around the onset day and analyze how climate change may have impacted these interactions by comparing the periods 1940\u0026ndash;1960 and 2002\u0026ndash;2023 in the month of May.\u003c/p\u003e\n\u003cp\u003eThe paper is organized as follows. After describing the data and methods used in the study in Section 2, the results are presented in Section 3. Having discussed the shift in large-scale stationary wave pattern associated with May (Section 3.1), the change in rainfall pattern over NEI is presented in Section 3.2. This is followed by a discussion on increase in Extra tropical Transient Rossby waves on onset process in recent decades (Section 3.3), we discuss change in Onset Dynamics process using ERA5 (Section 3.4) and finally change in air sea interaction with Bay of Bangle (BOB) (Section 3.5). The Conclusions and Discussions are summarized in Section 4.\u003c/p\u003e"},{"header":"Data and Methodology","content":"\u003cp\u003e\u003cstrong\u003e2.1 Data\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn this study, we have utilized data from the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation reanalysis dataset, ERA5\u003csup\u003e30\u003c/sup\u003e, to analyze winds, geopotential, rainfall, air temperature (from 200hPa to 600hPa), potential vorticity, and sea surface temperature for the periods 1940\u0026ndash;1960 and 2002\u0026ndash;2023. ERA5 reanalysis data is available at a horizontal resolution of 25 km. Moreover, ERA5 provides hourly data on a comprehensive range of atmospheric, land-surface, and oceanic parameters, offering high-resolution and detailed climate information essential for long-term climate studies.\u0026nbsp;The ERA5 rainfall data has been shown to represent both the amplitude and variability of observed rainfall over the North East India (NEI) region well when compared with Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) datasets for the period 1998\u0026ndash;2021\u003csup\u003e5\u003c/sup\u003e. Given its consistency and reliability across a broad set of atmospheric variables, ERA5 was selected as the primary dataset for all analyses in this study focused on the impacts of climate change on monsoon onset over the NEI region.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2 Methodology\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Tropospheric Temperature (TT), derived from the vertically averaged air temperature between 600hPa and 200hPa, has been widely used to develop indices for identifying monsoon onset and withdrawal over NEI. In this study, the TT gradient over NEI is calculated as the difference between the vertically averaged TT over the North Box (88\u003csup\u003e0\u003c/sup\u003eE\u0026ndash;100\u003csup\u003e0\u003c/sup\u003eE, 5\u003csup\u003e0\u003c/sup\u003eN\u0026ndash;35\u003csup\u003e0\u003c/sup\u003eN) and the South Box (40\u003csup\u003e0\u003c/sup\u003eE\u0026ndash;100\u003csup\u003e0\u003c/sup\u003eE, 15\u003csup\u003e0\u003c/sup\u003eS\u0026ndash;5\u003csup\u003e0\u003c/sup\u003eN) (Figure 2b). The date on which these gradient changes sign from negative to positive is defined as the monsoon onset date over NEI, while the date on which it changes from positive to negative is considered as the monsoon withdrawal date. Again, the stationary wave over Indian Summer Monsoon Region is obtained from geopotential and wind daily anomalies from which the zonal mean is removed and time averaged for May during both periods 1940-1960 and 2002-2023. To extract the Quasi-Biweekly Oscillation (QBWO) signals from wind anomalies, a two-dimensional (2D) Lanczos bandpass filter\u003csup\u003e31\u003c/sup\u003e with a 10\u0026ndash;20-day window is applied using the NCAR Command Language (NCL)\u003csup\u003e32\u003c/sup\u003e. To remove long-term trends, the SST data has been de-trended prior to analysis using Climate Data Operators (CDO)\u003csup\u003e33\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cstrong\u003e3.1 Shift in large-scale stationary wave pattern associated with May\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe monsoon onset over NEI is closely linked to the region\u0026apos;s orography, which acts as a facilitator for organizing a super-synoptic convective heat source through interaction with a quasi-stationary low-level vortex. This vortex drives orographic uplift, playing a critical role in initiating the \u0026apos;onset\u0026apos; over NEI\u003csup\u003e5\u003c/sup\u003e. To understand how climate change has influenced the onset dynamics in this region, we first analyze the global stationary Rossby wave-train \u0026lsquo;background\u0026rsquo; that dominates the May climate in the Northern Hemisphere during the two periods: 1940\u0026ndash;1960 and 2002\u0026ndash;2023 (Figure 1a, 1b, 1c, 1d). The stationary wave in May is constructed from daily anomalies of winds and geopotential from which the zonal mean is removed and time averaged over both periods at 200hPa and 500hPa. Interestingly, the wave train exhibits distinct change in stationary wave during May with the vorticity patterns at 200hPa over the South Asian monsoon region showing a pair of negative (cyclonic) and positive (anticylonic) vorticity during 1940\u0026ndash;1960 and a pair of positive and negative vorticity in the recent period (2002\u0026ndash;2023) (Figure 1a, 1b). Additionally, the vortex located to the east, spanning NEI and south/central China, appears to be barotropic in nature in both periods. Thus, a notable shift is observed in recent decades, as the vorticity at 200hPa and 500hPa over this region transitions from positive to negative keeping the barotropic nature same.\u003c/p\u003e\n\u003cp\u003eThese substantial changes in large-scale wind circulation patterns and Rossby waves between 1940\u0026ndash;1960 and 2002\u0026ndash;2023 have profound implications for monsoon dynamics over NEI. During the earlier period (1940\u0026ndash;1960), the climatological geopotential and winds at 200hPa (Figure 1a) reveal a stationary Rossby wave train in the Northern Hemisphere, characterized by a zonal wave number of 2\u0026ndash;3. In contrast, in recent decades (Figure 1b), this pattern has shifted to a zonal wave number of 3\u0026ndash;4, accompanied by a transition from negative to positive geopotential anomalies over the Indian Summer Monsoon region during May. These notable shifts in large-scale circulation and Rossby waves raise a critical scientific question: how have these changes influenced the monsoon onset and its dynamics over NEI? This study seeks to explore whether these altered atmospheric patterns have directly impacted the onset dynamics, offering valuable insights into the evolving relationship between climate and monsoon behavior over NEI.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.2 Change in rainfall pattern over NEI\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe climatological annual cycle of daily rainfall (Figure 2a), when compared across the two periods, suggests that the overall pattern of the annual rainfall cycle remains largely unchanged. However, the mean rainfall has decreased in the recent period, both annually and specifically in May. For both periods, more than 50% of the annual rainfall range exceeds in May itself, indicating that the rainy season begins in May\u003csup\u003e34\u003c/sup\u003e. Based on the objective definition of onset over NEI\u003csup\u003e5,8\u003c/sup\u003e, marked by the change in the sign of the north-south propagation of the TT gradient, the onset date in the earlier period is identified as May 14\u003csup\u003eth\u003c/sup\u003e, while in the recent period, it occurs around May 18\u003csup\u003eth\u003c/sup\u003e. Interestingly, the withdrawal date remains almost unchanged across the two periods (Figure 2c). The regions used for calculating the TT gradient are shown in Figure 2b. A noteworthy observation is the role of low-level vorticity at 850hPa, averaged over 88\u0026deg;\u0026ndash;100\u0026deg;E, 15\u0026deg;\u0026ndash;27\u0026deg;N, in the onset dynamics over NEI. In the recent period, the onset is strongly associated with an increase in low-level cyclonic vorticity (Figure 2c, cyan), which intensifies by a factor of 3\u0026ndash;4 on the onset day. In contrast, during the earlier period, the onset over NEI is not directly linked to such an increase in low-level cyclonic vorticity (Figure 2c, blue). Instead, the cyclonic vorticity intensifies by a similar factor of 3\u0026ndash;4 only in June. Despite the establishment of the heat source in May during the earlier period, the absence of a significant increase in low-level cyclonic vorticity raises critical questions. Did the earlier period produce synoptic-scale rain spells in the absence of such vorticity, or were the rainfall events primarily driven by more frequent localized meso-scale convection?\u003c/p\u003e\n\u003cp\u003eTo determine whether the rainfall patterns during May in the earlier period exhibit synoptic-scale characteristics like those observed in June in daily time scale, we analyzed daily rainfall anomalies averaged over the region 88\u003csup\u003e0\u003c/sup\u003e\u0026ndash;100\u003csup\u003e0\u003c/sup\u003eE, 22\u003csup\u003e0\u003c/sup\u003e\u0026ndash;29.5\u003csup\u003e0\u003c/sup\u003eN. The analysis reveals that in both periods, May rainfall occurs in distinct spells, with five-day spells being common and spells lasting up to 9\u0026ndash;10 days also observed (Figure 3). Notably, the earlier period shows a higher frequency of spells lasting more than 9\u0026ndash;10 days. To further explore the spatial distribution and persistence of daily wet rainfall anomalies, composite daily anomalies were constructed for all spells equal to or longer than five days. These composites were calculated for days 1 through 5 of such spells, focusing on rainfall anomalies exceeding the 0.75th quantile of their standard deviation. The composite spatial structure of rainfall and 850hPa winds associated with these wet spells is shown in Figure 4 and Figure S1, respectively. The results highlight that wet spells are organized and of synoptic scale in both periods, beginning with weak anomalies on day 1, intensifying by days 2 and 3, and maintaining their strength thereafter in both periods. In the recent period, it may be noted that the rain tends to get organized into two belts on the leeside of the two mountain ranges indicating the role of low-level wind induced uplift on the organization (Figure 4 and S2). On the other hand, during the early period the synoptic scale organization seems to arise from a simultaneous self-organization of several meso-scale convective clusters. This is consistent with composite of low-level wind anomalies at the time of onset (Figure 5b, 5d). It may be noted that, the wind anomalies, with a persistent cyclonic vortex over NEI during the spells bring in additional moisture from Bay of Bengal and organize the rain bands along the orographic ranges (Figure 5d). However, during the earlier period the low-level winds are dominated by a weak anticyclonic vortex (Figure 5b) and divergence of moisture from NEI. In addition to its inability to bring additional moisture to the region, they fail to generate wind induced uplift and contribute to the organization. As a result, in the earlier period, the spells are initiated simultaneously in both valleys, local meso-scale convective activity and self-organization as may be seen in Figure 4(e,g,i). Despite the absence of increased vorticity on the onset day in the earlier period, the region experiences persistent synoptic-scale rainfall spells in May, which cannot be classified as mere local thunderstorms. These findings suggest that the heat source necessary for generating such synoptic-scale spells is already established in May during both the periods. This raises a critical question: do the dynamics of monsoon onset remain consistent across the two periods, or have they undergone significant changes?\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.3 Increase in Extra tropical Transient Rossby waves on onset process in recent decades\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn both periods, the background barotropic anti-cyclonic and cyclonic vorticity over the east and NEI (Figure 1) play a crucial role in facilitating the climatological onset of the Indian monsoon over NEI, occurring on May 14\u003csup\u003eth\u003c/sup\u003e during 1940\u0026ndash;1960 and May 18\u003csup\u003eth\u003c/sup\u003e during 2002\u0026ndash;2023. The interaction of low-level cyclonic winds (Figure S1) with the region\u0026apos;s orography, combined with boundary layer convergence driven by cyclonic vorticity, has the potential to generate meso-scale weather systems of approximately 200 km in size on the southern lee side of the mountains, as seen in Figure 4. These systems are likely intensified through a CISK-type mechanism. Such an organized heat source can further attract moisture convergence from the Bay of Bengal, allowing the system to sustain itself and effectively establish the monsoon season in the region for both periods.\u003c/p\u003e\n\u003cp\u003eThe composite of wind anomalies at 200hPa and 850hPa (Figure 5) during the onset day reveals distinct differences between the two time periods. At the upper level, the anti-cyclonic vorticity (Figures 5a and 5c) exhibits a southeastward intrusion in recent decades, leading to the development of a low-pressure system over the Tibetan Plateau region (Figure 5c). In contrast, in the earlier period, this low-pressure system was positioned further north at the time of onset (Figure 5a). At the lower level (850hPa), the wind anomaly composite highlights the formation of a cyclonic vorticity over NEI in recent decades (Figure 5d), which likely plays a crucial role in triggering monsoon onset by creating an unstable atmosphere over the region. However, in the earlier period, a more asymmetric quasi-oscillatory pattern formed over the Bay of Bengal at the time of onset (Figure 5b). As we know that the climate of NEI is influenced by extra tropical transient Rossby waves through downward or southward intrusion\u003csup\u003e5\u003c/sup\u003e, the key question now is whether these changes in wind anomalies at the time of onset day have led to an increased influence of extra tropical transient Rossby waves in recent decades.\u003c/p\u003e\n\u003cp\u003eThe lead-lag composite of 850hPa relative vorticity, averaged over a broader region (88\u0026deg;E\u0026ndash;100\u0026deg;E, 15\u0026deg;N\u0026ndash;29\u0026deg;N) from seven days before to seven days after the onset (Figure 6a), reveals distinct differences between the two periods. In recent decades, large-scale low-level vorticity, initially around 2\u0026times;10\u003csup\u003e-6\u003c/sup\u003e\u0026thinsp;S\u003csup\u003e-1\u0026nbsp;\u003c/sup\u003eprior to onset, intensifies to approximately 5\u0026times;10\u003csup\u003e-6\u003c/sup\u003e\u0026thinsp;S\u003csup\u003e-1\u0026nbsp;\u003c/sup\u003eat the time of onset. In contrast, during the earlier period, low-level vorticity starts at a much weaker 0.3\u0026times;10\u003csup\u003e-6\u0026nbsp;\u003c/sup\u003eS\u003csup\u003e-1\u0026nbsp;\u003c/sup\u003ebefore onset and only strengthens to 4\u0026times;10\u003csup\u003e-6\u003c/sup\u003e\u0026thinsp;S\u003csup\u003e-1\u003c/sup\u003e two days after the onset. It is noted that the organized heat source during onset level of background vorticity prior to onset by (3.0-3.5) \u0026times;10\u003csup\u003e-6\u003c/sup\u003e\u0026thinsp;S\u003csup\u003e-1\u0026nbsp;\u003c/sup\u003eto the level after onset. Due to significantly higher level of background vorticity (2\u0026times;10\u003csup\u003e-6\u003c/sup\u003e\u0026thinsp;S\u003csup\u003e-1\u003c/sup\u003e), the transient wave vorticity could organize the convective activity 2 days before the onset, continue to intensity and reach steady level on onset day. On the other hand, the background vorticity in the early period is so low that the wave vorticity cannot generate enough large-scale updraft to generate organized convection on synoptic or larger scales. On the day of onset, several mesoscale convective events organize a heat source that through CISK type feedback intensifies the vorticity and in 2 days reaches the quasi-equilibrium level. The onset process also significantly increases average rainfall over the region. In recent decades, daily rainfall rises from ~6.5 mm/day before onset to nearly 3\u0026ndash;4 times that amount following onset. For the earlier period, a similar three- to four-fold increase occurs, but only after two days from the onset date.\u003c/p\u003e\n\u003cp\u003eGiven that eastward-propagating extra tropical Rossby waves can influence low-level relative vorticity in May through the eastward and downward intrusion of potential vorticity (PV)\u003csup\u003e5\u003c/sup\u003e, it is crucial to examine how this PV intrusion has evolved between the two periods. A lead-lag composite of PV anomalies, averaged over the same region and analyzed as a function of height from 50hPa to 1000hPa from seven days before to seven days after onset, shows that in recent decades, the downward intrusion of positive PV from the stratosphere triggers an increase in cyclonic relative vorticity at low levels (850-700hPa), intensifying the background cyclonic vorticity at the time of onset (Figure 6d). Additionally, the lead-lag composite of PV anomalies at 200hPa (Figure S3), spanning three days before to three days after onset, reveals a southeastward-propagating transient Rossby wave with a wavelength of approximately 4000 km in recent decades. The fact that this transient Rossby wave remains prominent even after compositing over 21 years highlights its crucial role in the monsoon onset process in recent decades. However, in the earlier period, no such downward PV intrusion occurs at the time of onset (Figure 6c) with only the background weak cyclonic vorticity at low levels (850-700hPa), nor is there any indication of a southeastward-propagating transient Rossby wave (Figure S4) as the travelling transient Rossby waves get dissipated at the time of onset over the NEI region at 200hPa. From this analysis, it is evident that the influence of extra tropical transient Rossby waves has significantly increased in recent decades compared to the earlier period. This raises an important question that if extra tropical transient Rossby waves did not play a significant role in the earlier period, what alternative mechanisms triggered the onset of the monsoon over NEI during that time and increase the low-level vorticity after two days of onset date?\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.4 Change in Onset Dynamics process\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSince the downward and southeastward intrusion of PV is not observed at the time of onset in the earlier period, we analyze the evolution of the quasi-biweekly oscillation (QBWO) over the Indian subcontinent concerning the onset day, from seven days before to seven days after. This is crucial as daily May rainfall over NEI is predominantly influenced by a 10\u0026ndash;20 day QBWO\u003csup\u003e5\u003c/sup\u003e. The QBWO, recognized as a westward-propagating, convectively unstable equatorial Rossby wave\u003csup\u003e20,35\u003c/sup\u003e, typically originates over the Western Pacific warm pool before propagating westward over the North Bay of Bengal or NEI. To investigate this, a 10\u0026ndash;20 day bandpass-filtered daily wind anomaly at 850hPa over the period 1940\u0026ndash;1960 was computed using Lanczos filter weights. A lead-lag composite of the filtered rainfall anomalies with respect to the onset day (Figure 7) reveals that an asymmetric QBWO travels toward NEI, and at the time of onset, these oscillations merge, generating anti-cyclonic vorticity over the Bay of Bengal. This vorticity pattern likely helps initiate the onset process over NEI during the earlier period. Interestingly, this anti-cyclonic vorticity later transitions into cyclonic vorticity over NEI (Figure 7), coinciding with increased vorticity and rainfall over the region two days after onset (Figure 6a, 6b).\u003c/p\u003e\n\u003cp\u003eFurthermore, a comparison of the annual cycle of convective available potential energy (CAPE) between the two periods (Figure S5) indicates that during the earlier period, CAPE increases by approximately 500 J/kg at the time of onset. In contrast, in recent decades, CAPE remains unchanged at the time of onset. This suggests that in the earlier period, the onset process over NEI was triggered by the traveling asymmetric QBWO, which, along with enhanced CAPE, facilitated the onset. Once initiated, the Matsuno\u0026ndash;Gill response\u003csup\u003e36\u003c/sup\u003e sustained elevated northward moisture transport during May and beyond. However, in recent decades, this traveling asymmetric QBWO is absent (Figure S6). Instead, a northward propagating wave appears to generate cyclonic vorticity over the Bay of Bengal at the time of onset. Again,\u0026nbsp;the spatial distribution of CAPE compared between the two periods (Figure S7) reveals that CAPE was more concentrated around the southern valley of the region in the earlier period. Since the influence of subtropical transient waves was weaker during this time, the initiation of wet rain spells was primarily observed in the southern valley (Figure 4). However, in recent decades, with an increased influence of subtropical winds, wet spells now initiate in both valleys, indicating a major shift in the dynamics of monsoon onset over NEI. The significance of the QBWO is further appreciated by the fact that, even after compositing over 21 years, its influence remains clearly visible during onset in the earlier period. These findings indicate that the monsoon onset process over NEI is evolving in a warming world, with a fundamental shift in the underlying mechanisms driving its initiation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.5 Change in air sea interaction with Bay of Bangle (BOB)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Indian Summer Monsoon is a coupled atmosphere-ocean system with intricate feedback mechanisms between atmospheric circulation and oceanic conditions\u003csup\u003e22\u003c/sup\u003e. However, the oceanic response over the Bay of Bengal (BOB) during the monsoon season in May, particularly in relation to monsoon onset over NEI, remains unclear. In this section, we explore how sea surface temperature (SST) over the BOB influences and in turn responds to the onset over NEI and examine how this relationship has evolved in a warming climate.\u003c/p\u003e\n\u003cp\u003eThe spatial structure of the lead-lag composite of de-trended SST anomalies across the North Indian Ocean (NIO) is shown in Figures S8 and S9 from 7-days before onset to 7-days after onset during recent and earlier decades respectively. It may be noted that the SST anomalies over the whole of NIO evolves coherently through the NEI onset till a week beyond onset. If we focus on the BOB (Figure 8b), during recent years, the positive SST anomalies persist over BOB till onset and become negative after the onset as could be seen from lead-lag composite SST anomalies averaged over BOB (Figure 8a). This is suggestive of an air-sea interaction on intra seasonal time scales where the increased moisture supply from higher than normal SST prior to the onset triggers the onset but invigoration of circulation or vorticity (Figure 6a) following the onset cools the ocean through increased evaporation. This air-sea interaction is rather weak compared to that associated with the Monsoon Intra Seasonal Oscillations (MISO)\u003csup\u003e37\u003c/sup\u003e but consistent that associated with the quasi-biweekly oscillation (QBWO)\u003csup\u003e17\u003c/sup\u003e. However, in the earlier period, the SST over BOB shows no significant evolution around the onset over NEI and remains close to zero throughout a two-week period around the NEI (Figure 8, S8) suggesting lack of air-sea interaction during NEI. These findings suggest that with the changing climate over NEI, the SST response over BOB has also evolved, aligning with shifts in the dynamics of monsoon onset over the region.\u003c/p\u003e"},{"header":"Conclusions and Discussions","content":"\u003cp\u003eThis study provides compelling evidence that the onset dynamics of the Indian Summer Monsoon over NEI have undergone significant changes between the periods 1940\u0026ndash;1960 and 2002\u0026ndash;2023. During both periods, the May rainfall over NEI is distinctly monsoonal, marked by its organized and persistent nature, occurring in active and break spells that help establish a significant monsoon heat source over the region. Also, neither the length of the rainy season (LRS) nor the climatological onset date changes significantly. However, this study reveals that in a warming world, the onset process over NEI and its persistence are undergoing notable changes. Examining both atmospheric and oceanic effects is necessary to comprehend how the monsoon onset dynamics over NEI are changing in a warming environment. Our analysis of lead lag composite of PV help confirms that the influence of extra tropical transient Rossby wave have increased in a worming world. Again with the expansion of jet stream, the anti-cyclonic vorticity at 200hPa has significantly changed its position at time of onset moving more towards the Tibetan plateau region in the recent decades and the intensity of the lower level winds at 850hPa at the time of onset is also seen to be increased over northern valley of the region (Figure 5) which may affect the large scale organization of the rainfall spells in recent decades. We specifically observe notable change in the dominant processes that drive onset during the two periods. While a moving anti-symmetric QBWO together with local increase in instability like CAPE initiates the onset over NEI in the earlier period (1940\u0026ndash;1960), in the recent decades (2002\u0026ndash;2023) we have seen an increasing impact of extra tropical transient Rossby waves to initiate the onset over the region. The downward and southeastward intrusion of PV from the stratosphere in recent decades has been a key driver of increased low-level cyclonic vorticity, accelerating the onset process. In contrast, in the earlier period this southeastward and downward intrusion of PV from upper atmosphere gets dissipated at the time of onset resulting in weaker relative vorticity over the region which exhibited a more gradual onset. The lack of help from extra tropical Rossy waves is compensated by an asymmetric QBWO contributed to the monsoon onset through a complex interaction of westward-propagating waves over the Bay of Bengal.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThis study also reveals significant shifts in dynamics of organized wet spells in May over the Northeast India as the first organized wet spell leads to the NEI onset. The wet spells initiation has changed from the early to recent. In recent decades the initiation of the wet spells are more organized at two sides of the u-shaped topography, which indicates the role of low-level wind induced uplift on the organization. However, in the earlier period, rainfall spells in both valleys were initiated simultaneously, driven by local meso-scale convection and self-organization. This is evident from the fact that the large scale 850hPa low-level vorticity is increased by 3\u0026ndash;4 times over the region in the month of June. For these large scale synoptic organization of vorticity at 850hPa in recent decades, the SST response with respect to the onset date is also seen over NIO as the SST over BOB regions gets cooler once the onset process start which suggests an intraseasonal air-sea interaction, where elevated SSTs prior to onset enhance moisture supply and trigger the monsoon onset, while strengthened circulation and vorticity after onset cool the ocean via increased evaporation. The compelling evidence presented here regarding the change in onset dynamic process over NEI suggests the need for a comprehensive reassessment of monsoon prediction models, improved climate adaptation strategies, and a deeper understanding of the evolving interactions between tropical and extra tropical systems in a warming world.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eConflict of interest/Competing interest:\u003c/strong\u003e No conflict of interest\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBNG and S Das designed and conceptualized the study and wrote the initial manuscript. S Das carried out all the analyses and generated the graphics. All the authors contributed to writing the final manuscript. All the authors read the final manuscript and approved it for submission.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eAvailability of data and material:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFifth-generation reanalysis (ERA5) data from the European Centre for Medium-Range Weather Forecasting (ECMWF) can be downloaded from https://climate.copernicus.eu/climate-reanalysis. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCode availability:\u003c/strong\u003e Used MATLAB, NCL, Python and climate data operator (CDO)\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eS. Das is grateful to Department of Science and Technology (DST), India for INSPIRE fellowship with grant No. DST/INSPIRE Fellowship/2020/IF200121 and thanks Cotton University for providing the research facilities. BNG thanks Science and Engineering Research Board (SERB), Government of India for supporting the computational facility for this work and Gauhati University for the Honorary Professor of Excellence. R.M. extends his appreciation to Cotton University for providing the essential infrastructure and resources necessary for conducting this research.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eShukla, J. Interannual variability of monsoons. \u003cem\u003eMonsoons\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 399\u0026ndash;464 (1987).\u003c/li\u003e\n\u003cli\u003eMishra, V., Smoliak, B. V., Lettenmaier, D. P. \u0026amp; Wallace, J. M. A prominent pattern of year-to-year variability in Indian Summer Monsoon Rainfall. \u003cem\u003eProc. Natl. Acad. Sci.\u003c/em\u003e \u003cstrong\u003e109\u003c/strong\u003e, 7213\u0026ndash;7217 (2012).\u003c/li\u003e\n\u003cli\u003eZahan, Y., Mahanta, R., Rajesh, P. V. \u0026amp; Goswami, B. N. 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(Springer Science \u0026amp; Business Media, 2011).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"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":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Northeast India (NEI), Onset, Potential Vorticity, QBWO, SST","lastPublishedDoi":"10.21203/rs.3.rs-6739417/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6739417/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"The planetary-scale onset of the Indian Summer Monsoon over Northeast India (NEI) typically occurs about two weeks earlier than the monsoon the onset over Kerala, extending the length of the rainy season in NEI to approximately 150 days. The dynamics of the May onset over NEI are strongly linked to interactions with extra tropical Rossby waves, which through downward and eastward propagation of potential vorticity (PV) drive low-level relative vorticity and initiate the onset process. Additionally, the quasi-biweekly oscillation (QBWO) also plays a significant role in modulating the onset dynamics over NEI. While climate change is known to weaken the seasonal mean monsoon over NEI, its role in amplifying extra tropical influences on the onset and the trend of early onset over NEI remains unclear. Here we assess these changes by examining tropical circulation patterns and extra tropical influences, specifically focusing on the intrusion of potential vorticity, over two distinct periods: 1940–1960 and 2002–2023. Our analysis reveals a notable increase in extra tropical influence in recent decades. In the 2002–2023 period, with clear evidence of downward PV intrusion and the generation of low-level cyclonic vorticity, both of which are largely absent during the earlier period. Lead-lag composites of PV anomalies at 200hPa further support these findings, showing prominent eastward-propagating PV anomalies around the onset in recent years. In contrast, during the 1940–1960 periods, monsoon onset over NEI was predominantly governed by local meso-scale convective activity and self-organization, as indicated by the strengthening of low-level cyclonic vorticity two days after the onset. During the earlier period, heat sources were more likely driven by local convective instability, as indicated by higher convective available potential energy, which was approximately 500 J/kg greater in May compared to recent decades. Furthermore, the study highlights the role of air-sea interactions in influencing the onset dynamics. In recent decades, the Bay of Bengal Sea surface temperature show a stronger correlation with the onset process, indicating that changes in SST patterns and associated heat fluxes play a crucial role in facilitating the extra tropical influences. These findings underscore a significant shift in the mechanisms driving monsoon onset over NEI, with increasing extra tropical influences and altered air-sea interactions in recent decades. This evolving dynamic, likely influenced by climate change, has critical implications for understanding and predicting monsoon variability in NEI.","manuscriptTitle":"Changing Northeast India Summer Monsoon Onset Dynamics in a Warming World","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-03 11:12:28","doi":"10.21203/rs.3.rs-6739417/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"3573ea8d-3d17-4b40-87d7-406cc838225d","owner":[],"postedDate":"June 3rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":49368821,"name":"Earth and environmental sciences/Climate sciences/Atmospheric science"},{"id":49368822,"name":"Earth and environmental sciences/Climate sciences/Climate change"}],"tags":[],"updatedAt":"2025-06-03T11:12:28+00:00","versionOfRecord":[],"versionCreatedAt":"2025-06-03 11:12:28","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6739417","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6739417","identity":"rs-6739417","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}