Phase-dependent modulation of the MJO during cross-equatorial northerly surges (CENS)

Phase-dependent modulation of the MJO during cross-equatorial northerly surges (CENS) | 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 Phase-dependent modulation of the MJO during cross-equatorial northerly surges (CENS) Qoosaku Moteki This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8342789/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 16 Mar, 2026 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract This study investigates phase-dependent associations between Cross-Equatorial Northerly Surges (CENS) and Madden-Julian Oscillation (MJO) precipitation and circulation patterns using 84 years (1940–2023). Approximately 79% of CENS days (76% of events) occurred during phases 4–7 of the active MJO. Composite analysis reveals distinct phase-dependent co-variations between CENS occurrence and MJO structure: during phases 4–5 (MJO over the Maritime Continent), CENS occurrence is associated with enhanced precipitation around Java and along the northern coast of Australia through enhanced meridional convergence. In contrast, during phases 6–7 (MJO over the Western Pacific), CENS-associated patterns are meridionally broader and vertically deeper, with enhanced precipitation over the western flank (rear) of the MJO. Notably, phases 6–7 exhibit significant positive sea surface temperature anomalies in the Western Pacific, a pattern that persists even when controlling for ENSO phase and MJO amplitude variations, consistent with a southward shift of the MJO propagation path. These results indicate that CENS are not merely passively affected by the MJO but are systematically associated with distinct phase-dependent differences in MJO precipitation patterns, vertical structure, and propagation characteristics. Earth and environmental sciences/Climate sciences Earth and environmental sciences/Ocean sciences Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction The Cross-Equatorial Northerly Surge (CENS) is an intermittent burst of northerly winds during boreal winter embedded within the East Asian Winter Monsoon. CENS frequently crosses the equator via the South China Sea, Karimata Strait, and Java Sea, modulating precipitation and lower-tropospheric circulation over the western Maritime Continent (MC) [ 1 , 2 , 3 ]. As CENS traverses warm tropical waters, they acquire moisture, often triggering multi-day heavy rainfall and flooding in western Indonesia and adjacent regions [ 3 ]. Superimposed on the synoptic disturbance of CENS is the Madden-Julian Oscillation (MJO), the dominant mode of intraseasonal variability in the tropics, which also strongly governs MC precipitation [ 4 ]. Reflecting the importance of their interaction, previous studies have demonstrated that the concurrent occurrence of CENS and the convective active phase of the MJO results in significantly amplified precipitation compared to either phenomenon occurring in isolation [ 5 , 6 ]. CENS can enhance precipitation by 200–400% when coincident with the MJO active phase [ 5 ]. Beyond this amplification effect, recent work by Lubis et al. [ 7 ] revealed that CENS enhance the southward detour of the MJO over the MC by strengthening moisture convergence in the Southern Hemisphere. This finding suggests that CENS can actively modify the propagation characteristics and intensity of the MJO, implying that the influence of CENS may be distinct across different MJO phases. Consistent with this possibility, observational studies have established that CENS events show a strong preference for specific MJO phases. For instance, Moteki [ 2 ] demonstrated that approximately 87% of CENS events occur during MJO phases 4–7, with particularly high frequency when the MJO convective envelope is located over the MC (phases 4–5) and the Western Pacific (phases 6–7). This phase-dependent distribution suggests that while the MJO creates environmental conditions favorable for CENS generation, the high frequency of CENS events in these phases is consistent with possible feedbacks between CENS and the MJO. The mechanisms behind these enhancements—including strengthened meridional convergence, modulated sea surface temperature (SST) patterns, and altered vertical circulation structures—are likely to vary depending on the MJO phase in which the CENS occurs [ 2 , 6 , 8 ]. Given these phase-dependent characteristics, key questions remain: How do CENS-associated precipitation and circulation patterns differ between MJO phases 4–5 and 6–7? Do CENS-related SST anomalies exhibit phase-dependent characteristics that reflect modified MJO propagation pathways? How does the vertical circulation structure differ systematically between CENS and non-CENS conditions across these phases? This study addresses these questions through a comprehensive analysis of the statistical association between CENS occurrence and MJO-related fields. To isolate potential CENS-associated patterns, we employ a composite approach comparing CENS days and non-CENS days within the same MJO phase, thereby controlling for the large-scale background state associated with MJO convective activity. While this approach documents systematic co-variation rather than directly establishing temporal causality, it reveals how MJO-related fields differ when CENS occurs. Specifically, we document: (1) distinct spatial patterns of CENS-associated precipitation anomalies between MJO phases 4–5 and 6–7; (2) phase-dependent characteristics of CENS-related SST anomalies consistent with modified MJO propagation pathways; (3) systematic differences in vertical circulation structure between CENS and non-CENS conditions. This study extends previous studies in several key ways: (1) Extended temporal coverage: While previous works were constrained by dataset limitations to periods of less than 50 years, this study extends the analysis to 84 years (1940–2023), providing a substantially larger sample size; (2) Three-dimensional circulation structure: This study documents the three-dimensional vertical circulation structure associated with CENS occurrence synchronized with different MJO phases, revealing phase-dependent differences in meridional and zonal circulation patterns, vertical extent of updrafts, and the organization of convective disturbances. Results Sample Characteristics and Statistical Overview To extend the work of Moteki [ 2 ], which performed composite analysis using 48 years of data (1974–2022) using Japanese 55-year reanalysis (JRA-55 [ 9 ]) and the Australian Bureau of Meteorology MJO index, this study investigated CENS events in January and February over 84 consecutive boreal winter seasons (1940–2023) using ERA5 [ 10 ]. Although CENS can occur with lower frequency in December and March, the analysis is restricted to January and February to exclude the effects of seasonality in atmospheric circulation fields and sea surface conditions. From a total of 4,976 days in January and February across the 84 analyzed seasons, 715 days (226 events) were identified as CENS days, defined as days when the area-averaged surface meridional wind speed over 105°E–110°E, 8°S–0° northerly wind speed exceeded 5 m/s, following Moteki [ 2 ]. As shown in Moteki [ 2 ], CENS occurrence frequency exhibited clear variations depending on the ENSO phase (Table 1 , Fig. 1 ). La Niña conditions showed the highest activity, with 291 CENS days (17.5% of all La Niña January–February days; 40.7% of total CENS days) and 95 events (5.72 events per 100 days). In contrast, El Niño conditions had only 94 CENS days (7.6% of El Niño days; 13.1% of total CENS) and 37 events (2.98 events per 100 days). Figure 1 shows the time series of CENS event frequency in January and February for each year from 1940 to 2023, categorized by ENSO phase (La Niña: light blue, El Niño: pink, Neutral: white). The time series visually confirms the trend of significantly more frequent CENS events during La Niña years compared to El Niño years (light blue background) and also indicates the magnitude of interannual variability. Statistical analysis using a chi-square test confirmed a significant association between ENSO phase and CENS occurrence (χ² = 64.33, p < 0.0001). Pairwise comparisons with Bonferroni correction revealed that La Niña had a significantly higher CENS occurrence rate than El Niño (17.5% vs 7.6%, p < 0.0001), and El Niño had a significantly lower rate than Neutral conditions (7.6% vs 15.9%, p < 0.0001). This characteristic of significantly higher CENS frequency during La Niña compared to El Niño is consistent with the results of Moteki [ 2 ] and shows a common feature with no difference between January and February. Table 1 CENS occurrence statistics stratified by ENSO phase. 'CENS Days (Jan)' and 'CENS Days (Feb)' denote the number of CENS days in January and February, respectively, in each ENSO category. 'Total Days in Phase' is the total number of January–February days in each ENSO category over the 84 winters. 'Total CENS Days' is the sum of CENS days in January and February. Note that the mean CENS event duration is approximately 3.2 days, so 'Number of CENS events' × 3.2 ≈ 'Total CENS Days'. ENSO Phase CENS Days (Jan) CENS Days (Feb) Total Days in Phase Total CENS Days % of Phase % of Total CENS Number of CENS events La Niña 165 126 1,660 291 17.5% 40.7% 95 El Niño 53 41 1,243 94 7.6% 13.1% 37 Neutral 196 134 2,073 330 15.9% 46.2% 94 Regarding the relationship between MJO activity and CENS frequency, out of 715 CENS days (226 events), 471 days (141 events) occurred during the MJO active phase (amplitude ≥ 1.0), while 244 days (85 events) occurred during the MJO inactive phase. The mean (median) duration of individual CENS events is about 3.2 (2.0) days, with a range from 1 to 20 days. Figure 2 a shows the phase distribution of CENS frequency during the MJO active phase, clearly confirming a significant increase in CENS days during phases 4–7. When classified by MJO phase, 41% of CENS days (41% of events) occurred in phases 4–5 and 38% of days (35% of events) in phases 6–7, consistent with Moteki [ 2 ]. On the other hand, during the MJO inactive phase (amplitude < 1.0) shown in Fig. 2 b, the phase dependence of CENS frequency is not as clear as in the active phase, but a relatively higher tendency is still observed in phases 5–7. Based on this phase-dependent CENS frequency distribution, this study performed composite analyses separately for phases 4–5 (MJO located over the MC) and phases 6–7 (MJO located over the Western Pacific). CENS-associated differences in precipitation To isolate potential CENS-associated patterns in MJO-related fields, we compare CENS days and non-CENS days within the same MJO phase, thereby controlling for the large-scale background state associated with the MJO. This approach allows us to document how precipitation, circulation, and SST patterns systematically differ when CENS occurs, although it does not directly establish temporal causality. Composite analysis reveals distinct precipitation responses to CENS depending on the MJO phase (Fig. 3 ). In phases 4–5, the CENS–non-CENS difference is particularly pronounced around Java and along the northern coast of Australia (Fig. 3 b). This spatial pattern is consistent with strengthened cross-equatorial northerlies and suggests enhanced low-level meridional moisture convergence that may locally amplify MJO-related convection over the Maritime Continent. In MJO phases 6–7, the MJO convective envelope is located over the Western Pacific, and the precipitation maximum shifts eastward toward the region off the northeast coast of Australia (Fig. 3 c). In this phase, the precipitation field extends farther east than in phases 4–5, consistent with the eastward propagation of the MJO. Notably, the CENS–non-CENS difference in phases 6–7 exhibits a meridionally broader pattern of statistically significant precipitation enhancement over the western flank (rear) of the MJO convective envelope (Fig. 3 d). In addition, a narrow, elongated band of significant positive anomalies extends along and offshore of the northeastern coast of the Philippines. This spatial distribution suggests that, during phases 6–7, CENS occurrence is associated not only with local precipitation enhancement near the Maritime Continent but also with broader modifications to disturbances and convective systems in the western rear of the MJO, resulting in more widespread precipitation increases. Comparison between phases 4–5 and phases 6–7 clarifies the phase dependence of CENS-associated precipitation patterns in the MJO. In phases 4–5, CENS occurrence is associated with localized amplification of MJO precipitation, with the most pronounced differences around Java and along the northern coast of Australia. On the other hand, in phases 6–7, CENS-associated patterns extend over a wider area, with meridionally broader enhancement over the western rear of the MJO and an additional significant signal near the northeastern Philippines. This spatial contrast suggests that the role of CENS in the MJO system could differ by phase: localized amplification of convective activity in phases 4–5 versus broader-scale co-variations in precipitation and circulation in phases 6–7. CENS-associated patterns in SST and SLP The distinct precipitation responses documented above raise a mechanistic question: what atmospheric and oceanic conditions are associated with CENS occurrence in each MJO phase? To address this, we examine SST and sea level pressure (SLP) fields. SST provides insight into both the thermodynamic environment supporting convection and potential air-sea interaction processes, while SLP reveals the large-scale pressure gradients that drive the cross-equatorial flow characteristic of CENS. As shown in Figs. 4 a and 4 c, in both MJO phases 4–5 and 6–7, a high SST region (> 27°C) is widely distributed in the latitude band from 20°S to 15°N. Meanwhile, in the South China Sea, a low SST region (< 26°C) is formed in response to strong northerly winds blowing out from the high SLP anomalies associated with the cold surge, clearly showing the characteristics of the so-called cold tongue [ 11 ]. During CENS occurrence in MJO phases 4–5 (Fig. 4 b), distinct SLP and SST anomaly patterns can be confirmed between 90°E–120°E. In the Northern Hemisphere, a high-SLP anomaly of about + 2 hPa is seen, and in the Southern Hemisphere, a low-SLP anomaly of about − 2 hPa is seen, indicating that the strengthened meridional pressure gradient supports the occurrence of CENS. Also noteworthy is the significant negative SST anomaly (-0.7 to -0.3°C) between 90°E–120°E. In MJO phases 6–7, a clear difference appears in the spatial pattern of CENS-associated contrasts (Fig. 4 d). Although negative SST anomalies in the South China Sea are seen as in phases 4–5, significant positive SST anomalies can be confirmed in the Western Pacific region. These positive SST anomalies are more distinct than in phases 4–5 and extend over a wider spatial area. Comparison with phases 4–5 (Fig. 4 b) reveals that while weak positive SST anomalies are also present over the Western Pacific in phases 4–5, the anomalies in phases 6–7 are substantially stronger and more extensive. Potential concerns regarding these SST patterns include that the positive anomalies might reflect (1) the La Niña background state (i.e., warm water accumulation in the Western Pacific) rather than a specific response to CENS, or (2) differences in MJO amplitude between CENS and non-CENS days, where stronger MJO phases might preferentially trigger CENS with concurrent structural modifications. To address both concerns, an analysis was performed using a subsample restricted to Neutral years with MJO amplitude in the range of 1.0–1.5, thereby controlling for both ENSO phase and MJO amplitude variations (Supplemental Fig. 1). This analysis reveals that the phase-dependent contrast persists even under these more stringent conditions: positive SST anomalies are present in both phases but are substantially stronger and more extensive in phases 6–7 compared to phases 4–5. This fact suggests that the enhanced positive SST anomalies in MJO phases 6–7 are not merely due to ENSO background state differences or MJO amplitude selection effects but are systematically associated with specific CENS-associated contrasts in MJO-related fields. Lubis et al. [ 7 ] pointed out that the occurrence of CENS shifts the eastward propagation path of the MJO further southward. The positive SST anomalies in the Western Pacific observed here could be consistent with two distinct, non-exclusive processes: (1) an active CENS effect where the southward shift of MJO propagation reduces cloudiness and enhances shortwave heating, leading to SST warming through ocean mixed layer processes (quantitatively, an SST increase of + 0.1 to + 0.2°C is reasonable for such a one-dimensional process [ 12 ]); or (2) pre-existing warm SST conditions that simultaneously favor both CENS generation and MJO intensification in these phases, with the warm anomalies representing oceanic background states rather than CENS-induced responses. The simultaneous composite approach employed here cannot distinguish between these processes, as it documents co-variation at a single time point rather than temporal evolution. Preliminary lead-lag analysis (Supplemental Fig. 2) provides suggestive evidence regarding the temporal sequence of air-sea interaction during CENS. The analysis shows that SLP begins increasing (indicating anticyclone extension) approximately 2–3 days before CENS occurrence, followed by near-surface air temperature decreases that precede SST cooling by approximately 1 day. This sequence—SLP rise, then air temperature drop, then SST cooling—is consistent with cold advection from continental highs driving the oceanic response rather than SST anomalies preceding atmospheric changes. However, given limited sample sizes when stratifying by both MJO phase and lag (approximately 20–30 events per phase), and the potential for aliasing between MJO propagation and CENS-related signals, the causal sequence of SST anomalies relative to CENS occurrence remains unresolved. Nevertheless, the observed temporal ordering provides a testable hypothesis for future studies with larger samples or model experiments. Phase Dependence of Vertical Circulation Structure While the surface analyses reveal distinct phase-dependent SST and SLP patterns, understanding how CENS modulates the full depth of the tropospheric circulation requires examination of the vertical structure. Analysis of the vertical meridional cross-section in the CENS occurrence region (105°E–110°E) reveals how the enhanced meridional flow associated with CENS projects onto the vertical circulation and how this projection differs between MJO phases. In the overall composite of MJO phases 4–5 shown in Fig. 5 a, a prominent updraft region can be confirmed south of the equator (near 5°S–10°S), which is consistent with the center of MJO convective activity being located on the Southern Hemisphere side of the Maritime Continent. The meridional wind field (contours) shows a pattern of inflow from the Northern Hemisphere crossing the equator into the Southern Hemisphere, with a clear meridional circulation cell structure of strong northerly winds in the lower layer and southerly winds in the upper layer. The zonal wind field (color) shows typical characteristics of the MJO, with remarkable westerly winds below 400 hPa between 15°S and the equator. The difference between CENS occurrence and non-occurrence (Fig. 5 b) shows how CENS modulates this vertical circulation. The most prominent feature is the enhanced updraft in the Southern Hemisphere. This updraft enhancement area spatially corresponds to the precipitation increase area shown in Fig. 3 b, supporting the mechanism that CENS occurrence is associated with enhanced convective activity. The meridional wind anomalies show a clear strengthening of the meridional circulation with enhanced northerly winds in the lower layer. The zonal wind anomalies show positive anomalies (westerly wind strengthening) in the Southern Hemisphere, indicating that the westerlies associated with the MJO are strengthened during CENS occurrence. The vertical structure in MJO phases 6–7 (Fig. 5 c) shows several important differences compared to phases 4–5. The updraft region is located near the equator and shifted slightly northward. The structure of the meridional circulation is similar to phases 4–5, but differences are seen in its intensity and vertical extent. In the difference during CENS occurrence (Fig. 5 d), updraft enhancement in the Southern Hemisphere can be confirmed as in phases 4–5. But the spatial distribution of positive zonal wind anomalies extends over a wider area and also on the Northern Hemisphere side compared to phases 4–5. This corresponds to the widening of the precipitation enhancement area in the Northern Hemisphere shown in Fig. 3 d. Analysis of these vertical structures reveals that while CENS occurrence is associated with increased precipitation and enhanced updrafts in the Southern Hemisphere in both phases, the vertical extent and horizontal spread of these associations are more pronounced in phases 6–7. This difference reflects the phase-dependent differences in the organization of the convective disturbances associated with the MJO. Vertical Circulation Structure Along the MJO Eastward Propagation Path Considering the southward shift of the MJO path due to CENS pointed out by Lubis et al. [ 7 ], the vertical zonal cross-section averaged between 10°S and 5°S was analyzed. In the overall composite of MJO phases 4–5 shown in Fig. 6 a, the characteristic vertical structure associated with the eastward propagation of the MJO can be clearly confirmed. Strong updraft regions of the MJO active convective area are seen, and westerly winds associated with westerly wind bursts are seen from the surface to 400 hPa. Looking at the meridional wind component shown by color shading, deep southerly winds are seen in the Indian Ocean west of 95°E, while northerly winds are seen below 850 hPa east of 95°E. The difference between CENS occurrence and non-occurrence (Fig. 6 b) provides important information on how CENS modulates the zonal circulation structure of the MJO. The most prominent feature is that updrafts are enhanced over 85°E–120°E, with significant lower-level northerly wind anomalies associated with CENS, and southerly wind anomalies above 250 hPa. In the vertical longitudinal structure in MJO phases 6–7 (Fig. 6 c), the westerly wind structure below 400 hPa has shifted eastward compared to phases 4–5, accompanying the eastward movement of the MJO convective system. In the difference during CENS occurrence (Fig. 6 d), the updraft enhancement area is distributed to the east of 90°E and is deeper compared to phases 4–5. On the other hand, the significant lower-level northerly wind anomalies associated with CENS are slightly shallower compared to phases 4–5, and upper-level southerly wind anomalies are seen corresponding to the northerly basic field. Analysis of these vertical zonal structures reveals that while CENS occurrence is associated with enhanced zonal circulation of the MJO in both phases, these associations extend over a wider area in phases 6–7, with patterns consistent with convective development in the downstream region of the MJO. This feature is consistent with the widening of the precipitation enhancement area in phases 6–7 shown in Fig. 3 d, supporting that CENS occurrence is systematically associated with modifications in the structure of the MJO. Summary and Conclusion This study comprehensively analyzed phase-dependent associations between CENS occurrence and precipitation and circulation patterns under different MJO phases using 27 years (1996–2023) of GPCP precipitation data and 84 years (1940–2023) of ERA5 reanalysis data. The primary novel contributions of this work relative to previous studies are: (1) Extended temporal coverage: By extending the analysis period from less than 50 years to 84 years, this study provides a substantially larger sample size (715 CENS days across 226 events) and enhanced statistical robustness for examining CENS-MJO associations, enabling more reliable characterization of phase-dependent patterns and their interannual variability; (2) Three-dimensional circulation structure: This study documents the three-dimensional vertical circulation structure associated with CENS occurrence synchronized with different MJO phases. Addressing the research questions raised in the introduction regarding phase-dependent co-variations between CENS and MJO-related precipitation and circulation patterns, this study presents the following findings based on composite analysis. First, CENS events show a strong preference for specific MJO phases, with 41% occurring in phases 4–5 and 35% in phases 6–7 out of 141 CENS events during the MJO active phase. This phase-dependent distribution suggests that not only does the MJO create environmental conditions favorable for CENS occurrence, but the systematic co-occurrence of CENS with modified MJO structures in these phases is consistent with potential bidirectional interactions, although confirmation requires lead-lag or event-tracking analyses. Second, the spatial distribution and mechanism of precipitation enhancement associated with CENS differ clearly by MJO phase. In MJO phases 4–5, CENS occurrence is associated with local amplification of convective activity in the waters surrounding Java, with enhanced precipitation area of the MJO itself. On the other hand, in phases 6–7, CENS-associated patterns extend over a wider area, bringing precipitation enhancement even in regions separated to the west from the main MJO convective area. This difference suggests that the association between CENS and precipitation enhancement appears to involve different mechanisms: localized amplification of convective activity in phases 4–5, and broader-scale circulation co-variations in phases 6–7. Third, analysis of the vertical circulation structure revealed that while CENS occurrence is associated with increased precipitation and enhanced updrafts in the middle troposphere in both phases, the vertical extent and horizontal spread of these associations are more pronounced in phases 6–7. Particularly in phases 6–7, association with deeper convective systems and patterns consistent with convective development in the MJO downstream region were confirmed. Fourth, SST analysis revealed significant positive SST anomalies in the Western Pacific region specifically in phases 6–7, a pattern that persists even when controlling for both ENSO phase and MJO amplitude variations (Supplemental Fig. 1). This finding is consistent with two non-exclusive interpretations: (1) the southward shift of the MJO propagation path, as proposed by Lubis et al. [ 7 ], resulting in reduced cloudiness and enhanced shortwave heating north of the displaced convection; or (2) pre-existing warm SST conditions that simultaneously favor both CENS generation and MJO intensification in these phases. The phase-specificity of these anomalies (substantially stronger and more extensive in phases 6–7 compared to phases 4–5) suggests processes beyond simple ENSO background effects, but establishing the causal sequence requires temporal evolution analyses beyond the scope of simultaneous composites. Taken together, these findings suggest a phase-dependent transition in how CENS and MJO covary: from localized convective amplification over the Maritime Continent (phases 4–5) to broader-scale structural associations as the MJO moves into the Western Pacific (phases 6–7). The consistency of this pattern across precipitation, SST, and three-dimensional circulation fields provides observational constraints for future process-oriented studies aimed at establishing causal mechanisms. These findings complement the conventional perspective of one-way modulation of CENS by the MJO by documenting systematic structural differences in MJO-related fields between CENS and non-CENS conditions. An important caveat is that the simultaneous composite approach employed here documents co-variation rather than establishing causal direction. The observed associations could reflect active CENS effects on MJO evolution, preferential triggering of CENS by stronger MJO phases, or pre-existing conditions that favor both phenomena. Distinguishing among these possibilities requires temporal evolution analyses such as lead-lag composites or MJO amplitude tracking, which we identify as a priority for future work. To develop the results of this study, several complementary approaches are warranted. First, since reanalysis-based diagnostic studies cannot completely separate ocean-atmosphere feedbacks, verification by coupled model experiments is required to clarify the SST-mediated association between CENS and MJO propagation. Second, lead-lag composite analysis targeting the cold anticyclone extension process could provide direct observational constraints on the temporal sequence of air-sea interaction during CENS. Preliminary analysis suggests that near-surface air temperature decreases prior to SST cooling, consistent with cold advection from the continental high leading the oceanic response. However, given the limited sample size when stratifying by both MJO phase and lag, such analyses require careful assessment of statistical robustness. Third, detailed case studies combining satellite observations, ship-based measurements, and high-resolution atmospheric reanalysis could document the mesoscale processes linking cold surge dynamics to SST evolution. These approaches would help distinguish whether the phase-dependent SST patterns documented here represent active CENS effects on air-sea heat exchange or pre-existing oceanic conditions that modulate CENS-MJO coupling. Methods The fifth generation ECMWF (European Centre for Medium-Range Weather Forecasts) atmospheric reanalysis (ERA5) data covers the period from 1940 to the present [ 10 ] and was used to investigate large-scale atmospheric environmental factors. The dataset has a horizontal resolution of 0.25 degrees, 37 levels from 1–1000 hPa, and daily intervals. Precipitation data from the Global Precipitation Climatology Project (GPCP) Version 1.3, with 1-degree daily resolution from multi-satellite observations covering the period from October 1996 to the present [ 13 ], was used to analyze precipitation patterns associated with CENS. CENS events were identified using the area average of surface meridional wind speed (10-m wind, ERA5 variable v10), calculated only at grid points located over the ocean in the region 105°E–110°E and 8°S–0°, with daily averages applied. A CENS day was defined when the daily average meridional wind speed was at or below − 5 m/s (i.e., v10 ≤ -5 m/s, corresponding to northerly wind speed ≥ 5 m/s). An event was defined as a continuous period during which the daily average meridional wind speed remained at or below − 5 m/s. Events were separated by at least one day when the threshold was not met. The Niño 3.4 index was obtained from the Global Climate Observing System Working Group on Surface Pressure ( https://psl.noaa.gov/data/timeseries/month/DS/Nino34/ ) on December 12, 2025. The Niño 3.4 index typically uses a 5-month running mean, and El Niño or La Niña events are defined when Niño 3.4 SSTs exceed +/- 0.4°C or +/- 0.5°C for a period of 6 months or more. In this study, to analyze trends in CENS occurrence, El Niño or La Niña characteristics were defined when Niño 3.4 SSTs exceeded +/- 0.5°C for 4 months (December, January, February, March). The ERA5 OLR MJO Index [ 14 ] was obtained from the National Oceanic and Atmospheric Administration (NOAA) ( https://psl.noaa.gov/mjo/mjoindex/ ) on December 12, 2025. It is the projection of 20–96 day filtered ERA5 OLR, including all eastward and westward wavenumbers, onto the daily spatial EOF patterns of 30–96 day eastward filtered OLR from the ERA5 dataset. This index is defined by the projection of 20–96 day filtered ERA5 OLR onto the daily spatial EOF patterns of 30–96 day eastward filtered OLR. This index relies solely on OLR data and does not include zonal wind fields at 200- or 850-hPa, ensuring independence from the circulation anomalies associated with CENS events. Each phase represents a rough geographic location on the globe. Specifically, phases 8 and 1 correspond to the Western Hemisphere and Africa, respectively; phases 2 and 3 correspond to the Western and Eastern Indian Ocean, respectively; phases 4 and 5 correspond to the Western and Eastern Maritime Continent, respectively; and phases 6 and 7 correspond to the Western and Central Pacific, respectively. The intensity of the MJO is approximated by its amplitude, with amplitudes less than 1 usually considered weak, incoherent, or inactive. Composite analysis was performed for January–February data, stratified by active MJO phase (4–5 vs. 6–7) and CENS occurrence. Statistical significance of the differences between CENS and non-CENS composites was assessed using a two-sided Student's t test at the 95% confidence level. It should be noted that CENS events typically persist for multiple consecutive days (mean duration 3.2 days), which introduces temporal autocorrelation in the daily samples. While this autocorrelation may lead to slightly optimistic significance estimates compared to fully independent samples, a simple assessment accounting for the effective sample size reduction suggests that the main signals remain statistically significant: even if the effective sample size is conservatively estimated as N/3 (where N is the number of days, accounting for the mean event duration of 3.2 days), the major spatial patterns shown in Figs. 3 – 6 —characterized by widespread coherent anomalies of the same sign—remain significant at the 5% level. The large sample sizes (ranging from 74 to 649 days per composite) and the robustness of the main spatial patterns therefore suggest that the qualitative conclusions are not substantially affected by temporal autocorrelation. Declarations Competing interests The author declares no competing interests. Author Contribution QM proposed the topic and conceived and designed the study. Acknowledgments The authors acknowledge members of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) for their valuable suggestions. Data Availability The data supporting the conclusions of this paper are available upon request to the corresponding author. ERA5 reanalysis data are available from the Copernicus Climate Data Store (https://cds.climate.copernicus.eu/). GPCP precipitation data are available from NASA Goddard Space Flight Center (https://precip.gsfc.nasa.gov/). The ERA5 OLR MJO Index is available from NOAA PSL (https://psl.noaa.gov/mjo/mjoindex/). References Hattori, M., Mori, S. & Matsumoto, J. The Cross-Equatorial Northerly Surge over the Maritime Continent and Its Relationship to Precipitation Patterns. J. Meteorol. Soc. Jpn Ser. II . 89A , 27–47 (2011). Moteki, Q. Vertical structure and occurrence patterns of the cross-equatorial northerly surge under different ENSO and MJO phases. Sci. Rep. 14 , 29116 (2024). Xavier, P. et al. Seasonal Dependence of Cold Surges and their Interaction with the Madden–Julian Oscillation over Southeast Asia. J. Clim. 33 , 2467–2482 (2020). Madden, R. A. & Julian, P. R. Detection of a 40–50 Day Oscillation in the Zonal Wind in the Tropical Pacific. J. Atmos. Sci. 28 (5), 702–708. https://doi.org/10.1175/1520-0469(1971)028%3C0702 (1971). :DOADOI>2.0.CO;2 Wijayanti, A. V., Hidayat, R., Faqih, A. & Alfahmi, F. The Impact of the Interaction between Madden-Julian Oscillation and Cold Surge, on Rainfall over Western Indonesia. Indones J. Geogr. 53 , (2021). Tan, I., Reeder, M. J., Singh, M. S., Birch, C. E. & Peatman, S. C. Wet and Dry Cold Surges Over the Maritime Continent. J. Geophys. Res. Atmos. 128 , (2023). e2022JD038196. Lubis, S. W., Hagos, S., Chang, C. C., Balaguru, K. & Leung, L. R. Cross-Equatorial Surges Boost MJO’s Southward Detour Over the Maritime Continent. Geophys. Res. Lett. 50, eGL104770 (2023). (2023). Hadi, T. W. et al. Cross-Equatorial Northerly Surges Associated with Extratropical Cold Surges and Tropical Variability over the Maritime Continent. https://doi.org/10.1175/JCLI-D-24-0132.1 doi: 10.1175/JCLI-D-24-0132.1 . (2025). Ebita, A. et al. Japanese 55-year Reanalysis JRA-55: Interim Rep. Sola 7 , 149–152 (2011). Hersbach, H. et al. The ERA5 global reanalysis. Q. J. R Meteorol. Soc. 146 , 1999–2049 (2020). Koseki, S., Koh, T. Y. & Teo, C. K. Effects of the cold tongue in the South China Sea on the monsoon, diurnal cycle and rainfall in the Maritime Continent. Q. J. R Meteorol. Soc. 139 , 1566–1582 (2013). Shinoda, T. & Hendon, H. H. Mixed layer modeling of intraseasonal variability in the tropical western Pacific and Indian Oceans. J. Clim. 11 , 2623–2643 (1998). Adler, R. F. et al. The Global Precipitation Climatology Project (GPCP) Monthly Analysis (New Version 2.3) and a Review of 2017 Global Precipitation. Atmosphere 9 , 138 (2018). Kiladis, G. N. et al. A Comparison of OLR and Circulation-Based Indices for Tracking the MJO. (2014). https://doi.org/10.1175/MWR-D-13-00301.1 Additional Declarations No competing interests reported. Supplementary Files Supplementaryinformation.docx Cite Share Download PDF Status: Published Journal Publication published 16 Mar, 2026 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 04 Feb, 2026 Reviews received at journal 18 Jan, 2026 Reviews received at journal 06 Jan, 2026 Reviewers agreed at journal 05 Jan, 2026 Reviewers agreed at journal 26 Dec, 2025 Reviewers invited by journal 26 Dec, 2025 Editor invited by journal 18 Dec, 2025 Editor assigned by journal 15 Dec, 2025 Submission checks completed at journal 15 Dec, 2025 First submitted to journal 12 Dec, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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04:28:42","extension":"html","order_by":20,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":83582,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8342789/v1/599244aec91780440c30cf27.html"},{"id":98188713,"identity":"dc486ff5-2c3a-4c60-97a9-f850d137e3d5","added_by":"auto","created_at":"2025-12-15 04:28:42","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":201586,"visible":true,"origin":"","legend":"\u003cp\u003eTime series of CENS event occurrence frequency by ENSO phase from 1940–2023. The number of CENS events in January and February of each year is shown by month (January: blue, February: green). Background colors represent ENSO phases: La Niña (light blue), El Niño (pink), and Neutral (white). A trend of significantly more frequent CENS events during La Niña years can be confirmed. All plots were generated using Python 3.10.13 (\u003ca href=\"http://www.python.org/\"\u003ehttp://www.python.org\u003c/a\u003e).\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8342789/v1/094d1520b7247e5cb5abc134.png"},{"id":98188716,"identity":"1f615bab-1c5e-458f-a32c-15214ce18a9e","added_by":"auto","created_at":"2025-12-15 04:28:42","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":76758,"visible":true,"origin":"","legend":"\u003cp\u003eDistribution of CENS occurrence frequency by MJO phase. Phase distribution of CENS days in January (blue) and February (green) based on 84 seasons of ERA5 data from 1940–2023 for (a) MJO active period (amplitude ≥ 1.0) and (b) MJO inactive period (amplitude \u0026lt; 1.0). All plots were generated using Python 3.10.13 (\u003ca href=\"http://www.python.org/\"\u003ehttp://www.python.org\u003c/a\u003e).\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8342789/v1/f4907f76f84d6e53fffc7380.png"},{"id":98188719,"identity":"468f1566-8ff2-4ae7-b8a6-1eefdd3f0a8a","added_by":"auto","created_at":"2025-12-15 04:28:42","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1635655,"visible":true,"origin":"","legend":"\u003cp\u003eComposite maps of precipitation (mm/day) by phase during MJO active periods. Based on GPCP precipitation data from 1996–2023. (a) Overall composite for MJO phases 4–5, (b) difference between CENS days (57 days) and non-CENS days (199 days) in MJO phases 4–5, (c) Overall composite for MJO phases 6–7, (d) difference between CENS days (62 days) and non-CENS days (185 days) in MJO phases 6–7. In (b) and (d), black stippling marks grid points where the CENS–non-CENS differences are statistically significant at the 95% confidence level. All plots were generated with GrADS v2.2.1 (http://cola.gmu.edu/grads/grads.php).\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8342789/v1/33ec0681f61dfe9db9c52d9a.jpeg"},{"id":98430641,"identity":"1a33d298-94f6-4550-a9cf-0cdfd42990af","added_by":"auto","created_at":"2025-12-17 16:45:57","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":2037818,"visible":true,"origin":"","legend":"\u003cp\u003eComposite maps of SST, SLP, and surface wind fields by phase during MJO active periods. Based on ERA5 reanalysis data (SST, surface wind, SLP) from 1940–2023. (a) Overall composite for MJO phases 4–5, (b) difference between CENS days (191 days) and non-CENS days (649 days) in MJO phases 4–5, (c) Overall composite for MJO phases 6–7, (d) difference between CENS days (180 days) and non-CENS days (563 days) in MJO phases 6–7. Color shading indicates SST (°C), contours indicate SLP (hPa), and arrows indicate surface wind vectors (m/s). Panels (b) and (d) show only differences significant at the 95% confidence level. All plots were generated with GrADS v2.2.1 (http://cola.gmu.edu/grads/grads.php).\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8342789/v1/e11e3537df3659bbf6480aa2.jpeg"},{"id":98432451,"identity":"fb6818a6-d60c-442b-9264-40413a93f9bc","added_by":"auto","created_at":"2025-12-17 16:49:34","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1896996,"visible":true,"origin":"","legend":"\u003cp\u003eVertical meridional cross-sections by phase during MJO active periods. Vertical meridional cross-sections averaged over 105°E–110°E based on ERA5 reanalysis data (zonal wind, meridional wind, vertical p-velocity) from 1940–2023. (a) Overall composite for MJO phases 4–5, (b) difference between CENS days and non-CENS days in MJO phases 4–5, (c) Overall composite for MJO phases 6–7, (d) difference between CENS days and non-CENS days in MJO phases 6–7. Color shading indicates zonal wind (m/s), blue contours indicate meridional wind (m/s), and streamlines are calculated from meridional wind and negative vertical p-velocity (Pa/s, multiplied by a scaling parameter of 100). Panels (b) and (d) show only differences significant at the 95% confidence level. All plots were generated with GrADS v2.2.1 (http://cola.gmu.edu/grads/grads.php).\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8342789/v1/c05b120bb76d49f8d7a677a8.jpeg"},{"id":98188720,"identity":"efc8268e-1547-4e01-b015-7e5f1f267c95","added_by":"auto","created_at":"2025-12-15 04:28:42","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1956469,"visible":true,"origin":"","legend":"\u003cp\u003eVertical longitudinal cross-sections by phase during MJO active periods. Vertical longitudinal cross-sections averaged over 10°S–5°S based on ERA5 reanalysis data (zonal wind, meridional wind, vertical p-velocity) from 1940–2023. (a) Overall composite for MJO phases 4–5, (b) difference between CENS days and non-CENS days in MJO phases 4–5, (c) Overall composite for MJO phases 6–7, (d) difference between CENS days and non-CENS days in MJO phases 6–7. Color shading indicates meridional wind (m/s), blue contours indicate zonal wind (m/s), and streamlines are calculated from zonal wind and negative vertical p-velocity (Pa/s, multiplied by a scaling parameter of 100). Panels (b) and (d) show only differences significant at the 95% confidence level. All plots were generated with GrADS v2.2.1 (http://cola.gmu.edu/grads/grads.php).\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8342789/v1/e65700ecfc53b1bfdfef4f8f.jpeg"},{"id":105223758,"identity":"98c13f8d-b4c1-4227-a696-86df1a950bc4","added_by":"auto","created_at":"2026-03-23 16:10:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8401338,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8342789/v1/70fcb3d6-3af4-480a-ae6c-06ed2c9f731f.pdf"},{"id":98431342,"identity":"419bb753-4c33-4bb7-b9e5-52e989a4bead","added_by":"auto","created_at":"2025-12-17 16:47:33","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1187414,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-8342789/v1/3274d4625722726b9ac8e730.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Phase-dependent modulation of the MJO during cross-equatorial northerly surges (CENS)","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe Cross-Equatorial Northerly Surge (CENS) is an intermittent burst of northerly winds during boreal winter embedded within the East Asian Winter Monsoon. CENS frequently crosses the equator via the South China Sea, Karimata Strait, and Java Sea, modulating precipitation and lower-tropospheric circulation over the western Maritime Continent (MC) [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. As CENS traverses warm tropical waters, they acquire moisture, often triggering multi-day heavy rainfall and flooding in western Indonesia and adjacent regions [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Superimposed on the synoptic disturbance of CENS is the Madden-Julian Oscillation (MJO), the dominant mode of intraseasonal variability in the tropics, which also strongly governs MC precipitation [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eReflecting the importance of their interaction, previous studies have demonstrated that the concurrent occurrence of CENS and the convective active phase of the MJO results in significantly amplified precipitation compared to either phenomenon occurring in isolation [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. CENS can enhance precipitation by 200\u0026ndash;400% when coincident with the MJO active phase [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Beyond this amplification effect, recent work by Lubis et al. [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] revealed that CENS enhance the southward detour of the MJO over the MC by strengthening moisture convergence in the Southern Hemisphere. This finding suggests that CENS can actively modify the propagation characteristics and intensity of the MJO, implying that the influence of CENS may be distinct across different MJO phases.\u003c/p\u003e \u003cp\u003eConsistent with this possibility, observational studies have established that CENS events show a strong preference for specific MJO phases. For instance, Moteki [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] demonstrated that approximately 87% of CENS events occur during MJO phases 4\u0026ndash;7, with particularly high frequency when the MJO convective envelope is located over the MC (phases 4\u0026ndash;5) and the Western Pacific (phases 6\u0026ndash;7). This phase-dependent distribution suggests that while the MJO creates environmental conditions favorable for CENS generation, the high frequency of CENS events in these phases is consistent with possible feedbacks between CENS and the MJO. The mechanisms behind these enhancements\u0026mdash;including strengthened meridional convergence, modulated sea surface temperature (SST) patterns, and altered vertical circulation structures\u0026mdash;are likely to vary depending on the MJO phase in which the CENS occurs [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eGiven these phase-dependent characteristics, key questions remain: How do CENS-associated precipitation and circulation patterns differ between MJO phases 4\u0026ndash;5 and 6\u0026ndash;7? Do CENS-related SST anomalies exhibit phase-dependent characteristics that reflect modified MJO propagation pathways? How does the vertical circulation structure differ systematically between CENS and non-CENS conditions across these phases? This study addresses these questions through a comprehensive analysis of the statistical association between CENS occurrence and MJO-related fields.\u003c/p\u003e \u003cp\u003eTo isolate potential CENS-associated patterns, we employ a composite approach comparing CENS days and non-CENS days within the same MJO phase, thereby controlling for the large-scale background state associated with MJO convective activity. While this approach documents systematic co-variation rather than directly establishing temporal causality, it reveals how MJO-related fields differ when CENS occurs. Specifically, we document: (1) distinct spatial patterns of CENS-associated precipitation anomalies between MJO phases 4\u0026ndash;5 and 6\u0026ndash;7; (2) phase-dependent characteristics of CENS-related SST anomalies consistent with modified MJO propagation pathways; (3) systematic differences in vertical circulation structure between CENS and non-CENS conditions.\u003c/p\u003e \u003cp\u003eThis study extends previous studies in several key ways: (1) Extended temporal coverage: While previous works were constrained by dataset limitations to periods of less than 50 years, this study extends the analysis to 84 years (1940\u0026ndash;2023), providing a substantially larger sample size; (2) Three-dimensional circulation structure: This study documents the three-dimensional vertical circulation structure associated with CENS occurrence synchronized with different MJO phases, revealing phase-dependent differences in meridional and zonal circulation patterns, vertical extent of updrafts, and the organization of convective disturbances.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSample Characteristics and Statistical Overview\u003c/h2\u003e \u003cp\u003eTo extend the work of Moteki [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], which performed composite analysis using 48 years of data (1974\u0026ndash;2022) using Japanese 55-year reanalysis (JRA-55 [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]) and the Australian Bureau of Meteorology MJO index, this study investigated CENS events in January and February over 84 consecutive boreal winter seasons (1940\u0026ndash;2023) using ERA5 [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Although CENS can occur with lower frequency in December and March, the analysis is restricted to January and February to exclude the effects of seasonality in atmospheric circulation fields and sea surface conditions. From a total of 4,976 days in January and February across the 84 analyzed seasons, 715 days (226 events) were identified as CENS days, defined as days when the area-averaged surface meridional wind speed over 105\u0026deg;E\u0026ndash;110\u0026deg;E, 8\u0026deg;S\u0026ndash;0\u0026deg; northerly wind speed exceeded 5 m/s, following Moteki [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAs shown in Moteki [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], CENS occurrence frequency exhibited clear variations depending on the ENSO phase (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). La Ni\u0026ntilde;a conditions showed the highest activity, with 291 CENS days (17.5% of all La Ni\u0026ntilde;a January\u0026ndash;February days; 40.7% of total CENS days) and 95 events (5.72 events per 100 days). In contrast, El Ni\u0026ntilde;o conditions had only 94 CENS days (7.6% of El Ni\u0026ntilde;o days; 13.1% of total CENS) and 37 events (2.98 events per 100 days). Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the time series of CENS event frequency in January and February for each year from 1940 to 2023, categorized by ENSO phase (La Ni\u0026ntilde;a: light blue, El Ni\u0026ntilde;o: pink, Neutral: white). The time series visually confirms the trend of significantly more frequent CENS events during La Ni\u0026ntilde;a years compared to El Ni\u0026ntilde;o years (light blue background) and also indicates the magnitude of interannual variability. Statistical analysis using a chi-square test confirmed a significant association between ENSO phase and CENS occurrence (χ\u0026sup2; = 64.33, p\u0026thinsp;\u0026lt;\u0026thinsp;0.0001). Pairwise comparisons with Bonferroni correction revealed that La Ni\u0026ntilde;a had a significantly higher CENS occurrence rate than El Ni\u0026ntilde;o (17.5% vs 7.6%, p\u0026thinsp;\u0026lt;\u0026thinsp;0.0001), and El Ni\u0026ntilde;o had a significantly lower rate than Neutral conditions (7.6% vs 15.9%, p\u0026thinsp;\u0026lt;\u0026thinsp;0.0001). This characteristic of significantly higher CENS frequency during La Ni\u0026ntilde;a compared to El Ni\u0026ntilde;o is consistent with the results of Moteki [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] and shows a common feature with no difference between January and February.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCENS occurrence statistics stratified by ENSO phase. 'CENS Days (Jan)' and 'CENS Days (Feb)' denote the number of CENS days in January and February, respectively, in each ENSO category. 'Total Days in Phase' is the total number of January\u0026ndash;February days in each ENSO category over the 84 winters. 'Total CENS Days' is the sum of CENS days in January and February. Note that the mean CENS event duration is approximately 3.2 days, so 'Number of CENS events' \u0026times; 3.2 \u0026asymp; 'Total CENS Days'.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eENSO Phase\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCENS Days (Jan)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCENS Days (Feb)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTotal Days in Phase\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTotal CENS Days\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e% of Phase\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e% of Total CENS\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eNumber of CENS events\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLa Ni\u0026ntilde;a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e165\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e126\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1,660\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e291\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e17.5%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e40.7%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e95\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEl Ni\u0026ntilde;o\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1,243\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e7.6%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e13.1%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e37\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNeutral\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e196\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e134\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2,073\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e330\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e15.9%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e46.2%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e94\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eRegarding the relationship between MJO activity and CENS frequency, out of 715 CENS days (226 events), 471 days (141 events) occurred during the MJO active phase (amplitude\u0026thinsp;\u0026ge;\u0026thinsp;1.0), while 244 days (85 events) occurred during the MJO inactive phase. The mean (median) duration of individual CENS events is about 3.2 (2.0) days, with a range from 1 to 20 days. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea shows the phase distribution of CENS frequency during the MJO active phase, clearly confirming a significant increase in CENS days during phases 4\u0026ndash;7. When classified by MJO phase, 41% of CENS days (41% of events) occurred in phases 4\u0026ndash;5 and 38% of days (35% of events) in phases 6\u0026ndash;7, consistent with Moteki [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. On the other hand, during the MJO inactive phase (amplitude\u0026thinsp;\u0026lt;\u0026thinsp;1.0) shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb, the phase dependence of CENS frequency is not as clear as in the active phase, but a relatively higher tendency is still observed in phases 5\u0026ndash;7. Based on this phase-dependent CENS frequency distribution, this study performed composite analyses separately for phases 4\u0026ndash;5 (MJO located over the MC) and phases 6\u0026ndash;7 (MJO located over the Western Pacific).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eCENS-associated differences in precipitation\u003c/h3\u003e\n\u003cp\u003eTo isolate potential CENS-associated patterns in MJO-related fields, we compare CENS days and non-CENS days within the same MJO phase, thereby controlling for the large-scale background state associated with the MJO. This approach allows us to document how precipitation, circulation, and SST patterns systematically differ when CENS occurs, although it does not directly establish temporal causality. Composite analysis reveals distinct precipitation responses to CENS depending on the MJO phase (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). In phases 4\u0026ndash;5, the CENS\u0026ndash;non-CENS difference is particularly pronounced around Java and along the northern coast of Australia (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). This spatial pattern is consistent with strengthened cross-equatorial northerlies and suggests enhanced low-level meridional moisture convergence that may locally amplify MJO-related convection over the Maritime Continent.\u003c/p\u003e \u003cp\u003eIn MJO phases 6\u0026ndash;7, the MJO convective envelope is located over the Western Pacific, and the precipitation maximum shifts eastward toward the region off the northeast coast of Australia (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). In this phase, the precipitation field extends farther east than in phases 4\u0026ndash;5, consistent with the eastward propagation of the MJO. Notably, the CENS\u0026ndash;non-CENS difference in phases 6\u0026ndash;7 exhibits a meridionally broader pattern of statistically significant precipitation enhancement over the western flank (rear) of the MJO convective envelope (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). In addition, a narrow, elongated band of significant positive anomalies extends along and offshore of the northeastern coast of the Philippines. This spatial distribution suggests that, during phases 6\u0026ndash;7, CENS occurrence is associated not only with local precipitation enhancement near the Maritime Continent but also with broader modifications to disturbances and convective systems in the western rear of the MJO, resulting in more widespread precipitation increases.\u003c/p\u003e \u003cp\u003eComparison between phases 4\u0026ndash;5 and phases 6\u0026ndash;7 clarifies the phase dependence of CENS-associated precipitation patterns in the MJO. In phases 4\u0026ndash;5, CENS occurrence is associated with localized amplification of MJO precipitation, with the most pronounced differences around Java and along the northern coast of Australia. On the other hand, in phases 6\u0026ndash;7, CENS-associated patterns extend over a wider area, with meridionally broader enhancement over the western rear of the MJO and an additional significant signal near the northeastern Philippines. This spatial contrast suggests that the role of CENS in the MJO system could differ by phase: localized amplification of convective activity in phases 4\u0026ndash;5 versus broader-scale co-variations in precipitation and circulation in phases 6\u0026ndash;7.\u003c/p\u003e\n\u003ch3\u003eCENS-associated patterns in SST and SLP\u003c/h3\u003e\n\u003cp\u003eThe distinct precipitation responses documented above raise a mechanistic question: what atmospheric and oceanic conditions are associated with CENS occurrence in each MJO phase? To address this, we examine SST and sea level pressure (SLP) fields. SST provides insight into both the thermodynamic environment supporting convection and potential air-sea interaction processes, while SLP reveals the large-scale pressure gradients that drive the cross-equatorial flow characteristic of CENS.\u003c/p\u003e \u003cp\u003eAs shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, in both MJO phases 4\u0026ndash;5 and 6\u0026ndash;7, a high SST region (\u0026gt;\u0026thinsp;27\u0026deg;C) is widely distributed in the latitude band from 20\u0026deg;S to 15\u0026deg;N. Meanwhile, in the South China Sea, a low SST region (\u0026lt;\u0026thinsp;26\u0026deg;C) is formed in response to strong northerly winds blowing out from the high SLP anomalies associated with the cold surge, clearly showing the characteristics of the so-called cold tongue [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDuring CENS occurrence in MJO phases 4\u0026ndash;5 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), distinct SLP and SST anomaly patterns can be confirmed between 90\u0026deg;E\u0026ndash;120\u0026deg;E. In the Northern Hemisphere, a high-SLP anomaly of about\u0026thinsp;+\u0026thinsp;2 hPa is seen, and in the Southern Hemisphere, a low-SLP anomaly of about \u0026minus;\u0026thinsp;2 hPa is seen, indicating that the strengthened meridional pressure gradient supports the occurrence of CENS. Also noteworthy is the significant negative SST anomaly (-0.7 to -0.3\u0026deg;C) between 90\u0026deg;E\u0026ndash;120\u0026deg;E.\u003c/p\u003e \u003cp\u003eIn MJO phases 6\u0026ndash;7, a clear difference appears in the spatial pattern of CENS-associated contrasts (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). Although negative SST anomalies in the South China Sea are seen as in phases 4\u0026ndash;5, significant positive SST anomalies can be confirmed in the Western Pacific region. These positive SST anomalies are more distinct than in phases 4\u0026ndash;5 and extend over a wider spatial area. Comparison with phases 4\u0026ndash;5 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) reveals that while weak positive SST anomalies are also present over the Western Pacific in phases 4\u0026ndash;5, the anomalies in phases 6\u0026ndash;7 are substantially stronger and more extensive. Potential concerns regarding these SST patterns include that the positive anomalies might reflect (1) the La Ni\u0026ntilde;a background state (i.e., warm water accumulation in the Western Pacific) rather than a specific response to CENS, or (2) differences in MJO amplitude between CENS and non-CENS days, where stronger MJO phases might preferentially trigger CENS with concurrent structural modifications. To address both concerns, an analysis was performed using a subsample restricted to Neutral years with MJO amplitude in the range of 1.0\u0026ndash;1.5, thereby controlling for both ENSO phase and MJO amplitude variations (Supplemental Fig.\u0026nbsp;1). This analysis reveals that the phase-dependent contrast persists even under these more stringent conditions: positive SST anomalies are present in both phases but are substantially stronger and more extensive in phases 6\u0026ndash;7 compared to phases 4\u0026ndash;5. This fact suggests that the enhanced positive SST anomalies in MJO phases 6\u0026ndash;7 are not merely due to ENSO background state differences or MJO amplitude selection effects but are systematically associated with specific CENS-associated contrasts in MJO-related fields.\u003c/p\u003e \u003cp\u003eLubis et al. [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] pointed out that the occurrence of CENS shifts the eastward propagation path of the MJO further southward. The positive SST anomalies in the Western Pacific observed here could be consistent with two distinct, non-exclusive processes: (1) an active CENS effect where the southward shift of MJO propagation reduces cloudiness and enhances shortwave heating, leading to SST warming through ocean mixed layer processes (quantitatively, an SST increase of +\u0026thinsp;0.1 to +\u0026thinsp;0.2\u0026deg;C is reasonable for such a one-dimensional process [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]); or (2) pre-existing warm SST conditions that simultaneously favor both CENS generation and MJO intensification in these phases, with the warm anomalies representing oceanic background states rather than CENS-induced responses. The simultaneous composite approach employed here cannot distinguish between these processes, as it documents co-variation at a single time point rather than temporal evolution.\u003c/p\u003e \u003cp\u003ePreliminary lead-lag analysis (Supplemental Fig.\u0026nbsp;2) provides suggestive evidence regarding the temporal sequence of air-sea interaction during CENS. The analysis shows that SLP begins increasing (indicating anticyclone extension) approximately 2\u0026ndash;3 days before CENS occurrence, followed by near-surface air temperature decreases that precede SST cooling by approximately 1 day. This sequence\u0026mdash;SLP rise, then air temperature drop, then SST cooling\u0026mdash;is consistent with cold advection from continental highs driving the oceanic response rather than SST anomalies preceding atmospheric changes. However, given limited sample sizes when stratifying by both MJO phase and lag (approximately 20\u0026ndash;30 events per phase), and the potential for aliasing between MJO propagation and CENS-related signals, the causal sequence of SST anomalies relative to CENS occurrence remains unresolved. Nevertheless, the observed temporal ordering provides a testable hypothesis for future studies with larger samples or model experiments.\u003c/p\u003e\n\u003ch3\u003ePhase Dependence of Vertical Circulation Structure\u003c/h3\u003e\n\u003cp\u003eWhile the surface analyses reveal distinct phase-dependent SST and SLP patterns, understanding how CENS modulates the full depth of the tropospheric circulation requires examination of the vertical structure. Analysis of the vertical meridional cross-section in the CENS occurrence region (105\u0026deg;E\u0026ndash;110\u0026deg;E) reveals how the enhanced meridional flow associated with CENS projects onto the vertical circulation and how this projection differs between MJO phases.\u003c/p\u003e \u003cp\u003eIn the overall composite of MJO phases 4\u0026ndash;5 shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, a prominent updraft region can be confirmed south of the equator (near 5\u0026deg;S\u0026ndash;10\u0026deg;S), which is consistent with the center of MJO convective activity being located on the Southern Hemisphere side of the Maritime Continent. The meridional wind field (contours) shows a pattern of inflow from the Northern Hemisphere crossing the equator into the Southern Hemisphere, with a clear meridional circulation cell structure of strong northerly winds in the lower layer and southerly winds in the upper layer. The zonal wind field (color) shows typical characteristics of the MJO, with remarkable westerly winds below 400 hPa between 15\u0026deg;S and the equator.\u003c/p\u003e \u003cp\u003eThe difference between CENS occurrence and non-occurrence (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb) shows how CENS modulates this vertical circulation. The most prominent feature is the enhanced updraft in the Southern Hemisphere. This updraft enhancement area spatially corresponds to the precipitation increase area shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, supporting the mechanism that CENS occurrence is associated with enhanced convective activity. The meridional wind anomalies show a clear strengthening of the meridional circulation with enhanced northerly winds in the lower layer. The zonal wind anomalies show positive anomalies (westerly wind strengthening) in the Southern Hemisphere, indicating that the westerlies associated with the MJO are strengthened during CENS occurrence.\u003c/p\u003e \u003cp\u003eThe vertical structure in MJO phases 6\u0026ndash;7 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec) shows several important differences compared to phases 4\u0026ndash;5. The updraft region is located near the equator and shifted slightly northward. The structure of the meridional circulation is similar to phases 4\u0026ndash;5, but differences are seen in its intensity and vertical extent. In the difference during CENS occurrence (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed), updraft enhancement in the Southern Hemisphere can be confirmed as in phases 4\u0026ndash;5. But the spatial distribution of positive zonal wind anomalies extends over a wider area and also on the Northern Hemisphere side compared to phases 4\u0026ndash;5. This corresponds to the widening of the precipitation enhancement area in the Northern Hemisphere shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed.\u003c/p\u003e \u003cp\u003eAnalysis of these vertical structures reveals that while CENS occurrence is associated with increased precipitation and enhanced updrafts in the Southern Hemisphere in both phases, the vertical extent and horizontal spread of these associations are more pronounced in phases 6\u0026ndash;7. This difference reflects the phase-dependent differences in the organization of the convective disturbances associated with the MJO.\u003c/p\u003e\n\u003ch3\u003eVertical Circulation Structure Along the MJO Eastward Propagation Path\u003c/h3\u003e\n\u003cp\u003eConsidering the southward shift of the MJO path due to CENS pointed out by Lubis et al. [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], the vertical zonal cross-section averaged between 10\u0026deg;S and 5\u0026deg;S was analyzed. In the overall composite of MJO phases 4\u0026ndash;5 shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, the characteristic vertical structure associated with the eastward propagation of the MJO can be clearly confirmed. Strong updraft regions of the MJO active convective area are seen, and westerly winds associated with westerly wind bursts are seen from the surface to 400 hPa. Looking at the meridional wind component shown by color shading, deep southerly winds are seen in the Indian Ocean west of 95\u0026deg;E, while northerly winds are seen below 850 hPa east of 95\u0026deg;E.\u003c/p\u003e \u003cp\u003eThe difference between CENS occurrence and non-occurrence (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb) provides important information on how CENS modulates the zonal circulation structure of the MJO. The most prominent feature is that updrafts are enhanced over 85\u0026deg;E\u0026ndash;120\u0026deg;E, with significant lower-level northerly wind anomalies associated with CENS, and southerly wind anomalies above 250 hPa.\u003c/p\u003e \u003cp\u003eIn the vertical longitudinal structure in MJO phases 6\u0026ndash;7 (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec), the westerly wind structure below 400 hPa has shifted eastward compared to phases 4\u0026ndash;5, accompanying the eastward movement of the MJO convective system. In the difference during CENS occurrence (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed), the updraft enhancement area is distributed to the east of 90\u0026deg;E and is deeper compared to phases 4\u0026ndash;5. On the other hand, the significant lower-level northerly wind anomalies associated with CENS are slightly shallower compared to phases 4\u0026ndash;5, and upper-level southerly wind anomalies are seen corresponding to the northerly basic field.\u003c/p\u003e \u003cp\u003eAnalysis of these vertical zonal structures reveals that while CENS occurrence is associated with enhanced zonal circulation of the MJO in both phases, these associations extend over a wider area in phases 6\u0026ndash;7, with patterns consistent with convective development in the downstream region of the MJO. This feature is consistent with the widening of the precipitation enhancement area in phases 6\u0026ndash;7 shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed, supporting that CENS occurrence is systematically associated with modifications in the structure of the MJO.\u003c/p\u003e "},{"header":"Summary and Conclusion","content":"\u003cp\u003eThis study comprehensively analyzed phase-dependent associations between CENS occurrence and precipitation and circulation patterns under different MJO phases using 27 years (1996\u0026ndash;2023) of GPCP precipitation data and 84 years (1940\u0026ndash;2023) of ERA5 reanalysis data. The primary novel contributions of this work relative to previous studies are: (1) Extended temporal coverage: By extending the analysis period from less than 50 years to 84 years, this study provides a substantially larger sample size (715 CENS days across 226 events) and enhanced statistical robustness for examining CENS-MJO associations, enabling more reliable characterization of phase-dependent patterns and their interannual variability; (2) Three-dimensional circulation structure: This study documents the three-dimensional vertical circulation structure associated with CENS occurrence synchronized with different MJO phases. Addressing the research questions raised in the introduction regarding phase-dependent co-variations between CENS and MJO-related precipitation and circulation patterns, this study presents the following findings based on composite analysis.\u003c/p\u003e \u003cp\u003eFirst, CENS events show a strong preference for specific MJO phases, with 41% occurring in phases 4\u0026ndash;5 and 35% in phases 6\u0026ndash;7 out of 141 CENS events during the MJO active phase. This phase-dependent distribution suggests that not only does the MJO create environmental conditions favorable for CENS occurrence, but the systematic co-occurrence of CENS with modified MJO structures in these phases is consistent with potential bidirectional interactions, although confirmation requires lead-lag or event-tracking analyses.\u003c/p\u003e \u003cp\u003eSecond, the spatial distribution and mechanism of precipitation enhancement associated with CENS differ clearly by MJO phase. In MJO phases 4\u0026ndash;5, CENS occurrence is associated with local amplification of convective activity in the waters surrounding Java, with enhanced precipitation area of the MJO itself. On the other hand, in phases 6\u0026ndash;7, CENS-associated patterns extend over a wider area, bringing precipitation enhancement even in regions separated to the west from the main MJO convective area. This difference suggests that the association between CENS and precipitation enhancement appears to involve different mechanisms: localized amplification of convective activity in phases 4\u0026ndash;5, and broader-scale circulation co-variations in phases 6\u0026ndash;7.\u003c/p\u003e \u003cp\u003eThird, analysis of the vertical circulation structure revealed that while CENS occurrence is associated with increased precipitation and enhanced updrafts in the middle troposphere in both phases, the vertical extent and horizontal spread of these associations are more pronounced in phases 6\u0026ndash;7. Particularly in phases 6\u0026ndash;7, association with deeper convective systems and patterns consistent with convective development in the MJO downstream region were confirmed.\u003c/p\u003e \u003cp\u003eFourth, SST analysis revealed significant positive SST anomalies in the Western Pacific region specifically in phases 6\u0026ndash;7, a pattern that persists even when controlling for both ENSO phase and MJO amplitude variations (Supplemental Fig.\u0026nbsp;1). This finding is consistent with two non-exclusive interpretations: (1) the southward shift of the MJO propagation path, as proposed by Lubis et al. [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], resulting in reduced cloudiness and enhanced shortwave heating north of the displaced convection; or (2) pre-existing warm SST conditions that simultaneously favor both CENS generation and MJO intensification in these phases. The phase-specificity of these anomalies (substantially stronger and more extensive in phases 6\u0026ndash;7 compared to phases 4\u0026ndash;5) suggests processes beyond simple ENSO background effects, but establishing the causal sequence requires temporal evolution analyses beyond the scope of simultaneous composites.\u003c/p\u003e \u003cp\u003eTaken together, these findings suggest a phase-dependent transition in how CENS and MJO covary: from localized convective amplification over the Maritime Continent (phases 4\u0026ndash;5) to broader-scale structural associations as the MJO moves into the Western Pacific (phases 6\u0026ndash;7). The consistency of this pattern across precipitation, SST, and three-dimensional circulation fields provides observational constraints for future process-oriented studies aimed at establishing causal mechanisms.\u003c/p\u003e \u003cp\u003eThese findings complement the conventional perspective of one-way modulation of CENS by the MJO by documenting systematic structural differences in MJO-related fields between CENS and non-CENS conditions. An important caveat is that the simultaneous composite approach employed here documents co-variation rather than establishing causal direction. The observed associations could reflect active CENS effects on MJO evolution, preferential triggering of CENS by stronger MJO phases, or pre-existing conditions that favor both phenomena. Distinguishing among these possibilities requires temporal evolution analyses such as lead-lag composites or MJO amplitude tracking, which we identify as a priority for future work.\u003c/p\u003e \u003cp\u003eTo develop the results of this study, several complementary approaches are warranted. First, since reanalysis-based diagnostic studies cannot completely separate ocean-atmosphere feedbacks, verification by coupled model experiments is required to clarify the SST-mediated association between CENS and MJO propagation. Second, lead-lag composite analysis targeting the cold anticyclone extension process could provide direct observational constraints on the temporal sequence of air-sea interaction during CENS. Preliminary analysis suggests that near-surface air temperature decreases prior to SST cooling, consistent with cold advection from the continental high leading the oceanic response. However, given the limited sample size when stratifying by both MJO phase and lag, such analyses require careful assessment of statistical robustness. Third, detailed case studies combining satellite observations, ship-based measurements, and high-resolution atmospheric reanalysis could document the mesoscale processes linking cold surge dynamics to SST evolution. These approaches would help distinguish whether the phase-dependent SST patterns documented here represent active CENS effects on air-sea heat exchange or pre-existing oceanic conditions that modulate CENS-MJO coupling.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eThe fifth generation ECMWF (European Centre for Medium-Range Weather Forecasts) atmospheric reanalysis (ERA5) data covers the period from 1940 to the present [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] and was used to investigate large-scale atmospheric environmental factors. The dataset has a horizontal resolution of 0.25 degrees, 37 levels from 1\u0026ndash;1000 hPa, and daily intervals.\u003c/p\u003e \u003cp\u003ePrecipitation data from the Global Precipitation Climatology Project (GPCP) Version 1.3, with 1-degree daily resolution from multi-satellite observations covering the period from October 1996 to the present [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], was used to analyze precipitation patterns associated with CENS.\u003c/p\u003e \u003cp\u003eCENS events were identified using the area average of surface meridional wind speed (10-m wind, ERA5 variable v10), calculated only at grid points located over the ocean in the region 105\u0026deg;E\u0026ndash;110\u0026deg;E and 8\u0026deg;S\u0026ndash;0\u0026deg;, with daily averages applied. A CENS day was defined when the daily average meridional wind speed was at or below \u0026minus;\u0026thinsp;5 m/s (i.e., v10 \u0026le; -5 m/s, corresponding to northerly wind speed\u0026thinsp;\u0026ge;\u0026thinsp;5 m/s). An event was defined as a continuous period during which the daily average meridional wind speed remained at or below \u0026minus;\u0026thinsp;5 m/s. Events were separated by at least one day when the threshold was not met.\u003c/p\u003e \u003cp\u003eThe Ni\u0026ntilde;o 3.4 index was obtained from the Global Climate Observing System Working Group on Surface Pressure (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://psl.noaa.gov/data/timeseries/month/DS/Nino34/\u003c/span\u003e\u003cspan address=\"https://psl.noaa.gov/data/timeseries/month/DS/Nino34/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) on December 12, 2025. The Ni\u0026ntilde;o 3.4 index typically uses a 5-month running mean, and El Ni\u0026ntilde;o or La Ni\u0026ntilde;a events are defined when Ni\u0026ntilde;o 3.4 SSTs exceed +/- 0.4\u0026deg;C or +/- 0.5\u0026deg;C for a period of 6 months or more. In this study, to analyze trends in CENS occurrence, El Ni\u0026ntilde;o or La Ni\u0026ntilde;a characteristics were defined when Ni\u0026ntilde;o 3.4 SSTs exceeded +/- 0.5\u0026deg;C for 4 months (December, January, February, March).\u003c/p\u003e \u003cp\u003eThe ERA5 OLR MJO Index [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] was obtained from the National Oceanic and Atmospheric Administration (NOAA) (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://psl.noaa.gov/mjo/mjoindex/\u003c/span\u003e\u003cspan address=\"https://psl.noaa.gov/mjo/mjoindex/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) on December 12, 2025. It is the projection of 20\u0026ndash;96 day filtered ERA5 OLR, including all eastward and westward wavenumbers, onto the daily spatial EOF patterns of 30\u0026ndash;96 day eastward filtered OLR from the ERA5 dataset. This index is defined by the projection of 20\u0026ndash;96 day filtered ERA5 OLR onto the daily spatial EOF patterns of 30\u0026ndash;96 day eastward filtered OLR. This index relies solely on OLR data and does not include zonal wind fields at 200- or 850-hPa, ensuring independence from the circulation anomalies associated with CENS events. Each phase represents a rough geographic location on the globe. Specifically, phases 8 and 1 correspond to the Western Hemisphere and Africa, respectively; phases 2 and 3 correspond to the Western and Eastern Indian Ocean, respectively; phases 4 and 5 correspond to the Western and Eastern Maritime Continent, respectively; and phases 6 and 7 correspond to the Western and Central Pacific, respectively. The intensity of the MJO is approximated by its amplitude, with amplitudes less than 1 usually considered weak, incoherent, or inactive.\u003c/p\u003e \u003cp\u003eComposite analysis was performed for January\u0026ndash;February data, stratified by active MJO phase (4\u0026ndash;5 vs. 6\u0026ndash;7) and CENS occurrence. Statistical significance of the differences between CENS and non-CENS composites was assessed using a two-sided Student's t test at the 95% confidence level. It should be noted that CENS events typically persist for multiple consecutive days (mean duration 3.2 days), which introduces temporal autocorrelation in the daily samples. While this autocorrelation may lead to slightly optimistic significance estimates compared to fully independent samples, a simple assessment accounting for the effective sample size reduction suggests that the main signals remain statistically significant: even if the effective sample size is conservatively estimated as N/3 (where N is the number of days, accounting for the mean event duration of 3.2 days), the major spatial patterns shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e\u0026mdash;characterized by widespread coherent anomalies of the same sign\u0026mdash;remain significant at the 5% level. The large sample sizes (ranging from 74 to 649 days per composite) and the robustness of the main spatial patterns therefore suggest that the qualitative conclusions are not substantially affected by temporal autocorrelation.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe author declares no competing interests.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eQM proposed the topic and conceived and designed the study.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThe authors acknowledge members of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) for their valuable suggestions.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data supporting the conclusions of this paper are available upon request to the corresponding author. ERA5 reanalysis data are available from the Copernicus Climate Data Store (https://cds.climate.copernicus.eu/). GPCP precipitation data are available from NASA Goddard Space Flight Center (https://precip.gsfc.nasa.gov/). The ERA5 OLR MJO Index is available from NOAA PSL (https://psl.noaa.gov/mjo/mjoindex/).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHattori, M., Mori, S. \u0026amp; Matsumoto, J. The Cross-Equatorial Northerly Surge over the Maritime Continent and Its Relationship to Precipitation Patterns. \u003cem\u003eJ. Meteorol. Soc. Jpn Ser. II\u003c/em\u003e. \u003cb\u003e89A\u003c/b\u003e, 27\u0026ndash;47 (2011).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMoteki, Q. Vertical structure and occurrence patterns of the cross-equatorial northerly surge under different ENSO and MJO phases. \u003cem\u003eSci. 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Mixed layer modeling of intraseasonal variability in the tropical western Pacific and Indian Oceans. \u003cem\u003eJ. Clim.\u003c/em\u003e \u003cb\u003e11\u003c/b\u003e, 2623\u0026ndash;2643 (1998).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAdler, R. F. et al. The Global Precipitation Climatology Project (GPCP) Monthly Analysis (New Version 2.3) and a Review of 2017 Global Precipitation. \u003cem\u003eAtmosphere\u003c/em\u003e \u003cb\u003e9\u003c/b\u003e, 138 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKiladis, G. N. et al. A Comparison of OLR and Circulation-Based Indices for Tracking the MJO. (2014). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1175/MWR-D-13-00301.1\u003c/span\u003e\u003cspan address=\"10.1175/MWR-D-13-00301.1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8342789/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8342789/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study investigates phase-dependent associations between Cross-Equatorial Northerly Surges (CENS) and Madden-Julian Oscillation (MJO) precipitation and circulation patterns using 84 years (1940–2023). Approximately 79% of CENS days (76% of events) occurred during phases 4–7 of the active MJO. Composite analysis reveals distinct phase-dependent co-variations between CENS occurrence and MJO structure: during phases 4–5 (MJO over the Maritime Continent), CENS occurrence is associated with enhanced precipitation around Java and along the northern coast of Australia through enhanced meridional convergence. In contrast, during phases 6–7 (MJO over the Western Pacific), CENS-associated patterns are meridionally broader and vertically deeper, with enhanced precipitation over the western flank (rear) of the MJO. Notably, phases 6–7 exhibit significant positive sea surface temperature anomalies in the Western Pacific, a pattern that persists even when controlling for ENSO phase and MJO amplitude variations, consistent with a southward shift of the MJO propagation path. These results indicate that CENS are not merely passively affected by the MJO but are systematically associated with distinct phase-dependent differences in MJO precipitation patterns, vertical structure, and propagation characteristics.\u003c/p\u003e","manuscriptTitle":"Phase-dependent modulation of the MJO during cross-equatorial northerly surges (CENS)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-15 04:28:37","doi":"10.21203/rs.3.rs-8342789/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-02-04T10:23:05+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-18T05:27:36+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-06T17:41:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"136308251135048187091555227843131567","date":"2026-01-06T02:55:43+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"200428014954486061968466903664257568275","date":"2025-12-26T18:52:09+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-26T13:30:40+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-12-18T08:58:24+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-15T11:35:24+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-12-15T11:33:40+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-12-12T07:01:06+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"f065f86f-0511-41de-b2ce-35b1c5548e44","owner":[],"postedDate":"December 15th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":59588180,"name":"Earth and environmental sciences/Climate sciences"},{"id":59588181,"name":"Earth and environmental sciences/Ocean sciences"}],"tags":[],"updatedAt":"2026-03-23T16:07:22+00:00","versionOfRecord":{"articleIdentity":"rs-8342789","link":"https://doi.org/10.1038/s41598-026-44735-7","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2026-03-16 15:59:01","publishedOnDateReadable":"March 16th, 2026"},"versionCreatedAt":"2025-12-15 04:28:37","video":"","vorDoi":"10.1038/s41598-026-44735-7","vorDoiUrl":"https://doi.org/10.1038/s41598-026-44735-7","workflowStages":[]},"version":"v1","identity":"rs-8342789","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8342789","identity":"rs-8342789","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}