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Article

Post-El Niño Influence on Summer Monsoon Rainfall in Sri Lanka

by
Pathmarasa Kajakokulan
1,* and
Vinay Kumar
2
1
Department of Oceanography and Marine Geology, Faculty of Fisheries and Marine Sciences & Technology, University of Ruhuna, Matara 81000, Sri Lanka
2
Department of Atmospheric Science, Environmental Science and Physics, University of the Incarnate Word, San Antonio, TX 78209, USA
*
Author to whom correspondence should be addressed.
Water 2025, 17(11), 1664; https://doi.org/10.3390/w17111664
Submission received: 28 April 2025 / Revised: 27 May 2025 / Accepted: 28 May 2025 / Published: 30 May 2025
(This article belongs to the Special Issue Impacts of Climate Change on Water Resources and Water Risks, 2nd Edition)
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Abstract

:
Sri Lanka typically experiences anomalously wet conditions during the summer following El Niño events, but this response varies due to El Niño complexity. This study investigates the impact of post-El Niño conditions on Sri Lanka’s Monsoon rainfall, contrasting summers after fast- and slow-decaying El Niño events. Results indicate that fast-decaying El Niño events lead to wet and cool summers while slow-decaying events result in dry and warm summers. These contrasting responses are linked to sea surface temperature (SST) changes in the central to eastern Pacific. During the fast-decaying El Niño, the transition to La Niña generates strong easterlies in the central and eastern Pacific, enhancing moisture convergence, upward motion, and cloud cover, resulting in wetter conditions over Sri Lanka. During the fast-decaying El Niño, enhanced precipitation over the Maritime Continent acts as a diabatic heating source, inducing Gill-type easterly wind anomalies over the tropical Pacific. These winds promote coupled feedbacks that accelerate the transition to La Niña, strengthening moisture convergence and upward motion over Sri Lanka. Conversely, slow-decaying El Niño events are associated with cooling in the western North Pacific and warming in the Indian Ocean, which promotes the development of the western North Pacific anticyclone, suppressing upward motion and reducing cloud cover, leading to conditions over Sri Lanka. Changes in the Walker circulation further contribute to these distinct rainfall patterns, highlighting its influence on regional climate dynamics. These findings enhance our understanding of the seasonal predictability of rainfall in Sri Lanka during post-El Niño Summers.

1. Introduction

The El Niño-Southern Oscillation (ENSO) is a significant climate phenomenon that affects global weather patterns [1,2,3]. It is characterized by periodic fluctuations in sea surface temperatures (SSTs) and atmospheric conditions over the central and eastern Pacific Ocean [4]. El Niño events are diverse, categorized by their intensity (strong/weak), spatial origin (central/eastern Pacific), and temporal characteristics (duration and decay rate) [5,6]. Some El Niño events are classified as strong or weak based on sea surface temperature anomalies, while others are categorized by their central or eastern Pacific origins [7,8,9]. The duration of these events can differ, with some persisting for only a few seasons and others lasting over a year [10]. Moreover, the rate at which El Niño conditions dissipate also varies, influencing their global climatic impacts [11]. El Niño events can be categorized into two types based on their intensity, and duration, thus, the rate at which the ENSO conditions decay [12]. The fast decaying, which shifts phase by the following summer, is known as fast-decaying El Niño [13]. On the other hand, the slow-decaying phase that extends into the following summer and autumn is referred to as a slow-decaying El Niño [14]. Moreover, fast-decaying El Niño events involve a rapid transition to La Niña conditions altering seasonal forecasts and disrupting agricultural planning. Meanwhile, slow-decaying events show a more gradual return to neutral conditions can extend anomalous weather conditions like droughts, cyclone frequency and heavy rainfall well into the next year [13]. Generally, a slow-decaying El Niño often delays monsoon onset, exacerbating drought and heatwave conditions [7]. These variations in El Niño characteristics can lead to distinct climate responses [15,16], which affect precipitation, temperature, and weather extremes in different regions [17]. Therefore, understanding the regional impacts of ENSO is crucial for effective climate prediction and management in the tropics.
Sri Lanka, a tropical island located close to the equator in the Indian Ocean, is particularly vulnerable to climatic variations [18]. As a result, Sri Lanka experiences two primary monsoon seasons, such as the summer monsoon (May to September) and the winter monsoon (December to February). However, the summer monsoon is particularly active and contributes to the country’s agriculture, water resources, and overall socio-economic stability [19,20]. The summer monsoon is particularly active, contributing ~60% of Sri Lanka’s annual rainfall, which sustains key crops like rice and tea, and replenishes reservoirs critical for hydropower and irrigation [19,20]. El Niño significantly influences the Indian Ocean and the surrounding South Asian countries, driving notable changes in rainfall patterns and ocean dynamics [21]. During El Niño years, the strength of the Summer Monsoon weakens, often leading to widespread droughts in countries such as India, Pakistan, and Bangladesh [22]. Conversely, La Niña events are typically associated with increased rainfall in certain regions, including southern India and Sri Lanka, which can result in localized flooding [23]. These climatic shifts profoundly impact agriculture, water resources, and marine ecosystems, underscoring the interconnected atmospheric and oceanic effects of El Niño and La Niña on the region.
It is crucial to understand how different phases of ENSO affect Sri Lanka’s summer rainfall. Some previous studies have explored the impact of El Niño conditions on Sri Lanka’s climate and present significant importance on rainfall patterns [24,25,26]. In addition, the correlation coefficient between SST anomalies in the eastern/central Pacific and Sri Lankan rainfall patterns during El Niño events typically ranges from −0.3 to −0.6 [26]. During El Niño events, Sri Lanka typically experiences anomalous dry conditions, particularly during the summer monsoon season [27]. Meteorologically, during El Niño years, the subsiding branch of the Walker circulation shifts toward the Indian subcontinent. As a result, Sri Lanka often experiences anomalously dry conditions due to the weakened Walker circulation and reduced moisture transport from the Pacific Ocean [27]. On the other hand, wet conditions, are observed during the decaying El Niño summer monsoon [23,28]. Thus, understanding El Niño events is key for predicting seasonal rainfall patterns in Sri Lanka.
While numerous studies have explored the impact of El Niño on Sri Lanka’s climate [29,30,31,32], the post-El Niño period, characterized by the decay phase of the event, remains relatively less investigated. Beyond SST-driven dynamics, aerosols also modulate monsoon variability. For instance, Arabian Sea aerosol loading can suppress rainfall, while ENSO phases alter aerosol transport pathways [33]. Recent studies highlight the need to integrate aerosol-ENSO interactions for robust monsoon predictions [34]. This study aims to delve deeper into the influence of post-El Niño conditions on Sri Lanka’s summer monsoon rainfall, specifically focusing on the differences between fast-decaying El Niño events. By examining the diverse characteristics of El Niño events, explicitly focusing on fast and slow decaying phases, we seek to unravel the complex mechanisms underlying the varying rainfall patterns observed during these periods. Our findings provide valuable insights into the predictability of summer monsoon rainfall in Sri Lanka, aiding in developing climate forecasting models and informing decision-making processes related to water resource management and agriculture.

2. Materials and Methods

The study used a dataset covering the period from 1981 to 2024. Otpimum Interpolated Sea surface temperature (OISST) data with a resolution of 2° × 2° were obtained from the National Oceanic and Atmospheric Administration (NOAA [35]). Additionally, ERA5 data, with a higher resolution of 0.25° × 0.25°, were sourced from the European Centre for Medium-Range Weather Forecasts (ECMWF), as detailed in [36]. The dataset features several variables, including rainfall, surface air temperature (SAT), wind data, shortwave radiation (SWR), omega (vertical velocity), total cloud cover (TCC), and divergence of wind. The satellite-based SAT data with a resolution of 0.5° × 0.5° were obtained from the gridded Climatic Research Unit (CRU) database at the University of East Anglia [37]. Furthermore, rainfall and SAT data for Sri Lanka were sourced from the Department of Meteorology, Sri Lanka (Figure S1).
The El Niño events were selected based on the December–January–February (DJF) Niño-3.4 index, defined as the SST anomalies averaged over the region 5° S–5° N and 120–170° W [38], which exhibited values exceeding 0.5 STD. Following the approach of [39], we selected thirteen El Niño events: 1982/83, 1986/87, 1987/88, 1991/92, 1994/95, 1997/98, 2002/03, 2004/05, 2006/07, 2009/10, 2015/16, 2018/19, and 2023/24. To distinguish between slow-decaying (SD) and fast-decaying (FD) El Niño events, we use the Niño3.4 index for the decaying summer (JJA). An FD El Niño is identified by a negative value of the Niño3.4 index in the decaying summer. At the same time, an SD event is characterized by a positive value of the Niño3.4 index in the decaying summer (Figure 1). To analyze the impact of FD and SD El Niño on the summer monsoon, we conduct a composite analysis in this study and calculate statistical significance using a two-tailed Student’s t-test. Composite analysis and t-tests robustly identify ENSO-phase differences but may overlook nonlinear interactions. ERA5’s assimilation system reduces biases but may underestimate extreme rainfall in complex terrains [36]. Thus, we analyzed the rainfall, SAT, wind, SWR, omega, TCC, and wind divergence. Additionally, we removed the climatological mean and annual cycle for the study period to obtain anomalies.

3. Results

3.1. Rainfall and SAT Climatology of Sri Lanka and Response During the Different El Niño Decay Phases

During the monsoon, cross-equatorial winds over the Indian Ocean led to increased rainfall over landmasses with dominant topography, such as the western coast of India, the Bay of Bengal, and the Himalayas. Sri Lanka also experiences an average of 1.67 mm/day of rainfall during the summer monsoon season. The climatology (Figure 2a) shows cross-equatorial winds from the southwest dominating the Indian Ocean, with strong westerly winds around Sri Lanka. In addition, mean rainfall (Figure 2b,c) over Sri Lanka exhibits similar patterns, with the southwestern side experiencing nearly 6.2 mm/day and a standard deviation exceeding 2 mm/day, largely due to the influence of topography. Surface air temperature (SAT) contours indicate cooler conditions in the southwest compared to the rest of the island, while the northern and eastern regions are drier and warmer. We first show the Scatter plot between Niño 3.4 index and summer monsoon rainfall over Sri Lanka. We found wetter conditions during FD El Niño events and drier conditions during SD El Niño events (Figure S2). To investigate further the rainfall and SAT responses over Sri Lanka to different El Niño decay phases, composite anomalies during the two distinct types of El Niño decaying summers are presented in Figure 3. Here, consistent with previous studies [40,41], we selected FD El Niño events (i.e., 1987/88, 1997/98, 2006/07, 2009/10), and SD El Niño events (i.e., 1982/83, 1986/87, 1991/92, 1994/95, 2002/03, 2004/05, 2015/16, 2018/19, 2023/24). Composite analyses indicate wetter and cooler conditions during FD El Niño events and drier and warmer conditions during SD El Niño events. It is noted that the rainfall anomalies in both cases are most prominent in the southwestern region (Figure 3a,b). Furthermore, the findings are broadly consistent with those of other sources, thereby supporting the reliability of the observed patterns (Figure S3).

3.2. Mechanisms for the Response of Rainfall and SAT in Sri Lanka to El Niño Decay Phases

Figure 4 depicts a composite map of precipitation and 850-hPa wind anomalies over the Indo-Western Pacific Ocean. During FD El Niño events (Figure 4a), a noticeable pattern of increased rainfall over the Indo-Pacific region, extending from the Maritime Continent to the Indian subcontinent, particularly over Sri Lanka. During FD El Niño, rainfall increases by 1.5–2.0 mm/day over Sri Lanka, with anomalies exceeding 3 mm/day in the southwest (Figure 4a). In contrast, a significant reduction in rainfall is observed across the equatorial Pacific, western Pacific, and South China Sea to the western Pacific and the South China Sea. For SD El Niño events (Figure 4b), there is a widespread general negative reduction in rainfall anomalies across the Indo-Pacific region, including suppressed rainfall over Sri Lanka. During FD El Niño events, the Indo-Pacific shows a positive SST pattern across the Indo-Pacific region, with and negative anomalies in the central-eastern Pacific (Figure 5a). These warm and cool SST gradient drives conditions lead to a dipole Gill-type response [42,43] characterized by a. enhanced convection over the Maritime Continent due to diabatic heating; b. easterly wind anomalies over the equatorial Pacific as a Kelvin wave response; and c. strengthened zonal winds across the Maritime Continent, reinforcing moisture convergence.
These dynamical adjustments initiate coupled ocean–atmosphere feedback, e.g., wind–evaporation–SST (WES) feedback and thermocline adjustments, that amplify the transition to La Niña. Consequently, the intensified easterlies further enhance ascending motion over Sri Lanka, explaining the wetter conditions observed during post-FD El Niño summers (Figure 4a). This in turn gives rise to notably stronger zonal winds across the Maritime Continent and the equatorial Pacific Ocean (Figure 4a).
On the other hand, SST anomalies during SD events (Figure 5b) are less intense compared to FD El Niño, with negative anomalies over the northwestern Pacific and the southern Indian Ocean and positive anomalies over the central Pacific, which is associated with the formation of the western North Pacific anticyclone (Figure 4b). In addition, we noted that the wind patterns are also less intense than FD events. This analysis suggests that the Walker circulation also plays a significant role in modulating rainfall over Sri Lanka. Therefore, we now look at the response of walker circulation, which is defined as vertical velocity averaged over the 10° S–10° N [44,45] to understand the vertical motion. During FD El Niño, the Walker circulation strengthens (Figure 5a), with a pronounced ascending branch over the eastern Indian Ocean and a descending branch over the eastern Pacific. In contrast, during SD El Niño, the Walker circulation weakens (Figure 5b), resulting in a less pronounced ascending branch over the Indian Ocean and a weakened descending branch near the Maritime Continent. Thus, we found that SST is a key factor for different rainfall changes by modifying wind patterns and walker circulation during the two types of El Niño transition.
To understand the atmospheric dynamics affecting the Indo-Pacific region [46], we examined moisture flux convergence (MFC) during different El Niño phases. FD El Niño shows positive MFC anomalies over Sri Lanka, strong upward motion at 500 hPa, and enhanced cloudiness and rainfall (Figure S4). In contrast, SD El Niño shows negative MFC anomalies, weaker 500 hPa upward motion, and reduced cloudiness, convection, and rainfall (Figures S4 and S5).

4. Discussion

The impact of El Niño phases on Sri Lanka’s summer monsoon rainfall emphasizes the crucial role of large-scale atmospheric dynamics, particularly the Walker circulation, in regulating regional climate patterns [47,48]. During the summer monsoon, cross-equatorial winds play a significant role in carrying moisture from the Indian Ocean toward landmasses with prominent topography such as the Western Ghats, Bay of Bengal, and the Himalayas (Figure 2a [41]). Sri Lanka receives an average of 1.67 mm/day of rainfall during this season (Figure 2b [49]). These winds, mainly from the southwest, drive strong westerlies across the Indian Ocean, affecting rainfall distribution across the region. Rainfall is particularly intense in the southwestern part of Sri Lanka, where topographic effects amplify precipitation, as observed in the climatology [20]. In contrast, the northern and eastern regions remain relatively drier and warmer.
Composite analyses of El Niño phases (Figure 3) reveal distinct differences in rainfall anomalies during FD and SD events. During FD El Niño phases, increased rainfall is observed from the Maritime Continent to the Indian subcontinent, including Sri Lanka (Figure 4a). This is driven by positive sea surface temperature (SST) anomalies in the Indo-Pacific region, while negative SST anomalies over the Central Pacific [50]. These SST anomalies intensify zonal wind patterns, enhancing moisture transport and rainfall over Sri Lanka. On the other hand, SD El Niño phases result in reduced rainfall over Sri Lanka, with weaker SST anomalies and zonal winds, and the formation of the western North Pacific anticyclone (Figure 4b, [11]). This variation can be attributed to the strength of the Walker circulation during each phase.
The Walker circulation plays a crucial role in these patterns [51], with a strong ascending branch over the eastern Indian Ocean and a descending branch over the eastern Pacific during FD El Niño events (Figure 5a), resulting in more robust moisture transport and rainfall over Sri Lanka. Conversely, SD El Niño phases exhibit a weaker Walker circulation, leading to less effective moisture transport (Figure 5b). Further analysis of MFC and vertical motion supports this finding.
During FD El Niño, there is a marked increase in MFC over Sri Lanka, associated with strong upward vertical motion at 500 hPa resulting in excessive cloudiness and rainfall anomalies (Figure S4). In contrast, SD El Niño shows negative MFC anomalies and downward vertical motion over Sri Lanka, leading to drier conditions, corroborated by reduced cloud cover (Figure S4). In addition, Figure 6a,b summarizes the diverse responses of SAT and rainfall to the underlying mechanisms that contribute to the two types of El Niño transition. Figure 6a illustrates the wet summer mechanism following FD El Niño events, initiated by strong easterly wind anomalies over the central-eastern Pacific that drive a Gill-type response: upwelling-enhanced SST cooling (blue shading) in the east and compensatory rising motion over the Maritime Continent (red convection). This Pacific convection anomaly excites a Rossby wave, establishing cyclonic circulation over the Bay of Bengal (curved arrow) that enhances westerly moisture transport (bold arrows) toward Sri Lanka. Converging moisture fluxes (dashed lines) over the island then trigger strong upward motion (vertical red arrow), increased precipitation (rain symbols), and cooler surface temperatures through enhanced evaporation and cloud shading—collectively explaining the observed wet anomalies. Figure 6b more clearly depicts the stronger anticyclonic circulation in the tropical Pacific, illustrating how surface easterly winds—through wind shear effects—induce pronounced anticyclonic flow on both flanks of the wind maximum. The revised schematic now explicitly includes the associated oceanic downwelling mechanism, showing (1) surface convergence, (2) subsequent downward motion, and (3) the resulting suppression of upward moisture transport, consistent with established ENSO dynamics [52,53]. While this anticyclonic circulation is prominent in the central Pacific, its influence weakens near Sri Lanka, where insufficient convergence leads to prevailing divergence conditions. This aligns with our original findings of reduced rainfall during SD El Niño events, as the large-scale downward motion fails to dominate locally, allowing dry anomalies to persist.
These findings suggest that FD El Niño phases have a stronger influence on Sri Lanka’s summer monsoon rainfall due to the enhanced Walker circulation and associated moisture transport. In contrast, SD El Niño phases lead to diminished impacts. These results align with previous studies on El Niño’s influence on South Asian monsoon dynamics [14,54,55]. Furthermore, a strong and statistically significant correlation was identified between the CHIRPS, CRU, and ERA5 datasets and observational data (Figures S6 and S7 [20]). These findings suggest that the datasets are reliable for studying rainfall and SAT variability in Sri Lanka. Based on these strong correlations, we selected CHIRPS as the primary dataset for rainfall and ERA5 for SAT, using the others as supplementary datasets to validate and support the findings in this study. Therefore, future research should continue to explore how these large-scale atmospheric processes, including the interaction between SST anomalies and zonal circulation, affect regional rainfall patterns under different climate change scenarios. While this study clarifies post-El Niño rainfall responses, limitations include the exclusion of aerosol effects and reliance on reanalysis data. Addressing these in future work will enhance predictability under climate change. This study focuses on SST-driven mechanisms; future work should integrate aerosol and land–atmosphere interactions. The 44-year record may not fully capture low-frequency ENSO variability. Model simulations could validate the observed teleconnections. Since our understanding remains unclear on SD and FD in the future, this could serve as an excellent point for further discussion.

5. Conclusions

This study has investigated the impact of post-El Niño on rainfall in Sri Lanka, particularly during the fast and slow decaying summers, using 44 years of reanalysis and observational data. We analyzed the composite results and found that the enhanced wet conditions are robust in FD El Niño summer, while dry conditions are in SD El Niño summer. These different responses are mainly associated with the changes in the sea surface temperature in the central to the eastern part of the Pacific Ocean with enhanced walker circulation. During the FD El Niño, the strong easterly winds over Sri Lanka from the central to the eastern Pacific Ocean due to the transition from El Niño to La Niña conditions result in increased moisture convergence, upward motion, and cloud cover, resulting in wet conditions over Sri Lanka. This effect is further amplified by Sri Lanka’s topography, which enhances precipitation, particularly in the southwestern areas. In contrast, the cooling of the western North Pacific and the warming of the Indian Ocean were associated with the formation of the western North Pacific anticyclone in the SD El Niño, which in turn reduced the upward motion and cloud cover, resulting in dry conditions over Sri Lanka. The weakened Walker circulation during SD events contributes to this reduction in rainfall, highlighting its pivotal role in modulating regional climate patterns. Future research should focus on understanding how SST anomalies and atmospheric circulation patterns are linked under greenhouse warming scenarios.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w17111664/s1, Figure S1: Topographic elevation map of Sri Lanka (m; GTOPO30). The blue circle denotes the location of meteorological stations over Sri Lanka; Figure S2: Scatter plot showing the relationship between post-Niño 3.4 index and (left panel) summer monsoon rainfall (right panel) SAT over Sri Lanka. Green points represent slower decay El Niño events, while red points indicate faster decay El Niño events. Figure S3: Same as Figure 3 but from CRU and ERA5 data. Figure S4: Convergence (shadings), wind at 850 hPa (vector) anomalies over the Indian Ocean during the summer for selected events from 1981 to 2024, (a) FD El Niño, (b) SD El Niño. Vertical velocity anomalies for selected events during summer, (c) FD El Niño, (d) SD El Niño, (e,f) same as (c,d), respectively, but for total cloud cover (TCC). The black dots highlight grid points where anomalies are statistically significant at the 0.1 level. Figure S5: SWR anomalies for selected events during summer, (a) FD El Niño, (b) SD El Niño. Furthermore, the black dots highlight the regions which are statistically significant at the 0.1 level. Figure S6: Correlation between rainfall observations and CHIRPS rainfall (left panel), ERA5 rainfall (right panel). Correlations are based on the detrended rainfall summer monsoon anomalies, which are calculated after removing the seasonal climatology and long-term linear trend. Figure S7. Correlation between SAT observations and ERA5 (left panel), CRU (right panel). Correlations are based on the detrended SAT anomalies, which are calculated after removing the seasonal climatology and long-term linear trend.

Author Contributions

Conceptualization, P.K.; methodology, P.K.; software, P.K.; validation, P.K. and V.K.; formal analysis, P.K.; investigation, V.K.; resources, V.K.; data curation, P.K.; writing—original draft preparation, P.K.; writing—review and editing, P.K. and V.K.; visualization, P.K.; supervision, V.K.; project administration, V.K.; funding acquisition, V.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

All the datasets used in this study are publicly available.

Acknowledgments

The authors are thankful to Jérôme Vialard for the valuable discussions leading to the improvement of the paper. Assistance and the facilities provided by the Department of Oceanography and Marine Geology, Faculty of Fisheries and Marine Sciences & Technology, University of Ruhuna, Sri Lanka, are highly acknowledged. All plots and analyses are carried out using Python v.3.9 including supportive packages.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Time-series of post-summer Niño3.4 index from 1981 to 2024. The years in red label indicate a fast-decaying (FD) of El Niño and the years in green color indicate a slow-decaying (SD) of El Niño.
Figure 1. Time-series of post-summer Niño3.4 index from 1981 to 2024. The years in red label indicate a fast-decaying (FD) of El Niño and the years in green color indicate a slow-decaying (SD) of El Niño.
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Figure 2. (a) Climatology of rainfall (color shading) and wind vectors over the Indian Ocean during the summer monsoon (June to August, JJA) for the period 1981–2024. In addition, the black box denotes Sri Lanka. (b) Climatology of rainfall (color shading), and surface air temperature (SAT, contour) climatology of Sri Lanka during the summer from 1981 to 2024. (c) as (b) but for standard deviation.
Figure 2. (a) Climatology of rainfall (color shading) and wind vectors over the Indian Ocean during the summer monsoon (June to August, JJA) for the period 1981–2024. In addition, the black box denotes Sri Lanka. (b) Climatology of rainfall (color shading), and surface air temperature (SAT, contour) climatology of Sri Lanka during the summer from 1981 to 2024. (c) as (b) but for standard deviation.
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Figure 3. Rainfall anomaly composite during the summer for selected El Niño events from 1981 to 2024, (a) FD El Niño, (b) SD El Niño, (c,d) represents surface air temperature (SAT) anomaly for FD El Niño and SD El Niño, respectively. The black dots highlight grid points where anomalies are statistically significant at the 0.1 level (Source: Rainfall; CHIRPS, SAT; ERA5).
Figure 3. Rainfall anomaly composite during the summer for selected El Niño events from 1981 to 2024, (a) FD El Niño, (b) SD El Niño, (c,d) represents surface air temperature (SAT) anomaly for FD El Niño and SD El Niño, respectively. The black dots highlight grid points where anomalies are statistically significant at the 0.1 level (Source: Rainfall; CHIRPS, SAT; ERA5).
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Figure 4. Rainfall (color shading, superimposed vectors are wind) anomaly over the Indian Ocean during the summer for selected composite events, (a) FD El Niño, (b) SD El Niño. The black dots highlight grid points where anomalies are statistically significant at the 0.1 level (Source: Rainfall; ERA5).
Figure 4. Rainfall (color shading, superimposed vectors are wind) anomaly over the Indian Ocean during the summer for selected composite events, (a) FD El Niño, (b) SD El Niño. The black dots highlight grid points where anomalies are statistically significant at the 0.1 level (Source: Rainfall; ERA5).
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Figure 5. SST anomalies for selected events during summer, (a) for FD El Niño and (b) for SD El Niño. Walker circulation over the Tropical Ocean during the summer (JJA) for selected events from 1981 to 2024. Anomalous omega (multiplied by −100) averaged over 10° S–10° N is shaded (c) for FD El Niño and (d) for SD El Niño, respectively. The black vector represented the composite climatology of wind vectors, while the green vectors showed anomalous wind vectors (multiplied by 15).
Figure 5. SST anomalies for selected events during summer, (a) for FD El Niño and (b) for SD El Niño. Walker circulation over the Tropical Ocean during the summer (JJA) for selected events from 1981 to 2024. Anomalous omega (multiplied by −100) averaged over 10° S–10° N is shaded (c) for FD El Niño and (d) for SD El Niño, respectively. The black vector represented the composite climatology of wind vectors, while the green vectors showed anomalous wind vectors (multiplied by 15).
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Figure 6. The schematic diagram denotes the mechanism for rainfall responses during (a) FD El Niño, and (b) SD El Niño.
Figure 6. The schematic diagram denotes the mechanism for rainfall responses during (a) FD El Niño, and (b) SD El Niño.
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Kajakokulan, P.; Kumar, V. Post-El Niño Influence on Summer Monsoon Rainfall in Sri Lanka. Water 2025, 17, 1664. https://doi.org/10.3390/w17111664

AMA Style

Kajakokulan P, Kumar V. Post-El Niño Influence on Summer Monsoon Rainfall in Sri Lanka. Water. 2025; 17(11):1664. https://doi.org/10.3390/w17111664

Chicago/Turabian Style

Kajakokulan, Pathmarasa, and Vinay Kumar. 2025. "Post-El Niño Influence on Summer Monsoon Rainfall in Sri Lanka" Water 17, no. 11: 1664. https://doi.org/10.3390/w17111664

APA Style

Kajakokulan, P., & Kumar, V. (2025). Post-El Niño Influence on Summer Monsoon Rainfall in Sri Lanka. Water, 17(11), 1664. https://doi.org/10.3390/w17111664

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