Distinct impacts of pure El Niño events on spring rainfall of Sri Lanka

Distinct impacts of pure El Niño events on spring rainfall of Sri Lanka | 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 Research Article Distinct impacts of pure El Niño events on spring rainfall of Sri Lanka Pathmarasa Kajakokulan, Gayan Pathirana, Xin Geng, Upul Premarathne This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4355490/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The El Niño Southern Oscillation (ENSO) strongly influences the climate of the tropical Indo-Pacific region, but the specific impact of pure El Niño events on Sri Lanka's rainfall remains largely unexplored. By analyzing observational and reanalysis datasets from 1981 to 2023, we investigate this relationship, particularly during the El Niño decaying spring season. Our results show that during pure Central Pacific (CP) El Niño events, Sri Lanka experiences enhanced spring rainfall due to warmer sea surface temperatures (SST) in the tropical Indian Ocean and strong westerly winds over the Arabian Sea, which favor moisture convergence and subsequent rainfall enhancement over Sri Lanka. Conversely, during pure Eastern Pacific (EP) El Niño events, spring rainfall is reduced due to cooler SST and stronger easterly winds inducing anti-cyclonic circulation over the Arabian Sea, resulting in moisture divergence and reduced rainfall. These contrasting responses highlight the distinct impacts of pure El Niño events on the rainfall of Sri Lanka and associated ocean-atmosphere dynamics, providing valuable insights for future climate projections and adaptation strategies in the country. Figures Figure 1 Figure 2 Figure 3 Figure 4 1 Introduction Sri Lanka is an island in the tropical Indian Ocean (IO) that experiences two distinct monsoon seasons each year (Jayawardena et al. 2023). The southwest monsoon, which occurs from May to September, brings heavy rains on to the wet zone of Sri Lanka (Kajakokulan et al. 2023b ). In contrast, rainfall during the northeast monsoon (from December to February) is relatively low (Nisansala et al. 2023 ). However, Sri Lanka experiences the highest rainfall during October, November, and December (OND) and the second highest peak rainfall during March, April, and May (MAM), which has a significant impact on agriculture, water resources, and irrigation in the country (Burt and Weerasinghe 2014 ; Kajakokulan et al. 2023a ). In addition, the secondary 'yala' cropping season begins with the onset of MAM rains and continues until September (Zubair 2002 ). An earlier study showed that local convective rainfall, which varies diurnally, is more active during the inter-monsoon period than during the primary monsoon period (Thambyahpillay 1954 ). The diurnal amplitude changes in atmospheric thermodynamic factors are greater in MAM (hereafter spring), thereby increasing spring rainfall over Sri Lanka (Huang et al. 2023 ). Therefore, given the importance of ecology and economy, understanding Sri Lanka's spring rainfall variability and the drivers of this variability, particularly the influence of the air-sea coupled climate phenomena such as Madden-Julian Oscillation (MJO), Indian Ocean Dipole (IOD), and El Niño Southern Oscillation (ENSO), is critical. One of the strongest air-sea coupled climate modes in the tropics is ENSO (Kug et al. 2006 ; Cai et al. 2015 ). In particular, the positive phase, El Niño, has a significant influence on annual rainfall variability over the Indo-Pacific regions due to modulations in convection and atmospheric circulation (Xie et al. 2009 ; Yang et al. 2018 ; Chen et al. 2019 ; Pathirana et al. 2023 ). ENSO has been reported to be coupled with IOD, but most occurrences are independent (Ham et al. 2017 ). For example, there are occurrences of independent El Niño, referred to as pure El Niño (Chakravorty et al. 2013 ). On the other hand, it is widely reported that there are two dominant types of El Niño events, known as Eastern Pacific (EP) El Niño and Central Pacific (CP) El Niño, which are defined based on the magnitude of sea surface temperature (SST) anomalies and the zonal location of SST maxima (Jin 2022 ; Heidemann et al. 2023 ; Zhang et al. 2022 ; Shin et al. 2022 ). It has been shown that there is a clear influence of EP and CP El Niño on precipitation in the tropical region (Yuan et al. 2012 ; Jin 2022 ). For example, Zhang et al. ( 2022 ) highlighted that the Northwest Pacific Anti-cyclonic Circulation (NWPA) leads to an enhancement (suppression) of precipitation over India during CP (EP) El Niño events. Moreover, Chowdary et al. ( 2017 ) also showed that excess Indian summer monsoon precipitation is associated with SST changes in the IO by CP El Niño events compared to EP El Niño events. Thus, EP and CP El Niño events have different effects on the tropical climate, including the Indian Ocean and surrounding regions (Wang et al. 2017 ; Jin 2022 ). Given the importance of the different impacts and their response to greenhouse warming, Shin et al. ( 2022 ) examined the response of EP and CP El Niño to global warming and reported that CP El Niño events are projected to become more frequent, while EP El Niño events are projected to become more intense. Therefore, given the importance of El Niño to the tropical climate, it is important to understand its impact on Sri Lanka. It is well known that El Niño exerts its effects on the spring rainfall of Sri Lanka through changes in the Walker circulation modulated by El Niño-induced SST anomalies in the IO (Deoras et al. 2023 ; Zubair 2002 ). For example, Zubair ( 2002 ) has documented a significant correlation between spring rainfall in Sri Lanka and El Niño. Furthermore, it is noteworthy that the spring rainfall of Sri Lanka shows a robust negative correlation with basin-wide tropical IO SST, which in turn is highly correlated with ENSO (Zubair et al. 2008 ). It has been found that during El Niño events, a low-level divergence zone often appears over the western Pacific that extends toward South Asia (Koralegedara et al. 2023 ). This results in moisture divergence towards Sri Lanka, leading to reduced spring rainfall during El Niño (Zubair et al. 2003 ). In addition, Zubair et al. ( 2008 ) showed that the spring rainfall of Sri Lanka is influenced by seasonal changes in the geography of the Walker circulation during El Niño years, these changes often lead to suppressed convection over Sri Lanka, resulting in reduced rainfall. Thus, El Niño has a substantial impact on the spring rainfall of Sri Lanka. Much emphasis has been placed on understanding the relationship between El Niño and rainfall in Sri Lanka, including their feedback and interactions (Suppiah 1989 ; Zubair 2002 ; Malmgren et al. 2003 ; Zubair et al. 2003 , 2008 ; Zubair and Ropelewski 2006 ; Koralegedara et al. 2023 ). However, there is a noticeable gap in the literature regarding the effects of pure El Niño events on spring rainfall of Sri Lanka, in particular the diverse effects of CP and EP El Niños. Given the critical importance of spring rainfall patterns to the ecology and economy of Sri Lanka (Burt and Weerasinghe 2014 ; Karunapala and Yoo 2023 ; Kajakokulan et al. 2023a ) and the diverse effects of different pure El Niño events on the IO climate (Pokhrel et al. 2012 ; Sambasivarao 2023 ; Shen and Yu 2023 ), it is important to distinguish the responses of spring rainfall of Sri Lanka to two different types of pure El Niño events. Therefore, the present study investigates the effects of pure CP and EP El Niño events to understand their impacts on Sri Lanka and to uncover the plausible physical mechanisms driving these responses. 2 Data and methods In the present study, the monthly extended reconstructed SST (ERSSTv5) dataset from National Oceanographic and Atmospheric Administration (NOAA, Huang et al. 2017 ), the daily precipitation data from Climate Hazards Group InfraRed Precipitation (CHIRPS, Funk et al. 2015 ), the monthly wind (850 hPa) data from National Center for Environmental Prediction (NCEP, Kalnay et al. 1996 ) were used. In addition, reanalysis (ERA5) data from the European Centre for Medium-Range Weather Forecasting (ECWMF, Dee et al. 2011 ) were used. This dataset includes precipitation, vertical velocity, vertically integrated moisture flux divergence (VIMD), and outgoing longwave radiation (OLR). The study period in the present study is limited to 43 years (1981–2023). Table 1 List of indices utilized in the present study. Index Definition Reference Niño3 Niño3 index is defined as the average SST anomalies in the equatorial eastern Pacific Ocean (5°S-5°N and 150°W-90°W). Ashok et al. 2007 Niño4 Niño4 index is defined as the area-averaged SST anomalies in the equatorial central Pacific (5°S-5°N and 160°E-150°W). Ashok et al. 2007 Niño3.4 Niño3.4 index is defined as the area-averaged SST anomalies in the central to eastern equatorial Pacific Ocean (5°S-5°N and 120°W-170°W). Ashok et al. 2007 DMI Dipole Mode Index (DMI) is defined as the difference between the SST anomalies in the western Indian Ocean (10°S-10°N and 50°E-70°E) and the eastern Indian Ocean (10°S-0°S and 90°E-110°E). Saji et al. 1999 The CHIRPS and ERA5 precipitation data were checked against observational data from the Sri Lanka Meteorological Department for justification (Fig. S1 ). In addition, ENSO indices were calculated based on the definitions given in Table 1 . In order to clearly identify the CP and EP El Niño, we first selected the El Niño events based on the Niño3.4 index. Thus, in the present study, an El Niño event is defined as December-January-February (DJF) Niño3.4 > 1 standard deviation. Second, we classified the selected El Niño events into CP and EP by comparing the Niño3 and Niño4 indices. Thus, if Niño3 > Niño4, the event was considered an EP event and vice versa. Finally, we compared the selected CP and EP events with the DMI to identify the pure CP and EP events. Here, a positive IOD event is defined when the DMI is greater than 1 standard deviation. In line with previous findings (Chakravorty et al. 2013 ; Jiang and Li 2022 ), we also identified the pure EP El Niño years as 1986/87 and 1991/92, and the pure CP El Niño years as 1987/88 and 2009/10 (Fig. S2). A composite analysis was then performed to assess the influence of pure EP and CP El Niño on Sri Lanka precipitation during the El Niño decaying spring. In addition, vertical velocity, low-level (850 hPa) wind, OLR, and vertically integrated moisture flux convergence (VIMFC) were examined to identify the ocean-atmosphere dynamics that influence the differences in precipitation. Multiple linear regressions based on Niño4, Niño3, and the DMI indices were also performed to validate the composite results. In addition, in the present study, all anomalies were calculated after removing both the seasonal climatology and the long-term linear trend. 3 Results Previous studies consistently show that Sri Lanka receives its second-highest rainfall and standard deviation during spring, indicating that inter-annual rainfall variability is most pronounced during spring (Karunapala and Yoo, 2020). Our analysis of the monthly climatology further confirms that Sri Lanka receives its second-highest rainfall during the spring season (Fig. 1 a), and among these, the southwestern region (wet zone) of Sri Lanka receives the highest rainfall, exceeding 8 mm/day (Fig. 1 b). Next, to investigate the effects of pure CP and EP El Niño events on El Niño-induced rainfall during the El Niño decaying spring (hereafter spring) in Sri Lanka, first, we performed a multiple linear regression analysis based on Niño4, Niño3, and DMI indices to examine the sensitivity of spring rainfall of Sri Lanka (Fig. S3). It is found that the rainfall responses to CP and EP El Niño events are different, indicating that CP El Niño is likely to favor a wet spring, while EP El Niño is likely to favor a dry spring in Sri Lanka. Therefore, given the importance of these different effects, we secondly performed a composite analysis (Fig. 1 and S4). Since our interest in the present study is in pure El Niño events, we specifically selected two pure EP El Niño events (1986/87 and 1991/92) and two pure CP events (1987/88 and 2009/10), and the selected events are consistent with Zhang et al. ( 2022 ). Consistent with regression analysis (Fig. S3) it is found that during pure CP El Niño events, precipitation in the El Niño decaying spring is enhanced over Sri Lanka, leading to wetter conditions, especially in the country's wet zone (Fig. 1 c). However, during pure EP events, spring rainfall is anomalously weakened, leading to drier conditions, especially over the wet zone of the country (Fig. 1 d). Thus, pure CP (EP) El Niño events appear to be associated with a wet (dry) spring in Sri Lanka. This contrast in spring rainfall responses to pure El Niño events highlights two distinct effects. We also examined the differences in spring rainfall using ERA5 data and found consistent results with CHIRPS (Fig. S4). In terms of spatial variability, the study also shows substantial changes in rainfall patterns within the wet zone during the spring of both pure EP and CP El Niño events. These variations in spring rainfall across Sri Lanka in response to different types of pure El Niño events underline differences in the associated ocean-atmosphere dynamics. Therefore, we further investigate these differences in ocean-atmosphere dynamics to identify the reasons for the different responses of spring rainfall to CP and EP El Niño events. We next examined the rainfall over the tropical Indo-Pacific region during the peak winter (here after winter) and spring of pure CP and EP El Niño events (Figs. S5 and S6), and the composite difference (pure CP minus pure EP El Niño) is shown in Fig. 2 . Comparing the composite difference in two types of El Niño, it is clear that the positive rainfall anomaly over India and Sri Lanka is stronger during pure CP El Niño than during EP El Niño in winter (Fig. 2 a). It is also clear that there is a positive precipitation anomaly over India and Sri Lanka in spring, which is stronger than in winter (Fig. 2 b). Thus, it can be seen that pure CP El Niño events appear to be accompanied by strong wet conditions in spring than in winter over Sri Lanka and India. Previous studies have shown that El Niño events have a significant impact on precipitation through SST changes in the tropical Indo-Pacific region (Feng et al. 2011 ; Rifai et al. 2019). Therefore, we investigated the importance of El Niño-associated SST and lower-tropospheric circulation in the tropical Indo-Pacific during pure CP and EP El Niño events. We find that during winter, the CP El Niño-induced SST in the tropical IO is weakly positive and associated with weak cyclonic circulation (Fig. 2 c). However, during spring, both the SST and the cyclonic circulation become stronger than in winter (Fig. 2 d), favoring a strong increase in precipitation over the Sri Lanka, leading to an enhanced wetter condition. To investigate the response of precipitation and SST to El Niño during spring over the IO, we also performed a multiple linear regression analysis based on Niño4, Niño3, and DMI indices (Fig. S7). We found that the responses of precipitation and SST to CP and EP El Niño events are similar to those shown in Figs. S5 and S6. Thus, it is clear that CP and EP El Niño have a pronounced effect on the rainfall of Sri Lanka during the El Niño decaying spring. Given the importance of moisture supply for rainfall of Sri Lanka (Karunapala and Yoo 2023 ; Kajakokulan et al. 2023b ), and to understand the marked changes in spring rainfall, we examined the vertically integrated moisture flux convergence (VIMFC) and 850hPa wind anomalies associated with pure CP and EP El Niño over the IO (Fig. 3 ). In Fig. 3 , positive values of VIMFC indicate convergence and negative values divergence. It can be seen that strong cyclonic circulation over the Arabian Sea (AS) during the pure CP El Niño causes anomalous moisture flux convergence, leading to increased precipitation over the Sri Lanka in spring (Fig. 3 a). Conversely, moisture flux convergence anomalies become negative (divergent) due to the presence of anti-cyclonic circulation over the AS during pure EP El Niño events, suppressing precipitation over the Sri Lanka (Fig. 3 b). Furthermore, it is found that during spring, there is a notable difference in the lower-level circulation between the pure CP and EP El Niño events, that is in agreement with previous studies by Chen et al. ( 2023 ), which is due to the presence of strong convergence over South Asia. Furthermore, the composite difference map also suggests that moisture convergence is higher during pure CP El Niño compared to pure EP El Niño in spring (Fig. 3 c). The composite analysis of outgoing longwave radiation also reinforces the findings from the precipitation response in CP and EP El Niño events (Fig. S8). Thus, the present results confirm that pure CP El Niño events are typically associated with enhanced precipitation over Sri Lanka during spring, whereas pure EP El Niño events are associated with suppressed precipitation, and that these differences are associated with changes in SST, low-level circulation and moisture availability. The SST contrast between the CP and EP El Niño can lead to different responses of the Walker circulation, affecting the large-scale descending and ascending motions over the tropical region (Xu et al. 2013 ). Therefore, to investigate the potential link between Sri Lanka rainfall and changes in the Walker circulation, we analyzed composites of vertical velocity anomalies during the pure CP and EP El Niño during El Niño decaying spring (Fig. 4 ). In Fig. 4 , positive (negative) vertical velocity values indicate downward (upward) motion. It can be seen that the upward motion over the IO in spring is stronger during the pure CP El Niño, and therefore, the upward motion over Sri Lanka (79–82°E) is stronger during CP El Niño (Fig. 4 a). However, during pure EP El Niño events, the upward motion in the IO becomes weaker and the subsidence over the Sri Lanka becomes stronger (Fig. 4 b). Such vertical motion responses and the moisture availability are likely to lead to an enhancement (suppression) of spring rainfall during pure CP (EP) El Niño events. The composite difference map of vertical velocity also shows that the upward motion is positive during the pure CP El Niño compared to the pure EP El Niño in spring (Fig. 4 c). As shown in Fig. 2 , the presence of basin-wide warmer SST during pure CP El Niño events is consistent with a single Walker cell over IO, whereas the cooler SST during pure EP El Niño events is consistent with a double Walker cell. Yu et al. ( 2021 ) also investigated the Walker circulation cell in the IO and found that a single (double) cell is associated with the CP (EP) El Niño. Thus, our results show that there is an important relationship between SST changes and the Walker circulation in the tropical IO, which may influence the rainfall of Sri Lanka during pure El Niño decaying spring. 4 Discussion The present study examines the impact of pure El Niño on spring rainfall of Sri Lanka using observational/reanalysis data over 43 years (1981–2023). We find that El Niño has a robust effect on rainfall of Sri Lanka, especially in the El Niño decaying spring. It is evident from the results that the CP El Niño is likely to accompany a wet spring, while the EP El Niño leads to a dry spring in Sri Lanka. Such different responses of rainfall of Sri Lanka in El Niño decaying spring are likely to result from the differences in El Niño-induced SST and associated atmospheric circulation and moisture availability. For example, during CP El Niño, the IO SST is warmer in spring, which favors a single Walker cell, facilitating upward motion. Also, the presence of strong cyclonic circulation in the AS region during CP El Niño leads to moisture convergence extending to Sri Lanka. Such changes in moisture convergence and circulation lead to a wet spring during CP El Niño. However, the scenario changes during EP El Niño, as the cooler SST favors more subsidence in the tropical IO covering Sri Lanka during the El Niño decaying spring. Furthermore, despite the wet (dry) spring conditions associated with pure CP (EP) El Niño years, the effect on the wet zone of Sri Lanka is robust. This is in line with Koralegedara et al. ( 2023 ), who show that Niño3 and Niño4 indices are good predictors of spring rainfall of Sri Lanka, especially over the wet zone. Thus, our results highlight that pure El Niño has a substantial impact on spring rainfall of Sri Lanka. In the present study, based on the Niño3 and Niño4 indices, we have selected 1986/87 and 1991/92 as pure EP El Niño events, and 1987/88 and 2009/10 as pure CP El Niño events. These four events were selected considering that the Niño3.4 index is greater than 1 standard deviation. Therefore, to complement the limitation of the number of events, we have performed a multiple linear regression analysis. We also used a 0.5 standard deviation to select more events for the composite analysis. It should be noted that all the results are consistent and show that pure CP El Niño events are associated with wet spring and pure EP El Niño events are associated with dry spring. Furthermore, recent studies suggest that strong wet (dry) spring conditions in Sri Lanka are associated with negative (positive) geopotential height and sea level pressure over the entire IO in 1987/88 and 2009/10 respectively (Ranaweera and Kamae 2024 ). We also examined the precipitation response during EP and CP El Niño with positive IOD years and found that the responses were reversed. Our study focused solely on assessing the response of pure El Niño spring precipitation in Sri Lanka using reanalysis data, and future analyses integrating observational data at the seasonal scale may provide greater clarity on the impact of EP and CP El Niño with positive IOD events on Sri Lanka precipitation patterns. On the other hand, recent studies suggest that different phases of ENSO, including El Niño and La Niña, are associated with climate variability on a seasonal timescale (Lv et al. 2019 ; Hasan et al. 2023 ; Yin et al. 2022 ; Koralegedara et al. 2023 ; Wijeratne et al. 2023 ). For example, Liu et al. ( 2023 ) highlight the impact of CP La Niña and EP La Niña on Indian Ocean rim countries. Ma and Chen ( 2023 ) also emphasize that the separation of CP El Niño into CP-I and CP-II types is necessary for tropical climates due to the different spatial patterns of SST anomalies. Therefore, it will be crucial for future research to investigate the precipitation of Sri Lanka during La Niña in the EP, CP-I, and CP-II regions. Therefore, more detailed studies in the future will provide more insight into the multiple effects of ENSO on Sri Lankan precipitation. Nevertheless, the results of our study provide valuable insights into the influence of a pure El Niño on spring rainfall in Sri Lanka. Nevertheless, the present study offers a potentially valuable perspective on the differential impact of El Niño events on Sri Lanka's spring rainfall. During a pure CP El Niño, water availability in Sri Lanka is expected to increase, whereas, during a pure EP El Niño, water availability is expected to decrease, which may affect important sectors such as hydropower, irrigation, and ecosystem functions (Chowdary et al. 2014 ; Karunapala and Yoo 2020; Jayawardena et al. 2023). However, it is important to be aware of the potential impacts of increased (decreased) water availability, including the formation of floods (droughts), which can be a severe ecological and economic challenge in Sri Lanka, especially over the wet zone of Sri Lanka (Kajakokulan 2023b, Dilini et al. 2014 ; Sharma 2023 ). Thus, Sri Lanka's spring rainfall responses to different El Niño events provide critical insights into regional impacts and aid ecological and economic management. 5 Conclusion To investigate the impact of pure El Niño events on spring rainfall in SL, we conducted an analysis using 43 years of observational and reanalysis data from 1981 to 2023. Our study identified two different types of El Niño events: pure EP El Niño (1987/88 and 1991/92) and pure CP El Niño (1987/88 and 2009/10). Through composite analysis, we identified the contrasting effects of El Niño on SL precipitation patterns, particularly during the El Niño decaying spring. During the spring of the CP El Niño, the IO SST is relatively warmer, and an anomalous cyclonic circulation prevails over the AS, leading to a large-scale convergence of moisture fluxes and enhanced precipitation extending into the SL. Conversely, during the spring of a pure EP El Niño, the SL experiences dry conditions. This is associated with relatively cooler SST in the IO and anti-cyclonic circulation over the AS, which promotes moisture flux divergence and contributes to reduced precipitation over the SL. Further analysis of the IO Walker circulation revealed a significant upward motion over the SL during the spring of pure CP El Niño events, which reinforces the observed increase in precipitation. Thus, our study highlights the importance of understanding the effect of pure El Niño on spring rainfall in SL. Given the changes in El Niño characteristics in recent decades, we recommend further research to fully understand its significant influence on SL climate. Such research will improve the predictive ability of the SL climate. Statements & Declarations Acknowledgement The Department of Oceanography and Marine Geology at the Faculty of Fisheries and Marine Sciences & Technology, University of Ruhuna provided valuable support and facilities, which we acknowledge. We also acknowledge the use of open access data sources in this study. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Competing Interests The authors have no relevant financial or non-financial interests to disclose. Author Contributions Material prepeartion, data collection, and analysis were performed by Pathmarasa Kajakokulan. Gayan Pathirana contributed to the study conception, design, and supervision. The first draft of the manuscript was written by Pathmarasa Kajakokulan, and all authors discussed the study results and reviewed the previous versions of the manuscript. All authors read and approved the final manuscript. Data Availability The ERSSTV5 dataset is available at http://apdrc.soest.hawaii.edu/las/v6/. NCEP/NCAR dataset is from https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.pressure.html. The CHIRPS data is available online at https://data.chc.ucsb.edu/products/CHIRPS-2.0/. The ERA5 data is available at https://cds.climate.copernicus.eu/#!/. The insitu observation data is available from Department of Meteorology, Sri Lanka, and the data can be accessed upon request. References Ashok K, Behera SK, Rao SA, et al (2007) El Niño Modoki and its possible teleconnection. J Geophys Res Ocean 112:1–27. https://doi.org/10.1029/2006JC003798 Burt TP, Weerasinghe KDN (2014) Rainfall distributions in Sri Lanka in time and space: An analysis based on daily rainfall data. Climate 2:242–263. https://doi.org/10.3390/cli2040242 Cai W, Santoso A, Wang G, et al (2015) ENSO and greenhouse warming. 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Front Earth Sci 11:1–14. https://doi.org/10.3389/feart.2023.1098517 Malmgren BA, Hulugalla R, Hayashi Y, Mikami T (2003) Precipitatioon trends in Sri Lanka since the 1870s and relationships to El Niño-southern oscillation. Int J Climatol 23:1235–1252. https://doi.org/10.1002/joc.921 Nisansala WDS, Abeysingha NS, Islam A, Bandara AMKR (2023) Recent rainfall trend over Sri Lanka (1987–2017). Int J Climatol 40:3417–3435. https://doi.org/10.1002/joc.6405 Pathirana G, Oh JH, Cai W, et al (2023) Increase in convective extreme El Niño events in a CO2 removal scenario. Sci Adv 9:. https://doi.org/10.1126/sciadv.adh2412 Pokhrel S, Chaudhari HS, Saha SK, et al (2012) ENSO, IOD and Indian Summer Monsoon in NCEP climate forecast system. Clim Dyn 39:2143–2165. https://doi.org/10.1007/s00382-012-1349-5 Ranaweera KRKDN, Kamae Y (2024) Impact of El Niño Southern Oscillation on the first inter-monsoon rainfall over Sri Lanka in the post-El Niño years. https://doi.org/10.3389/fclim.2024.1361322 Rifai SW, Li S, Malhi Y (2019) Coupling of El Niño events and long-term warming leads to pervasive climate extremes in the terrestrial tropics. Environ Res Lett 14:. https://doi.org/10.1088/1748-9326/ab402f Saji NH, Goswami BN, Vinayachandran PN, Yamagata T (1999) A dipole mode in the tropical Indian ocean. Nature 401:360–363. https://doi.org/10.1038/43854 Sambasivarao V (2023) Delayed impact of El Niño on the spring Surface Air Temperature over India Sharma T (2023) Modulation of Indian Summer Monsoon Rainfall response to ENSO in the recent decades and its large-scale dynamics Shen MH, Yu JY (2023) Changes in El Niño characteristics and air–sea feedback mechanisms under progressive global warming. Terr Atmos Ocean Sci 34:. https://doi.org/10.1007/s44195-023-00051-5 Shin NY, Kug JS, Stuecker MF, et al (2022) More frequent central Pacific El Niño and stronger eastern pacific El Niño in a warmer climate. npj Clim Atmos Sci 5:1–8. https://doi.org/10.1038/s41612-022-00324-9 Shiromani Priyanthika Jayawardena IM, Wheeler MC, Sumathipala WL, Basnayake BRSB (2023) Impacts of the Madden-Julian oscillation (Mjo) on rainfall in Sri Lanka. Mausam 71:405–422 Suppiah R (1989) Relationships between the Southern Oscillation and the rainfall of Sri Lanka. Int J Climatol 9:601–618. https://doi.org/10.1002/joc.3370090605 Thambyahpillay G (1954) The rainfall rhythm in Ceylon. University of Ceylon Review. Wang SYS, Yoon JH, Funk CC, Gillies RR (2017) Climate Extremes: Patterns and Mechanisms. Clim Extrem Patterns Mech 1–386. https://doi.org/10.1002/9781119068020 Wijeratne VPIS, Li G, Mehmood MS, Abbas A (2023) Assessing the Impact of Long-Term ENSO, SST, and IOD Dynamics on Extreme Hydrological Events (EHEs) in the Kelani River Basin (KRB), Sri Lanka. Atmosphere (Basel) 14:1–24. https://doi.org/10.3390/atmos14010079 Xie SP, Hu K, Hafner J, et al (2009) Indian Ocean capacitor effect on Indo-Western pacific climate during the summer following El Niño. J Clim 22:730–747. https://doi.org/10.1175/2008JCLI2544.1 Xu K, Zhu C, He J (2013) Two types of El Niño-related Southern Oscillation and their different impacts on global land precipitation. Adv Atmos Sci 30:1743–1757. https://doi.org/10.1007/s00376-013-2272-3 Yang S, Li Z, Yu JY, et al (2018) El Niño-Southern Oscillation and its impact in the changing climate. Natl Sci Rev 5:840–857. https://doi.org/10.1093/nsr/nwy046 Yin H, Wu Z, Fowler HJ, et al (2022) The Combined Impacts of ENSO and IOD on Global Seasonal Droughts. Atmosphere (Basel) 13:1–17. https://doi.org/10.3390/atmos13101673 Yu T, Feng J, Chen W, Wang X (2021) Persistence and breakdown of the western North Pacific anomalous anticyclone during the EP and CP El Niño decaying spring. Clim Dyn 57:3529–3544. https://doi.org/10.1007/s00382-021-05882-x Yuan Y, Yang S, Zhang Z (2012) Different evolutions of the philippine sea anticyclone between the eastern and central Pacific El Niño: Possible effects of Indian Ocean SST. J Clim 25:7867–7883. https://doi.org/10.1175/JCLI-D-12-00004.1 Zhang X, Xu K, Wang W, He Z (2022) Revisiting the different responses of the following Indian summer monsoon rainfall to the diversity of El Niño events. Front Earth Sci 10:. https://doi.org/10.3389/feart.2022.978509 Zubair L (2002) El Niño-Southern oscillation influences on rice production in Sri Lanka. Int J Climatol 22:249–260. https://doi.org/10.1002/joc.714 Zubair L, Rao SA, Yamagata T (2003) Modulation of Sri Lankan Maha rainfall by the Indian Ocean Dipole. Geophys Res Lett 30:. https://doi.org/10.1029/2002GL015639 Zubair L, Ropelewski CF (2006) The strengthening relationship between ENSO and northeast monsoon rainfall over Sri Lanka and southern India. J Clim 19:1567–1575. https://doi.org/10.1175/JCLI3670.1 Zubair L, Siriwardhana M, Chandimala J, Yahiya Z (2008) Predictability of Sri Lankan rainfall based on ENSO. Int J Climatol 28:91–101. https://doi.org/10.1002/joc.1514 Additional Declarations No competing interests reported. Supplementary Files SupplementaryTAC.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4355490","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":299045779,"identity":"184fb189-18fb-4b14-aaf1-6384ee9e00b4","order_by":0,"name":"Pathmarasa Kajakokulan","email":"","orcid":"","institution":"University of Ruhuna","correspondingAuthor":false,"prefix":"","firstName":"Pathmarasa","middleName":"","lastName":"Kajakokulan","suffix":""},{"id":299045781,"identity":"5119f433-ec7f-407c-bece-7785daaed15c","order_by":1,"name":"Gayan Pathirana","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7ElEQVRIiWNgGAWjYPCCAzxg6gMQs7ETpSMBooVxBkgLM5FawBQzWCMhLQa32689+Pnjjox5+9mDn21+bZPnY2Zg/PAxB4+WO2fKDXsSnvHInMlLls7tu23YxszALDlzGx4tN3LSJHgSDvNIMOQYSOf23GYEamFj5iWgRfIPSAv/G+Pflj237YnQkn5MGmyLRI6ZNMOP24kEtUjeOcNuLJMG0vLGzLK34XZyGzNjM16/8N1uf/bwjc1hewn+HOMbP/7ctp3f3nzww0c8WhgkeMwQHMY2MNmARz1IC/szJN4f/IpHwSgYBaNgZAIAyTJRkZrLD48AAAAASUVORK5CYII=","orcid":"","institution":"University of Ruhuna","correspondingAuthor":true,"prefix":"","firstName":"Gayan","middleName":"","lastName":"Pathirana","suffix":""},{"id":299045783,"identity":"154cbe34-3e3a-46de-a6cc-1f31a37796a2","order_by":2,"name":"Xin Geng","email":"","orcid":"","institution":"CIC-FEMD/ILCEC, Nanjing University of Information Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Xin","middleName":"","lastName":"Geng","suffix":""},{"id":299045785,"identity":"04fca53a-c9a2-4105-b9cd-1cc968d62311","order_by":3,"name":"Upul Premarathne","email":"","orcid":"","institution":"University of Ruhuna","correspondingAuthor":false,"prefix":"","firstName":"Upul","middleName":"","lastName":"Premarathne","suffix":""}],"badges":[],"createdAt":"2024-05-01 17:53:46","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4355490/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4355490/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":56053251,"identity":"bbe32d68-1ee6-43c6-a914-57cdd1e69f40","added_by":"auto","created_at":"2024-05-08 02:27:34","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":403551,"visible":true,"origin":"","legend":"\u003cp\u003ea) Monthly climatology of rainfall of Sri Lanka (ERA5: green, CHIRPS: blue) for the period of 1981 to 2023. The error bar denotes the standard deviation spread. b) Climatology of rainfall of Sri Lanka for spring, composite of precipitation anomalies (El Niño decaying spring) for pure c) CP El Niño events of 1987/88, and 2009/10, and d) EP El Niño events of 1986/87, and 1991/92.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4355490/v1/062b7eeb232c7ff436f2205a.png"},{"id":56052679,"identity":"8ad58016-84a3-4fd1-b35d-ccd1017692ee","added_by":"auto","created_at":"2024-05-08 02:19:34","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":793375,"visible":true,"origin":"","legend":"\u003cp\u003eComposite difference of rainfall anomalies for pure CP El Niño events of 1987/88, and 2009/10 and EP El Niño events of 1986/87, and 1991/92, a) El Niño peak winter, and b) El Niño decaying spring. Composite difference of SST anomalies (color shading, \u003csup\u003eo\u003c/sup\u003eC), and low-level wind (850 hPa, vector) for pure CP El Niño events of 1987/88, and 2009/10 and EP El Niño events of 1986/87, and 1991/92, c) El Niño peak winter, and d) El Niño decaying spring.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4355490/v1/8f3791972fb9e6b60d274adc.png"},{"id":56052681,"identity":"177b30f0-f8d7-48fe-8ecc-653f89dfc3be","added_by":"auto","created_at":"2024-05-08 02:19:34","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1561727,"visible":true,"origin":"","legend":"\u003cp\u003eComposite of VIMFC (color shading), and low-level wind (850 hPa, vector) anomalies during El Niño decaying spring for pure, a) CP El Niño events of 1987/88, and 2009/10, and b) EP El Niño event of 1986/87, and 1991/92 for the period of 1981 to 2023. c) Composite difference between pure CP and EP El Niño events.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4355490/v1/02d41d23b701a13cd8dbc902.png"},{"id":56052683,"identity":"be48c99b-21eb-4d34-b762-b75e85d006cc","added_by":"auto","created_at":"2024-05-08 02:19:34","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":359837,"visible":true,"origin":"","legend":"\u003cp\u003eComposite of vertical velocity anomalies (averaged over 10°S-10°N) during El Niño decaying spring for pure a) CP El Niño events of 1987/88, and 2009/10, and b) EP El Niño event of 1986/87, and 1991/92. c) Composite difference between pure CP, and EP El Niño events. The contour lines indicate the zero level, and the longitude band of Sri Lanka is marked with solid vertical lines.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4355490/v1/2d423096811fa39b5f48d291.png"},{"id":56054173,"identity":"554e50d5-98c9-4dd4-b4de-63c6a774ee23","added_by":"auto","created_at":"2024-05-08 02:43:37","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3735476,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4355490/v1/7618bb94-8331-4d42-9e9e-fe63d3eb303c.pdf"},{"id":56052682,"identity":"4af87ba4-ffbb-47a9-bc29-6d45ebd5ceb0","added_by":"auto","created_at":"2024-05-08 02:19:34","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2927337,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryTAC.docx","url":"https://assets-eu.researchsquare.com/files/rs-4355490/v1/41c3958b311a85c9aec77ae9.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Distinct impacts of pure El Niño events on spring rainfall of Sri Lanka","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eSri Lanka is an island in the tropical Indian Ocean (IO) that experiences two distinct monsoon seasons each year (Jayawardena et al. 2023). The southwest monsoon, which occurs from May to September, brings heavy rains on to the wet zone of Sri Lanka (Kajakokulan et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2023b\u003c/span\u003e). In contrast, rainfall during the northeast monsoon (from December to February) is relatively low (Nisansala et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). However, Sri Lanka experiences the highest rainfall during October, November, and December (OND) and the second highest peak rainfall during March, April, and May (MAM), which has a significant impact on agriculture, water resources, and irrigation in the country (Burt and Weerasinghe \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Kajakokulan et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e). In addition, the secondary 'yala' cropping season begins with the onset of MAM rains and continues until September (Zubair \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). An earlier study showed that local convective rainfall, which varies diurnally, is more active during the inter-monsoon period than during the primary monsoon period (Thambyahpillay \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e1954\u003c/span\u003e). The diurnal amplitude changes in atmospheric thermodynamic factors are greater in MAM (hereafter spring), thereby increasing spring rainfall over Sri Lanka (Huang et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Therefore, given the importance of ecology and economy, understanding Sri Lanka's spring rainfall variability and the drivers of this variability, particularly the influence of the air-sea coupled climate phenomena such as Madden-Julian Oscillation (MJO), Indian Ocean Dipole (IOD), and El Ni\u0026ntilde;o Southern Oscillation (ENSO), is critical.\u003c/p\u003e \u003cp\u003eOne of the strongest air-sea coupled climate modes in the tropics is ENSO (Kug et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Cai et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). In particular, the positive phase, El Ni\u0026ntilde;o, has a significant influence on annual rainfall variability over the Indo-Pacific regions due to modulations in convection and atmospheric circulation (Xie et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Yang et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Chen et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Pathirana et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). ENSO has been reported to be coupled with IOD, but most occurrences are independent (Ham et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). For example, there are occurrences of independent El Ni\u0026ntilde;o, referred to as pure El Ni\u0026ntilde;o (Chakravorty et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). On the other hand, it is widely reported that there are two dominant types of El Ni\u0026ntilde;o events, known as Eastern Pacific (EP) El Ni\u0026ntilde;o and Central Pacific (CP) El Ni\u0026ntilde;o, which are defined based on the magnitude of sea surface temperature (SST) anomalies and the zonal location of SST maxima (Jin \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Heidemann et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Zhang et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Shin et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). It has been shown that there is a clear influence of EP and CP El Ni\u0026ntilde;o on precipitation in the tropical region (Yuan et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Jin \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). For example, Zhang et al. (\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) highlighted that the Northwest Pacific Anti-cyclonic Circulation (NWPA) leads to an enhancement (suppression) of precipitation over India during CP (EP) El Ni\u0026ntilde;o events. Moreover, Chowdary et al. (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) also showed that excess Indian summer monsoon precipitation is associated with SST changes in the IO by CP El Ni\u0026ntilde;o events compared to EP El Ni\u0026ntilde;o events. Thus, EP and CP El Ni\u0026ntilde;o events have different effects on the tropical climate, including the Indian Ocean and surrounding regions (Wang et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Jin \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Given the importance of the different impacts and their response to greenhouse warming, Shin et al. (\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) examined the response of EP and CP El Ni\u0026ntilde;o to global warming and reported that CP El Ni\u0026ntilde;o events are projected to become more frequent, while EP El Ni\u0026ntilde;o events are projected to become more intense. Therefore, given the importance of El Ni\u0026ntilde;o to the tropical climate, it is important to understand its impact on Sri Lanka.\u003c/p\u003e \u003cp\u003eIt is well known that El Ni\u0026ntilde;o exerts its effects on the spring rainfall of Sri Lanka through changes in the Walker circulation modulated by El Ni\u0026ntilde;o-induced SST anomalies in the IO (Deoras et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Zubair \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). For example, Zubair (\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) has documented a significant correlation between spring rainfall in Sri Lanka and El Ni\u0026ntilde;o. Furthermore, it is noteworthy that the spring rainfall of Sri Lanka shows a robust negative correlation with basin-wide tropical IO SST, which in turn is highly correlated with ENSO (Zubair et al. \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). It has been found that during El Ni\u0026ntilde;o events, a low-level divergence zone often appears over the western Pacific that extends toward South Asia (Koralegedara et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). This results in moisture divergence towards Sri Lanka, leading to reduced spring rainfall during El Ni\u0026ntilde;o (Zubair et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). In addition, Zubair et al. (\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) showed that the spring rainfall of Sri Lanka is influenced by seasonal changes in the geography of the Walker circulation during El Ni\u0026ntilde;o years, these changes often lead to suppressed convection over Sri Lanka, resulting in reduced rainfall. Thus, El Ni\u0026ntilde;o has a substantial impact on the spring rainfall of Sri Lanka.\u003c/p\u003e \u003cp\u003eMuch emphasis has been placed on understanding the relationship between El Ni\u0026ntilde;o and rainfall in Sri Lanka, including their feedback and interactions (Suppiah \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e1989\u003c/span\u003e; Zubair \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Malmgren et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Zubair et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2003\u003c/span\u003e, \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Zubair and Ropelewski \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Koralegedara et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). However, there is a noticeable gap in the literature regarding the effects of pure El Ni\u0026ntilde;o events on spring rainfall of Sri Lanka, in particular the diverse effects of CP and EP El Ni\u0026ntilde;os. Given the critical importance of spring rainfall patterns to the ecology and economy of Sri Lanka (Burt and Weerasinghe \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Karunapala and Yoo \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Kajakokulan et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e) and the diverse effects of different pure El Ni\u0026ntilde;o events on the IO climate (Pokhrel et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Sambasivarao \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Shen and Yu \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), it is important to distinguish the responses of spring rainfall of Sri Lanka to two different types of pure El Ni\u0026ntilde;o events. Therefore, the present study investigates the effects of pure CP and EP El Ni\u0026ntilde;o events to understand their impacts on Sri Lanka and to uncover the plausible physical mechanisms driving these responses.\u003c/p\u003e"},{"header":"2 Data and methods","content":"\u003cp\u003eIn the present study, the monthly extended reconstructed SST (ERSSTv5) dataset from National Oceanographic and Atmospheric Administration (NOAA, Huang et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), the daily precipitation data from Climate Hazards Group InfraRed Precipitation (CHIRPS, Funk et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), the monthly wind (850 hPa) data from National Center for Environmental Prediction (NCEP, Kalnay et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) were used. In addition, reanalysis (ERA5) data from the European Centre for Medium-Range Weather Forecasting (ECWMF, Dee et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2011\u003c/span\u003e) were used. This dataset includes precipitation, vertical velocity, vertically integrated moisture flux divergence (VIMD), and outgoing longwave radiation (OLR). The study period in the present study is limited to 43 years (1981\u0026ndash;2023).\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\u003eList of indices utilized in the present study.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIndex\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDefinition\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eReference\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNi\u0026ntilde;o3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNi\u0026ntilde;o3 index is defined as the average SST anomalies in the equatorial eastern Pacific Ocean (5\u0026deg;S-5\u0026deg;N and 150\u0026deg;W-90\u0026deg;W).\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAshok et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2007\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNi\u0026ntilde;o4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNi\u0026ntilde;o4 index is defined as the area-averaged SST anomalies in the equatorial central Pacific (5\u0026deg;S-5\u0026deg;N and 160\u0026deg;E-150\u0026deg;W).\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAshok et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2007\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNi\u0026ntilde;o3.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNi\u0026ntilde;o3.4 index is defined as the area-averaged SST anomalies in the central to eastern equatorial Pacific Ocean (5\u0026deg;S-5\u0026deg;N and 120\u0026deg;W-170\u0026deg;W).\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAshok et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2007\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDMI\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDipole Mode Index (DMI) is defined as the difference between the SST anomalies in the western Indian Ocean (10\u0026deg;S-10\u0026deg;N and 50\u0026deg;E-70\u0026deg;E) and the eastern Indian Ocean (10\u0026deg;S-0\u0026deg;S and 90\u0026deg;E-110\u0026deg;E).\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSaji et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e1999\u003c/span\u003e\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\u003eThe CHIRPS and ERA5 precipitation data were checked against observational data from the Sri Lanka Meteorological Department for justification (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). In addition, ENSO indices were calculated based on the definitions given in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. In order to clearly identify the CP and EP El Ni\u0026ntilde;o, we first selected the El Ni\u0026ntilde;o events based on the Ni\u0026ntilde;o3.4 index. Thus, in the present study, an El Ni\u0026ntilde;o event is defined as December-January-February (DJF) Ni\u0026ntilde;o3.4\u0026thinsp;\u0026gt;\u0026thinsp;1 standard deviation. Second, we classified the selected El Ni\u0026ntilde;o events into CP and EP by comparing the Ni\u0026ntilde;o3 and Ni\u0026ntilde;o4 indices. Thus, if Ni\u0026ntilde;o3\u0026thinsp;\u0026gt;\u0026thinsp;Ni\u0026ntilde;o4, the event was considered an EP event and vice versa. Finally, we compared the selected CP and EP events with the DMI to identify the pure CP and EP events. Here, a positive IOD event is defined when the DMI is greater than 1 standard deviation. In line with previous findings (Chakravorty et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Jiang and Li \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), we also identified the pure EP El Ni\u0026ntilde;o years as 1986/87 and 1991/92, and the pure CP El Ni\u0026ntilde;o years as 1987/88 and 2009/10 (Fig. S2). A composite analysis was then performed to assess the influence of pure EP and CP El Ni\u0026ntilde;o on Sri Lanka precipitation during the El Ni\u0026ntilde;o decaying spring. In addition, vertical velocity, low-level (850 hPa) wind, OLR, and vertically integrated moisture flux convergence (VIMFC) were examined to identify the ocean-atmosphere dynamics that influence the differences in precipitation. Multiple linear regressions based on Ni\u0026ntilde;o4, Ni\u0026ntilde;o3, and the DMI indices were also performed to validate the composite results. In addition, in the present study, all anomalies were calculated after removing both the seasonal climatology and the long-term linear trend.\u003c/p\u003e"},{"header":"3 Results","content":"\u003cp\u003e \u003c/p\u003e \u003cp\u003ePrevious studies consistently show that Sri Lanka receives its second-highest rainfall and standard deviation during spring, indicating that inter-annual rainfall variability is most pronounced during spring (Karunapala and Yoo, 2020). Our analysis of the monthly climatology further confirms that Sri Lanka receives its second-highest rainfall during the spring season (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea), and among these, the southwestern region (wet zone) of Sri Lanka receives the highest rainfall, exceeding 8 mm/day (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Next, to investigate the effects of pure CP and EP El Ni\u0026ntilde;o events on El Ni\u0026ntilde;o-induced rainfall during the El Ni\u0026ntilde;o decaying spring (hereafter spring) in Sri Lanka, first, we performed a multiple linear regression analysis based on Ni\u0026ntilde;o4, Ni\u0026ntilde;o3, and DMI indices to examine the sensitivity of spring rainfall of Sri Lanka (Fig. S3). It is found that the rainfall responses to CP and EP El Ni\u0026ntilde;o events are different, indicating that CP El Ni\u0026ntilde;o is likely to favor a wet spring, while EP El Ni\u0026ntilde;o is likely to favor a dry spring in Sri Lanka. Therefore, given the importance of these different effects, we secondly performed a composite analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and S4). Since our interest in the present study is in pure El Ni\u0026ntilde;o events, we specifically selected two pure EP El Ni\u0026ntilde;o events (1986/87 and 1991/92) and two pure CP events (1987/88 and 2009/10), and the selected events are consistent with Zhang et al. (\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Consistent with regression analysis (Fig. S3) it is found that during pure CP El Ni\u0026ntilde;o events, precipitation in the El Ni\u0026ntilde;o decaying spring is enhanced over Sri Lanka, leading to wetter conditions, especially in the country's wet zone (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). However, during pure EP events, spring rainfall is anomalously weakened, leading to drier conditions, especially over the wet zone of the country (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed). Thus, pure CP (EP) El Ni\u0026ntilde;o events appear to be associated with a wet (dry) spring in Sri Lanka. This contrast in spring rainfall responses to pure El Ni\u0026ntilde;o events highlights two distinct effects. We also examined the differences in spring rainfall using ERA5 data and found consistent results with CHIRPS (Fig. S4). In terms of spatial variability, the study also shows substantial changes in rainfall patterns within the wet zone during the spring of both pure EP and CP El Ni\u0026ntilde;o events. These variations in spring rainfall across Sri Lanka in response to different types of pure El Ni\u0026ntilde;o events underline differences in the associated ocean-atmosphere dynamics. Therefore, we further investigate these differences in ocean-atmosphere dynamics to identify the reasons for the different responses of spring rainfall to CP and EP El Ni\u0026ntilde;o events.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWe next examined the rainfall over the tropical Indo-Pacific region during the peak winter (here after winter) and spring of pure CP and EP El Ni\u0026ntilde;o events (Figs. S5 and S6), and the composite difference (pure CP minus pure EP El Ni\u0026ntilde;o) is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Comparing the composite difference in two types of El Ni\u0026ntilde;o, it is clear that the positive rainfall anomaly over India and Sri Lanka is stronger during pure CP El Ni\u0026ntilde;o than during EP El Ni\u0026ntilde;o in winter (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). It is also clear that there is a positive precipitation anomaly over India and Sri Lanka in spring, which is stronger than in winter (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). Thus, it can be seen that pure CP El Ni\u0026ntilde;o events appear to be accompanied by strong wet conditions in spring than in winter over Sri Lanka and India. Previous studies have shown that El Ni\u0026ntilde;o events have a significant impact on precipitation through SST changes in the tropical Indo-Pacific region (Feng et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Rifai et al. 2019). Therefore, we investigated the importance of El Ni\u0026ntilde;o-associated SST and lower-tropospheric circulation in the tropical Indo-Pacific during pure CP and EP El Ni\u0026ntilde;o events. We find that during winter, the CP El Ni\u0026ntilde;o-induced SST in the tropical IO is weakly positive and associated with weak cyclonic circulation (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). However, during spring, both the SST and the cyclonic circulation become stronger than in winter (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed), favoring a strong increase in precipitation over the Sri Lanka, leading to an enhanced wetter condition. To investigate the response of precipitation and SST to El Ni\u0026ntilde;o during spring over the IO, we also performed a multiple linear regression analysis based on Ni\u0026ntilde;o4, Ni\u0026ntilde;o3, and DMI indices (Fig. S7). We found that the responses of precipitation and SST to CP and EP El Ni\u0026ntilde;o events are similar to those shown in Figs. S5 and S6. Thus, it is clear that CP and EP El Ni\u0026ntilde;o have a pronounced effect on the rainfall of Sri Lanka during the El Ni\u0026ntilde;o decaying spring.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eGiven the importance of moisture supply for rainfall of Sri Lanka (Karunapala and Yoo \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Kajakokulan et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2023b\u003c/span\u003e), and to understand the marked changes in spring rainfall, we examined the vertically integrated moisture flux convergence (VIMFC) and 850hPa wind anomalies associated with pure CP and EP El Ni\u0026ntilde;o over the IO (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). In Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, positive values of VIMFC indicate convergence and negative values divergence. It can be seen that strong cyclonic circulation over the Arabian Sea (AS) during the pure CP El Ni\u0026ntilde;o causes anomalous moisture flux convergence, leading to increased precipitation over the Sri Lanka in spring (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Conversely, moisture flux convergence anomalies become negative (divergent) due to the presence of anti-cyclonic circulation over the AS during pure EP El Ni\u0026ntilde;o events, suppressing precipitation over the Sri Lanka (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). Furthermore, it is found that during spring, there is a notable difference in the lower-level circulation between the pure CP and EP El Ni\u0026ntilde;o events, that is in agreement with previous studies by Chen et al. (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), which is due to the presence of strong convergence over South Asia. Furthermore, the composite difference map also suggests that moisture convergence is higher during pure CP El Ni\u0026ntilde;o compared to pure EP El Ni\u0026ntilde;o in spring (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). The composite analysis of outgoing longwave radiation also reinforces the findings from the precipitation response in CP and EP El Ni\u0026ntilde;o events (Fig. S8). Thus, the present results confirm that pure CP El Ni\u0026ntilde;o events are typically associated with enhanced precipitation over Sri Lanka during spring, whereas pure EP El Ni\u0026ntilde;o events are associated with suppressed precipitation, and that these differences are associated with changes in SST, low-level circulation and moisture availability.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe SST contrast between the CP and EP El Ni\u0026ntilde;o can lead to different responses of the Walker circulation, affecting the large-scale descending and ascending motions over the tropical region (Xu et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Therefore, to investigate the potential link between Sri Lanka rainfall and changes in the Walker circulation, we analyzed composites of vertical velocity anomalies during the pure CP and EP El Ni\u0026ntilde;o during El Ni\u0026ntilde;o decaying spring (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, positive (negative) vertical velocity values indicate downward (upward) motion. It can be seen that the upward motion over the IO in spring is stronger during the pure CP El Ni\u0026ntilde;o, and therefore, the upward motion over Sri Lanka (79\u0026ndash;82\u0026deg;E) is stronger during CP El Ni\u0026ntilde;o (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). However, during pure EP El Ni\u0026ntilde;o events, the upward motion in the IO becomes weaker and the subsidence over the Sri Lanka becomes stronger (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). Such vertical motion responses and the moisture availability are likely to lead to an enhancement (suppression) of spring rainfall during pure CP (EP) El Ni\u0026ntilde;o events. The composite difference map of vertical velocity also shows that the upward motion is positive during the pure CP El Ni\u0026ntilde;o compared to the pure EP El Ni\u0026ntilde;o in spring (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the presence of basin-wide warmer SST during pure CP El Ni\u0026ntilde;o events is consistent with a single Walker cell over IO, whereas the cooler SST during pure EP El Ni\u0026ntilde;o events is consistent with a double Walker cell. Yu et al. (\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) also investigated the Walker circulation cell in the IO and found that a single (double) cell is associated with the CP (EP) El Ni\u0026ntilde;o. Thus, our results show that there is an important relationship between SST changes and the Walker circulation in the tropical IO, which may influence the rainfall of Sri Lanka during pure El Ni\u0026ntilde;o decaying spring.\u003c/p\u003e"},{"header":"4 Discussion","content":"\u003cp\u003eThe present study examines the impact of pure El Ni\u0026ntilde;o on spring rainfall of Sri Lanka using observational/reanalysis data over 43 years (1981\u0026ndash;2023). We find that El Ni\u0026ntilde;o has a robust effect on rainfall of Sri Lanka, especially in the El Ni\u0026ntilde;o decaying spring. It is evident from the results that the CP El Ni\u0026ntilde;o is likely to accompany a wet spring, while the EP El Ni\u0026ntilde;o leads to a dry spring in Sri Lanka. Such different responses of rainfall of Sri Lanka in El Ni\u0026ntilde;o decaying spring are likely to result from the differences in El Ni\u0026ntilde;o-induced SST and associated atmospheric circulation and moisture availability. For example, during CP El Ni\u0026ntilde;o, the IO SST is warmer in spring, which favors a single Walker cell, facilitating upward motion. Also, the presence of strong cyclonic circulation in the AS region during CP El Ni\u0026ntilde;o leads to moisture convergence extending to Sri Lanka. Such changes in moisture convergence and circulation lead to a wet spring during CP El Ni\u0026ntilde;o. However, the scenario changes during EP El Ni\u0026ntilde;o, as the cooler SST favors more subsidence in the tropical IO covering Sri Lanka during the El Ni\u0026ntilde;o decaying spring. Furthermore, despite the wet (dry) spring conditions associated with pure CP (EP) El Ni\u0026ntilde;o years, the effect on the wet zone of Sri Lanka is robust. This is in line with Koralegedara et al. (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), who show that Ni\u0026ntilde;o3 and Ni\u0026ntilde;o4 indices are good predictors of spring rainfall of Sri Lanka, especially over the wet zone. Thus, our results highlight that pure El Ni\u0026ntilde;o has a substantial impact on spring rainfall of Sri Lanka.\u003c/p\u003e \u003cp\u003eIn the present study, based on the Ni\u0026ntilde;o3 and Ni\u0026ntilde;o4 indices, we have selected 1986/87 and 1991/92 as pure EP El Ni\u0026ntilde;o events, and 1987/88 and 2009/10 as pure CP El Ni\u0026ntilde;o events. These four events were selected considering that the Ni\u0026ntilde;o3.4 index is greater than 1 standard deviation. Therefore, to complement the limitation of the number of events, we have performed a multiple linear regression analysis. We also used a 0.5 standard deviation to select more events for the composite analysis. It should be noted that all the results are consistent and show that pure CP El Ni\u0026ntilde;o events are associated with wet spring and pure EP El Ni\u0026ntilde;o events are associated with dry spring. Furthermore, recent studies suggest that strong wet (dry) spring conditions in Sri Lanka are associated with negative (positive) geopotential height and sea level pressure over the entire IO in 1987/88 and 2009/10 respectively (Ranaweera and Kamae \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). We also examined the precipitation response during EP and CP El Ni\u0026ntilde;o with positive IOD years and found that the responses were reversed. Our study focused solely on assessing the response of pure El Ni\u0026ntilde;o spring precipitation in Sri Lanka using reanalysis data, and future analyses integrating observational data at the seasonal scale may provide greater clarity on the impact of EP and CP El Ni\u0026ntilde;o with positive IOD events on Sri Lanka precipitation patterns. On the other hand, recent studies suggest that different phases of ENSO, including El Ni\u0026ntilde;o and La Ni\u0026ntilde;a, are associated with climate variability on a seasonal timescale (Lv et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Hasan et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Yin et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Koralegedara et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Wijeratne et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). For example, Liu et al. (\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) highlight the impact of CP La Ni\u0026ntilde;a and EP La Ni\u0026ntilde;a on Indian Ocean rim countries. Ma and Chen (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) also emphasize that the separation of CP El Ni\u0026ntilde;o into CP-I and CP-II types is necessary for tropical climates due to the different spatial patterns of SST anomalies. Therefore, it will be crucial for future research to investigate the precipitation of Sri Lanka during La Ni\u0026ntilde;a in the EP, CP-I, and CP-II regions. Therefore, more detailed studies in the future will provide more insight into the multiple effects of ENSO on Sri Lankan precipitation. Nevertheless, the results of our study provide valuable insights into the influence of a pure El Ni\u0026ntilde;o on spring rainfall in Sri Lanka.\u003c/p\u003e \u003cp\u003eNevertheless, the present study offers a potentially valuable perspective on the differential impact of El Ni\u0026ntilde;o events on Sri Lanka's spring rainfall. During a pure CP El Ni\u0026ntilde;o, water availability in Sri Lanka is expected to increase, whereas, during a pure EP El Ni\u0026ntilde;o, water availability is expected to decrease, which may affect important sectors such as hydropower, irrigation, and ecosystem functions (Chowdary et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Karunapala and Yoo 2020; Jayawardena et al. 2023). However, it is important to be aware of the potential impacts of increased (decreased) water availability, including the formation of floods (droughts), which can be a severe ecological and economic challenge in Sri Lanka, especially over the wet zone of Sri Lanka (Kajakokulan 2023b, Dilini et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Sharma \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Thus, Sri Lanka's spring rainfall responses to different El Ni\u0026ntilde;o events provide critical insights into regional impacts and aid ecological and economic management.\u003c/p\u003e"},{"header":"5 Conclusion","content":"\u003cp\u003eTo investigate the impact of pure El Ni\u0026ntilde;o events on spring rainfall in SL, we conducted an analysis using 43 years of observational and reanalysis data from 1981 to 2023. Our study identified two different types of El Ni\u0026ntilde;o events: pure EP El Ni\u0026ntilde;o (1987/88 and 1991/92) and pure CP El Ni\u0026ntilde;o (1987/88 and 2009/10). Through composite analysis, we identified the contrasting effects of El Ni\u0026ntilde;o on SL precipitation patterns, particularly during the El Ni\u0026ntilde;o decaying spring. During the spring of the CP El Ni\u0026ntilde;o, the IO SST is relatively warmer, and an anomalous cyclonic circulation prevails over the AS, leading to a large-scale convergence of moisture fluxes and enhanced precipitation extending into the SL. Conversely, during the spring of a pure EP El Ni\u0026ntilde;o, the SL experiences dry conditions. This is associated with relatively cooler SST in the IO and anti-cyclonic circulation over the AS, which promotes moisture flux divergence and contributes to reduced precipitation over the SL. Further analysis of the IO Walker circulation revealed a significant upward motion over the SL during the spring of pure CP El Ni\u0026ntilde;o events, which reinforces the observed increase in precipitation. Thus, our study highlights the importance of understanding the effect of pure El Ni\u0026ntilde;o on spring rainfall in SL. Given the changes in El Ni\u0026ntilde;o characteristics in recent decades, we recommend further research to fully understand its significant influence on SL climate. Such research will improve the predictive ability of the SL climate.\u003c/p\u003e"},{"header":"Statements \u0026 Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Department of Oceanography and Marine Geology at the Faculty of Fisheries and Marine Sciences \u0026amp; Technology, University of Ruhuna provided valuable support and facilities, which we acknowledge. We also acknowledge the use of open access data sources in this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMaterial prepeartion, data collection, and analysis were performed by Pathmarasa Kajakokulan. Gayan Pathirana contributed to the study conception, design, and supervision. The first draft of the manuscript was written by Pathmarasa Kajakokulan, and all authors discussed the study results and reviewed the previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe ERSSTV5 dataset is available at http://apdrc.soest.hawaii.edu/las/v6/. NCEP/NCAR dataset is from https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.pressure.html. The CHIRPS data is available online at https://data.chc.ucsb.edu/products/CHIRPS-2.0/. The ERA5 data is available at https://cds.climate.copernicus.eu/#!/. The insitu observation data is available from Department of Meteorology, Sri Lanka, and the data can be accessed upon request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAshok K, Behera SK, Rao SA, et al (2007) El Ni\u0026ntilde;o Modoki and its possible teleconnection. 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Int J Climatol 28:91\u0026ndash;101. https://doi.org/10.1002/joc.1514\u003cbr\u003e\u0026nbsp;\u0026nbsp;\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4355490/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4355490/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe El Ni\u0026ntilde;o Southern Oscillation (ENSO) strongly influences the climate of the tropical Indo-Pacific region, but the specific impact of pure El Ni\u0026ntilde;o events on Sri Lanka's rainfall remains largely unexplored. By analyzing observational and reanalysis datasets from 1981 to 2023, we investigate this relationship, particularly during the El Ni\u0026ntilde;o decaying spring season. Our results show that during pure Central Pacific (CP) El Ni\u0026ntilde;o events, Sri Lanka experiences enhanced spring rainfall due to warmer sea surface temperatures (SST) in the tropical Indian Ocean and strong westerly winds over the Arabian Sea, which favor moisture convergence and subsequent rainfall enhancement over Sri Lanka. Conversely, during pure Eastern Pacific (EP) El Ni\u0026ntilde;o events, spring rainfall is reduced due to cooler SST and stronger easterly winds inducing anti-cyclonic circulation over the Arabian Sea, resulting in moisture divergence and reduced rainfall. These contrasting responses highlight the distinct impacts of pure El Ni\u0026ntilde;o events on the rainfall of Sri Lanka and associated ocean-atmosphere dynamics, providing valuable insights for future climate projections and adaptation strategies in the country.\u003c/p\u003e","manuscriptTitle":"Distinct impacts of pure El Niño events on spring rainfall of Sri Lanka","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-05-08 02:19:29","doi":"10.21203/rs.3.rs-4355490/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"d84b4611-f935-4b2d-a7a5-06f3d6db52c5","owner":[],"postedDate":"May 8th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-05-08T02:19:31+00:00","versionOfRecord":[],"versionCreatedAt":"2024-05-08 02:19:29","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4355490","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4355490","identity":"rs-4355490","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}