Effect of El Niño Southern Oscillation (ENSO) events on inter-seasonal variability of rainfall in Wet and Intermediate zones of Sri Lanka

Ocean-atmospheric teleconnections such as El Niño Southern Oscillation (ENSO) are important climatic determinants in studying inter-and intra-seasonal variability of rainfall. The ENSO takes place in the central eastern equatorial Pacific Ocean. Warm and cold phases of ENSO cycle are termed as El Niño and La Niña, respectively. As it influences the global climate, the present study was carried out to find out such teleconnections in the seasonal rainfall climatology of Wet and Intermediate zones of Sri Lanka during four rainfall seasons using daily rainfall time series from 1976-2018 (February) for 15 locations scattered throughout Wet and Intermediate zones. El Niño, La Niña and neutral years during the study period were identified using Oceanic Nino Index (ONI). The mean cumulative rainfall of ENSO years were statistically compared with the remaining neutral years. The results clearly revealed a significant reduction of rainfall during both First Inter Monsoon (FIM) and North East Monsoon (NEM) seasons during El Niño years. The Second Inter Monsoon (SIM) season showed a positive anomaly of rainfall during the El Niño years. However, the effect of El Niño condition on the Southwest Monsoon season (SWM) was not consistent across both climatic zones under review. During La Niña conditions, an above normal rainfall was observed in the FIM, SWM and NEM seasons where the strongest correlation was evident during the NEM season. The SIM season has shown a below normal rainfall during the La Niña period. These teleconnections can be effectively used as a forecasting tool of seasonal rainfall in the two climatic zones of the country and thereby minimizing climate-related risks in agricultural and hydrological operations.