Asymmetric response of Sri Lanka Northeast Monsoon rainfall to El Niño/La Niña

Many studies have highlighted an asymmetry in rainfall anomalies over India between the warm phase (El Niño) and the cold phase (La Niña) of the El Niño-Southern Oscillation (ENSO). However, the extent to which the distinctive phases of ENSO influence the asymmetry pattern of Sri Lanka rainfall remains unclear. In this study, utilizing observational/reanalysis datasets for the period 1981–2022, we found that Sri Lanka's winter rainfall response to El Niño/La Niña is asymmetric, with a significant response during El Niño. During the El Niño peak winter, the presence of PSAC (Philippine Sea anomalous anticyclone) results in the prevailing anticyclone over the Bay of Bengal (BoB), suppressing moisture convergence and rainfall over Sri Lanka. On the other hand, the PSCC (Philippine Sea anomalous cyclone), which has shifted westward during the La Niña. This shift enhances cyclone over the BoB, resulting in enhanced moisture convergence and rainfall over Sri Lanka, with a magnitude that is weaker than that of the El Niño-induced PSAC. This results in the emergence of asymmetric rainfall anomaly patterns in Sri Lanka in the El Niño and La Niña peak phases. Thus, this study highlights that the asymmetric circulation of PSAC/PSCC during the ENSO phenomenon contributes to the observed asymmetry in rainfall anomalies between El Niño and La Niña events and has important implications for seasonal forecasting.