El Niño-Southern Oscillation Modulation of Springtime Diurnal Rainfall Over a Tropical Indian Ocean Island

This study analyzes the interannual variability of boreal springtime (March and April) diurnal rainfall (hereafter SDR) over the second biggest Island of the Indian Ocean, Sri Lanka (SL), and expresses the possible effects of the El Niño Southern Oscillation (ENSO) events in modulating this variability. The analysis is primarily based on high spatiotemporal resolution satellite precipitation estimates and reanalysis data from 2001 to 2019. Results indicate that the SDR in SL exhibited a consistent afternoon peak throughout the study period. In contrast, the SDR's amplitude consists of a notable 2 to 4-year oscillation period, similar to the oscillation period of ENSO events. Further analysis revealed that the interannual variation of the SDR's amplitude in SL has a negative relationship with the springtime and previous winter sea surface temperature changes in the central and eastern tropical Pacific Ocean covering NINO3.4 and NINO3 regions (referred to as ENSO indices). When there is a winter La Niña (El Niño), the following SDR in SL is more (less) active. A possible explanation is that during La Niña (El Niño) years, the region around SL experienced an enhanced (suppressed) cyclonic circulation, enhanced (suppressed) large-scale moisture flux convergence, and enhanced (suppressed) local-scale diurnal varying moisture flux convergence, leading to an increase (decrease) in the potential for rainfall and more (less) active SDR over SL. These findings highlight the potential to use ENSO indices to predict the interannual variation of SDR activities over SL.