Abstract
Kerala has witnessed a surge in heavy rainfall events (HRE) during August in recent years. This study examines the influence of ocean-atmospheric conditions in the Southern Hemisphere on the variability of HRE in Kerala during August. The study finds that the changing ocean-atmospheric conditions in the South Indian Ocean such as uneven sea surface temperature (SST) trends (warming near Madagascar Island and cooling to the south of Madagascar Island, north-south SST gradient), weakening of the Mascarene High, strengthening of the Australian High and associated circulation changes significantly impact the recent trend in HRE over Kerala. A significant negative correlation exists between the April Southern Annular Mode (SAM) index and August HRE. Negative April SAM induces warm SST in the southwest Pacific Ocean (SWPO SST), which persist until August. April SWPO SST is positively associated with the north-south SST gradient in the southwest Indian Ocean in August. The intensification of cyclonic circulation over the southwest Indian Ocean and anticyclonic circulation over the southeast Indian Ocean may be the atmospheric response to April SAM, facilitated through SWPO SST. This cyclonic circulation over the southwest Indian Ocean may enhance divergent winds from the cool eastern Indian Ocean, and increase the moisture transport from the central equatorial Indian Ocean to the Kerala region and causes HRE over Kerala. A positive correlation between April SWPO SST and enhanced regional Hadley circulation over Kerala in August emphasizes this hypothesis. Therefore, negative SAM and warming in the SWPO in April can be considered as precursory factors for HRE occurrence over Kerala in August.