The Impact of the Bay of Bengal Sea Surface Temperature Gradient on Indian Summer Monsoon Rainfall

Abstract

Indian summer monsoon rainfall (ISMR), which occurs between June and September, holds a pivotal role in both the agriculture and economy of the Indian subcontinent and the Greater Mekong Subregion (GMS). Our findings suggest that the combined influence of local SST warming and Indian Ocean Dipole (IOD) events can account for a substantial portion of historical meridional SST gradients (SSTG) in the Bay of Bengal (BOB), and thus contribute to the ISMR anomalies over Central India, Northeast India, and the northwestern GMS. The meridional SSTG in the BOB exert oceanic feedback on the atmosphere by modulating anomalous planetary boundary layer winds via the sea level pressure adjustment mechanism, vertical momentum mixing mechanism, and the thermal wind mechanism. The evolution of deep convection aligns with that of the meridional SSTG, thereby intensifying the downward transport occurred over the BOB north of 15°N and the upward transport occurred over the near-equator BOB. Both the temperature and pressure anomalies contribute to the anomalous easterlies at 850 and 500 hPa, and thus leads to the significant weakening of the Indian summer monsoon (ISM). Numerical experiments provide robust evidence supporting the proposed modulating influence of the meridional SSTG on the reduction of ISM rainfall over the BOB. The results highlight the combined impacts of local SST warming and IOD events.