Drivers and Variability of Marine Heatwaves in the North Indian Ocean and their Impacts on South Asian Monsoon Rainfall

Abstract. Our planet is warming rapidly, and, with it, the frequency and intensity of marine heatwaves (MHWs) are increasing. While MHWs disrupt marine ecosystems, they also significantly influence regional climate systems, including the Asian monsoon. This study investigates the variability, drivers, and monsoon impacts of MHWs in the North Indian Ocean using detrended sea surface temperature anomalies from 1982 to 2024. An Empirical Orthogonal Function (EOF) analysis of MHW intensity reveals two leading modes. The first mode (PC1), explaining 22 % of the variance, shows widespread MHWs with stronger intensity in the Arabian Sea. It is associated with anomalously high pressure over the North Indian Ocean and low pressure in the Southern Hemisphere, which weakens monsoon winds, reduces evaporation and cloud cover, and increases shortwave radiation, thereby warming the upper ocean. The second mode (PC2), explaining 8 % of the variance, displays a dipole pattern, with MHWs in the Bay of Bengal and suppressed activity in the Arabian Sea during its positive phase, and the reverse during its negative phase. Large-scale climate modes modulate MHW development. El Niño combined with the transition phase of MISO (from break to active) triggers basin-wide MHWs (PC1), while La Niña during a similar MISO phase promotes PC2-like warming in the Bay of Bengal. These modes influence rainfall as well. PC1 and PC2+ are linked to wetter conditions in southern India and drier conditions in the north, while PC2- corresponds to widespread dryness. MHW termination can enhance rainfall through the revival of monsoon winds and heat release. These findings suggest potential feedback between MHWs and MISO, with implications for improved monsoon prediction under climate change.