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, accompanied by an increase in the frequency and intensity of marine heatwaves (MHWs). Beyond their impacts on marine ecosystems, MHWs can also modulate regional climate systems, including the Asian monsoon. Here, we investigate the variability, drivers, and monsoon impacts of MHWs in the North Indian Ocean using detrended sea surface temperature anomalies over the period 1982–2024. An Empirical Orthogonal Function (EOF) analysis of MHW intensity identifies two leading modes of variability. The first mode (PC1), explaining 22% of the variance, is characterized by basin-wide MHWs with enhanced intensity in the Arabian Sea and is associated with weakened monsoon winds, reduced evaporation and cloud cover, and enhanced shortwave radiation, leading to upper-ocean warming. The second mode (PC2), accounting for 8% of the variance, exhibits a dipole structure, with intensified MHWs in the Bay of Bengal and suppressed activity in the Arabian Sea during its positive phase, and the opposite pattern during its negative phase. Large-scale climate modes modulate these patterns. Basin-wide MHWs resembling PC1 are usually associated with the mature phase of El Niño, coinciding with the transition from active to break phases of the Monsoon Intraseasonal Oscillation (MISO). Under similar MISO conditions, La Niña tends to favor PC2+-type warming. These modes are accompanied by distinct rainfall responses: PC1 and PC2+ are linked to wetter conditions over southern India and drier conditions in the north, whereas PC2− corresponds to more widespread dryness. The termination of MHWs is associated with the re-intensification of monsoon winds, which both suppresses further ocean warming and enhances rainfall through increased evaporation and moisture transport. Together, these results point to a potential interaction between MHWs, monsoon intraseasonal variability, and ENSO and suggest that certain climate conditions may favor the transition of MISO-related ocean warming into marine heatwaves, with implications for monsoon predictability in a warming climate.

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Identifiers

ORCID for Ligin Joseph: orcid.org/0000-0001-6637-0475

ORCID for Nikolaos Skliris: orcid.org/0000-0002-2473-2586

ORCID for Dipanjan Dey: orcid.org/0000-0002-9588-4042

ORCID for Robert Marsh: orcid.org/0000-0002-1051-8749

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