Observed surface and subsurface Marine Heat Waves in the Bay of Bengal from in-situ and high-resolution satellite data

Marine Heat Waves (MHWs) can cause significant distress to marine environment and modulate air-sea interaction, which in turn can have economic and societal impacts. This study aims to identify surface and subsurface MHWs in the Bay of Bengal using available sea-surface temperature data and buoy observations spanning four decades. The results show significant increase in the number, frequency, duration, and intensity of surface MHWs in recent years. To better understand the relationship between MHW occurrences and different phases of two climate phenomena, namely, the Indian Ocean Dipole (IOD) and the El Niño-Southern Oscillation, multiple extreme MHW event years between 2008 and 2018 are analyzed. The findings show that the surface MHWs become more prominent during El Niño/positive IOD events due to weakened winds, increased net heat flux input towards the ocean, increased stratification and warming tendency through vertical processes in the presence of inversion. This, in turn, affects the surface biological productivity in the region. Additionally, surface MHWs were also found to be driven by surface currents and eddies. On the other hand, subsurface MHWs develop during negative IOD/La Niña conditions due to the deepening of the thermocline, which is forced by coastally trapped downwelling Kelvin waves and reflected downwelling Rossby waves, and is followed by substantial freshening of deeper layers. This study provides a deeper understanding of the causes of surface and subsurface MHWs in the Bay of Bengal, which is a crucial basin in influencing monsoon and cyclonic events affecting its surrounding landmasses.