Dipole variability of subsurface marine heatwaves in the Bay of Bengal

Marine heatwaves (MHWs) are ocean temperature extremes that can occur at any ocean depth. Surface features and drivers of MHWs have been extensively explored based on satellite observations; however, their subsurface features and drivers remain unclear. This study investigates the characteristics and drivers of subsurface MHWs near the thermocline in the Bay of Bengal (BoB) from 1993 to 2024 using high-resolution ocean reanalysis datasets. The subsurface MHW days exhibit a dipole pattern in response to the El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). During El Niño or positive IOD, the anticyclonic mesoscale eddies in the western BoB are favorable for MHW generation in this region, related to the anomalous anticyclonic winds and currents over the BoB. Meanwhile, the equatorial easterly anomalies drive upwelling Kelvin waves to propagate eastward into the eastern BoB, inhibiting MHW formation in that area. Thus, the subsurface MHW days in the BoB increase in the west but decrease in the east during El Niño or positive IOD events. Over the past decades, a significant increasing trend in the subsurface MHW days has been observed due to the rise in mean temperature over the BoB. This study highlights the inconsistent spatial responses of subsurface MHWs to distinct ocean dynamics induced by ENSO and IOD.