Future Summertime Marine Heatwaves in the Indian Ocean in Response to Enhanced Variability of the Western North Pacific Subtropical High Under Warming Climate

Marine heatwaves (MHWs) pose significant threats to marine ecosystems and associated services, necessitating a deeper understanding of their driving mechanism. This study examines how the intensification of the Western North Pacific Subtropical High (WNPSH) influences future summer MHW occurrences in the Indian Ocean through complex ocean-atmosphere coupling. Over two thirds of CMIP6 models project more frequent an intense strong WNPSH years by the end of the 21st century, resulting in prolonged and extreme summer MHWs in the future. Westward extension of stronger WNPSH generates pronounced anomalous anticyclonic circulation, producing easterly winds that extend into the north and equatorial Indian Ocean and oppose climatological monsoon winds. While these anomalous easterlies suppress key cooling mechanisms, such as wind driven evaporative cooling and upwelling, the westward propagating downwelling Rossby waves dynamically precondition the warming in the western Indian Ocean by deepening the thermocline. This coupled system creates sustained surface and subsurface warming extending from preceding seasons into summer. Regional differences emerge in future summer MHWs through cloud-sea surface temperature (SST) feedback mechanisms. The central and northeastern Indian Ocean experiences more extreme MHWs due to reduced cloud cover, enhanced solar radiation exposure, and suppressed evaporative cooling through positive low cloud-SST feedback. Conversely, the western Indian Ocean exhibits enhanced convection and cloud formation, moderating extreme warming through negative SST-cloud feedback, exposing the region only to strong-moderate MHWs. These findings highlight the critical role of multiseasonal, coupled ocean-atmospheric interactions in shaping future summer MHW patterns, emphasizing the enhanced vulnerability of marine ecosystems.