Regime Shifts in Marine Heat Extremes in the Northern Indian Ocean

Sea Surface Temperature (SST) extreme, known as the Marine Heat Extremes (MHEs), has become more frequent and intense over the years in the Northern Indian Ocean (NIO), leading to increased uncertainty in monsoon and cyclones. In this study, we characterised the evolution of MHEs utilising the monthly Hadley Centre Sea Ice and SST (HadISST) for 1900–2020 over the NIO. For a comparative analysis of evolution of MHEs, the region was further divided into Eastern Equatorial IO (EEIO), Western Equatorial IO (WEIO), Arabian Sea (AS), and Bay of Bengal (BoB). A MHE event is defined when the SST crosses the monthly varying 98th percentile threshold corresponding to the fixed climatological baseline of 1901–1950. Two normalized indices, i.e., Normalized Extreme Frequency Index and Heat Index, have been utilized to understand the spatio-temporal characteristics of intensity and frequency, respectively. Both the indices show a non-linear exponential increment. Moreover, the area fraction experiencing MHEs was also found to increase swiftly, following a sigmoidal curve. Frequent mean regime-shifts in these quantities have been observed, increasing the unpredictability of the climate system. Moreover, statistical tests revealed that the MHE attributes are increasing because of the increasing mean SST rather than its variance.  A mixed layer heat budget analysis shows that the MHE attributes have been increasing more rapidly over the WEIO, followed by EEIO, AS, and lastly the BoB, majorly due to the net heat flux followed by the horizontal advection. These findings underscore the non-linear escalation of thermal stress on marine ecosystems and the broader climate, emphasizing the need to develop mitigation strategies.