Characteristics of Marine Heat Extreme Evolution in the Northern Indian Ocean

Abstract

Marine Heat Extremes (MHEs) are events of anomalously high Sea Surface Temperature (SST) during which SST values exceed a certain pre-defined threshold. These MHEs have profound influence over weather patterns, air-sea interaction and the health of marine ecosystems. This study investigates the long-term evolution of MHEs in the Northern Indian Ocean (NIO) from 1900 to 2020. We utilised two normalised indices, the Normalised Extreme Frequency Index (NEFI) for frequency and the Normalised Extreme Heat Index (NEHI) for the intensity of MHEs, to objectively compare the MHE attributes across different periods and regions of the NIO. The analysis reveals non-linearly increasing NEFI, with the Western Equatorial Indian Ocean (WEIO) experiencing the fastest rise, followed by the Eastern Equatorial Indian Ocean (EEIO), Arabian Sea (AS) and Bay of Bengal (BoB). MHE intensity shows exponential growth, with its mean-based regimes becoming shorter and shifting more frequently. A new regime has been emerging since the last decade. Analysis of the spatial extent of the MHEs indicates that the WEIO is the fastest-growing region of the NIO. Similar observations were found upon removing sub-decadal variabilities, which include the potential effects of El Niño-Southern Oscillation and Indian Ocean Dipole, highlighting the long-term warming associated with global warming. The study also links the increasing mean SST to the rising frequency and intensity of MHEs, which is predominantly driven by the net surface heat-flux, which is a combined effect of local and pantropical air-sea interaction. The surface warming is outpacing subsurface warming, thereby strengthening thermal stratification over time, potentially impacting vertical mixing and upwelling, which can, in turn, lead to further surface warming.