Progression of marine heat wave events over the tropical Indian Ocean and its underlying mechanisms

Marine Heat Waves (MHW) are devastating extreme oceanic events that have severe and destructive effects on the marine and coastal ecosystems. In the present study, the generation and advancement of MHW events during the last four decades over the tropical Indian Ocean (IO) and its primary regulating factors are investigated. Multiple MHW events have been detected over the tropical IO in the past decade. It was found that between 2011 and 2021, the tropical Indian Ocean observed a significant increase in both the mean annual number of MHW days and the frequency of MHW occurrences. This coincided with a substantial rise in sea surface temperatures (SST) in the region during the same period. Long-running events were detected for the years 2015 over the Somali coast, 2016 over the Java-Sumatra Coast, and for 2019, and 2020 over the Seychelles Dome. Event-specific analysis revealed that a decline in wind speed was observed during the second phase of the 2015 MHW event along the Somali coast which resulted in the subsidence of upwelling, similar observations were also made around the Java-Sumatra coast. Subsequently, the role of planetary waves in the sustenance of the long-running MHW events is analysed. Positive sea level anomaly values were observed around Somali and Sumatra for 2015 and 2016, which in turn signify the presence of downwelling planetary waves. These planetary waves play an important role in oceanic surface and sub-surface warming and mixing by deepening thermocline and consequentially inhibiting the upwelling and entrainment. An analysis of the mixed layer heat budget terms over the active regions of MHW events in 2015, 2016, 2019, and 2020 shows that the primary contributor influencing these MHW events is net heat flux over the majority of the affected areas which is followed by the dominant role of vertical advection over the Somali region.