Subsurface marine heat waves and coral bleaching in the southern red sea linked to remote forcing

Research on marine heat waves (MHWs) in the Red Sea has focused on the surface signatures of these extreme warm events, such as the sea surface temperature (SST). This focus may potentially neglect the detrimental effects of subsurface MHWs. The unprecedented coral bleaching event observed in the southern Red Sea in 2015, despite less intense SSTs than in the MHW in 2002, highlights this oversight. A high-resolution regional reanalysis of the Red Sea reveals that 2015 and 2002 were characterized by subsurface heat content anomalies of opposite signs at depths up to 100 m, with positive anomalies in 2015 and negative anomalies in 2002. A heat budget analysis suggests that the primary heat source is advection from the southern boundary connecting with the Gulf of Aden (GoA). The advection of negative temperature anomalies from the GoA contributed to decreased subsurface heat in 2002, and the advection of positive temperature anomalies from the GoA contributed to increased subsurface heat in 2015. The total increase in the subsurface heat observed in 2015 is linked to the reduction in Red Sea surface water (RSSW) and GoA intermediate water (GAIW). The higher sea surface height (SSH) and deeper 25 σ isopycnal in GoA during 2015 resulted in horizontal pressure differences between the southern Red Sea and GoA, corresponding to the reduced flow of RSSW and GAIW that year. The primary factor contributing to the elevated SSH and deeper 25 σ isopycnal is the presence of an anticyclonic eddy (Somali current ring) along the western shores of the GoA. The probable cause for the stronger anticyclonic eddy in 2015 compared to 2002 is the decreased intensity of the westward-propagating upwelling Rossby waves that originated from as far away as the Arabian Sea and the western coasts of India.