Investigating the role of air-sea heat flux for marine heatwaves in the Mediterranean Sea

Abstract

The Mediterranean Sea (MS) has been experiencing progressively intensified Marine heatwave (MHW) conditions over the past decades, associated with severe environmental and socioeconomic impacts. Building upon prior research on physical mechanisms underlying the occurrence of MHWs, here we assess the relative role of air-sea heat exchange in driving the onset and decline phases of surface MHWs in the basin, utilizing remote sensing and reanalysis data for the period 1993-2022. Although contributing positively to the SST evolution during most MHWs, surface heat flux is identified as the primary driver in less than half of the onset/decline MHW phases. This finding suggests that oceanic processes play a crucial role in driving SST anomalies during MHWs in the basin. The role of surface heat flux becomes more pronounced during onset periods and warmer seasons, with the latent heat being the most significant heat flux component in modulating SST anomalies during both MHW phases and across all seasons. Heat flux emerges as the major driver of most onset phases in the Adriatic and the Aegean Seas. Onset/decline phases shorter than 5 days exhibit a weaker heat flux contribution compared to longer phases. Moreover, an inverse relationship between event severity and heat flux contribution is observed. At the subsurface, mixed layer shoaling is observed over the entire duration of most events, particularly for those of shorter duration. Therefore, the surface cooling right after the peak intensity day is likely not associated with vertical mixing in such cases. After the MHW end day, a significant mixed layer deepening in most cases suggests that further dissipation of heat is commonly driven by vertical mixing. This study emphasizes the need for considering subsurface information for MHW studies and accounting for limitations associated with the definitions employed for MHW phases. Clearly articulating such choices, tailored to the specific contexts of individual studies, is vital for precise interpretation and meaningful comparisons across different studies on MHW drivers.