Air–Sea Interactions and Biogeochemical Responses to Medicane Daniel

Abstract. Medicane Daniel, formed on 4–12 September 2023, has stood out as the deadliest recorded storm in Mediterranean history. In this study, we investigate the role of sea features as contributors to the intensification of the Medicane Daniel. Our findings reveal the presence of a warm core eddy (WCE), high ocean heat content, and a moderate marine heat wave (MHW) at the location where Medicane Daniel intensified. These features were situated near the coastal region, facilitating the Medicane's intensification close to the coast. Consequently, the Medicane did not weaken significantly after landfall, leading to severe damage along the coast of Libya. These conditions favoured the Medicane's intensification and, due to high moisture convergence, contributed to significant precipitation at the eddy and MHW position. Importantly, observations from the high-resolution Surface Water and Ocean Topography (SWOT) satellite captured the WCE more accurately or in finer detail. This allowed for attribution of changes in biogeochemical properties –namely, chlorophyll, phytoplankton, nutrients, and dissolved oxygen concentrations due to eddy-induced vertical mixing and upwelling. The biogeochemical properties tend to increase over the WCE and MHW locations due to mixing and upwelling induced by the presence of the WCE and MHW. Our case-study analysis suggests that, under atmospheric cyclone conditions, subsurface mixing may be more influential within CCEs than upwelling driven by Ekman pumping, which, by contrast, may play a more prominent role within WCEs.