Air–Sea Interactions and Biogeochemical Responses to Medicane Daniel

Abstract. Medicane Daniel, formed on 4–12 September 2023, stands out as the deadliest recorded storm in Mediterranean history. In this study, we investigate the role of sea features in the intensification of the medicane Daniel and the response of biogeochemical properties to the storm. Our results show that medicane Daniel intensified immediately prior to landfall in a coastal environment characterized by the co-occurrence of a warm-core eddy (WCE), elevated ocean heat content, and a moderate marine heatwave (MHW), suggesting that sea anomalies may have supported or modulated the intensification under favorable atmospheric forcing. Additionally, observations from the high-resolution Surface Water and Ocean Topography (SWOT) satellite reveal a larger anticyclonic eddy than that depicted in lower-resolution products, thereby further supporting the hypothesis of sea-induced intensification. The favorable conditions at the sea before landfall were accompanied by moisture convergence and moisture supply in the atmosphere above, leading to intense precipitation in this region. Biogeochemical properties were strongly affected by cyclone-induced subsurface vertical mixing and upwelling. Focusing on two eddies in the vicinity of the maximum cyclone intensity, we found that the observed vertical displacement of the deep chlorophyll maximum exceeds that expected by direct wind-driven upwelling alone, suggesting additional contribution from a structural isopycnal adjustment triggered by the neutralization of eddy vorticity. We propose that the medicane destabilizes the eddies' internal balance, leading to a large-scale reorganization of the water column that persists longer in the WCE than the transient response observed in the cold-core eddy (CCE).