Subsurface warming derived from Argo floats during the 2022 Mediterranean marine heat wave

Abstract. The Mediterranean marine heat wave (MHW) during the warm season (May–September) and the fall period (October–December) of 2022 is analyzed using Argo float in situ observations in the upper 2000 m of depth. The five study regions (North Western Mediterranean, South Western Mediterranean, central Ionian Sea, Pelops Gyre, and South Adriatic Pit) most affected by warming in different layers were selected and investigated. The primary goal is to provide insights into how the water column responds to the onset and progression of the MHW during the warming period, characterized by peak stratification and reduced vertical mixing. Additionally, this study aims to examine how the heat accumulated in the upper layers is redistributed to deep layers within regions with different dynamic characteristics through advection and/or mixing during the subsequent fall period. Temperature anomaly profiles, Ta (z), computed for each area and for both periods were divided into three categories based on vertical heat penetration: Category 1 (shallow, 0–150 m), Category 2 (intermediate, 150–700 m), and Category 3 (deep, >700 m). During the warm season, Category 1 profiles had a temperature anomaly near zero or slightly negative in a thin layer between 50 and 150 m depth, while warming was observed in the 0–50 m layer and below the middle layer. Profiles characterized by greater vertical heat penetration (categories 2 and 3) were mainly in mesoscale or subbasin structures and showed the largest positive temperature anomaly in the surface and intermediate layers. All profile categories showed a warming between 200 and 800 m depth. This increase is roughly split, with half attributed to the impact of the 2022 MHW and the other half linked to the ongoing long-term trend in ocean temperatures. During the fall period and in the layer below 200 m depth, the shapes of the Ta profiles are similar for all sectors with the exception of the South Adriatic Pit, which depicts a +0.5 °C warming at 800 m depth. The present work highlights the warming characteristics throughout the entire water column across different regions of the Mediterranean Sea, and it seeks to connect the impacts of the warm season on the cold period with oceanic dynamic processes, such as dense water formation, upwelling, or water column stratification. These regions are characterized by dynamic activities (e.g., dense water formation, upwelling); therefore, any variation in these ocean processes can influence the thermohaline circulation and, consequently, the climate system.