Extreme marine summers in the Mediterranean Sea

The Mediterranean Sea (MS) has been experiencing significant surface warming over the past decades, greater than the global ocean and particularly higher during summers. The present study proposes the concept of Extreme Marine Summers (EMS) and investigates their characteristics in the MS in a climatological framework, based on ECMWF ERA5 daily sea surface temperature (SST) data for the period 1950-2020. Main objectives are to explore the SST substructures within EMSs, the contribution of Marine Heatwaves (MHW) during EMSs and the driving role of air-sea heat fluxes in the EMS formation.

EMSs are defined as the summers (July-August-September) exhibiting a mean SST greater than the 95^th percentile of the mean summer SST values within the study period. A marine summer may evolve as extreme under different SST substructures within the season, e.g., due to uniformly increased SST values throughout the summer or due to higher than usual SSTs of a specific part of the SST distribution during the season. Results suggest that EMSs identified in the greatest part of the basin are formed due to the warmest part of the ranked daily SST distribution being warmer than normal. SSTs within EMSs are organised under high dependency on the climatological SST variability: locations where the warmest (coldest) part of the ranked daily SST distribution is more variable climatologically, experience EMSs primarily due to the contribution of the warmest (coldest) part of the SST distribution.

MHWs in EMSs present greater intensity, duration and occurrence frequency with respect to mean MHW conditions, in the northern flanks of the Mediterranean basin and particularly in the Aegean and Adriatic Seas. Although the north-western part of the basin experiences the most intense EMSs and summer MHWs, the role of MHWs in the formation of EMSs appears more pronounced in the central and eastern MS. In the rest of the basin, and particularly in southern MS regions, MHWs in EMSs are less intense but longer lasting and more frequent than usual.

To quantify the driving role of the net surface heat flux (Q_net) in the EMS formation, a metric is proposed based on the surface heat budget equation. The proposed metric represents the mean contribution of Q_net during summer sub-periods within which SST is kept above climatology via a) faster warming or b) slower cooling compared to the corresponding climatological period. Results show that EMSs are largely driven by Q_net in the northern MS regions: a latitudinal gradient is generally observed in the basin with increasing contribution percentages while moving northerly. In areas where the observed SST anomalies are not entirely explained by surface heat fluxes, negative wind speed and mixed layer depth seasonal anomalies relative to climatology are commonly observed, suggesting that wind-induced mixed layer shoaling is a complementary EMS contributing mechanism. Moreover, results reveal a strong link between MHW properties and surface heat fluxes during EMSs, suggesting that Q_net modulates particularly the intensity of MHWs.