Recent salinification, warming and stratification changes impacting deep water formation in the Mediterranean Sea

The Mediterranean Sea undergoes a long-term salinification and warming with strong implications for its overturning circulation and subsequently for its regional climate. In this study ERA5 atmospheric reanalysis and En4 hydrographic monthly datasets were used to investigate the impacts of changing air-sea fluxes and stratification on deep water formation of the Mediterranean Sea over 1979-2023. Results indicate non-significant long-term changes in winter net heat flux controlling deep water formation at the major formation sites of the two sub-basins. However, winter sensible and latent heat fluxes show a pronounced decreasing trend resulting in a strong reduction of winter ocean heat loss in the Gulf of Lions over the last two decades. This is contrasted by increased turbulent air-sea heat fluxes driving enhanced winter ocean heat loss in the Aegean Sea during the same period. Large salinity increases are obtained across the basin mainly induced by warming-driven evaporation increases over 1979-2023. This excessive salinification may have strongly enhanced salt preconditioning in all major dense water formation sites, sustaining deep water formation despite of the increasingly warming climate.  Results reveal that temperature and salinity increases largely counteract each other resulting in relatively small changes in density in most parts of the basin. However, the density contrast is significantly increasing between the upper layer which becomes lighter due to excessive surface warming and the deeper layers affected by large amounts of saltier/denser water masses mainly formed during strong climatic transient events in both sub-basins under anomalous winter surface cooling over one or two severe winters, particularly during the East Mediterranean Transient (EMT) in the early 1990’s and the Western Mediterranean Transition (WMT) in the mid-2000s. The combined effects of increased water-column stratification and decreasing winter ocean heat loss can have dramatic impacts on deep water formation as evidenced in the Gulf of Lions over the last decade.