Declining winter heat loss threatens continuing ocean convection at a Mediterranean dense water formation site

A major change in winter sea surface heat loss between two key Mediterranean dense water formation sites, the North-West Mediterranean (NWMed) and the Aegean Sea, since 1950 is revealed using atmospheric reanalyses. The NWMed heat loss has weakened considerably (from −154 Wm−2 in 1951-1985 to −137 Wm−2 in 1986-2020) primarily because of reduced latent heat flux. This long-term weakening threatens continued dense water formation, and we show by evaluation of historical observations that winter-time ocean convection in the NWMed has declined by 40% from 1969 to 2018. Extension of the heat flux analysis reveals changes at other key dense water formation sites that favour an eastward shift in the locus of Mediterranean convection towards the Aegean Sea (where heat loss has remained unchanged at −172 Wm−2). The contrasting behaviour is due to differing time evolution of sea-air humidity and temperature gradients. These gradients have weakened in the NWMed due to more rapid warming of the air than the sea surface but remain near-constant in the Aegean. The different time evolution reflects the combined effects of global heating and atmospheric circulation changes which tend to offset heating in the Aegean but not the NWMed. The shift in heat loss has potentially significant consequences for dense water formation at these two sites and outflow to the Atlantic. Our observation of differential changes in heat loss has implications for temporal variations in the balance of convection elsewhere e.g. the Labrador-Irminger-Nordic Seas nexus of high latitude Atlantic dense water formation sites.