Reduction in Winter Surface Heat Loss over the Past 70 Years Threatens North-West Mediterranean Convection

Over the past 70 years, a major change in winter air-sea heat loss between the North-west Mediterranean (NWMed) and the Aegean Sea,  is revealed using the ERA5 amd 20CRv3 atmospheric reanalyses. The NWMed heat loss weakens from -154 Wm^-2 in 1951-1985 to -137 Wm^-2 in 1986-2020 as a results of weaker latent heat loss. 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. The process of differential heat loss change in the Mediterranean Sea has implications for temporal variations in the balance of convection elsewhere e.g., the  high latitude Atlantic/Arctic margin dense water formation sites (Labrador-Irminger-Nordic Seas).