Deep open-ocean convection in the Southern Ocean is projected to weaken and possibly stop due to climate change, suggests a study published online in Nature Climate Change this week. If this is the case, it has implications for bottom water formation, which contributes to global overturning circulation, as well as the storage of heat and carbon in the ocean.

Casimir de Lavergne and colleagues analysed observations and model simulations to investigate deep convection changes in the Southern Ocean, where the Weddell and Ross Sea ocean current systems are the main open-ocean sites. They found that freshening of the surface ocean has increased salinity stratification since the 1950s, which inhibits deep mixing. The authors note that 25 climate models, which show that significant convection occurred under pre-industrial conditions, found a decrease in its strength over the period 1900 to 2100 under a high-emissions scenario. In seven of these models, deep circulation was completely stopped by 2030.

This finding suggests that deep convection was more active historically and that climate change has caused it to weaken and will continue to do so.