Southern Ocean Deep Convection in Global Climate Models: A Driver of Sub-polar Gyre Strength and Drake Passage Transport Variability on Decadal Timescales

In this study, dependencies between Southern Ocean state variables, such as sea-ice cover, deep convection, bottom water production, sub-polar gyre strength, and the Drake Passage transport, on decadal to multi-decadal time scales are investigated by an inter-comparison of a global ocean and various coupled climate models incl. observations. Most of the coupled climate models show large deficiencies in representing the present day Southern Ocean – model biases of the sea surface temperature, Antarctic Bottom Water formation, sea-ice trends and open-ocean convection are ubiquitous. Open-ocean deep convection events are a large source for decadal to multi-decadal variability in Southern Ocean state variables. Analyses of an ensemble of climate model simulations using the National Institute of Water and Atmospheric Research UK Chemistry and Aerosols (NIWA-UKCA) model show that these intense mixing events can be triggered by a surface freshwater perturbations affecting the oceanic stratification and a slight phase shift between freshwater and heat content south of 60°S. Regardless of whether models are open-ocean convective or not, stronger convection south of 60°S -- and thus increased bottom water formation -- increases the meridional density gradient and subsequently leads to larger Drake Passage transport. Expanding sea-ice cover causes the opposite effect. Many coupled climate models indicate a positive coupling between the strength of sub-polar gyres and sea-ice cover, which is not solely triggered by anomalous wind stress curl.