Antarctic Convective Oscillations in Climate Models

Numerous climate models display large-amplitude, long-period variability in the Southern Ocean, but the mechanisms for such variability are poorly understood. In this paper we use empirical orthogonal functions of sea surface temperature and salinity to identify feedback loops that can generate such oscillations within a suite of Earth System Models. In one loop convective events in the Weddell Sea generate lower salinities nearby (T’ correlates negatively with dS’/dt). The resulting low salinity anomalies propagate into the convective regions, shut off convection and cause cooling (S’ correlates positively with dT’/dt). In another loop changes in temperature drive changes in winds, which in turn drive changes in advection. We examine the term balances that drive the changes in salinity and temperature in the centers of action and show that they involve both advection and eddy mixing. We then compare these modes across a suite of models with different eddy mixing parameterizations that generate different oscillations, demonstrating that the structure and interrelationship between modes are modulated by the parameterization of the effects of these mesoscale eddies. Convection is found in models for which the mesoscale eddy mixing is relatively weak- a state that may be representative of the preindustrial Southern Ocean.

The figure below shows principal oscillation patterns associated with the dominant mode in a low-mixing version of the GFDL ESM2Mc model. "In phase" (top row) is the pattern of SST and SSS associated with convection in the Weddell Sea. "Out of phase" (bottom row) shows the patterns of anomalous SST and SSS leading peak convection by a quarter cycle.