The impact of topography and eddy parameterization on the simulated Southern Ocean response to changes in surface wind stress

Hailu Kong, University of Chicago, Chicago, IL, United States and Malte Jansen, University of Chicago, Department of the Geophysical Sciences, Chicago, IL, United States
Surface wind stress over the Southern Ocean has been increasing for decades, yet the insensitive response of Southern Ocean circulation has not been fully understood. Meso-scale eddies are crucial in this response but in GCMs they can only be parameterized due to their small size. These eddies can be resolved in idealized models but few idealized modeling studies have systematically tested the effectiveness of state-of-the-art eddy parameterizations against an eddy-resolving simulation under the same model configuration. Moreover, while topography has been shown to be critical in maintaining the current ocean state, its effect on the parameterization problem remains largely unexplored.

In this work we use a hierarchy of idealized models of the Southern Ocean to investigate its response to changes in surface wind stress. The Model hierarchy is designed to include topography of increasing complexity, so as to allow us to investigate the role of topography. For each topographic configuration, we compare several state-of-the-art eddy parameterizations against a high-resolution simulation under varying surface wind stress.

We find that: 1) in the presence of complex topography, stationary eddies dominate over transient eddies in counteraction the response of the Southern Ocean circulation to wind stress change; 2) coarse resolution simulations with parameterized eddies capture the response of the meridional overturning circulation reasonably well, but they systematically overestimate the sensitivity of the ACC transport; 3) in the absence of topography, this overestimate can be explained by insufficient sensitivity of the parameterized transient eddy diffusivity to wind-stress changes, but when topography is present, the overestimate is mainly caused by the coarse resolution model’s inability to capture the full effect of stationary meanders. Our results highlight the importance of topography in modulating the response of the Southern Ocean circulation to changes in surface wind stress, which requires more attention in future developments of eddy parameterizations.