Quantifying the role of the eddy transfer coefficient in simulating the response of the Southern Ocean Meridional Overturning Circulation to enhanced westerlies in a coarse-resolution model

The ability of a coarse-resolution ocean model to simulate the response of the Southern Ocean Meridional Overturning Circulation (MOC) to enhanced westerlies is evaluated as a function of the eddy transfer coefficient (κ), which is commonly used to parameterize the bolus velocities induced by unresolved eddies. The strongest eddy-induced MOC response, accounting for 82% of the reference eddy-resolving simulation, is achieved using a stratification-dependent κ with spatiotemporal variability. By decomposing the eddy-induced velocity into its vertical variation (VV) and spatial structure (SS) components, we find that the intensified eddy compensation response is primarily driven by the enhanced SS term, while the introduced VV term weakens the response. Additionally, the temporal variation of the stratification-dependent κ plays a key role in strengthening the eddy compensation response to intensified westerlies. The stronger eddy compensation response in the experiment with stratification-dependent κ than the constant κ can be attributed to the structure of κ and the vertical variation of the density slope. These findings highlight the significance of accurately representing κ for capturing the response of the Southern Ocean MOC and emphasize the role of the isopycnal slope in modulating the eddy compensation mechanism.