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

This study assesses the capability of a coarse-resolution ocean model to replicate the response of the Southern Ocean Meridional Overturning Circulation (MOC) to intensified westerlies, focusing on the role of the eddy transfer coefficient (kappa). kappa is a parameter commonly used to represent the velocities induced by unresolved eddies. Our findings reveal that a stratification-dependent kappa, incorporating spatiotemporal variability, leads to the most robust eddy-induced MOC response, capturing 82% of the reference eddy-resolving simulation. Decomposing the eddy-induced velocity into its vertical variation (VV) and spatial structure (SS) components unveils that the enhanced eddy compensation response primarily stems from an augmented SS term, while the introduced VV term weakens the response. Furthermore, the temporal variability of the stratification-dependent kappa emerges as a key factor in enhancing the eddy compensation response to intensified westerlies. The experiment with stratification-dependent kappa exhibits a more potent eddy compensation response compared to the constant kappa, attributed to the structure of kappa and the vertical variation of the density slope. These results underscore the critical role of accurately representing kappa in capturing the response of the Southern Ocean MOC and emphasize the significance of the isopycnal slope in modulating the eddy compensation mechanism.