EAPS

Title: Eddy-Driven Jet-Scale Overturning Circulations and Their Impact on Mixed Layer Formation in the Southern Ocean

The dynamics of the interaction between Antarctic Circumpolar Current (ACC) jets and eddy fluxes in the Indo-western Pacific Southern Ocean is investigated using a global 1/10° ocean model. In this study, it is demonstrated that there exist jet-scale overturning circulations (JSOCs) in ocean sectors where the ACC jets are relatively zonal and persistent. Each JSOC is driven by eddy momentum flux convergence with its descending branch on the warmer equatorward flank of each jet. Each JSOC is only about 2-degrees wide but its depth extends well below 2-km depth. Argo-float trajectory analyses indicate that the JSOCs also exist in nature.

The deepest winter mixed layer in the Southern Ocean is collocated with the descending branch of the JSOC associated with the Subantarctic Front (SAF). The mixed layer initially develops ~1° north of the SAF. This narrow mixed layer wedge develops above a de-stratified water column. Buoyancy budget analysis indicates that this de-stratification can be attributed to the downwelling branch of the JSOC which transports warm, buoyant water downward from the surface during the warm season. The model-computed net air-sea heat flux indicates that the net heat flux contributes to the development of the MLD wedge. An analysis of subgrid-scale vertical heat diffusion indicates that this feature is also preconditioned by the JSOC. Ekman advection contributes to the formation of the mixed layer, but it occurs farther north of the region where the mixed layer initially deepens. This analysis sheds light on why MLD is not properly simulated by coarse resolution models.