The Impact of Eddy Fluxes and Jet-Scale Overturning Circulations on the Mixed Layer Formation in the Indo-Western Pacific Southern Ocean

The dynamics of the interaction between Antarctic Circumpolar Current (ACC) jets and eddy fluxes in the Indo-western Pacific Southern Ocean (90°E-145°E) is investigated using an eddy-resolving model. In ocean sectors where the ACC jets are relatively well defined, there exist jet-scale overturning circulations (JSOCs) with sinking motion on the equatorward flank, and rising motion on the poleward flank of the jets. The JSOC is driven by eddy momentum flux convergence centered at the jet core. The structure of the JSOC resembles the Ferrel cell in the atmosphere.

In this region, the early winter mixed layer is remarkably narrow and deep, and occurs immediately north of the Subantarctic Front (SAF), the primary and northernmost ACC jet. The mixed layer initially develops in June, centered at 47.5°S, with a meridional scale of only ~2° latitude. Its center is located ~1° north of the model’s SAF. Consistent with previous studies, Ekman advection contributes to the formation of the mixed layer, but it occurs farther north of where the mixed layer initially forms. Similarly, the net air-sea heat flux also contributes, but it does not initiate the formation of the narrow mixed layer. The formation of the narrow mixed layer coincides with positive buoyancy transport at the mixed layer base. The buoyancy transport is carried out by the aforementioned JSOC and by an equatorward eddy buoyancy flux, which coincides with an equatorward meridional density gradient confined to a narrow region just north of the SAF. These findings suggest that eddy fluxes associated with the SAF play an important role in initiating the narrow, deep mixed layer that forms north of the SAF.