Eddy heat diffusion and Subantarctic Mode Water formation

Subantarctic mode waters (SAMW) form in the deep winter mixed layers occuring north of the Subantarctic Front (SAF). The recent increase of hydrographic and surface drifter data in the Southern Ocean allows a better spatial representation of the distinct regions of SAMW formation. This study focuses on the thermodynamical processes acting on the winter mixed layer heat budget. Eddy heat diffusion play a substantial role in the local heat balance, whereas its action vanishes with large‐scale averaging. South of the western boundary currents and north of the SAF, the eddy heating plays an important role in specific regions, counterbalancing the cooling of the mixed layer by Ekman advection and air‐sea fluxes. Specifically, the eddy diffusion term reduces the tendency for mixed layer destabilisation north of the SAF in the Western Indian Ocean downstream of the Agulhas Retroflection and in the Western Pacific downstream of Campbell Plateau. This role for mixed layer eddy fluxes emphasizes a large‐scale control of mixed layer properties by topography and mesoscale processes in the Southern Ocean.