Heat budget of the surface mixed layer south of Africa
ARGO hydrographic profiles, two hydrographic transects and satellite measurements of air–sea exchange parameters were used to characterize the properties and seasonal heat budget variations of the Surface Mixed Layer (SML) south of Africa. The analysis distinguishes the Subtropical domain (STZ) and the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ) and Antarctic Zone (AZ) of the Antarctic Circumpolar Current. While no Subantarctic Mode Water forms in that region, occurrences of deep SML (up to ∼450 m) are observed in the SAZ in anticyclones detached from the Agulhas Current retroflection or Agulhas Return Current. These are present latitudinally throughout the SAZ, but preferentially at longitudes 10–20° E where, according to previous results, the Subtropical Front is interrupted. Likely owing to this exchange window and to transfers at the Subantarctic Front also enhanced by the anticyclones, the SAZ shows a wide range of properties largely encroaching upon those of the neighbouring domains. Heat budget computations in each zone reveal significant meridional changes of regime. While air–sea heat fluxes dictate the heat budget seasonal variability everywhere, heat is mostly brought through lateral geostrophic advection by the Agulhas Current in the STZ, through lateral diffusion in the SAZ and through air–sea fluxes in the PFZ and AZ. The cooling contributions are by Ekman advection everywhere, lateral diffusion in the STZ (also favoured by the ∼10° breach in the Subtropical Front) and geostrophic advection in the SAZ. The latter likely reflects an eastward draining of water warmed through mixing of the subtropical eddies.
KeywordsOcean surface mixed layer Heat budget Southern Ocean Agulhas Current retroflection
This contribution to the CLIVAR/GoodHope programme was supported by the IFREMER programme “Circulation Océanique”, by INSU (Institut National des Sciences de l’Univers), by the CNRS (Centre National de la Recherche Scientifique) and by the Université de Bretagne Occidentale. V. Faure’s contribution was done while under post-doctorate studies at the Laboratoire de Physique des Océans, supported by a grant from the CNRS. S. Gladyshev’s participation was benefited by a contract for bilateral cooperation between the CNRS and the Russian Academy of Sciences. We are thankful to J.B. Sallée for providing us with his surface diffusivity values and to C. Messager for the useful discussions on the air–sea heat fluxes. The aid of A. Prigent in the preparation of some figures is acknowledged.
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