Seasonal and spatial variations of Southern Ocean diapycnal mixing from Argo profiling floats

Abstract

in the large-scale ocean circulation and climate 4‐6 . Here we use high-resolution hydrographic profiles from Argo floats in combination with the Iridium communications system to investigate diapycnal mixing in the Southern Ocean. We find that the spatial distribution of turbulent diapycnal mixing in the Southern Ocean at depths between 300 and 1,800 m is controlled by the topography, by means of its interaction with the Antarctic Circumpolar Current. The seasonal variation of this mixing can largely be attributed to the seasonal cycle of surface wind stress and is more pronounced in the upper ocean over flat topography. We suggest that additional highresolution profiles from Argo floats will serve to advance our understanding of mixing processes in the global ocean interior. The Southern Ocean is an important source of bottom water in the world’s oceans and is a key upwelling site of deep water, pointing to diapycnal mixing (that is, mixing across surfaces of equal density) in the region as an important controlling factor of the global meridional overturning circulation (MOC; refs 7,8). Modelling studies have demonstrated that the global strength of the MOC depends critically on the details of the representation of mixing processes in the Southern Ocean interior 9 . A few existing observations 1014 in this region have revealed dramatic spatial heterogeneity in diapycnal mixing, with values of diffusivity over smooth topography comparable to those in the mid-latitude ocean interior (of the order of 10 5 m 2 s 1 ), and enhanced mixing (of the order of 10 4 m 2 s 1 or larger) extending far from rough topography. However, discerning the nature of diapycnal mixing over the entire Southern Ocean is still a formidable challenge because of the very limited sampling of the necessary parameters in the region. Furthermore, in general the temporal variability of diapycnal mixing remains poorly assessed and understood throughout most of the world’s oceans, but in some cases the causes of such variability might be reasonably straightforward to document.Asanexample,recentanalysisofhistoricalhydrographic data reveals that the subsurface turbulent diapycnal mixing in the Northwestern Pacific displays a pronounced seasonal variability stirred by surface wind stress 15 . The International Argo Program has created the first global array for observing the subsurface ocean. A subset of Argo floats deployed are designed to use the Iridium communications system (henceforth referred to as Argo Iridium floats), and are able to provide high-resolution (2m) profiles that are able to resolve fine-scale (tens to hundreds of meters) strain. These floats