Southern ocean winter mixed layer

Abstract

Austral winter 1986 observations from the Polarstern along the Greenwich meridian from the ice edge to the Antarctic margin show the mixed layer beneath the winter sea ice cover to be significantly depressed in oxygen saturation. Incorporation of Weddell Deep Water (WDW) into the winter mixed layer, responsible for this undersaturation, also introduces heat and salinity into the surface layer which strongly influences the mixed layer, sea‐air exchanges and sea ice formation processes. The total WDW transfer into the mixed layer averages 45 myr−1, implying a residence time for the surface water of 2.5 years. The associated winter heat flux is 41 W m−2, which limits ice thickness to about 0.55 m, agreeing quite well with observations. The air temperatures during the cruise are just sufficient to remove the WDW heat input in the presence of observed ice thickness and concentration. This suggests that the sea ice cover and WDW heat input into the mixed layer are in approximate balance by midwinter. The annual heat flux from WDW to the surface layer, and hence into the atmosphere, is estimated as 16 W m−2. Extrapolation of the Greenwich meridian WDW entrainment value to the full circumpolar 60°–70°S belt yields total up welling of 24×10 6 m 3 s−1. Similar extrapolation of the heat flux value gives a circumpolar total of 2.8 × 10 14 W. As a consequence of circulation/topography interaction, the Maud Rise water column stands out as an anomaly relative to the surrounding region, with a significantly more saline and dense mixed layer. Below the mixed layer the water column over the crest of the rise is identical to that over the flanks if the latter water column is upwelled by 400 m. This uplifting is believed to be a response of the upstream flow encountering the rise. Increased upstream flow would be expected to increase Maud Rise upwelling and the dependent salinity (density) of the mixed layer. Slight increases in the mixed layer density could trigger a convective mode and generation of a polynya. It is hypothesized that spin‐up of the Weddell Gyre's barotropic circulation induced by an increase of the regional wind stress curl would enhance the probability of polynya development over Maud Rise.