Seasonal evolution of upper ocean thermal structure between Tasmania and Antarctica

We describe the upper ocean thermal structure between Tasmania and Antarctica based on thirteen repeat temperature sections occupied between 1991 and 1994. The sections cross three main fronts. The subtropical front is found between Tasmania and the South Tasman Rise in each of the sections. The subantarctic front (SAF) is composed of two parts, which have distinct thermohaline signatures and behave somewhat independently: the northern part, associated with the 6-8°C isotherms, is characterised by large meridional gradients of both temperature and salinity; the southern part is associated with a weaker meridional temperature gradient and negligible salinity gradient between the 3°and 5°C isotherms. The northern part of the SAF is located between 50°S and 51°S in each of the sections, but the position of the southern part of the SAF is more variable with time. A cold core eddy or meander is found north of the SAF throughout the 1993-1994 austral summer. The polar front (PF) is found near 53°S in all sections. Dynamic height is estimated for each of the XBT sections by exploiting the tight correlation in this region between vertically-integrated temperature and dynamic height. Dynamic height decreases relatively smoothly with latitude between 50°S and 53°S, so that the SAF, PF and the water between the two fronts forms a broad belt of eastward flow relative to a deeper level. The difference in dynamic height at the sea surface relative to 2000 m is 1.03 dyn m between 47°S and 60°S and is constant through the 1993-1994 austral summer to within the accuracy of the method (rms error ≃0.07 dyn m). The dynamic height expression of the cold core eddy reaches a maximum of 0.23 dyn m in February 1994. The upper 100 m of the water column warms by about 1.6°C between December and March south of 54°S, corresponding to an average warming rate of 95 W m-2. Changes in heat content at other latitudes are dominated by meridional shifts of the fronts, and no clear seasonal trend can be identified.