Summary
The Southern Ocean is vitally important in understanding climate change, both regionally and globally, due to factors such as the presence of the Antarctic Circumpolar Current, seasonal sea-ice and strong Southern Hemisphere westerly winds. Also, the upper branch of the Circumpolar Deep Water (UCDW), which is warm relative to Antarctic waters, as well as being high in dissolved CO2 and nutrients, in particular iron, upwells in this region. UCDW contributes to carbon dioxide regulation by out-gassing of CO2 and through its effects on solubility, primary production and the biological pump. This project examines long-term trends (1958-2007) in the meridional circulation of UCDW and in upper-ocean structure, primarily in the Australian region (110-160„aE, 40-70„aS) of the Southern Ocean. This is achieved by the use of the Simple Ocean Data Assimilation (SODA) 2.0.2/2.0.4 reanalysis of ocean climate variability, which combines output from an ocean global circulation model with observational data. In addition, trends in chlorophyll-a and its inter-annual variability are examined for 1997-2007 and correlated with physical factors. Results show that, unlike the trends in mixed layer variables, trends in UCDW seem to operate on longer than decadal time-scales. Recent trends in sea-surface temperature and salinity are outside the bounds of earlier variability, which seems to indicate there has been a shift in the state of the Southern Ocean in the last ten to fifteen years.