The Southern Ocean is vitally important in understanding climate change, both regionally and globally. This is due partly to physical factors such as the Antarctic Circumpolar Current (ACC), which facilitates the interchange of heat, nutrients and carbon dioxide between the world oceans. These physical factors also include the seasonal variation in sea-ice around the Antarctic continent and the strong Southern Hemisphere westerly winds, which are responsible for the divergence-driven upwelling of deep water to the surface, south of the Polar Front. The Upper branch of the Circumpolar Deep Water (UCDW) is warm, relative to Antarctic waters, and is high in dissolved inorganic carbon and nutrients, in particular iron. It contributes to carbon dioxide regulation, therefore, through temperature effects on solubility, the out-gassing of carbon dioxide and also via its effect on primary production and the biological pump. This project examines long-term trends in the meridional circulation of UCDW and in upper-ocean structure, primarily in the Australian region (110- 160E, 40-70S) 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, through a sequential data estimation scheme. Trends are produced for 1958-2007, in five-degree latitudinal bands that approximate the ACC frontal zones, for UCDW and mixed layer hydrodynamic variables and these are related to trends in Southern Hemisphere winds and global climate indices. In addition, similar trends are also produced for the shorter period 1997-2007, for which satellite data are available, and these trends are related to trends in chlorophyll-a, a proxy for phytoplankton biomass, and primary production. As well as these trends, interannual variability in chlorophyll-a in spring and summer, and its controls, are also studied.