Interannual variability in estimated biological productivity in the Australian sector of the Southern Ocean in 1997-2007

Changes in biological productivity in the Southern Ocean have the potential to have a significant effect on world climate. Here we use a combination of satellite, model and model reanalysis data to examine climate variability in the Australian sector of the Southern Ocean (110-160 degrees E, 40-70 degrees S) to identify the controls on chlorophyll-a (a proxy for phytoplankton biomass) and primary productivity and evaluate trends in these controls over the period 1997-2007. In summer, in the 65-70 degrees S zone, sea-ice concentration together with the Southern Annular Mode explains 51% of the variance in chlorophyll-a, while mean wind stress and sea-surface temperature explains 55% of the variance in the 60-65 degrees S zone. Further north, key controls are photosynthetically active radiation, sea-surface temperature, mixed layer depth and stratification. Trends in hydrodynamic variables are found to often be opposite in sign and up to an order of magnitude larger than those previously identified in the same sector for 1958-2005. Allowing for the effect of shorter time series on the magnitude of the trends, many recent trends seem to be outside the range of previous variability. These results are consistent with a shift in the ocean state in the past 10-15 yr, in response to a shift in climate.