Estimating carbonate concentrations south of the Polar Front

Friday, 19 December 2014
Sara E Mikaloff Fletcher, Helen Clare Bostock and Michael Williams, NIWA National Institute of Water and Atmospheric Research, Wellington, New Zealand
A major gap in our ability to understand the present state of ocean acidification in the Southern Ocean has been the limited number of observations of carbonate species in these regions. We use multiple linear regressions (MLR) to estimate alkalinity and DIC from the common hydrographic parameters; temperature, salinity, depth/pressure and oxygen both for the Southern Ocean and more specifically for the Ross Sea. We find distinct regimes based on water masses, where the alkalinity and DIC have different relationships with the hydrographic parameters. The aim of this work is to use all the hydrographic data for the region to produce detailed maps of the carbonate parameters: pCO2, pH, [CO32-], aragonite saturation, calcite saturation, that take into account local currents, especially around complex topography.
This approach was initially applied to the Southern Ocean (south of 25S) using observations from the GLODAP (1990s), CARINA and PACIFICA (2000s) global datasets. Then, we applied this approach to look more specifically at the region south of the Polar Front (approximated at 60S), where the strong role of sea ice and interaction between sea ice and biogeochemistry leads to different relationships between hydrographic parameters and carbonate species. In this regime, biology drives strong temporal variability between observations and non-linear relationships in surface water. We use tracer based techniques to separate the physical and biological contributions and compare the results with CMIP-5 models.