Impact of historical climate change on the Southern Ocean carbon cycle

International audienceClimate change over the last several decades is suggested to cause a decrease in the magnitude of the uptake of CO$_2$ by the Southern Ocean (Le Quere et al.). In this study, the atmospheric fields from NCEPR1 for the years 1948–2003 are used to drive an ocean biogeochemical model to probe how changes in the heat and freshwater fluxes and in the winds affect the Southern Ocean’s uptake of carbon. Over this period,the model simulations herein show that the increases in heat and freshwater fluxes drive a net increase in Southern Ocean uptake (south of 40°S) while the increases in wind stresses drive a net decrease in uptake.The total Southern Ocean response is nearly identical with the simulation without climate change because the heat and freshwater flux response is approximately both equal and opposite to the wind stress response.It is also shown that any change in the Southern Ocean anthropogenic carbon uptake is always opposed by a much larger change in the natural carbon air–sea exchange. For the 1948–2003 period, the changes in thenatural carbon cycle dominate the Southern Ocean carbon uptake response to climate change. However, it is shown with a simple box model that when atmospheric CO$_2$ levels exceed the partial pressure of carbondioxide (pCO$_2$) of the upwelled Circumpolar Deep Water ($\approx$450 $\mu$atm) the Southern Ocean uptake response will be dominated by the changes in anthropogenic carbon uptake. Therefore, the suggestion that the Southern Ocean carbon uptake is a positive feedback to global warming is only a transient response that will change to a negative feedback in the near future if the present climate trend continues.Associated with the increased outgassing of carbon from the natural carbon cycle was a reduction in the aragonite saturation state of the high-latitude Southern Ocean (south of 60°S). In the simulation with justwind stress changes, the reduction in the high-latitude Southern Ocean aragonite saturation state ($\approx$0.2) was comparable to the magnitude of the decline in the aragonite saturation state over the last 4 decades because of rising atmospheric CO$_2$ levels ($\approx$0.2). The simulation showed that climate change could significantly impact aragonite saturation state in the Southern Ocean