Implied Nutrient Transport into the Southern Ocean in IPCC-AR4 Coupled Climate Models

The ventilation of Southern Ocean deep water regulates climate through carbon and heat uptake and transport, yet published Southern Ocean transport estimates from data differ widely. Simulations of the biogeochemical cycling of the Southern Ocean vary widely and are especially sensitive to the underlying physical circulation. We address this uncertainty by analyzing coupled climate model simulations of the late 20th century associated with the Intergovernmental Panel on Climate Change’s Fourth Assessment Report (IPCC-AR4) in terms of their volume, heat and nutrient transport across 30°S, into and out of the Southern Ocean. We quantify these transports in each of the observationally based layers of the water column and compare them to published inverse model analyses of hydrographic data. IPCC-AR4 coupled climate models do not directly simulate most nutrients (oxygen, nitrate, silica, etc.) so we calculate each model’s “implied transport” by mapping observed nutrient concentrations from the Global Ocean Data Analysis Project (GLODAP) onto model density and velocity fields. The strength and position of the sub-tropical gyres controls the accuracy and precision of the heat transport, but intermediate and deep ocean exchanges are most important for nutrient transports. This study has broad implications for future simulations of the biogeochemistry of the Southern Ocean. The potential impacts of a changing circulation on marine ecosystems will be addressed.