The import & export of carbon & nutrients from the Weddell Gyre.

The physical circulation and biogeochemistry of the Southern Ocean has proved crucial to understanding the sensitivity of global climate. The ventilation of deep water, rich in carbon and nutrients throughout the subpolar Southern Ocean is usually framed in terms of the residual overturning. This places the emphasis on the up- and down-welling of different water masses. However, for the Weddell Gyre it has been proposed that casting the carbon cycle in terms of the horizontal gyre circulation may be more informative (MacGilchrist et al., 2019). This emphasises the role of remineralisation at mid-depth of organic carbon and the differential transport in/out of the Weddell Sea in the longitudinal direction.

 

Using MITgcm as an idealised two-basin model with a Weddell Sea at the southern boundary of the Atlantic basin, we examine the physical controls of the import/export of carbon & nutrients from the Weddell Sea. The idealised nature of the model allows us to easily change the surface forcing and bathymetry. By perturbing the idealised model's Scotia Ridge and Weddell Sea wind stress curl, we are able to influence the connection between the Weddell Gyre and the rest of the Southern Ocean. Other perturbation experiments, including the diapycnal diffusivity at depth, are used to perturb the overturning circulation. Using simple biogeochemistry and a carbon pump decomposition we are able to see how individual reservoirs are altered and the role of their transport in the overall carbon budget of the Weddell Sea. In particular, we are able to use Reynolds averaging to split the import/export of carbon & nutrients into the Weddell Gyre into components due to overturning and gyre circulations. Our experiments allow us to consider the physical aspects that control the relative strength of these components.