Parameterization of tracer transport by geostrophic eddies in the southern ocean

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Abstract Eddy diffusion is the actual mixing caused by turbulent processes associated with ocean eddies, which vary from transient microscales (cm) to stable features (hundreds of km) such as subtropical gyres or even hemispheric scale currents. The eddy component of the ocean circulation, clearly a first order mixing term, is not directly driven by the wind stress forcings that climate models typically project. A problem arises - how can the lateral mixing produced by eddies be parameterized? An approach to improving a widely used coarse resolution ocean climate model (MITgcm) of the Southern Ocean using a residual-mean formulation in which eddy potential vorticity fluxes appear as forcing terms in the residual-mean momentum equations, will be investigated. Prior application of residual-mean theory has suggested that values of the eddy diffusivity (K) is enhanced in the vicinity of critical levels where eddies and mean flows move at the same speed. Some scientific problems that the resulting model will be used to better understand and more reliably predict are: (i) the response of the Southern Ocean and its meridional overturning circulation to changes in prevailing winds and in the larger sense changing climate(ii) associated changes in the air-sea flux of heat, CO2, biogeochemical fluxes, including the ability of the Southern Ocean to enable the deep storage of anthropogenic carbon (iii) how ocean tracer transports depend on the underlying physics, dynamics and biogeochemistry of the Southern Ocean.