April 23, 2007: Thermodynamic analysis of ocean circulation

Jonas Nycander, Department of Meteorology, Stockholm University, Sweden

Calculating a stream function as function of depth and density is proposed as a new way of analysing the thermodynamic character of the overturning circulation in the global ocean. The sign of an overturning cell in this stream function directly shows whether it is driven mechanically by large-scale wind stress, or ''thermally'' by heat conduction and small scale mixing. It is also shown that the integral of this stream function gives the thermodynamic work performed by the fluid. The analysis is also valid for the Boussinesq equations, although the thermodynamic work formally vanishes for an incompressible fluid. The proposed method is applied both to an idealized coarse-resolution three-dimensional numerical ocean model, and to the realistic high-resolution OCCAM model. It is shown that the overturning circulation in OCCAM between 200 m and 1000 m depth is dominated by a thermally indirect cell of 24 Sv, forced by Ekman pumping. In the densenst and deepest waters there is a thermally direct cell of 18 Sv, which requires a forcing by around 100 GW of parameterized small-scale mixing.