Eddy-mean-flow interaction in a barotropic ocean model

The role of eddies in the maintenance of idealized, antisymmetrically forced double gyres is studied using a barotropic ocean circulation model. A diagnosis in terms of q , the quasi-conserved absolute vorticity , enables an understanding of the effect of the eddies on the time-mean flow. The unstable model Gulf Stream, partitioning counter-rotating gyres, is the confluence region where q contours from widely differing latitudes become concentrated, interwoven and irreversibly deformed. Here there is a strong enstrophy cascade, large eddy fluxes and eddy flux divergences, and significant driving of mean flow by the eddies. This barotropic instability of the internal jet results in a lateral transfer of vorticity sufficient to balance the net forcing of the subtropical gyre by the wind-stress curl. The separation of the eddy flux into non-divergent ‘advection’ and divergent ‘conversion’ contributions, using the eddy enstrophy equation as a dynamical reference, makes transparent the sense of the local eddy q flux and its systematic effect on time-mean flow.