Quasi-geostrophic oceanic adjustment in the presence of mean currents

The effect of mean currents on the adjustment of the ocean to a change in the winds is studied using a quasigeostrophic shallow water model. In an inviscid ocean the eigenmodes that affect the oceanic adjustment fall into two groups: a finite discrete number of Rossby waves with speeds greater than that of the mean flow, and a continuum of modes that have critical layers where the wave speed equals that of the mean current. In the case of a baroclinic mean current there exists large-scale latitudinal modes that propagate at the speed of long non-dispersive Rossby waves. Because of their presence, the adjustment in the presence of mean baroclinic currents is very similar to the adjustment in the absence of any currents provided that the forcing has large latitudinal scale. If the continuum of modes with critical layers is important in the adjustment-this is the case if the mean current is barotropic-then the adjustment occurs at approximately the Doppler-shifted Rossby wave speed.