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Description
The coupling of the upper mixed layer with the thermocline is investigated in a planetary geostrophic model. Assuming that submesoscale instabilities lead to a rectified effect of geostrophic eddies on the (late winter) mixed layer depth D, and that the mesoscale eddy field is stronger where the thermocline (depth G) is larger, a parameter (r) coupling D to G is identified. Inclusion of this parameter in the Pedlosky and Young (1983) model leads, given an idealised wind forcing, to two modes, each valid over both subtropical and subpolar gyres. One has a mixed layer of uniform depth, the other a mixed layer deepening westward with a maximum in the subpolar gyre. The latter mode is particularly interesting as it displays an ``overlapping'' between the surface and subsurface gyres (i.e., an expanded subpolar gyre at the surface and an expanded subtropical gyre at depth), cross-gyre exchange, asymmetry between subduction and obduction, as well as Eastern boundary jets. It is suggested that this mode is relevant to the structure of the North Pacific thermocline.
