LAUSR

Abstract This paper examines quasigeostrophic flows in an ocean that can be subdivided into an upper active layer (AL) and a lower passive layer (PL), with the flow and density stratification mainly confined to the former. Under this assumption, an asymptotic model is derived parameterizing the effect of the PL on the AL. The model depends only on the PL’s depth, whereas its Vaisala–Brunt frequency turns out to be unimportant (as long as it is small). Under an additional assumption—that the potential vorticity field in the PL is well-diffused and, thus, uniform—the derived model reduces to a simple boundary condition. This condition is to be applied at the AL/PL interface, after which the PL can be excluded from consideration.