Frictionally Induced Asymmetries in Wind-Driven Flows

Abstract

The effect of the parameterization of lateral friction on the separation of western boundary currents is addressed in an idealized context. The study is motivated by a puzzling issue that arises from the nonlinear theory of the wind-driven double-gyre circulation in shallow-water models. Subtle changes in the representation of the lateral friction in these models have a substantial effect on both steady-state and transient flows. The aim of this paper is to explain how lateral friction introduces a north?south asymmetry in the steady double-gyre flows and why the degree of this asymmetry depends on the type of frictional parameterization. A more conceptual model of a zonal jet in a channel turns out to be very useful to determine the dynamical processes behind the asymmetries. It is also shown that the north?south asymmetries have an impact on the low-frequency variability of the time-dependent flows. This is caused by changes in stability behavior of the steady-state flows.