A Three-Layer Model for Wind-Driven Circulation in a Subtropical–Subpolar Basin. Part I: Model Formulation and the Subcritical State

Abstract

A three-layer model is formulated and run numerically in order to understand the highly nonlinear structure of the wind-driven circulation in a basin including both a subtropical and a subpolar gyre. The third layer is assumed to be infinitely deep and motionless. We emphasize the strong nonlinearity associated with large layer thickness change. A broad range of parameters has been tested to explore possible flow patterns. For cases of weak wind forcing and large amounts of water in the upper layer, the upper layer covers the whole basin. The second layer is stagnant, except near the western boundary. For stronger forcing, closed potential vorticity isolines appear within the second layer and there is a strong nonlinear coupling between the first and second layers. The numerical results show some flow patterns highly asymmetric with respect to the zero wind-curl line. The potential vorticity distribution in the second layer shows a strong competition between potential vorticity advection, interfacial friction, and along-isopycnal mixing. With only two moving layers, the model produces some structures which are very similar to the results from the primitive equation model developed at the Geophysical Fluid Dynamics Laboratory and to observations.