The Influence of Boundary Conditions on Midlatitude Jet Separation in Ocean Numerical Models

Abstract

The separation Point of a midlatitude jet from the western boundary in ocean numerical models depends upon both the governing equations and the vertical coordinate used. Systematic differences in the point of separation between level and layer models are shown. In level models, the separation usually occurs poleward of the zero wind-stress curl line, whereas, in layer models, it usually occurs equatorward. These differences are caused by two aspects of the numerical implementation. First, the wind forcing is usually assumed to act as a body force over the upper layer or level in the models, and this corresponds to a different physical assumption. Second, the free-slip boundary condition is imposed as zero vorticity in both models. This is an inconsistency because vorticity is not the same quantity when the governing equations are formulated in physical (level model) and isopycnal (layer model) coordinates. The effects on separation of these numerical implementation differences are illustrated using analytical solutions of linear models and numerical solutions of several nonlinear models.