Effects of Alongshore Wind Stress at the Eastern Boundary on Circulation in the Ocean Pycnocline

Abstract

An alongshore wind stress is applied at the eastern boundary of an ocean model, and its effects on the structure of the circulation in the ventilated pycnocline are studied. The offshore surface Ekman flow associated with such a wind is taken to be balanced by onshore geostrophic transport distributed over the surface and subsurface layers. Hence, the layer interfaces at the boundary slope downward and equatorward, giving nonzero thicknesses. Because of this, some geostrophic contours (lines of constant potential vorticity) emerge from the eastern wall and strike northward, while some strike southward, giving rise to northern and southern limbs of the eastern shadow zones described by Luyten et al. The position of the layer-interface outcrop that forms the northern boundary of the northern limb of a shadow zone may not be freely chosen, but is determined so as to be consistent with the eastern boundary conditions, whose constraints are transmitted along the geostrophic contours. The westward extent of the northern shadow zone is usually quite modest, O(100 km) for typical alongshore winds. For stronger alongshore winds, or weaker interior forcing, greater westward penetration is possible. The shadow zones furnish a means for sharply turning midocean currents and density outcrops to the south as they approach the eastern boundary, and forming alongshore boundary currents, which partly feed the wind-driven boundary upwelling and partly recirculate into the ocean interior. When the subsurface layer is allowed to carry part of the onshore transport at the eastern boundary, motion is permitted in what would otherwise be a stagnant shadow zone. The potential vorticity of this layer is determined at the eastern boundary and conserved along geostrophic contours even against the sense of the circulation?a form of upstream influence.