A forcing mechanism for the poleward flow off the southern California coast

It is shown that when the wind distribution along a coast is anisotropic, such that its cross-shore scale is smaller than its alongshore scale, the coastal sea level (or the upper layer anamoly) “A” of an ocean forced by both wind and wind curl is governed by a modified (nondimensionalized) Kelvin wave equation: ­A/­t* 1 ­A/­ y* 5 k0(0, y*, t*) 1 * ­k1/­ y* dtL where k0 and k1 are wind stress and wind stress curl at the coast, respectively, y is the alongshore distance, and t is the time. Numerical experiments, from a simple reduced-gravity type with idealized forcing and coastline to a three-dimensional primitive equation model with a realistic coastline, bottom topography of the Southern California Bight and the Santa Barbara Channel, and observed wind stresses, were carried out to show that the observed near-coast near-surface poleward flow in the region is primarily forced by the equatorward weakening of the wind curl, (­k1/­ y* .0), in the bight. Beta provides natural damping by weakening and widening the current through westward propagating Rossby waves and causes the current to lead the coastal pressure field by 1–2 months, which improves the agreement with observations of the phasing of the modeled currents but is otherwise not required in forcing the poleward flow.