On the Momentum, Vorticity and Mass Balance on the Oregon Shelf

Abstract

Velocity measurements from the continental shelf off Oregon taken during the Coastal Upwelling Experiment CUE-2 in the summer of 1973 are utilized to investigate momentum, vorticity and mass balance relationships for subinertial frequency (? < 0.6 cpd) current fluctuations. Measurements from stations in water of depths of 54, 100 and 200 m are utilized. By a comparison of the magnitude of terms involving horizontal velocities in the linear momentum and in the nonlinear, depth-integrated momentum equations, support is found for the linear geostrophic balance of the alongshore velocity in the onshore-offshore momentum equation and, in the depth range 100 m ≤ H ≤ 200 m, for a linear ageostrophic balance in the alongshore momentum equation. Evidence is also found to support the validity of a linear depth-integrated vorticity balance, again for depths 100 m ≤ H ≤ 200 m. In this balance, which is similar to that in the theory for continental shelf waves, the interaction of the onshore velocity with the onshore-offshore bottom slope of the continental shelf forms the primary vortex stretching mechanism. The mass balance equation from idealized two-dimensional coastal upwelling models, wherein the depth integral of the interior, inviscid onshore velocity U equals the offshore Ekman layer transport ?τ/?0F, where τ is the alongshore component of the wind stress, is investigated by comparing the time-dependent behavior of U and τ/?0f. It is found that the correlation of U and τ/rho;0f is of the proper sign to support this relation and that, in general, the magnitudes of these two terms are similar, but that the correlation is not especially high, presumably due to three-dimensional effects.