Barotropic eulerian residual circulation in the Gulf of California due to the M2 tide and wind stress

Abstract

A vertically integrated, non-linear numerical model in finite differences is used to analyze two forcing mechanisms of the mean barotropic circulation in the Gulf of California: topographic rectification due to tidal currents (M₂) and wind stress. Under tidal forcing the nonlinearities of the momentum equations induce unorganized strong tidal induced residual currents (u_e > 5 cm s-1) in the channels between the islands, and along-isobath anticyclonic circulation in the Northern Gulf, with speeds U_e < 2.5 cm s^-1 over the edge of Delfín Basin. These numerical results are in agreement with analytical results, which indicate that the tidal-induced currents are mostly due to the advective terms, and that continuity and the Coriolis term (but regulated by bottom friction) are responsible for the along-isobath flow. The quadratic bottom friction plays a role in generating mean currents only in the very shallow area off the Colorado River Delta. The effect of wind stress was modeled by imposing upon the running M₂ model a constant surface stress (r = 0.016 Pa), from the NW for winter conditions and from the SE for summer conditions. The wind-induced circulation was obtained by averaging over a tidal cycle and then subtracting the tidal residuals. The two wind directions produce almost identical circulation patterns, but with opposite directions. For the NW wind stress, the main features of the predicted circulation are: (a) In the Northern Gulf an anticyclonic circulation pattern, with the strongest currents (up to ~ 10 cm s^-1) following the bathymetry of the rim of Delfín Basin, Wagner Basin and the mainland coast off Bahía Adair and Bahía San Jorge. There is also a southward flow along the peninsula coast, from the Colorado River to Bahía San Luis Gonzaga. (b) In the Southern Gulf, there is a strong flow (~ 10 to 15 cm s^-1) to the SE over the continental shelf along the mainland coast. A somewhat less well-defined return flow (~ 5 cm s^-1) from Santa Rosalía to Isla San Lorenzo complete an anticyclonic circulation in the area immediately to the SE of the islands. Some of the predicted features of the wind-driven flow in the Northern Gulf of California are in qualitative agreement with current meter observations, but the model underestimates them which may be attributable to baroclinity, not included in the model.