Residual Flow and Mixing in the Large Islands Region of the Central Gulf of California

Abstract

A three-dimensional numerical model is used to describe mixing and tidal and residual flow in the archipelago of the central Gulf of California. The model is forced at the gulf’s opening by prescribing the sea surface elevation (tidal, annual and semiannual) and the climatological (annual and semiannual) fields of temperature and salinity on the cross-section at the mouth. At the sea surface, forcing is with the climatological annual variations of the wind and of the heat and freshwater fluxes. The tidal currents dominate the instantaneous circulation, which floods and ebbs twice a day with speeds as large as 60 cm s−1; the currents present a strong fortnightly modulation, with springs tidal currents being twice than those during neaps. The tidal flow over bottom features (like sills) generate internal tidal currents, which although less energetic than the barotropic currents, imprint their spatial variability on the total current field. Strong mixing is caused by the tidal flow, both by friction against the bottom and in the interior of the fluid. Mixing is tidally modulated, at the diurnal, semidiurnal and fortnightly frequencies. The strongest mixing occurs over and close to the sills, even during neap tides. The residual circulation in the upper layers reverses sign along the year and is rich in gyres over basins and sills. Over San Pedro Basin, a permanent anticyclonic gyre is produced. In the San Esteban and San Lorenzo sills the near-bottom current is always up-gulf, which causes a vertical structure that consists of two layers in winter and three layers in summer. The latter consists of a surface layer that flows into the northern gulf, a middle out-flowing layer, and the permanent near-bottom inflow. The residual currents are fortnightly modulated, with faster currents in springs than in neaps. The dynamics of the currents are such that in the transversal direction the dominant balance is geostrophic with some contribution of the advective terms, while in the along gulf direction allthe terms are equally important, reflecting the non-linear and diffusive character of the area.