Modelado de alta resolución para el estudio de la respuesta oceánica al forzamiento del viento en el Estrecho de Gibraltar

Abstract

This research work makes an incremental step in understanding the role of the wind-stress and the associated dynamic ocean response of the barotropic and baroclinic exchange fluctuations by means of a wind-forced ocean model within the Strait of Gibraltar (south of Iberian Peninsula). The limited-area, non-hydrostatic atmospheric model MM5 was configured to study the mesoscale atmospheric processes in the area of interest and to provide realistic wind-stress forcing to the two-layer, two-dimensional, nonlinear, boundary-fitted coordinate hydrostatic model UCA2.5D at high-resolution, also forced by tidal motions. A statistical evaluation of the MM5 model performance is undertaken by means of pairwise comparison of observations and model output, showing good skills in reproducing the mesoscale flows over the complex Andalusian orography, sea-land breeze circulation and mountain-valley breezes. However, some discrepancies between observations and predictions have been found and their main causes have been discussed. Most importantly, comparison of model results with an extensive oceanographic data set collected in situ shows that the wind-forced ocean model provides better agreement with the observations than the ocean-alone model with tidal forcing. Results show the effect of winds on the dynamics in the Strait of Gibraltar with and without tidal forcing, pointing out the essential sensitivity of the area to different forcings. The hydrodynamic patterns induced by wind stress are successfully simulated showing that prevailing easterly (westerly) winds originate downwelling (upwelling) phenomena in the Spanish coast of the Strait of Gibraltar, which subsequently will induce an increase (reduction) of the upper layer thickness, deepening (rising up) the interface and rising (dropping) the sea level. During persistent and intense easterly (westerly) wind episodes, the accumulation of water in the northern (southern) part reduces (reinforces) the downwards south-to-north sea surface slope, which induces a geostrophic current westward (eastward). Under easterlies, the geostrophic current reinforces the semi-permanent westward countercurrent along the northern coast of the Strait, on the one hand, and reduces the intensity of the inflow, on the other, being able to reverse it. Westerlies increase the upper layer current intensity, which is almost twice as intense as that of the easterly experiment. Specifically, it is discussed the fluctuations at the interface due to wind forcing and its consequent impact on the location of the hydraulic controls, being able to activate extra supercritical regions (under westerly episodes) or to inhibit the already existing ones (under easterly episodes). Changes in the intensity of the barotropic flow and fluctuations of the interface depth, due to wind forcing, induce modifications of the hydraulic jump generation at the lee side of Camarinal Sill and of the internal bore speed. Estimations of Atlantic inflow and Mediterranean outflow noticeably improved when including wind-stress in the hydrodynamic model, particularly during the intense easterly wind events.