A numerical study of wind forcing in the eastern boundary current system off Portugal

Abstract

A high resolution, multi-level, primitive equation ocean model is used to examine the response to wind forcing of an idealized, flat-bottomed oceanic regime on a ß-plane, along the eastern ocean boundary off the west coast of the Iberian Peninsula. Three experiments are conducted to investigate the role of different types of prescribed wind forcing. In experiment 1, a band of equatorward winds which are uniform alongshore but contain zonal variability (i.e., large negative wind stress curl near the coast), results in an equatorward coastal surface current nearshore and a poleward surface current offshore. With time, the currents become unstable and anticyclonic warm core eddies develop in the region of negative wind stress curl. In experiment 2, the model is forced with poleward wind stress for 20 days followed by a sudden change to equatorward wind stress. A 1-2 day transition from downwelling and a poleward surface current near the coast to upwelling and an equatorward surface jet occurs. With time the upwelled isotherms are displaced offshore, the equatorward surface jet widens and gets shallower nearshore, and the poleward undercurrent moves closer to the surface (depth ~ 120 m at the coast). For experiment 3, a time series of 6-hourly values of the north/south component of the wind, computed from Fleet Numerical Oceanography Center synoptic surface pressure analyses are used to investigate the response of the coastal current system to time dependent wind forcing. Equatorward wind events increase the vertical extent and cross-section of the surface equatorward jet and reduce the poleward undercurrent. Relaxations reduce the vertical extent of the equatorward surface coastal current, increase the cross-sectional area of the poleward undercurrent, and the undercurrent tends to shoal nearshore. Poleward winds displace the equatorward current offshore and create a poleward surface current nearshore. By the middle of the upwelling season, eddies are generated. By the end of the upwelling season (with the seasonal reversal to poleward winds), the eddies and equatorward surface current weaken or disappear and a poleward surface current develops nearshore. The results from the experiments support the hypothesis that wind forcing, and particularly time dependent winds, can be a significant generation mechanism for eddies, jets, fronts and upwelling filaments off the west coast of the Iberian Peninsula.