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

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Author
Lopes da Costa, Carlos N.
Date
1989-09Advisor
Batteen, M.L.
Second Reader
Nelson, C.S.
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Show full item recordAbstract
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.
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