A two-layer primitive equation model of an idealized Strait of Gibraltar connected to an eastern basin

We describe the solutions of a numerical two-layer primitive equation model of an idealized Strait of Gibraltar and an adjacent eastward basin. The quasi-steady circulation pattern in the basin features an anticyclonic gyre southward of the strait exit and an eastward boundary current attached to the southern boundary of the basin. A variation of the initial upper-layer depth and the target interface height at the eastern boundary causes minor changes of the upper-layer circulation in a basin with rectangular coastline and larger changes when a cape is added to the coastline of the southern boundary.     

Filament generation off the Strait of Gibraltar in response to Gap winds

We present a case study of the generation of a cold filament rooted off the southwestern edge of the Strait of Gibraltar (Atlantic side) during the summer of 2000. The event is successfully simulated using high-resolution atmospheric and oceanic numerical models. It is shown that a sharp filament may develop oceanwards with little modification of the Atlantic inflow into the Mediterranean, contrary to usual expectations. The filament is essentially driven by the surface layer response to Gap winds occurring during Levanter conditions. The easterly wind funnelling in the Strait generates a strong wind jet and intense wind curl which impacts the oceanic surface layer through Ekman pumping and mixing processes. The generation and fate of the filament is very similar to the Gulf of Tehuantepec case, where strong Gap wind events produce asymmetric deformation and erosion of the thermocline that tends to favour anticyclonic mesoscale circulations. Our observations and model results from both realistic and idealized experiments suggest that similar phenomena are present in the Gulf of Cadiz, but they are altered by the persisting Atlantic inflow, so that the response to Gap winds is not as dramatic.     

The Characteristics of Near-surface Velocity During the Upwelling Season on the Northern Portugal Shelf

Observations made on the northern Portugal mid-shelf between May 13 and June 15,2002 were used to characterise the near-surface velocity during one upwelling season. It was found that in the surface mixed layer,the 'tidal current' was diurnal,but the tidal elevation was semi-diurnal. Both the residual current and the major axes of all tidal constituents were nearly perpendicular to the isobaths and the tidal current ellipses rotated clockwise;the major axis of the major tidal ellipse was about 3 cm s-1. The extremely strong diurnal current in the surface layer was probably due to diurnal heating,cooling,and wind mixing that induced diurnal oscillations,including the diurnal oscillation of wind stress. This is a case different from the results measured in the other layers in this area. The near-inertial spectral peaks occurred with periods ranging from 1 047 min to 1 170 min,the longest periods being observed in deeper layers,and the shortest in the surface layer. Weak inertial events appeared during strong upwelling events,while strong inertial events appeared during downwelling or weak subinertial events. The near-inertial currents were out of phase between 5 m and 35 m layers for almost the entire measurement period,but such relationship was very weak during periods of irregular weak wind. Strong persistent southerly wind blew from May 12 to 17 and forced a significant water transport onshore and established a strong barotropic poleward jet with a surface speed exceeding 20 cm s-1. The subinertial current was related to wind variation,especially in the middle layers of 15 m and 35 m,the maximum correlation between alongshore current and alongshore wind was about 0.5 at the 5 m layer and 0.8 at the 35 m layer. The alongshore current reacted more rapidly than the cross-shore current. The strongest correlation was found at a time lag of 20 h in the upper layer and of 30 h in the deeper layer. The wind-driven surface velocity obtained from the PWP model had maximum amplitude of about 7 cm s-1,corresponding to a wind stress at 0.1 Pa,and the horizontal velocity shear due to thermal wind balance had the order of 3 cm s-1. So the local wind and thermal wind would only explain a part of the strong surface velocity variations.     

The upper mixed layer during coastal upwelling events on the northern portugal shelf

The upper mixed layer (UML) depth obtained from temperature is very close to that from density:the maximum is about 15m. This indicates that temperature is a good indicator of mixed layer during measurements. When the surface heat flux is balanced by a cross-shore heat flux, the surface mixed layer depth obtained from the WM model (Weatherly and Martin, 1978),hPRT, is roughly the same as observed. The mixed layer depth calculated from the PWP model (Price, Weller and Pinkel, 1986) is close to the depth obtained from thermistor chain temperature data. The results show that both the WM model and PWP model can provide a good estimate of stratification in the study area during the cruise. The value of log( h/u3) is about 9.5 in the study area, which shows that the study area is strongly stratified in summer. Observations on the northern Portugal shelf reveal high variability in stability, giving rise to semi-diurnal, semi-monthly and diurnal oscillations, and long term variations. The fortnightly oscillatio