Wind‐driven upwelling in the vicinity of Cape Finisterre, Spain

Abstract

Observations and numerical simulations of upwelling along the Galician coast of Spain during April 1982 are presented. In situ measurements include shipboard determinations of hydrographic and biological parameters from a grid of stations covering the continental shelf from Cape Finisterre to Ria de Vigo, sea level data from Vigo and La Corufia, and wind stress estimates derived from the ship winds and from surface pressure charts. Sea surface temperature information and pigment concentration information have been obtained from a sequence of satellite images from the NOAA 7 advanced very high resolution radiometer and the Nimbus 7 coastal zone color scanner, respectively. Since the Cape Finisterre sector of the Iberian peninsula is characterized by an abrupt change in coastline orientation, wind-driven upwelling can occur in that region over a 270 ø range of wind direction. These data document the evolution of upwelling and the resultant coastal circulation in response to two wind events that occurred over a 10-day period. Salient features of the circulation include a southward coastal jet and a northward flow further offshore along the western coast. Numerical simulations of the coastal currents, the vertical excursion of a density interface from a static equilibrium position, and coastal sea level are conducted using a wind patch characterized by constant direction and negative curl. The simulations show that during these wind events, the greatest upwelling will occur either at Cape Finisterre or along the northern coast as was observed in this case and as has been reported by others. It is suggested on the basis of the analysis of the sea level records and on the numerical simulations that wave disturbances propagate northward along the coast at a speed of 120-160 km/day. Finally, it is speculated that much of the organic material formed during upwelling events north of the Cape Finisterre is advected out to sea northwest of the cape. INTRODUCTION The western coast of the Iberian peninsula is the site of seasonal wind-driven upwelling from March through October [Wooster et al., 1976]. This upwelling is part of a general system that extends southward to about 15øN. Along the Galician coast of Spain (Figure 1), the upwelling is most intense from April to August [Fraga, 1981] with the region between capes Finisterre and Ortegal exhibiting the coldest surface temperatures. During the remainder of the year, the winds are predominantly from the south, or downwelling favorable. The irregular configuration of the Galician coastline and the general circulation of the offshore water masses add complexity to the response of the coastal waters to the wind. For instance, south of Cape Finisterre are five embayments, or rias, which cause enhancement of the upwelling through current-bathymetry interactions [Blanton et al., 1984]. Also, the coastline abruptly ends its north-south orientation at Cape Finisterre and continues in an east-west direction east of Cape Ortegal. Thus upwelling favorable winds span a 270 ø range in di...

Mesoscale distribution patterns of diatoms in surface sediments as tracers of coastal upwelling of the Galician Shelf (NW Iberian Peninsula)

“…The effect of winds on upwelling differs between the shelf sectors. North winds intensify upwelling in the western shelf (south of Cape Finisterre) while southeast winds do so on the northern shelf (north of Cape Finisterre) (McClain et al, 1986). It has also been considered that south of Cape Finisterre upwelling is more intense and closer to the coast while in the north it is discontinuous and keeps close to the margin (Prego and Bao, 1997).…”

“…The upwelling system off the Galician coast is associated with the north Atlantic anticyclonic gyre (Wooster et al, 1976). The general hydrographical effects of upwelling off the Galician coast have been studied during the last two decades (Fraga, 1981;Fraga et al, 1982;Blanton et al, 1984;McClain et al, 1986;Tenore et al, 1995) but a detailed mesoscale description of upwelling patterns in the Galician coast has still not been considered until recently (Prego and Bao, 1997). Using the hydrodynamical and biological characteristics of upwelling, the Galician shelf waters can hypothetically be divided into two contrasting sectors south (western shelf) and north (northern shelf) of Cape Finisterre (Estrada, 1984;Tenore et al, 1995;Prego and Bao, 1997).…”

Vertical biogenic particle flux in a western Galician ria (NW Iberian Peninsula)

“…In the 4 western Galician rias, (Rías Baixas), upwelling events are common from April to October, with a stronger intensity during summer (McClain et al 1986, Blanton et al 1987, Tilstone et al 1994). This phenomenon is commonly attributed to the action of the northerly winds on the shelf, which produce, by Coriolis effect, an Ekman transport of offshore surface water, which is then replaced by deeper, colder, and nutrient-rich water (Wooster et al 1976).…”

Ubiquitous imposex and organotin bioaccumulation in gastropods Nucella lapillus from Galicia (NW Spain):a possible effect of nearshore shipping

“…Prego & Bao 1997). This upwelling interacts with the irregular littoral configuration, the shifting wind fields and the varying estuarine runoff to result in a complex oceanic circulation pattern over the narrow continental shelf (McClain et al 1986, Tenore et al 1995. As a result of it all, nearshore waters are thoroughly mixed (R. Prego pers, comm.).…”