Observations of a poleward surface current off the coasts of Portugal and Spain during winter

Abstract

Evidence of a warm, salty surface current flowing poleward along the Iberian Peninsula is presented using a sequence of satellite infrared images and concomitant in situ hydrographic data obtained during the winter of 1983–1984. The current, which flows over 1500 km along the upper continental slope‐shelf break zone off western Portugal, northwest and northern Spain, and southwest France, is 25–40 km wide, about 200 m deep, and characterized by velocities of 0.2–0.3 m s−1. According to the hydrographic data acquired during late November and early December 1983, the current's salinity signal off Portugal is about 0.2 practical salinity units, and its waters are ∼0.5°C warmer than the surrounding ones. The satellite observations, however, which span a longer time period and cover a much larger area, indicate that the current's typical thermal signature is 1°–1.5°C. The current's associated geostrophic volume transports show an increase from about 300×10 3 m 3 s−1 near 38°3′N to 500–700×10 3 m 3 s−1 at 41°–42°N. The origin of this poleward flow and the causes for its increasing transport off western Iberia are investigated. Onshore Ekman convergence induced by southerly winds along the Portuguese west coast provides about one fifth of the computed transports in the correct direction. A mechanism giving better quantitative agreement with the observations is the geostrophic adjustment of the eastward oceanic flow driven by the large‐scale meridional baroclinic pressure gradient in the eastern North Atlantic as the flow reaches the continental slope of the western Iberian Peninsula. Topographic trapping by the bathymetric step existing along the shelf break explains both the width and the path of the observed current. The role of “dam break” type mechanisms is discarded owing to strong discrepancies between the available models and the present observations. Since satellite images reveal that similar situations occurred during many winters, the flow identified here appears as a characteristic feature of the winter circulation off southwest Europe. Furthermore, the occurrence of analogous poleward flows in eastern boundary layers of the subtropical and mid‐latitude oceans suggests that these currents are typical features of those regions' winter circulation.