Water mass exchange in the Gulf of Cadiz

Abstract

Velocities inferred from several closed hydrographic sections in the Gulf of Cadiz are used to estimate the volume transport and the heat and freshwater fluxes associated with the exchange of Atlantic and Mediterranean waters through the Strait of Gibraltar. Five different sets of constraints are imposed to obtain absolute flow fields from the baroclinic shear determined by the thermal wind equation. The resultant transports and fluxes through a given section are similar for all five cases. Transports and fluxes from sections near the strait underestimate the actual exchange. This is attributed to ageostrophic effects associated with the gravity-driven downslope flow that is characteristic of the deep Mediterranean outflow near the strait. West of 7°W, the estimated fluxes are larger, comparable to those deduced from climatological observations over the Mediterranean. In this region, the outflow appears to have no significant downslope component, so that transports calculated from closed sections are independent of the orientation relative to the direction of the outflow.Exchange between the Atlantic and Mediterranean can be characterized in several ways. A common measure used is volume transport, usually that of the outflow, observed somewhere near the sill. This is not an adequate definition of exchange, as transport may be altered by recirculation or entrainment without changing the net flux of properties. Thus, volume transport is a function of position; fluxes of freshwater, salt and heat, on the other hand, should be independent of position in the absence of local sources and sinks. In these observations, the total volume transport exchanged through each section increases from 1 Sv at 6°30'W to nearly 7 Sv at 8°W. The part of the total exchange that does not recirculate along density surfaces increases from 0.5 Sv near the strait to 2.2 Sv through sections at 7°30'W and 8°W. Part of this increase is due to entrainment, and part to the ageostrophic character of the outflow near the strait. Freshwater flux is estimated to be the equivalent of 0.53 m y I of evaporation excess over the Mediterranean. Heat flux was higher in the autumn of 1986 than in the spring (6.0 W m-2 vs 2.2 W m-2) , due to warmer surface temperatures in the autumn. For comparison with earlier estimates, the observed outflow transport may be converted to an equivalent transport of "pure" Mediterranean water of 0.7 Sv, assuming salinities of 38.4 and 35.6 for Mediterranean and Atlantic waters. This implicit flux, while useful for comparisons with observations where true fluxes cannot be estimated, involves several approximations, and should not, in general, be used to characterize the exchange if fluxes can be calculated directly.