The thermohaline forcing of the Gibraltar exchange

A conceptual framework for understanding the exchange through Gibraltar and its thermohaline forcing is presented. The Mediterranean Sea annually prod uces a dense water mass that sinks and accumulates above the level of the s ill until the internal pressure gradient generated through the Strait is su fficiently strong to force it out at a rate equal to the rate of its mean i nterannual production, The dense water forced out creates a sea-level drop through the Strait that drives a compensatory inflow of surface Atlantic wa ter. The two-way exchange can be calculated geostrophically by requiring th at the baroclinic outflow equal the opposing barotropic inflow plus the net water balance of the Basin. Bottom friction acts as a retarding force for the outflow and reduces the geostrophic flow by roughly a half. The exchang e was calculated from the steric heights derived from a series of historica l hydrographic transects across the western Alboran Sea and the eastern Gul f of Cadiz. Bottom Ekman frictional parameters were estimated from the curr ent-meter data of the Gibraltar Experiment. The mean outflow determined fro m these data was similar to 0.84 +/- 0.3 Sv. It is shown that time-dependen t fluctuations of the sea level can generate an additional, net mass exchan ge through a 'barotropic pumping' mechanism that increases the outflow by s imilar to 50% to 1.26 Sv. This fluctuating flow component is susceptible to hydraulic control during the percentage of the time that the combined outf low (or inflow) achieves a supercritical state. This combined outflow sugge sts an interannual mean value of similar to 96 cm/yr for the internal water balance the annual value of which has little direct effect on the exchange due to the similar to 9-year e-folding time for draining the reservoir of dense water accumulated to similar to 180 m above the depth of the sill. Th is relatively stable accumulation of dense water provides the steady force for the exchange from seasonal to interannual time scales. However, signifi cant variability in the exchange on weekly to seasonal time scales exists o wing to the variability in the Basin's internal circulations, that supply t he dense water to and evacuate the Atlantic water from the western Alboran, together with the variability in the sea-level fluctuations that drive bar otropic-pumping exchange. Ln addition, variations in the amplitude of the e xchange are damped by negative feedback loops that exist due to the interde pendency between the exchange and the force generating it. This interpretat ion of an exchange buffered from the variability in its meteorological forc ing and responsive to the variability in local potential energy suggests th at any objective to detect a response to climatic trends in the Strait of G ibraltar should be coordinated with observations of the sea level, internal potential energy, water-mass characteristics, and air-sea interaction both locally and within the Basin and its sub-basins. (C) 1999 Elsevier Science B.V. All rights reserved.