The thermohaline forcing of the Gibraltar exchange

Abstract

A conceptual framework for understanding the exchange through Gibraltar and its thermohaline forcing is presented. The Mediterranean Sea annually produces a dense water mass that sinks and accumulates above the level of the sill until the internal pressure gradient generated through the Strait is sufficiently strong to force it out at a rate equal to the rate of its mean interannual production. The dense water forced out creates a sea-level drop through the Strait that drives a compensatory inflow of surface Atlantic water. The two-way exchange can be calculated geostrophically by requiring that 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 exchange was calculated from the steric heights derived from a series of historical hydrographic transects across the western Alboran Sea and the eastern Gulf of Cadiz. Bottom Ekman frictional parameters were estimated from the current-meter data of the Gibraltar Experiment. The mean outflow determined from these data was ∼0.84±0.3 Sv. It is shown that time-dependent fluctuations of the sea level can generate an additional, net mass exchange through a `barotropic pumping' mechanism that increases the outflow by ∼50% to 1.26 Sv. This fluctuating flow component is susceptible to hydraulic control during the percentage of the time that the combined outflow (or inflow) achieves a supercritical state. This combined outflow suggests an interannual mean value of ∼96 cm/yr for the internal water balance the annual value of which has little direct effect on the exchange due to the ∼9-year e-folding time for draining the reservoir of dense water accumulated to ∼180 m above the depth of the sill. This relatively stable accumulation of dense water provides the steady force for the exchange from seasonal to interannual time scales. However, significant variability in the exchange on weekly to seasonal time scales exists owing to the variability in the Basin's internal circulations, that supply the dense water to and evacuate the Atlantic water from the western Alboran, together with the variability in the sea-level fluctuations that drive barotropic-pumping exchange. In addition, variations in the amplitude of the exchange are damped by negative feedback loops that exist due to the interdependency between the exchange and the force generating it. This interpretation of an exchange buffered from the variability in its meteorological forcing and responsive to the variability in local potential energy suggests that any objective to detect a response to climatic trends in the Strait of Gibraltar 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.