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.