TIDES IN THE STRAIT OF GIBRALTAR

The Strait of Gibraltar is where the transition occurs between two distinct tidal regimes; the North Atlantic, where tidal ranges are in excess of 2 m, and the western Mediterranean, where tidal ranges are less than 1 m. Within the strait the tide is principally semidiurnal: observations indicate that on average, 96% of the bottom pressure and 74% of the current variability are contained in the semidiurnal band (2 cpd). The structure of the local cotidal chart of the M2 tidal constituent is complicated but can be explained in terms of the along- and across-strait momentum balances. Along the strait, the pressure gradient is mainly balanced by the acceleration of the flow, although friction can be of appreciable magnitude, corresponding to a decay time scale of 3-16 hours. Across the strait the momentum balance appears mostly geostrophic, but better observations would be required to explain residual terms. Available observations indicate that the M2 tide has no net energy flux through the strait. At the sill the tidal transport in the upper layer exceeds that in the lower layer by nearly a factor of 2. At the eastern end of the strait, between Algeciras and Ceuta, the tidal transport is larger in the lower layer. This apparent shift in the location of maximum tidal transport is explained in terms of the kinematics of the interface between Atlantic and Mediterranean waters. A mean transport of 0.21 Sv, due to subinertial and tidal flows, carries Atlantic water through the strait, compensated by an approximately equal return in the lower layer. © 1990 by the American Geophysical Union.