Generation and Kinematics of the Internal Tide in the Strait of Gibraltar

Abstract

Observations from moorings and hydrographic surveys during the Gibraltar Experiment are used to describe the structure in space and time of the internal tide in the strait. By internal tide is meant those fluctuations of the interface (separating inflowing Atlantic from outflowing Mediterranean waters) that have a persistent and demonstrable phase relationship to the barotropic tide. Two modes of fluctuation dominate the variance at semi-diurnal frequency: rising and falling of the interface throughout the strait, which explains about half of the total variance, and fluctuations in the cross-strait slope of the interface, which explains about 25% of the variance. The largest amplitude fluctuations are found at the Camarinal sill. The interface rises and falls approximately in quadrature with the barotropic current, and a kinematic argument for why that should occur is presented. The cross-strait slope of the interface changes approximately in phase with the barotropic tidal currents, corresponding to changes in the vertical shear, in what may be an incomplete geostrophic adjustment in the cross-strait direction.