Internal Tides and Currents in the Gibraltar and Bab-el-Mandeb Straits

E. G. Morozov

We compare moored measurements of currents and temperature in the Gibraltar and Bab-el-Mandeb straits. In both straits the vertical displacements caused by semidiurnal internal waves are extremely high. Both straits are characterized by permanent strong two-layer currents of opposing directions. The hydrological regimes of the Gibraltar and Bab-el-Mandeb straits are very similar. Each strait connects an inland sea with the ocean. Strong evaporation exists in both seas and the runoff of fresh water by rivers is small in both seas. Evaporation is compensated by the surface flow of water from the ocean. Due to evaporation in the seas the saltier and dense water descends to the deep layers of the seas and then flows to the ocean as a bottom current of the opposite direction to the surface flow. A barotropic tidal wave is superimposed on this current system and generates a tidal internal wave during the flow of the currents over the sills in the straits. Internal tides near the sills are characterized by the amplitudes of vertical displacements that reach 160—200 m. Internal tides propagate to both sides of the sills rapidly losing their energy. Internal tide is generated over the background of two-layer shear current, which intensifies the amplitude of internal waves. The amplitudes of internal tides propagating opposite to the mean flow intensify due to the decrease in the wave length.

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