Observations on the characteristics of tides in the Bab el Mandab Strait

摘要

While well known for its multilayered exchange flow, the Bab el Mandab Strait is also of interest as a transitional region between two distinctly different tidal regimes: the Gulf of Aden, where tides are mixed and have a range in excess of 2 m, and the Red Sea, where tidal fluctuations are principally semidiurnal and their range is generally less than 1 m. Observations collected in the strait indicate that dominant constituents for both elevations and currents are the K 1 and M 2. The vertical structure of the tidal currents differs between semidiurnal and diurnal constituents and depends on the location and stratification. The major part of the tidal signal is barotropic, but energetic baroclinic currents mainly of the diurnal period are observed near Perim Narrows and the Hanish Sill during winter months. The relationship between the barotropic currents and surface tides near Perim is fairly well explained by a simple momentum balance. To first order, a balance exists between local acceleration and the elevation gradient in the along-strait direction, while in the cross-strait direction, a balance between local acceleration, Coriolis term, and elevation gradient is observed. At the same location the barotropic transport fluctuations reach amplitudes of 1.6 Sv. The K 1 and M 2 barotropic energy fluxes are small and are directed into the strait at Perim Narrows and the Hanish Sill for the M 2 constituent and into the strait near Perim and out of the strait near Hanish for the K 1 constituent.