Tidal dynamics in the Bab el Mandab Strait.

摘要

The Bab el Mandab Strait is where the transition occurs between two noticeably different tidal regimes: the Gulf of Aden, where tidal fluctuations are mixed and have a range in excess of 2 m, and the Red Sea, where the tides are principally semidiurnal and their range is less than 1 m. Within the Strait, observations collected between May of 1995 and July of 1997 indicate that tidal currents are a mixed type and dominant constituents are the K₁ and M ₂. The vertical structure of the tidal currents is complicated, differs between semidiurnal and diurnal constituents, and depends on the location and stratification. In addition, the stratification impacts more the vertical distribution of the diurnal tidal currents. The major part of this signal is barotropic but energetic baroclinic currents are observed near Perim Narrows and the Hanish Sill during the winter stratification. Results of the EOF analysis and internal velocity modes estimated from observed density profiles indicate that the baroclinic currents of diurnal frequency have vertical structure, which is a combination of the second and third baroclinic modes near Perim Narrows and of the first and second modes near the Hanish Sill.; To examine in more detail the barotropic tides in the Strait, a two-dimensional finite element model (ADCIRC-2DDI) was implemented. Results from this model indicate that the largest elevations and most energetic currents are observed in the region located between Perim Narrows and Assab-Mocha line. They also imply that residual circulation induced by the barotropic tides is generally weak in the Strait and consists of mean and fluctuating components. Average barotropic energy fluxes over a tidal period are small and their direction depends on the constituent. The K₁ component has one source of energy, which is the flux from the Gulf of Aden, while there are two sources of energy for the M₂: one from the Gulf of Aden and another from the Red Sea. In addition, these results show that the major part of the tidal energy for both constituents is dissipated within the Strait itself.