Hydraulic Interpretation of Direct Velocity Measurements in the Bab al Mandab

Abstract

Acoustic Doppler Current Profiler velocity measurements in the Bab al Mandab during the period June 1995?March 1996 are used to assess the hydraulic character of the exchange flow. The strait is 150 km long and contains two distinct geometrical choke points: the Hanish sill and Perim narrows. The authors use a three-layer approximation of the monthly mean velocity and density structure at the sill and narrows to calculate the phase speeds of the first and second internal, long gravity waves. The first (second) mode is generally characterized by in-phase (out-of-phase) motions of the two interfaces. The calculations take cross-strait topographic variations into consideration by using a piecewise linear representation of the actual bottom. The resulting phase speeds are used to determine whether the flow is subcritical, supercritical, or critical with respect to the first and second modes. Subcritical (supercritical) means that the two wave pairs corresponding to a given mode propagate in opposite (the same) directions, whereas ?critical? means that one or both members of a pair has zero phase speed. Critical or supercritical conditions are indications of hydraulic control and imply that signal propagation through the strait associated with a particular mode can occur in only one direction, or perhaps not at all. During the summer months, the velocity profiles indicate a ?three-layer? structure, with surface water outflowing from the Red Sea, intermediate water inflowing from the Gulf of Aden, and Red Sea Water outflowing at the bottom. During this time period, the flow is found to be subcritical with respect to both internal modes, although tides and mesoscale disturbances are potentially strong enough to temporarily create critical or supercritical conditions, particularly with respect to the second internal mode at the narrows. During the winter and transitional months the velocity has a two-layer character with inflowing surface water and outflowing Red Sea Water. However, the outflowing Red Sea Water can further be partitioned into an intermediate layer originating from the Red Sea thermocline and a deeper, homogeneous layer originating from below the thermocline. A subtle three-layer character therefore exists and the three-layer model is configured accordingly. Surprisingly, the monthly mean narrows flow during this time period is found to be substantially subcritical with respect to the first baroclinic mode. At the Hanish sill the flow is marginally to moderately subcritical with respect to the first mode, suggesting the possibility of proximity to a section of critical control. It is possible that friction may be strong enough to shift the control section to the south of the sill. With respect to the second mode, the flow at both the Hanish sill and Perim narrows are found to be very close to the critical speed with respect to the second internal mode, suggesting hydraulic control. The wave whose propagation is arrested is one attempting to move from the Gulf of Aden into the Red Sea. The vertical structure of this wave suggests a role in determining how much Upper Red Sea Deep Water is able to pass through the Bab al Mandab and into the Gulf of Aden. The strength of tides and mesoscale disturbances in the strait suggest that this upstream influence may be intermittent. Estimation of internal Rossby radii of deformation for the first and second internal modes indicates that rotation (which is neglected in our wave speed calculations) is only moderately weak. Nevertheless, the errors in calculated propagation speeds due to the neglect of rotation are estimated to be quite small.