Hydraulic control of three-layer exchange flows: application to the Bab al Mandab

Two-layer hydraulic models have proven to be extremely useful in understanding a number of exchange flows through straits, for example, the Strait of Gibraltar. There are, though, some strait flows that cannot be represented by a two-layer model. A very striking example is the Bab al Mandab. In the exchange between the Red Sea and the Gulf of Aden, three distinct water masses are observed in the strait during summer. The wintertime flow is, though, very different, with only two layers observed in the strait.In this paper a three-layer model of hydraulically controlled exchange flows is described The model can represent exchange flows that are controlled with respect to either the first or second (or both) internal modes. A number of different examples are discussed.The model is able to explain the qualitative features of the seasonal variation of the flow in the Bab al Mandab. In particular, the reversal of the direction of flow of the surface layer and the intrusion into the Red Sea of cold intermediate water from the Gulf of Aden during the summer months can be explained by upwelling in the Gulf of Aden forced by the southwest monsoon.