Hydraulic control of three-layer exchange flows: Application to the Bah alMandab

Two-layer hydraulic models have proven to be extremely useful in understand ing a number of exchange flows through straits, for example, the Strait of Gibraltar. There are, though, some strait Rows that cannot be represented b y a two-layer model. A very striking example is the Bab al Mandab. In the e xchange between the Red Sea and the Gulf of Aden, three distinct water mass es are observed in the strait during summer. The wintertime Row is, though, very different, with only two layers observed in the strait. In this paper a three-layer model of hydraulically controlled exchange flow s is described The model can represent exchange flows that are controlled w ith respect to either the first or second (or both) internal modes. A numbe r of different examples are discussed. The model is able to explain the qualitative features of the seasonal varia tion of the flow in the Bab al Mandab. In particular, the reversal of the d irection 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 mons oon.