Recent Progress in Strait Dynamics

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A major change in the understanding of the dynamics of flow through straits has occurred over the last few years with the development of two layer hydraulic control models and their application to new observations in the Strait of Gibraltar. Observations of the flow through the Strait of Gibraltar suggest that there are at least two locations of critical flow where the Fronde number equals one, at the sill and at the narrowest section of the Strait, as required by hydraulic theory for maximal exchange through the Strait. Predictions of the inflow and outflow transports and the salinity difference between them from hydraulic theory are in agreement with the recent observations. The physical configuration of the Strait of Gibraltar does indeed appear to control the amount of exchange between the Mediterranean and Atlantic basins.New observational techniques utilized in the Gibraltar Experiment have dramatically portrayed the physical processes operating in the two layer exchange. Acoustic backscatter and Doppler current profiling have characterized hydraulic jumps and mixing, bore formation and propagation of internal solitary wave packets as no conventional measurements have done before. Time series current and, most importantly, salinity measurements on moorings have led to quantitative estimates of the inflow and outflow transports far superior to historical measurements. The new transports throught the Strait of Gibraltar are 30% smaller than historical estimates and, furthermore, tidal oscillations contribute nearly half of the total transport. Microstructure measurements have allowed regions of intense mixing in the Strait to be identified and the amount of mixing to be quantified. Finally the first conclusive evidence of the direct aspiration of deep water up and over a sill has been found in nearly all time series CTD stations at the Gibraltar sill.In this progress report and review, hydraulic control theory and Gibraltar observations are discussed. Then the effects of strait flows on the circulation in the adjoining seas and results from studies of other straits are described. Following a discussion on the generation and propagation of solitary waves in straits, a brief summary and list of outstanding problems are presented, with particular emphasis on the need to develop a fully time‐dependent hydraulic theory for the exchange through a strait.