Rotating exchange flows through straits with multiple channels: Preliminary results of laboratory studies

The deep basins of the world’s oceans are separated by ridges and coasts, and the exchange between basins occurs in flow over sills and through straits. Dynamical processes in the straits and sills control the value of the exchanges. Thus these topographic features have a considerable impact upon the large-scale thermohaline circulation of the ocean, and hence upon climate. Many wide straits, where rotation is expected to be important are composed of multiple channels separated by islands. The project objectives are: 1) To quantify the exchange flux through a flat-bottomed strait and compare with existing theory. 2) Determine the qualitative and quantitative effects of an island on the exchange flow. Rotating hydraulic flows are more complicated than the non-rotating equivalent; in particular with respect to hydraulic control. Widening of the channel leads to a greater flux for high Rossby numbers (ratio of Rossby radius to channel width). Experiments to model a rotating two-layer strait exchange were realised on the large Coriolis turntable in Grenoble. For Rossby numbers less than unity, we confirm previous findings that the flux is approximately independent of channel width and only constrained by rotation. The island significantly influences this behaviour by, in particular, by narrowing the strait width with respect to the Rossby number scaling and thus increasing the flux relative to the simple channel equivalent at the equal rotation. Furthermore, many qualitative differences were found in the flow field between the island and simple channel cases, in particular with respect to the path of the exchanging current within the channel.