Stratification and mixing in sea straits

Quantifying the transport through relatively narrow straits connecting basins with differing fluid properties is an important issue in both surface and abyssal flows. These flows range from large-scale flows in ocean straits connecting the major ocean basins down to small-scale regional flows, such as in estuaries and lagoons connected to the coastal ocean by relatively narrow straits or channels. The exchange rate through these straits is dependent on the topography, mixing and stratification, and for steady flows the interplay between these three effects can be quantified using the dimensionless parameter GrTA2, where the Grashoff number GrT and the aspect ratio A are evaluated within the strait. When GrTA2 is large, the exchange is determined by the well-known hydraulic solution. As GrTA2 becomes small the exchange rates progressively decrease below the hydraulic predictions toward values determined by diffusive control and characterized by strong vertical mixing. For systems with simple topographic controls, we show how the exchange rate in the strait may be linked to the external basin scale forcing characteristics. We review recent work on extending this picture to consider the effects of unsteadiness in two forms, time-dependent forcing in the basin due to seasonal effects and tidal forcing with an associated barotropic forcing in the strait itself.