Exchange flows between ocean basins
Tjipto J. Prastowo1, Ross W. Griffiths1, Graham O. Hughes1, Andrew McC. Hogg1
1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
Ocean straits and sills act as valves that control the rate of transport of water around the deep oceans and flow between shallow seas and the oceans. The transport commonly occurs through straits and over bottom sills, such as the Denmark Strait between the Arctic and North Atlantic Oceans, or a host of channels and sills through the mid-ocean ridges connecting abyssal basins. The exchange flows involve strong velocity and density gradients between the water flowing in different directions, and the flow becomes turbulent. Using laboratory experiments we have measured both the amount of mixing that occurs and the rate at which water is exchanged.
This year we have clarified the effects of different geometries, such as short or long straits, and straits with sills. Mixing in the flow reduces the exchange rate by approximately 16% relative to that predicted by inviscid hydraulic theory. Friction leads to a further reduction that depends on the length of the channel. However, our main focus is the mixing, which we describe in terms of a mixing efficiency. The mixing efficiency is defined as the proportion of the available potential energy released by the flow that is used to raise the centre of mass by vertical mixing, and is independent of whether the strait is narrow or wide, or short or long. For large Reynolds numbers the mixing efficiency takes a constant value of 11±1%. The mixing is also only weakly dependent on the presence of a smooth sill or a steep-sided ridge, the smallest value measured being 8.4% when there is a sill with gently sloping sides (figure 1).
These measurements are in agreement with our scaling theories that predict both the observed exchange rate and the mixing efficiency. We expect these results will be useful in estimating the contribution of exchange flows to the total interior mixing in the oceans.
Figure 1. Exchange flow over a sill in the laboratory, showing shear instability and mixing.
Prastowo, T. J., Griffiths, R. W., Hughes, G. O. and Hogg, A. McC. (2007) Mixing efficiency in controlled flows. J. Fluid Mech., submitted.