Is the Faroe Bank Channel a hydraulically-controlled overflow?
Abstract
The overflow of dense water from the Nordic Seas through the Faroe Bank Channel (FBC) has attributes suggesting hydraulic control, including an asymmetry across the sill reminiscent of flow over a dam. However, because of the influence of the earth's rotation, as well as the presence of continuous gradients in velocity and density, the standard approach of looking for a Froude number (v/√ {g'd}) of unity to diagnose criticality is not adequate. Of primary importance is the nature and speed of information-carrying waves---the flow is subcritical if any of these waves can travel upstream, supercritical if no waves can travel upstream, and critical if the fastest waves are arrested by the flow. We present a comparison of several different techniques for assessing the hydraulic criticality of overflows applied to data from a set of velocity and density sections across the FBC. These include: 1) modifications to the (non-rotating) local Froude number to account for shear and stratification in the flow; 2) rotating hydraulic solutions using a constant potential vorticity layer in a channel of parabolic cross-section; and 3) direct computation of shallow water wave speeds from the observed overflow structure, using a newly-developed generalized hydraulic condition and multiple-streamtube approach. Two of these three methods give similar answers, suggesting the location of control to be 60-100 km downstream of the sill and not at the sill itself. We discuss the implications of these results for hydraulic predictions of overflow transport and variability, as well as reasons for the failure of the parabolic model.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFMOS12B..06G
- Keywords:
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- 4512 Currents;
- 4544 Internal and inertial waves;
- 3220 Nonlinear dynamics;
- 1635 Oceans (4203)