Effect of barotropic tidal rectification on low‐frequency circulation near the shelf break in the northern Bay of Biscay

Abstract

Current meter data collected in the Bay of Biscay are used with a barotropic model to study the effect of the tidal rectification on the low‐frequency circulation near the shelf break. Data collected at two different locations are analyzed and show that the tidal forcing terms and the low‐frequency circulation are completely different at the two locations. In order to obtain a quantitative interpretation of these differences, a tidal forcing with both M2 and S2 waves is introduced in a barotropic tidal rectification model. We focus on the rectification process due to the “stretching of the water columns” alone, in the realistic case of large‐amplitude topography. A first theoretical result is that the bichromatic forcing generates a nonlinear variation of the residual current which depends on only one nondimensional parameter. This parameter is a function of the barotropic flow and of the wave frequencies. A second result is that the low‐frequency circulation generated by such a rectification process (including the two waves) is twice as large in spring tide than in neap tide. Finally, the model applied to both moorings confirms that the tidal rectification is much weaker at one mooring, the northwest one, than at the other, where the tidal rectification can explain 50% of the observed low‐frequency circulation.