Three-dimensional structure in a tidal convergence front

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Lagrangian measurements of the structure of a tidal convergence zone were made in Haro Strait, on the western boundary of the U.S. and Canada. Acoustic imaging of the ocean surface from below was used in combination with a sub-surface, acoustically tracked, neutrally buoyant float and several CTDs. Generally, regions of high acoustic back-scatter were correlated with surface convergence and downwelling, while regions of low back-scatter corresponded to divergence and upwelling. The large scale velocity gradients which appear to form this front were concentrated in a shear zone a few 10 s of m wide with vertical vorticity up to 0.15 s −1 . Associated with this is a 100 m wide region of roughly 0.1 m s −1 average downwelling and maximum downwellings of 0.2 m s −1. A rich structure of three-dimensional eddies and front-like features occurs throughout the tidal mixing region. One common feature, appearing on the surface as a circular boil, extended at least 50 m deep and consisted of a 100–150 m region of upwelling surrounded by a much narrower ring of more intense downwelling. These eddies have a turbulent Froude number near one, so their size is constrained by the stratification. As a whole, the convergence zone acted to mix the waters brought together by the converging tidal streams. Although our data are very limited, the turbulence intensities which we observed are potentially large enough to support all of the mixing required by the overall circulation in Haro Strait.