(Janzen, Simpson, Inall and Cottier, 2005, Across-sill circulation near a tidal mixing front in  a broad fjord,  in press CSR (as of June 2005))

One objective of our Clyde Sea research (as part of the EU OAERRE project) is to estimate horizontal fluxes and cross sill exchanges, both tidal and non-tidal.  We are investigating the spatial and temporal variability of the Clyde Sea sill front, which is important to the occurrence of intermediate as well as deep water renewal in the Clyde. Both shipboard hydrographic and ADCP observations from May 2000 are examined, along with mooring observations taken along the sill to better understand the dynamics of the cross sill exchange and the controls on subtidal circulation within the Clyde Sea itself.  We have confirmed the presence of a high gradient front with steeply sloping (1:250) isopycnals near the surface epxression of the front.  Along the sill,  a sloping interface (1:700) seperates heavy, high-salinity water in the lower water column at the southeast portion of the sill from  fresher water towards the northwest, indicating a gravitational circulation modified by rotation. A geostrophic interpretation of the observed density field suggests strong along-frontal velocity differences between surface and bottom of up to 16 cm/s in the strongest frontal gradients, and an across-sill exhcange flow of ~ 10 cm/s.  The along-frontal flows have been observed in previous studies, and agree with with those predited by geostrophy in our observations. However,  in both shipboard ADCP and moored current measurements, we observe across-sill barotropic flows additional to the geostrophic, across-sill circulation predicted by the density structure.  These barotropic flows are not coherent between moorings, and are an order of magnitude larger than those required to account fo the observed variations in mean sea level. The forcing of the barotropic motions is yet unclear, but are of interest, as these motions are likely responsible for non-tidal, across-sill excursions of the front, which control renewal processes in the Clyde Sea.

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