Density and flow structure in the Clyde Sea front

A well-defined front in temperature and salinity separates the stratified C lyde Sea water from the vertically well mixed water of the North Channel. T he detailed structure of the front was observed in autumn 1990 by a combina tion of, repeated crossings of the front using a shipborne ADCP and a towed undulating CTD system, and the deployment of a fixed mooring system with t emperature, salinity and velocity sensors for a period of 12 days. The resu lts show that the front was situated on the Great Plateau near a contour of log(10)(H/U-2(3)) = 2.7 similar to 3.7 where H is the water depth and U-2 the amplitude of M-2 tidal velocity. The temperature structure in the Clyde Sea was inverted and the Clyde Sea surface temperature was lower than that of the vertically well mixed water in the North Channel. Since the salinit y gradient was stronger than the temperature gradient with fresher water on the surface, the density structure was predominantly controlled by salinit y. There were indications of warm and saline bottom water upwelling on the mixed side of the front during spring tides. This upwelling disappeared and the salinity and temperature structure at the front was more diffuse durin g the neap tide period. A jet-like along-front residual current was observe d flowing to the northwest in the surface layer with a counter flow to the southeast in the bottom layer. The vertical difference in velocity was abou t 9 cm s(-1) and was approximately consistent with the shear determined fro m the thermal wind relation. Both cross- and along-front components of the current observed at the mooring station varied in response to the advection of the front, although both components had large variations with periods o f less than one day and several days. The front was advected past the moori ng system by a mean flow from the North Channel to the inner basin, while o scillating 3-5 km back and forth with the tidal currents. From the velocity at a current meter mooring and CTD data, the front was estimated to have m oved up to 20 km during the observational period and the cross frontal velo city was inferred to be 3-4 cm s(-1). (C) 1999 Elsevier Science Ltd. All ri ghts reserved.