Seasonal cycle of vertical structure and deep water renewal in the Clyde Sea

Two strings of moored current meters deployed between March 1993 and May 1994, together with monthly CTD surveys, provide the first comprehensive set of observations over the seasonal cycle in the Clyde Sea. In the summer, a strong thermal stratification maintained a partial isolation of the deep waters. In winter, the stratification was weaker, and a 1degreesC temperature inversion was persistent from November to the end of March. Rapid inflow of dense water from the North Channel of the Irish Sea served to re-establish the strong stratification in the spring. The mean rate of exchange was estimated from the salinity (practical salinity scale) and mass budgets to be similar to1.1 X 10(4) M-3 s(-1), indicating an average flushing time for the Clyde Sea of 3-4 months. Episodic increases in deep water salinity indicated that bottom water renewal occurred throughout the winter. Intense renewal events were observed in March 1993 and February 1994, when the North Channel density was near its seasonal maximum, and were coincident with periods of high wind stress. In the month prior to these rapid spring inflows, the basin bottom salinity reached its seasonal minimum, indicating that the effects of mixing dominated over renewal at this time. A marked inflow in the summer was inferred from the salinity budget, and observed as a salinity increase at a depth of 90 m. A 2-layer flow was observed in the Arran Deep basin throughout the year, the surface flow forming part of a clockwise circulation about Arran, with an opposing bottom layer circulation. This surface circulation prevents freshwater from entering the Kilbrannan Sound, leaving this area relatively susceptible to deep water mixing by the wind. At a station in the north of the basin, the internal tidal current was observed to have an amplitude of 2-3 cm s(-1), which is half the amplitude of the barotropic tide. The energy available to mix the water column mixing associated with the internal tide at this position is estimated to be similar to0.01 mWm(-2), which is 2 orders of magnitude less than wind mixing. The kinetic energy density in the Clyde Sea was found to be predominantly in low frequency oscillations (<1.0 cycles per day), the seasonal variation exhibiting some correlation with the wind. (C) 2001 Academic Press.