The influence of tidal currents and wind on mixing in the surface layers of the sea

The data from a series of continuous plume diffusion experiments conducted in the southern North Sea have been analysed to extract both Fickian and non-Fickian estimates of the lateral and vertical diffusivities, and the dependence of the mixing rates on the wind and tidal conditions investigated. Various power laws, expressing the diffusivities as functions of the environmental conditions, were fitted to the data but little improvement could be achieved over that obtained using a linear function of wind and current speed. The data displayed considerable scatter and multiple regression could only explain about 50% of the variance in the observed lateral and vertical diffusivities. Empirical formulae relating the mixing to the tidal currents and winds have been derived for use in applied studies.