The cycle of turbulent dissipation in the presence of tidal straining

In regions of large horizontal density gradient, tidal straining acts to pr oduce a periodic component of stratification that interacts with turbulent mixing to control water column structure and flow. A 25-h series of measure ments of the rate of dissipation of turbulent kinetic energy (epsilon) in t he Liverpool Bay region of freshwater influence (ROFI) have revealed the fo rm of this interaction and indicate substantial differences from regions wh ere horizontal gradients are weak. In the ROFI system there is a pronounced difference between flood and ebb regimes. During the ebb the water column stratifies and strong dissipation is confined to the lower half of the wate r column. By contrast, during the flood, stratification is eroded with comp lete vertical mixing occurring at high water and high values of dissipation (3 mW m(-3)) extending throughout the water column. The cycle of dissipati on is therefore predominantly semidiurnal in the upper layers whereas, near the bottom boundary, the principal variation is at the M-4 frequency as ob served in regions of horizontal uniformity. Toward the end of the flood pha se of the cycle, tidal straining produces instabilities in the water column that release additional energy for convective mixing. Confirmation of incr eased vertical motions throughout the water column during the late flood an d at high water is provided by measurements of vertical velocity and the er ror velocity from a bottom-mounted acoustic Doppler current profiler.