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Wind and tidal straining are proposed as key mechanisms influencing the magnitude and timing of the horizontal flux of freshwater across regions of freshwater influence (ROFIs). Evidence for this hypothesis is presented in estimates of the tidally averaged residual current profile, obtained from 5 years of continuous acoustic doppler current profiler measurements in the Liverpool Bay ROFI. The modified horizontal Richardson number (R-x(wt)), which includes both the tidal and the wind forcing, was assessed as a measure of stratification. R-x(wt) was found to be a good indicator of the timing of the evolution and destruction of stratification, but was not as successful as an indicator of the magnitude of stratification, both enduring and periodic. The observed mean residual velocities are compared to those predicted by a classical solution, and the eddy viscosity (N-z) is shown to be a control on differences between the observed and predicted circulation. Principal component analysis is used to show that the strongest residual currents occur when the water column periodically alternates between a well-mixed and stratified state, a consequence of straining, rather than simply related to the density gradient. Evidence of wind straining was found in the correspondence between the wind direction and the near surface and near bed residual current direction.

Wind and tidal straining are proposed as key mechanisms influencing the magnitude and timing of the horizontal flux of freshwater across regions of freshwater influence (ROFIs). Evidence for this hypothesis is presented in estimates of the tidally averaged residual current profile, obtained from 5 years of continuous acoustic doppler current profiler measurements in the Liverpool Bay ROFI. The modified horizontal Richardson number (R-x(wt)), which includes both the tidal and the wind forcing, was assessed as a measure of stratification. R-x(wt) was found to be a good indicator of the timing of the evolution and destruction of stratification, but was not as successful as an indicator of the magnitude of stratification, both enduring and periodic. The observed mean residual velocities are compared to those predicted by a classical solution, and the eddy viscosity (N-z) is shown to be a control on differences between the observed and predicted circulation. Principal component analysis is used to show that the strongest residual currents occur when the water column periodically alternates between a well-mixed and stratified state, a consequence of straining, rather than simply related to the density gradient. Evidence of wind straining was found in the correspondence between the wind direction and the near surface and near bed residual current direction.

Tidal straining is proposed as a key mechanism influencing the magnitude and timing of the horizontal mass flux of nutrients, terrestrial carbon and anthropogenic contaminants across Regions of Freshwater Influence (ROFIs), the critical interface between estuaries and continental shelf seas. Evidence for this hypothesis is presented in estimates of the long-term residual current profile, obtained from five years of continuous ADCP measurements, taken at the Proudman Oceanographic Laboratory Coastal Observatory mooring in Liverpool Bay. The observed mean residual velocities are found to be three times larger than those predicted by Heaps’ (1972) ‘classical’ solution. The strongest residual currents from the coastal observatory mooring are shown to occur when the water column periodically alternates between a well mixed and stratified state, a consequence of tidal straining, rather than simply related to the density gradient. These results are used for the further development of the POLCOMS 3D numerical model to improve the simulation of physical processes that drive the off-shore flux of freshwater.

Tidal straining is proposed as a key mechanism influencing the magnitude and timing of the horizontal mass flux of nutrients, terrestrial carbon and anthropogenic contaminants across Regions of Freshwater Influence (ROFIs), the critical interface between estuaries and continental shelf seas. Evidence for this hypothesis is presented in estimates of the long-term residual current profile, obtained from five years of continuous ADCP measurements, taken at the Proudman Oceanographic Laboratory Coastal Observatory mooring in Liverpool Bay. The observed mean residual velocities are found to be three times larger than those predicted by Heaps’ (1972) ‘classical’ solution. The strongest residual currents from the coastal observatory mooring are shown to occur when the water column periodically alternates between a well mixed and stratified state, a consequence of tidal straining, rather than simply related to the density gradient. These results are used for the further development of the POLCOMS 3D numerical model to improve the simulation of physical processes that drive the off-shore flux of freshwater.