The modification of current ellipses by stratification in the Liverpool Bay ROFI

Abstract

Understanding the fate of freshwater runoff and corresponding nutrient and pollution loads is of critical importance for the development of accurate predictive models and coastal management tools. A key element of such studies is the identification and understanding of the interaction between stratification and current structure. This paper presents a new series of measurements made in the Liverpool Bay region of freshwater influence (ROFI) during spring 2004 where freshwater-maintained horizontal density gradients and strong tidal currents interact to produce strain-induced periodic stratification (SIPS). During stratification, tidal current profiles are significantly modified such that the tidal flow deviates from the otherwise rectilinear E–W axis generating counter rotating upper and lower mixed layers. This feature has often been reported for the Rhine ROFI but not previously identified in Liverpool Bay despite previous investigation at this site. Investigation of an ongoing long-term dataset collected nearby reveals this process to be a common feature throughout the year. Liverpool Bay is shown to maintain three different regimes, long term mixed, long term stratified, and a transitional state when SIPS occurs. The phase of SIPS relative to the tide results in a residual flow away from the Welsh coastline in the upper water column of 2.3–3.6 cm s−1 with a counterflow in the lower layer of 2.8–3.1 cm s−1 towards the coast.