Semi-diurnal and longer period stability cycles in the Liverpool Bay region of freshwater influence

Detailed observations from a 12.5 h anchor station and a 45 day mooring deployment in the Liverpool Bay region of freshwater influence are presented. The anchor station data, showing vertical and temporal variations of current, density and gradient Richardson number, demonstrate a semi-diurnal stratification cycle driven by tidal straining of a freshwater-induced horizontal density gradient. The current meter mooring observations demonstrate the regular repetition of this cycle of semi-diurnal stratification and also indicate a period of more enduring stratification following a neap tide. This longer period of stability is caused by the decrease in tidal mixing energy allowing the gravitational relaxation of the horizontal density structure, with the increasing mixing towards the next spring tide returning the region to a vertically mixed state. It is suggested that the periods of post-neaps stability in Liverpool Bay will not occur more frequently than monthly, due to an N2 modulation of the spring-neaps mixing cycle. dimensional model incorporating a level 2 turbulence closure scheme reproduces the main features of the observations and is used to investigate the residual current behaviour. The model indicates that pulses in the residual flow are associated with periods of low turbulence at each slackwater, as the reduced frictional coupling through the water column allows the acceleration of the density-driven flow for a brief period. Similarly the general reduction in turbulent intensities at neaps should result, according to the model, in increased current speeds in directions closer to that of the isopycnals as the residual flow approaches geostrophy. Neither of these predictions of the model can be identified convincingly in the data because of other sources of variability.