Temporal variability of the gravitational circulation in a partially mixed estuary

Ribeiro, Cesar Henrique de Assis (2004) Temporal variability of the gravitational circulation in a partially mixed estuary. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The ecology of estuarine systems depends on the water exchange with the coastal region, and net mean, non-tidal, circulation driven by the longitudinal density gradient plays an important role in this exchange. This project investigated the responses of the gravitational flow to changes in vertical mixing on different time-scales. Three long term deployments of seabed-mounted Acoustic Doppler Current Meter and surface and seabed salinity-temperature loggers collected current profiles and surface and bottom density time series for a total of 268 days, between March 2001 and April 2002. Additionally, a vertical 1-D numerical model with a turbulence closure scheme was used to provide further insight into the mixing effects on the gravitational circulation. Analysis of the tidally averaged velocities suggested that the spring-neap fluctuation of the tidal mixing modulates the gravitational circulation. Mean velocities increased to 0.05 m s^-1 at the bottom and 0.07 m s^-1 at the surface during a typical weak neap tide. A critical bottom current of 0.4 m s^-1 was found for the typical longitudinal density gradient of 2.5 x 10^-4 kg m^-4, above which tidal mixing appeared to prevent this increased gravitational flow. This observed critical velocity agreed with the Hansen-Rattray solution for Southampton Water. Results also suggested that for weaker/stronger density gradients the critical velocity is lower/higher. Intra- and inter-annual fluctuations of precipitation also affect the gravitational flow by altering for longitudinal density gradient. No clear evidence of a modulation of the gravitational circulation on semi-diurnal time scales was found, although the mean flow did vary within a tidal cycle. The harmonic method may have removed possible semi-diurnal signal present on the gravitational circulation time series. It seems that the full development of a gravitational flow takes longer (1-2 days) than the semi-diurnal variability of the tidal mixing. Although inconclusively, it seems that barotropic mechanisms do not control gravitational circulation in Southampton Water, contrarily to San Francisco Bay (Stacey et al., 2001). This may be caused by the weaker tidal staining and stronger longitudinal density gradient found in Southampton Water. Wind speed, direction and persistence all seemed to affect the gravitational flow. Onshore wind speed of 8 m s^-1 was found to be sufficient to prevent the neap tide pulse of gravitational flow. Besides the surface stress, onshore winds can cause mixing by inducing vertical instability. Offshore winds aid seaward flow of the surface component of the gravitational circulation. No significant effects of transversal winds on the density-driven flow were found, probably due to the limited fetch. Data showed that local winds stress can not explain significant changes on the barotropic balance.