Abstract
The long-term variability of the non-tidal circulation in Southampton Water, a partially mixed estuary, was investigated using 71-day acoustic Doppler current profiler (ADCP) time series. The data show evidence that the spring–neap tidal variability of the turbulent mixing modulates the strength of the non-tidal residual circulation, with subtidal neap tide surface flows reaching 0.12 m s−1 compared to <0.05 m s−1 at spring tides. The amplitude of the neap-tide events in this non-tidal circulation is shown to be related to a critical value of the tidal currents, illustrating the strong dependence on tidal mixing. The results suggest that the dominant mechanism for generating these neap-tide circulation events is the baroclinic forcing of the horizontal density gradient, rather than barotropic forcing associated with ebb-induced periodic stratification. While tidal turbulence is thought to be the dominant control on this gravitational circulation, there is evidence of the additional effect of wind-driven mixing, including the effects of wind fetch and possibly wave development with along-estuary winds being more efficient at mixing the estuary than across-estuary winds. Rapid changes in atmospheric pressure also coincided with fluctuations in the gravitational circulation. The observed subtidal flows are shown to be capable of rapidly flushing buoyant material out of the estuary and into the coastal sea at neap tides.
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Acknowledgements.
We are grateful to the crew of R.V. Bill Conway for their invaluable support during the fieldwork. We thank William Heaps of Associated British Ports for the tide gauge data and the Environment Agency for the river discharge data. We also would like to thank the two anonymous reviewers for their helpful comments and suggestions. This work was supported by Natural Environment Research Council, NERC (grant no. GR3/12736) and C. H. A. Ribeiro had a personal grant from Conselho Nacional de Desenvolvimento Científico e Tecnológico, CNPq.
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Ribeiro, C., Waniek, J. & Sharples, J. Observations of the spring–neap modulation of the gravitational circulation in a partially mixed estuary. Ocean Dynamics 54, 299–306 (2004). https://doi.org/10.1007/s10236-003-0086-z
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DOI: https://doi.org/10.1007/s10236-003-0086-z