Modulated mixing and frontogenesis in shallow seas and estuaries

doi:10.1016/0278-4343(88)90015-5

Continental Shelf Research

Volume 8, Issue 10, October 1988, Pages 1107-1127

https://doi.org/10.1016/0278-4343(88)90015-5 Get rights and content

Abstract

This paper describes some laboratory experiments on the circulation produced in a fluid containing a horizontal density gradient in the presence of environmental turbulence whose level varies periodically with time. The experiments are intended to model the effects of turbulence generated by tidal flow in a shallow sea or estuary. At times of high tidal flow the water column is vertically mixed and the baroclinically driven circulation is weak. At low turbulence levels the baroclinic circulation accelerates and the water column stratifies. The longitudinal flux of density is measured and an effective longitudinal dispersion coefficient K is calculated. The value of K increases with the horizontal density gradient and also with the period of the turbulence modulation. This latter increase is a result of the enhanced flux driven by the baroclinic circulation at low turbulence levels. The results of these experiments are compared with some recent measurements of episodic mixing events in Liverpool Bay, U.K., Spencer Gulf, South Australia, and the Columbia River Estuary, U.S.A.

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The speed of the flow, followed by the halt of further offshore progression, suggests initial relaxation of the horizontal density gradient triggered by the spring thermal stratification switching off mixing between surface and bottom waters. This is akin to the ‘estuarine-style’ baroclinic circulation described by Linden and Simpson (1988) for a number of shallow seas and estuaries worldwide. As fresh water from the Bristol Channel is allowed to extend towards the northern Celtic Sea it is continually mixed with (and freshens) surrounding water, a non-reversible process that extends the southward influence of the Bristol Channel.
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Citation Excerpt :
Enhanced periods of gravitational circulation in inverse estuaries are associated with pulses of dense saline water flowing out of the estuary along the seabed, referred to here as ‘dense water outflows’. It is usually during periods of reduced mixing that dense water outflows develop (Linden and Simpson, 1988). Much knowledge of these processes has been gained through idealized laboratory experiments (e.g. Linden and Simpson, 1986, 1988) and through extensive observational studies in areas such as Spencer Gulf and Gulf St. Vincent in South Australia (Nunes and Lennon, 1986; Nunes and Lennon, 1987; Samarasinghe and Lennon, 1987).
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Modulated mixing and frontogenesis in shallow seas and estuaries

Abstract

Continental Shelf Research

Continental Shelf Research

Deep-Sea Research

The effect of river discharge on the residual circulation in the Eastern Irish Sea

Continental Shelf Research

New dimensions in estuary classification

Limnology and Oceanography

Formation of thermoclines in zero-mean-shear turbulence subjected to a stabilizing buoyancy flux

Journal of Fluid Mechanics

Residual circulation and classification of shallow, stratified estuaries