Abstract

The effects of salt finger and diffusive interface transports on the thermohaline properties of an oceanic intrusion are modelled by considering the time evolution of a well-mixed layer with a single diffusive interface below it and a single finger interface above. Both cause the T-S anomaly to decrease. At short times, the diffusive interface buoyancy flux dominates, and can cause the central layer to overturn the finger interface. Initial conditions are deduced for which the central layer must lose its S contrast while remaining intermediate in density between the top and bottom layers. Under these conditions, the diffusive interface buoyancy flux is large initially, but at long time, the finger fluxes prevail. The balance between the two transports provides a 'focussing' such that the final density of the central layer is approximately independent of its initial density. Thus the time scale of run-down and the final state are predictable.The conditions sufficient to guarantee stable run-down are plotted for the analogue sugar-salt system. Too little is known about the relative buoyancy fluxes of the heat-salt finger and diffusive interfaces at low stability to permit plotting of the stability boundaries of the heat-salt system. However, the general features of the theory are expected to hold.Two laboratory experiments were performed demonstrating stable and unstable evolution in the sugar-salt system. The measured time variations in layer properties agreed well with those expected from numerical integration of the governing equations.

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