Abstract

A series of exploratory laboratory experiments has been carried out to model an 'inverse estuary,' in which density differences and outflows were generated by heating and evaporation on a shallow shelf. This shelf was connected to a deep, long region of uniform depth through a steep slope. The experimental tank was filled with homogeneous salt solution, and the shelf region heated with infrared lamps to produce temperature and salinity anomalies; fresh water was added at the far end of the tank to keep the depth (and the mean salinity) constant. The emphasis in these experiments was on the influence in a closed region of horizontal differences of T and S together; i.e., on the double-diffusive effects, which are quite different from those due to a simple source of buoyancy in the shallow region. The vertical density difference increased over time, and this and the internal circulations became quasi-steady after several days. Two distinct regions of circulation and types of layering were revealed by dye streaks and detailed profile measurements. Near the surface there were strong salt fingers, driven by a hot saline layer spreading away from the shelf, with a counterflow of cooler, fresher water below. The warm salty water deposited by the fingers near the top of the slope formed a gravity current which flowed to the bottom and extended away from the slope, building up a stratification with salt and heat now distributed in the reversed 'diffusive' sense. There was also evidence for a second mechanism of 'bottom water' formation, the intermittent flow of hot very salty water off the shelf and down the slope. This latter process could be initiated more predictably by turning off the heater lamps.

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