Gravity currents and the release of salt from an inverse estuary

Abstract

Spencer Gulf, South Australia, forms a remarkable natural system in which marine gravity currents are regularly observed. The most intense of these forms every autumn, when the gulf waters, which are of high salinity after summer evaporation, are cooled to the point where they are much denser than the waters on the shelf. They then slip out of the gulf along the sea floor to be replaced by a surface inflow of less saline shelf water. Here we report the first complete survey of this outflow during a period when salt exchange was both intense and well-defined. The gravity current has been traced across the floor of the continental shelf to the point where it cascades over the shelf break until it finds its own density level at a depth of 250 m. The speed of the current is estimated to be in the region of 0.1 ms−1, and at that rate it would have to flow for three months to remove the salt accumulated in the gulf during summer. There is evidence that the flow is unsteady in character, and is possibly released in regular pulses.