Wind effects on dense shelf water cascades in south-west Australia

Abstract

Evaporation and cooling during autumn and winter in south-western Australia results in denser water near the coast and a cross-shelf density gradient. These conditions drive a gravitational circulation, with offshore transport of higher density water along the seabed, defined as dense shelf water cascades (DSWCs). A field experiment with data collected from multiple platforms is described to identify the temporal variability and controlling mechanisms of DSWCs during the austral autumn and winter (May–June 2015) when DSWCs were most prevalent. Data analysis confirmed that both the wind intensity and direction were critical parameters to: (1) inhibit DSWC formation through vertical mixing; and, (2) enhance DSWCs, depending on the wind direction. An intense and persistent DSWC was initiated through the passage of a winter cold front with strong onshore winds. Here, onshore winds caused downwelling, resulting in onshore movement of surface water and offshore movement in the bottom layer, enhancing the cross-shelf density gradient and the cascade. Vertical mixing of the water column and shutdown of the DSWC occurred when the winds were strong and had an offshore component or were upwelling-favourable, which resulted in the advection of higher density water offshore at the surface and vertical mixing.