Wind and tidal mixing controls on stratification and dense water outflows in a large hypersaline bay

Abstract

In Shark Bay, a large inverse estuary in Western Australia, longitudinal density gradients establish a gravitational circulation that is important for Bay-ocean exchange and transport of biological material such as larvae. The relative contributions of energy from wind and tidal mixing on the control of vertical stratification and gravitational circulation were investigated using the three-dimensional baroclinic ocean circulation model (General Estuarine Transport Model, GETM). In this large inverse estuary, the effects of the winds and tides on vertical mixing were found to be of similar magnitude. A critical depth of 15 m was identified that determined whether winds or tides or a combination of the two was required to create vertically mixed conditions. Where it was shallower than the critical depth, either the wind or tide could fully mix the water column. In contrast, a combination of both winds and tides was required to mix the deeper channels. Density-driven circulation peaked 0-3 days after periods of maximum stratification, resulting in a fortnightly modulation of dense water outflows along the seabed following the tidal stage. Salt flux calculations provided new evidence for the predominance of outflow through the deeper northern entrance channel where outflows persisted through all stages of the tide. In contrast, outflows through the western channel were more intermittent with a stronger tidal component. Wind driven lateral circulation between the entrances was also important and could temporarily reverse the circulation during northerly wind events.