Exchange Flow Variability between Hypersaline Shark Bay and the Ocean

Yasha Hetzel,Hrvoje Mihanović,Charitha Pattiaratchi

doi:10.3390/jmse6020065

Abstract

In Shark Bay, a large hypersaline bay in Western Australia, longitudinal density gradients force gravitational circulation that is important for Bay-ocean exchange. First-time observations of vertical stratification and velocity are presented, confirming the presence of a steady, near-bed dense water outflow from Shark Bay’s northern Geographe Channel that persisted through all stages of the tide. Outflow velocities were 2–3 times stronger than the outflows recorded previously in Naturaliste Channel (in the west), and were more resistant to breakdown by tidal mixing. Estimates of turbulent kinetic energy production derived from the variance method showed a more complex structure in the Geographe Channel, due to shear between surface and bottom layers. Turbulence varied between flood and ebb tide, with peak levels of turbulence occurring during reversal of tidal flows. For both channels, the main source of turbulence was tidal flow along the seabed, with the bottom current speed cubed, |Ub3|, providing a reasonable proxy for tidal mixing and prediction of dense water outflows from Shark Bay majority of the time. Orientation and deeper water of the Geographe Channel along the main axis of the longitudinal density gradient provided an explanation for the predominant outflow from the Bay’s northern entrance. These density-driven currents could potentially influence recruitment of commercially fished scallops and prawns through the dispersal and flushing of larvae.

Highlights

Horizontal density gradients in coastal bays and estuaries can drive gravitational circulation that is important for the exchange of water, salt and material between bay and ocean
Density profiles were collected on 23 August 2011, along a 35 km east-west transect across Geographe Channel (T1 in Figure 1a) at the northern end of Shark Bay
The CTD transect measured during a weak ebb equatorial tide across Geographe Channel at the start of the field experiment on 23 August (Figure 2) showed a warmer, fresher layer of water over a cooler, more saline bottom layer

Summary

Introduction

Horizontal density gradients in coastal bays and estuaries can drive gravitational circulation that is important for the exchange of water, salt and material between bay and ocean. In places where evaporation exceeds precipitation, elevated nearshore salinities and/or cooling can cause the water in the bay to become denser than the surrounding ocean water (hypersaline Bay), creating a ‘negative’ horizontal density gradient. This causes the bay to behave as a negative, or ‘inverse’ estuary, with outflow of dense saline water along the seabed, and an influx of fresher oceanic water at the surface [1]. Inverse estuaries and dense water outflows have been documented around Australia on a variety of scales (e.g., [2,3,4,5,6,7]), as well as globally (e.g., [8,9,10,11]), but have been studied much less than positive freshwater estuaries

Methods

Results

Discussion

Conclusion