Three-dimensional numerical simulation of circulation and vertical temperature structure during summer in Cockburn Sound

Abstract

Cockburn Sound is an important embayment close to a metropolitan area in Western Australia. In this paper, we study summer circulation patterns, volume balance, and mixing processes in Cockburn Sound and adjacent shelf using in-situ oceanographic surveys made during the 2016–2017 austral summer and numerical simulations using the Finite Volume Coastal Ocean Model (FVCOM). The model results revealed that the local wind variability played a primary role in regulating the water volume in the embayment, with decreasing volume associated with south-southeasterly winds. In Cockburn Sound, a diel stratification cycle during summer was caused by accumulation of heat in the surface layer due to net air–sea heat flux (mostly solar radiation) during the daytime, and wind mixing due to sea-breezes in the afternoon and convective mixing due to nightly heat losses. The regularity of well-mixed nature can be interrupted with intermittent periods of vertical stratification. During March 2017, very strong vertical stratification that coincided with low oxygen conditions was observed in the near bottom water. By analyzing the model results, we show that high air temperature, weak winds, and the southward compensating colder inflow at depth from the coastal waters to the north caused the sustained temperature difference between surface and bottom layers during March 2017. We conclude that this thermal stratification restricted ventilation of the bottom water, explaining the observed low oxygen content.Plain language SummaryLow oxygen conditions were observed in the bottom water of Cockburn Sound (a semi-enclosed coastal embayment) during the austral summer of 2017. This was unexpected in view of the shallow water depth and strong local wind conditions which should ensure that the water-column is well ventilated. However, modeled temperature profiles showed that thermal stratification lasting for up to a week can sometimes occur during late summer, potentially restricting oxygen ventilation. The stratification events are caused by a combination of high air temperature (especially when it remained high at night), low wind speed and inflow of cooler sub-surface water. Global warming is expected to enhance thermal stratification, reduce oxygen solubility and accelerate biological oxygen demand increasing the future risk of hypoxia in the area.