Wind-driven circulation of Cockburn Sound

Abstract

The water circulation within the Cockburn Sound embayment in Western Australia is predominantly wind-driven. Observations by moored current meters showed the mean velocities to be less than 0.05 m s^-1 within Cockburn Sound and 0.07 m s^-1 in the adjacent open coastal waters. Maximum tidal currents were very low, with an amplitude of the order of 0.01 m s^-1. The importance of the wind-forcing was evident in the time-history data collected by the wind and current meters, particularly under storm conditions when winds greater than 15 m s^-1 produce currents, within the Sound, between 0.10 and 0.25 m s^-1. The wind records from Fremantle were examined and recurring patterns, such as sea breeze on and winter storms, were identified and their annual frequency of occurrence was estimated. A time- dependent, vertically integrated wind-driven numerical model was used to simulate the various water circulation patterns of each wind category. The circulation Froude number was shown to be of the order of thus allowing a description in terms of the stream function. Correlation between the data collected by the moored current meters and predicted by the model ranged between 0.05 and 0.76. Profiling data showed that under near calm wind conditions (<2 m s^-1), local horizontal density gradients, caused by evaporation and heating, produced currents of up to 0.1 m s^-1. This inverse estuary nature of the flow appears to play a significant role in the dynamics at low wind speeds. Under these conditions there was little or no volume exchange with the open sea. By contrast, the wind-driven circulation model showed that under a sea-breeze pattern 1.4 × 10⁷ m³ was typically exchanged diurnally, and a 2-day winter storm may exchange about 1.1 × 10⁸ m³, which is small compared with the volume of the Soand (1.2 × 10⁹ m³). The bathymetry is such that the Sound acts mainly as a closed system.