A numerical modelling study of storm surges in Bass Strait

The relationship between severe weather events and storm surges in Bass Strait is investigated. The sustained westerly or southwesterly winds that accompany cold fronts along the south coast are found to be the most common cause of storm surges, although the intensification of low pressure systems in Bass Strait can also produce surges in this region. Two events, caused by cold fronts, are modelled using a high-resolution coastal ocean model. The storm surges produced by meteorological forcing show close agreement with observations at stations west of Bass Strait. In Bass Strait, measured sealevel residuals during strong westerly wind events exhibit a semi-diurnal oscillation resulting from an approximate 20 minute delay between the measured high tides and the predicted tides used to extract the sea-level residuals. This delay can be reproduced by the model when run with atmospheric and tidal forcing and the tides subsequently removed, indicating that the enhanced westerly current during the surge event interacts with the tidal currents to produce a temporary phase delay in the tides in western Bass Strait, although the physical mechanism behind the response is not clear. The contribution to storm surge height due to atmospheric pressure is found to be only around 10 per cent of the inverse barometer effect indicating that wind stress is by far the dominant component of the storm surge height in this region. The role of remote and local wind forcing in Bass Strait is investigated by performing model simulations where the western boundary of the model domain is moved progressively eastwards. Exclusion of the narrow shelf region immediately to the west of Bass Strait reduces the peak surge within Bass Strait by between 50 and 80 per cent in broad agreement with theoretical studies. An investigation of the effect of wind speed changes on storm surge height reveals that storm surge height responds linearly to changes in wind strength with a proportionality coefficient of around two.