Assessing the ability of storm surge models to simulate coastal trapped waves around Australia

Extreme sea levels result from the combined effects of a range of factors including long term sea level variability, astronomical tides, storm surges due to pressure and wind, coastal trapped waves (CTWs), and wave breaking. Coastal trapped waves are longer period ( days) sea level fluctuations caused by setup of water levels at the coast due to tropical cyclones and winter cold fronts and the subsequent propagation of the surge parallel to the coast and decreasing offshore. CTWs are commonly observed to propagate anticlockwise for thousands of kilometres along the western, southern, and eastern coasts of Australia. These CTWs can be both forced and free waves and can cause important current and water level oscillations in the region, particularly when elevated water levels from CTWs coincide with locally generated storm surges and high tides. CTWs can also impact port and shipping operations through negative surges that cause decreased channel depth when CTW troughs pass. We present results from ROMS model sensitivity studies for tropical cyclones Bianca (2011) and Jacob (1996) in Western Australia (WA) that generated CTWs in order to highlight requirements for simulating CTWs, the extent to which events are forced or free waves, and the influence of bathymetry and density stratification on propagation. In the case of TC Bianca the CTW was not a pure 'free wave' but was continuously 'forced' by wind and pressure along its trajectory. Results indicated that both 2D and 3D barotropic models were unable to reproduce freely propagating coastal trapped waves encountering changing continental shelf widths, despite the fact that these waves have been assumed to be mostly barotropic. On the other hand, the 3D model with vertical density stratification reproduced more realistic CTWs for all cases that travelled to the south coast of WA, generating current velocities greater than 0.5 m s-1 and water level anomalies up to 0.5 m more than a thousand kilometres from the source region.