As tropical and extratropical storm systems approach the western and southern coasts of Australia, wind and pressure increase the sea level at the coastline, generating Coastally Trapped Waves (CTW) that significantly impact coastal sea levels, circulation, mixing, and flushing. These long-period water level fluctuations (~days) propagate anti-clockwise around the coast for thousands of kilometres, often combining with local forcing to generate extreme sea levels and currents.

 The emergence of coastal observing systems and detailed atmospheric hindcasts over the past decade has allowed insight into how CTW influence coastal systems in the region. For this study we analysed IMOS (Integrated Marine Observing System) coastal HF radar, mooring, ocean glider, tide gauge and atmospheric reanalysis data in order to define conditions where significant CTW events occurred, and document their dynamics.

 We found that wave amplitude (up to ~1m) was highly sensitive to the storm track. CTW could be either ‘free’ or ‘forced’ waves, with strongest events occurring when storm systems moved parallel to the coastline at similar speeds to the CTW, inputting energy along the way. Tropical systems were more likely to generate free waves, whilst most extratropical low-pressure systems resulted in forced waves across the south coast. In some cases, individual CTW were subject to all of these conditions, and were influenced by multiple storm systems through their lifecycle. HF radar revealed that the passage of CTW reversed coastal currents, induced eddies, and inhibited upwelling. Environmental and human impacts from CTW in this region have been both negative, including fish kills, coastal erosion, and inundation; and positive such as flushing of lagoons with restricted connections to the ocean. The study showed that CTW were subject to range of complex factors and it was beneficial to investigate each event individually. Studies that generalise and ignore these factors may draw incorrect conclusions.