A classification of wave generation characteristics during large wave events on the Southern Australian margin

Abstract

The Southern Australian continental margin is exposed to some of the largest waves of the global ocean, having been generated by Southern Ocean extra-tropical storms. Waverider buoy data from four buoys which span the full 3000 km length of the Southern Australian continental margin, supplemented with 10 years of numerical model output from the WAVEWATCH III wave model, have been used to describe the directional wave climate of the region. The buoy records include two records at the western end of the margin (a 10-year record from Rottnest Island, and an 8-year record from Cape Naturaliste); a 6-year record from Cape de Couedic in the central portion of the margin; and a record spanning 20 years from Cape Sorell, at the southeastern end of the margin. Cape Sorell exhibits the steepest waves, largest mean significant wave heights, and most westerly wave directions, indicating increased proximity to Southern Ocean wave generating storms. Rottnest exhibits the least steep, smallest mean significant wave heights, and most southerly wave directions, reflecting the greater distance from the Southern Ocean generating storms. All sites demonstrate highly energetic extreme events, particularly at Cape Sorell, and a predominance of swell over wind sea states. Spatial coherence between simultaneous buoy records show that wave events are easily tracked in the wave record as they propagate eastwards along the Southern Australian margin. Maximum correlation between records indicates that wave events at Cape Sorell lag those at Rottnest by approximately 80 h, and at Cape de Couedic by approximately 7 h. The speed of propagation of events between records is therefore consistent with the propagation speeds of extra-tropical cyclones along the Southern Australian margin. Synoptic typing of six-hourly mean sea level pressure (MSLP) patterns obtained from the NCEP–NCAR reanalysis has been carried out to determine the main synoptic types associated with large wave events at the Cape Sorell waverider buoy site. Significant differences occur in wave characteristics between synoptic types. At Cape Sorell, wave event magnitude shows a significant correlation to the intensity (as measured by the depth) of the forcing cyclone, with larger wave events occurring during more intense storms in the Southern Ocean. At the other buoy sites, the magnitude of wave events shows little correlation to the intensity of the storm, but is more closely related to the location of the storm, with larger events occurring when storms are closer to the Australian continent. These relationships are of importance for determining how the wave climate might change in this region (and how Australia's coastline might respond) in response to changes in atmospheric forcing which are currently being observed.