Characteristics of coastal trapped waves along the southern and eastern coasts of Australia

Abstract

The spatial structures and propagation characteristics of coastal trapped waves (CTWs) along the southern and eastern coasts of Australia are investigated using observed daily mean sea level data and results from a high-resolution ocean general circulation model (OGCM), and by conducting sensitivity studies with idealized numerical models. The results obtained from the sea level observations show that shortterm variations, with a typical period of 1 to 2 weeks, dominate the sea level variability in the southern half of Australia. The signal propagates anticlockwise around Australia with a propagation speed of 4.5 m/s or faster in the western and southern coasts and 2.1 to 3.6 m/s in the eastern coast. Strong seasonality of the wave activity, with large amplitude during austral winter, is also observed. It turns out that the waves are mainly generated by synoptic weather disturbances in the southwestern and southeastern regions. The numerical experiment with idealized wind forcing and realistic topography confirms that the propagating signals have characteristics of the CTW both in the southern and eastern coasts. Sensitivity experiments demonstrate that the difference in the phase speed between the coasts and reduction of the amplitude of the waves in the eastern coast are attributed to the different shape of the continental shelf in each region. The structures and the propagation characteristics of the CTWs around Australia are well reproduced in OFES (OGCM for the Earth Simulator) with dominant contribution from the first mode, although meso-scale eddies may modify the structure of the CTWs in the eastern coast. It is also found that generation or reinforcement of the waves by the wind forcing in the southern part of the eastern coast is necessary to obtain realistically large amplitude of the CTWs in the eastern coast.