A Study of Low-Frequency, Wind-Driven, Coastal-Trapped Waves along the Southeast Coast of Australia

Abstract

AbstractCoastal-trapped waves (CTWs) along the southeast coast of Australia were investigated based on a frictional, wind-driven long-wave theory. It was found that low-frequency sea level anomalies (SLAs) were continuously propagating from the south coast up along the east coast as CTWs, mainly forced by the alongshore wind stress. Three main subinertial peaks existed in the spectral characteristics of the SLAs, with periods of 14.2, 10.2, and 7.8 days, respectively. Power spectral density distributions of the peaks showed that the CTW amplitudes varied significantly along the southeast coast. For idealized linear and exponential shelves, a theoretical analysis indicated that the fundamental factor influencing the eigenvector of mode 1, and therefore the CTW amplitude, was the offshore water depth. This theoretical work was well supported by eight sensitivity cases. Four additional cases were conducted, and time-averaged energy fluxes were calculated to identify the energy source of the CTWs in the Australian Coastal Experiment (ACE) region. It was shown that both the local wind stress and the wind stress in Bass Strait contributed to the CTWs in the ACE region, with the latter playing a more important role. The remaining CTW energy came from remote forcing farther west of Bass Strait. The energy flux calculation also showed that the CTW energy flux was almost constant along the investigated coast because of the balance between frictional dissipation and the energy gain from the alongshore wind stress; the significant variations in the power spectral density (PSD) of the subinertial peaks were mainly due to the variations in the modal eigenvectors caused by the shelf geometry.