Diurnal Sea Breezes Force Near-Inertial Waves along Rottnest Continental Shelf, Southwestern Australia

Abstract

bservations of upper-ocean dynamics close to the critical latitude (ratio of the local inertial to diurnal frequency is 0.94) from a range of platforms (surface currents using HF radar, moored instruments, and satellite remote sensing data) off southwest Australia indicated the presence of energetic, near-inertial waves generated through the diurnal?inertial resonance. During the austral summer, when southerly winds and land?sea breeze (LSB) system dominated the wind regime, strong counterclockwise diurnal motions (amplitudes surpassing 0.3 m s?1) penetrated to 300-m depth with diurnal vertical isotherm fluctuations up to 60 m. The upward phase propagation speed of ~140 m day?1, deep penetration of diurnal currents below the mixed layer, and the ~180° phase difference between the upper and lower water column suggested that the local LSB system caused the resonant diurnal motions. Relative vorticity fluctuations along two cross-shore transects indicated changes to the local effective Coriolis frequency by more than 50% (±0.5f). In the presence of strong and relatively consistent cross-shore diurnal wind forcing in the study area, the main factors that controlled the observed energetic but sporadic near-inertial oscillations were the Leeuwin Current strength and spatial?temporal variations. These variations controlled the effective Coriolis frequency and enabled the effective pumping of diurnal wind energy into the ocean particularly when the effective Coriolis frequency was ~24 h.