Response of internal waves to 2005 Typhoon Damrey over the northwestern shelf of the South China Sea

Abstract

Continuous observation of sea water temperature and current was made at Wenchang Station (19°35′N, 112°E) in 2005. The data collected indicate vigorous internal waves of both short periods and tidal and near-inertial periods. The temperature and current time series during 18–30 September were examined to describe the upper ocean internal wave field response to Typhoon Damrey (0518). The strong wind associated with the typhoon, which passed over the sea area about 45 km south of Wenchang Station on 25 September, deepened the mixed layer depth remarkably. It decreased the mixed layer temperature while increasing the deep layer temperature, and intensified the near-inertial and high-frequency fluctuations of temperature and current. Power spectra of temperature and current time series indicate significant deviations from those obtained by using the deep ocean internal wave models characterized by a power law. The frequency spectra were dominated by three energetic bands: around the inertial frequency (7.75×10⁻⁶ Hz), tidal frequencies (1.0×10⁻⁵ to 2.4×10⁻⁵ Hz), and between 1.4×10⁻⁴ and 8.3×10⁻⁴ Hz. Dividing the field data into three phases (before, during and after the typhoon), we found that the typhoon enhanced the kinetic energy in nearly all the frequency bands, especially in the surface water. The passage of Damrey made a major contribution to the horizontal kinetic energy of the total surface current variances. The vertical energy density distribution, with its peak value at the surface, was an indication that the energy injected by the strong wind into the surface current could penetrate downward to the thermocline.