Shoaling of large-amplitude nonlinear internal waves at Dongsha Atoll in the northern South China Sea

Abstract

Shoaling of large-amplitude (~100 m) nonlinear internal waves over a steep slope (~3°) in water depths between 100 m and 285 m near Dongsha Atoll in the northern South China Sea is examined with an intensive array of thermistor moorings and a bottom mounted Acoustic Doppler Current Profiler. During the 44 h study period in May 5–7, 2008, there were four groups of large internal waves with semidiurnal modulation. In each wave group a rapid transition occurred during the shoaling, such that the front face of the leading depression wave elongated and plunged to the bottom and the rear face steepened and transformed into a bottom-trapped elevation wave. The transitions occur in water depths of 200 m and deeper, and represent the largest documented internal wave shoaling events. The observations repeatedly capture the detailed temperature and velocity structures of the incident plunging waves. Strong horizontal convergence and intense upward motion are found at the leading edge of transformed elevation waves, suggesting flow separation near the bottom. The observations are compared with the previous observations and model studies. The implication of the shoaling internal waves on coral reef ecology also is discussed.