Abstract:

Shoaling of large-amplitude nonlinear internal waves over a steep slope in water depths between 100 m and 285 m near Dongsha Atoll in the northern South China Sea is examined withan 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 withsemidiurnal modulation. In each wave group a rapid transition occurred during the shoaling, such thatthe front face of the leading depression wave elongated and plunged to the bottom and the rear facesteepened and transformed into a bottom-trapped elevation wave. Strong horizontal convergence and intense upward motion were found at the leadingedge of transformed elevation waves, suggesting flow separation near the bottom (Fu et al., 2012).

During the two-day experiment, four wave trains consistingof 78 solitons at five sites were identified. The parameters related to soliton transformationwere formulated by applying the extended Korteweg–de Vries (eKdV) theory. The resultsindicated that the soliton energy dispersed dramatically along the sharp bottom slope,likely as a result of bottom friction and turbulence mixing. Large-amplitude depressionwaves wereobserved to transform into elevation waves before the thickness of the lowerlayer became equal to that of the upper layer. The ratio of the wave amplitude to thelower-layer thickness is found to be a good indicator of wave deformation. The criticalconditions for the transition of the internal waves (IWs) occurred at the ratio approximatelyequal to 0.66 ± 0.2. This means that a bottom-trapped elevation wave could formbefore passing through the theoretical critical point. The solitons felt the bottom anddeformed when their amplitudes approached half of the lower-layer thickness. The solitonsexisted in the form of elevation waves when the waves were in contact with the bottom (Fu et al., 2016).