||Vertical microstructure profiler (VMP-250 and Microrider), shipboard ADCP, and echo sounder were used to investigate the turbulent characteristics and mixing processes of nonlinear internal waves (NLIW) in the South China Sea during three cruises of 2011, 2014, and 2015. The survey area is in the continental shelves northeast of Dongsha, between isobaths of 250 and 600 m. A total of 11 NLIW were observed, with about two to six VMP profiles were conducted respectively for each NLIW to derive the dissipation rate of turbulent kinetic energy (ε). The purpose of this study is to get a better understanding of turbulence distribution in the fore, middle, and aft parts of NLIW.|
During the Cruise 1082 of R/V OR1 in 2014, 30 hours of continuous observations in five clusters of depression NLIW were conducted at a site of 250-m depth. Our results indicate that the first internal wave always has a larger amplitude, propagation speed (~1.5 m/s), and dissipated power, which is followed by several smaller, slower (0.3-0.5 m/s), and weaker internal waves. It took about 3 to 8 hours for the whole wave packet to pass by. The first and the strongest internal wave is usually accompanied by significant down-welling in the fore and upwelling in the aft parts. Vertically integrated dissipation level is about twenty times higher for the first internal wave than that of the following waves. Observations from the Cruise 1859 of OR3 in 2015 also indicate that ε in the fore and aft parts of NLIW is about 10-8 and 10-6~10-5 W/kg, respectively. Values of ε in the quiescent ocean without the disruption of NLIW are about 10-9~10-8 W/kg, and the integrated dissipated level is about 1.8 mWm-2, which is about 1/175 that of the aft part of NLIW. The depression internal waves lead to the sinking of surface waters, so that marked temperature variations of 6~12 oC in the same depth could be observed in a short time as the waves passed by. The water temperature in the core of NLIW is the highest. The depth of maximum chlorophyll concentration also moves downward then upward as the waves passed by. Finally, huge NLIW with an astonishing 170-m amplitude, 2.6 m/s propagation speed, 0.7 m/s vertical velocity, 11oC temperature variation, and 210-m mixed-layer depth were discovered during the Cruise 1541 of OR3 in 2011.