The inference of internal solitary waves in the northern South China Sea from data acquired by underwater gliders

Abstract. Internal solitary waves (ISWs) are typical large-amplitude nonlinear waves occurring in stratified oceans. The in situ observations of ISWs are needed to improve the regimes of nonlinear internal wave theories. There is violent mixing of water mass in the horizontal and vertical directions during the propagation of ISWs, which generally lasts for a short period at a fixed position. However, an underwater glider, with the features of low-speed and sawtooth motion, cannot obtain a complete thermohaline stratification before and after the ISWs arrival. Those thermohaline data collected in situ by gliders, which vary synchronously at spatial-temporal scales, raise challenges for identifying the ISWs. Four Petrel-II gliders are deployed in the active region of ISWs in the South China Sea. This paper estimates vertical water velocity from glider flight data and kinematic model, analyzes the sensitivity of parameters in the glider kinematic model, and adopts a standard nonlinear search method to calibrate the parameters insensitive to the vertical velocity. The depth-keeping experiment is performed to verify the effectiveness of the optimized results. The standard deviation of vertical water velocity in the eastern Dongsha Atoll is revealed, and its distribution indirectly reflects that the strength of vertical water activity increases gradually at the same latitude along the east-west direction. Using observations of vertical water velocity fluctuations and isothermal surface vertical displacements, single- and multiple-wave packets can be identified. The availability of this method is tested by comparison with a MODIS image. Such an analysis provides a basis for the application of glider in the observation of ISWs.