Eulerian and Lagrangian Statistics in the South China Sea as Deduced from Surface Drifters

Abstract

ulerian and Lagrangian statistics of the surface flows over the South China Sea (SCS) are investigated based on observations of satellite-tracked Lagrangian drifters of the Surface Velocity Program. Although the dataset contains the most recent observations up to 2011, the spatial and temporal distributions of the data are quite inhomogeneous. The Luzon Strait as well as the northern and western parts of the SCS are heavily sampled by drifters whereas the internal SCS is less sampled. The overall Eulerian mean circulation derived from all drifter observations resembles the wintertime circulations because of the seasonal sampling bias toward wintertime. It is found that the calculation of the diffusivity may be misled by the low-frequency temporal variabilities such as annual cycle, which could be removed using the Gauss?Markov decomposition. In the regions around Luzon Strait, typical values of diffusivity, time scale, and length scale are 3.7?11.9 ? 107 cm2 s?1, 0.8?1.9 days, and 16.4?44.4 km, respectively. In the regions around Hainan Island and the western boundary of the SCS, the typical estimated diffusivity and length scale are slightly smaller, that is, 2.3?6.9 ? 107 cm2 s?1 and 12.8?33.7 km while the time scale (0.8?2.0 days) is approximately the same as that in the regions around Luzon Strait. Although these regions are close to the coast, the ?flux versus gradient? relationship is still valid in these coastal regions.