Near-surface dynamic structure in the northern South China Sea and Northwest Pacific revealed using Lagrangian data

The Northwest Pacific region is one of the most energetic in the near-inertial frequency band. Owing to the impacts of significant monsoon and the Kuroshio, the local dynamic environment of the northern South China Sea (SCS) and Northwest Pacific is complicated, and the inertial motions exhibit temporal and spatial variabilities. By fitting the complex velocity spectrum of the Lagrangian time series derived from the Global Drifter Program data using a Lagrangian stochastic model, which combines the complex-valued Ornstein–Uhlenbeck and Matérn processes, the main characteristics of near-inertial oscillations (NIOs) and mesoscale turbulence were extracted and analyzed. The integral near-inertial variance (NIV) is related to the amplitude and damping parameters. Fitted high energy spectrum is determined by small amplitude and large damping parameter or large amplitude and small damping parameter. It was found that the pattern of NIV of NIOs is consistent with the strength of wind fluctuation, and the strongest NIV present in the Luzon strait and northern SCS. The frequency shift in NIOs is related to background vorticity and damping timescale. The large geostrophic turbulent amplitude presents along with the Kuroshio path. The decorrelation timescale in the northern SCS is lower than that in the Philippine Sea, and the motions in northern SCS are more approaching to white noise. The isotropic diffusivity is higher along the Kuroshio path, accompanied with the energetic geostrophic turbulent.