70 / 2023-07-26 22:18:14

Richardson Dispersion in the South China Sea via Helmholtz-Hodge decomposition

Helmholtz-Hodge decomposition,Richardson dispersion,Seasonal variation

The complex dynamics of the South China Sea (SCS) make it a region of significant interest. Understanding these dynamics is crucial for pollution management, marine resource exploration, and climate modeling. This study uses Fast-Fourier-Transform based Helmholtz-Hodge decomposition on the velocity data from MITgcm LLC4320 to analyze ocean currents, separating them into divergence-free (vortical flow), curl-free (wavy motion), and harmonic components while conserving momentum. The numerical results reveal that wavy motion in the Luzon Strait exhibits a semi-lunar cycle, mainly due to tidal motion, despite the diurnal cycle, while the vortical flow does not show this pattern. To better understand the complex dynamics in the SCS, we explore Richardson dispersion, which refers to the spreading and mixing of fluid parcels due to various flow motions. By considering monsoon activity, two time periods are examined: the northeast monsoon (autumn–winter, September 2011 to February 2012) and the southwest monsoon (spring–summer, March 2012 to August 2012). A significantly seasonal variation is evident when fluid parcels are released in the Luzon Strait. More precisely, the mean square displacement (MSD) is estimated individually for different flow components with the initial separation distance below 10 km. The MSD advected by the combined effect of vortical flow and wavy motion has been enhanced by roughly 4 to 10 times during the southwest monsoon, while it has been suppressed by 4 to 10 times during the northeast monsoon. In either monsoon case, the contribution of MSD advected by wavy motion or the harmonic component alone can be ignored. These results suggest that the wavy motion and vortical flow play important roles in the Richardson dispersion: it suppresses the dispersion in the northeast monsoon, while it enhances the dispersion in the southwest monsoon. This should be taken into account further in dispersion models.