Surface floating objects moving from the Pearl River Estuary to Hainan Island: An observational and model study

Abstract

A group of self-developed surface drifters were released near the Pearl River Estuary (PRE) from November to December 2021, and it is found that most of the drifters were finally stranded off the coast around Hainan Island after 10–30 days. This result makes us want to find out whether the surface floating objects around the PRE are converging on Hainan Island and what factors dominate this process. The velocity of the drifters not only displays prominent characteristics of tidal currents but is also highly correlated to winds associated with winter monsoon. A particle tracking model is employed to investigate the fate of the drifters and detailed process. Model simulation results show that the most important factor is winds, which affect the movement of floating objects through the direct way (direct wind drag) and indirect ways (wind-driven currents and the wave-induced Stokes drift). The influence of non-wind-driven flows (e.g., geostrophic currents, density-driven currents) is relatively limited. Tidal currents can significantly influence trajectory details within a tidal cycle. Extended model experiments with a large number of surface floaters released in October and January 2021 show that ∼34% of the surface floaters are stranded near the PRE, ∼30% are stranded to the east of Hainan Island, and ∼ 25% move to the west of Hainan Island. Our results indicate that a high proportion of the surface floaters from the PRE would converge around Hainan Island in the autumn and winter, of which the environmental impact warrants further investigation.