Tracking the drifting of pelagic Sargassum rafts in the East China Sea and Yellow Sea using a coastal ocean modeling system

Abstract

Drifts of massive Sargassum patches were observed in the East China Sea (ECS) and Yellow Sea (YS) by the Geostationary Ocean Color Imager in May 2017, which was the largest brown macroalgae bloom ever. Three-dimensional circulation modeling and Lagrangian particle tracking simulations were conducted to trace drifting trajectories of the macroalgae patches in the ECS and YS. The Lagrangian TRANSport model (LTRANS) was used with the Regional Ocean Modeling Systems (ROMS). The model domain included the ECS, YS, and the Korea Strait. The ensemble Kalman filter was used to assimilate sea surface temperature and profiles of temperature and salinity from January to December 2017. The trajectories of the macroalgae patches were controlled by winds and surface currents. A windage factor of 1% was chosen for Sargassum rafts based on sensitivity simulations. Southerly winds in May 2017 contributed to farther northward intrusion of the brown macroalgae into the YS. Without windage effect, the northeastward movement of the brown macroalgae was slower than the observation and the rafts of brown macroalgae did not enter the YS. When satellites were unable to capture all patches because of clouds and sea fog in the ECS and YS, the Lagrangian particle tracking model helped to track and restore the missing patches in satellite images. This study suggests that satellite observation and numerical ocean prediction systems are complementary to make accurate tracking of macroalgae patches in the ECS and YS. In this system, the biology of brown macroalgae was not taken into account and biological aspects, such as growing and sinking, of macroalgae need to be added for long-term particle tracking and prediction.