Interannual variations of Sargassum blooms in the Yellow Sea and East China Sea during 2017–2021

•Five yearly cycles of Sargassum blooms in the Yellow Sea and East China were studied.•Sargassum rafts were firstly detected in the middle or northern YS, moved southward through winter, then northerly in the following spring and disappeared in early summer.•The floating biomass was mostly bloomed in early spring and confined to an SST range of 10–16 ℃.•The inferred drifting pathways were congruent with the prevailing wind trajectory and coastal currents.•The floating S. horneri populations exhibited homogenous and conservative genetic structures among years.