Bifurcated Upshelf Extension of the Yangtze River Plume

Upshelf extension describes a river plume propagating in the opposite direction of the Kelvin wave that develops under various mechanisms. In this study, we observed two simultaneous upshelf branches in the Yangtze River (YR) plume in field monitoring, which were reproduced and investigated using a three-dimensional hydrodynamic model. A 12-year (2010–2021) realistic simulation indicates that the inshore branch appears on average 88% of the time correlated with river discharge, while the offshore branch occurs as several single events of 4–10 days in summer. Momentum and vorticity budget analyses and numerical experiments suggest that the homogeneous inshore branch develops in the absence of external forcing (e.g., winds, tides, ambient currents). Within the trapping depth of the bottom-advected YR plume, the upshelf motion tends to cross the isobaths into shallower inshore areas, facilitated by the barotropic pressure gradient and slope-induced inertia. Beyond the trapping depth, the plume expands radially offshore at the surface. The realistic plume oscillates with tidal mixing and trapping depth between these two modes in the spring-neap cycle. In comparison, the offshore branch results from wind entrainment during upwelling-favorable events, but is only formed around neap tides, when the wind-driven surface and tide-induced bottom Ekman layers are decoupled. Sufficient wind stress and duration and essential tide conditions to force the offshore branch were quantified. By contrasting properties and drivers of the two upshelf branches, this study sheds light on potential physical controls of upshelf motion in river plumes and has local implications for surrounding coastal and shelf waters.