Variability of Circulation in the Pearl River Estuary: Observation and FVCOM Modeling

A cruise measurement was implemented in 3-11 May 2014 in the Pearl River estuary and the plume area to explore the estuary circulation and dynamical properties. The ADCP data show that the flood tide causes divergence in the deep channel in the cross estuary direction, while the ebb tide causes convergence. The unstructured grid finite-volume coastal ocean model (FVCOM) with higher spatial resolution is implemented to explore details of the variability of circulations in the Pearl River Estuary, validated by cruise observations and tidal gauge seal level data. Modeling results suggest that the residual surface currents flow seaward and the bottom current flow landward due to the density gradients effects. Under the northeasterly wind, the stratification is weak and the plume turns westward along the coast with an anti-cyclonic bulge, enhancing the seawater intrusion more inside in the estuary, whereas under the southwesterly wind, the plume water spreads eastward, thins, and displays a density gradient current structure that enhances the stratification. With only freshwater discharge forcing, there are the sub-tidal lateral circulations with convergence at the surface and divergence near the bottom of the deep channel. Combined with the tidal effect, lateral circulation is clockwise in the center of the deep channel. Under the northeasterly wind, there is a westward lateral flow at the surface and eastward at the bottom, whereas the lateral circulation is in the opposite direction under the southwesterly wind. The cross-estuary momentum analysis based on the modeling suggests that one main mechanism driving the secondary flow in PRE is Ekman forcing, which represents a dynamical balance between friction and the Coriolis acceleration. The other one is cross-channel baroclinic pressure gradients that arise from differential advection of the longitudinal density gradient. The magnitude of the momentum terms of the experiments considering the tidal effect is ten times larger than non-tidal effect experiments. The northeasterly wind strengthens the nonlinear advection effect more and the southwesterly wind strengthens the friction of the ocean surface.