On the role ofwind and tide in generating variability of Pearl River plume during summer in a coupled wide estuary and shelf system

摘要

A numerical simulation of the buoyant river plume over the Pearl River Estuary (PRE) and adjacent shelf during a typical upwelling favorablewind period of the summermonsoon is utilized to explore the responses of the plume to wind and tide forcing. The model is forced with time-dependent river discharge, wind and tide, and it shows reasonable ability to capture the basic structure and responses of the plume. Additional numerical experiments that are forced without either wind or tide are used to evaluate the relative importance of wind and tide in generating plume variability. Results show that the vertical structure of the plume and the strength of the stratification in the estuary are determined by the combination of the buoyancy forcing associated with river discharge and tidal forcing, and vary with the advection process, while the horizontal shape and spreading of the plume over the shelf are highly influenced by the wind-driven coastal current, and are more susceptible to the change of vertical mixing.Mechanical energy analysis in each dynamical region (upper,middle, lower estuary, and shelf) reveals that this is because the systemmainly gains energy from tide (wind) in the estuary (shelf), and loses energy to the bottom friction (internal-shear mixing) in the estuary (shelf). The largest forcing and dissipation terms in the middle PRE, and at the entrances of smaller estuaries such asHuang Mao Hai, are due to tidal forcing, which enables themiddle PRE to serve dynamically as the entrance of an estuary,where the transition of the river plume into coastal buoyancy current usually takes place. In addition, themixing efficiency increases from upper PRE to the shelf and fromstrong to weak mixing period, thus the plume in thewell-mixed upper estuary is not as sensitive to the changes of wind and tide as that over the highly stratified shelf.