Abstract: 

The river plume-front plays an important role in regulating energy, water exchange and materials transport in the estuaries. In this study, we use a high-resolution process-oriented numerical model to investigate the characteristics and instabilities of two heterogeneous plume-fronts off Pearl River Estuary (PRE) in the Greater Bay Area ocean. During upwelling favorable wind (UFW), the southeastward-spreading plume current from the PRE forms two fronts in the offshore and inshore areas. These fronts eventually reach a quasi-stable state as they propagate seaward. Submesoscale processes are highly active but dynamically different at both fronts: the offshore front is characterized by convergence-induced subduction along isopycnals, whereas the inshore front is dominated by intense lateral velocity shear from the alongshore plume jet. Spatially, these submesoscale processes peak upstream and gradually weaken downstream across both fronts. The UFW acts as an up-front (down-front) wind, generating a negative (positive) Ekman buoyancy flux that restratifies (destratifies) the water column and injects positive (negative) Ertel potential vorticity (EPV) at the offshore (inshore) front. Energy budget analysis reveals that both fronts convert mean kinetic energy into eddy kinetic energy (EKE) through intense vertical velocity shear induced by the plume and wind. Furthermore, baroclinic instability is generated at the offshore front, converting mean potential energy into EKE. The negative EPV at the inshore front triggers symmetric instability to drive a rapid conversion of MKE into EKE.