Impact of Offshore Winds on a Buoyant River Plume System

摘要

Idealized numerical simulations utilizing the Regional Ocean Modeling System (ROMS) are carried out to examine the response of buoyant river plume systems to offshore-directed wind stresses. It is found that after a few inertial periods of wind forcing the plume becomes detached from the coast and reaches a steady state in terms of the plume's offshore position, width, and plume-averaged depth, salinity, and velocity. The steady-state offshore position of the plume is a balance between the cross-shore advection driven by the estuarine outflow and the alongshore advection driven by the Ekman velocities, and is described using the ratio of the outflow Froude number and the plume Froude number. The steady-state salinity structure is maintained by a balance between the cross-shore advection of salt creating stratification, the turbulent vertical mixing, and the downstream transport of freshwater continually resetting the system. Plume mixing is also analyzed using a salinity coordinate system to track the changes in freshwater volume in salinity space and time. A dynamical plume region classification is developed with use of a Richardson number-based critical mixing salinity criterion in salinity space. This salinity class-based classification agrees well with a classification based on an alongshore analysis of the salt flux equation. For this classification the near field is dominated by large cross-shore fluxes and the midfield by a diminishing cross-shore salt flux, and in the far field there is a balance between the alongshore salt flux and turbulent mixing.