Response of a river plume during an upwelling favorable wind event

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Abstract
The response of a surface‐trapped river plume to an upwelling favorable wind is studied using a three‐dimensional model in a simple, rectangular domain. Model simulations demonstrate that the plume thins and is advected offshore by the cross‐shore Ekman transport. The thinned plume is susceptible to significant mixing because of the vertically sheared horizontal currents. The Ekman dynamics and shear‐induced mixing cause the plume to evolve to a quasi‐steady uniform thickness, which can be estimated by a critical Richardson number criterion. Although the mixing rate decreases slowly in time, mixing continues under a sustained upwelling wind until the plume is destroyed. Mixing persists at the seaward plume front because of an Ekman straining mechanism in which there is a balance between the advection of cross‐shore salinity gradients and vertical mixing. The plume mixing rate observed is similar to the mixing law obtained by previous studies of one‐dimensional mixing, although the river plume mixing is essentially two‐dimensional.