Cross-Shelf Plumes April 2023

A study of the dynamics of a coastal buoyant plume, fed by a riverine discharge, that crosses a wide continental shelf at an oblique angle and with minimal lateral spreading. This type of plume delivers coastal low-salinity water offshore, possibly all the way to the shelf-break, where it can be entrained by major oceanic currents (e.g., the Gulf Stream on the US east coast). The plume’s formation is the result of tidally-modulated estuarine outflow and a balance between upwelling wind and buoyancy forcing, something not uncommon in a variety of environments around the globe, and yet it deviates from the traditional plume types commonly referred to in the literature. We hypothesize that under such wind/buoyancy conditions the tidal pulses of brackish water produce individual tidal sub-plumes, which are subsequently advected offshore by Ekman drift without being dispersed or mixed by excessive wind-induced shear stresses. The freshwater discharge is assumed to be sufficiently high so that geostrophic adjustment can occur within the tidal pulse of buoyant water. Under this scenario, the width of the resulting plume is simply the scale of the individual tidal sub-plume, while its total length is a sum of multiple tidal sub-plumes, roughly aligned with the surface wind-driven currents. The result of this process is a filament-like plume with an unusually high length-to-width aspect ratio, which breaks away from coastal trapping and protrudes far offshore. A number of field experiments will resolve mixing, wind and other forcings near the sea surface to develop an accurate description of the conditions favoring the development and persistence of these dynamics.