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Abstract River plumes are a dominant forcing agent in the coastal ocean, transporting tracers and nutrients offshore and interacting with coastal circulation. In this study we characterize the novel “cross-shelf” regime of freshwater river plumes. Rather than remaining coastally trapped (a well-established regime), a wind-driven cross-shelf plume propagates for tens to over 100 km offshore of the river mouth while remaining coherent. We perform a suite of high-resolution idealized numerical experiments that offer insight into how the cross-shelf regime comes about and the parameter space it occupies. The wind-driven shelf flow comprising the geostrophic along-shelf and the Ekman cross-shelf transport advects the plume momentum and precludes geostrophic adjustment within the plume, leading to continuous generation of internal solitons in the offshore and upstream segment of the plume. The solitons propagate into the plume interior, transporting mass within the plume and suppressing plume widening. We examine an additional ultra-high-resolution case that resolves submesoscale dynamics. This case is dynamically consistent with the lower-resolution simulations, but additionally captures vigorous inertial-symmetric instability leading to frontal erosion and lateral mixing. We support these findings with observations of the Winyah Bay plume, where the cross-shelf regime is observed under analogous forcing conditions to the model. The study offers an in-depth introduction to the cross-shelf plume regime and a look into the submesoscale mixing phenomena arising in estuarine plumes. Significance StatementIn this study, we characterize a novel regime of freshwater river plumes. Rather than spreading near to or along the coast, under certain conditions river plumes may propagate away from the coast and remain coherent for tens to over 100 km offshore. Cross-shelf plumes provide a mechanism by which freshwater and river-borne materials may be transported into the open ocean, especially across wide continental shelves. Such plumes carry nutrients critical for biological productivity offshore and interact with large-scale oceanic features such as the Gulf Stream. We use high-resolution numerical modeling to examine how the cross-shelf regime arises and support our findings with observational evidence. We also study the mixing phenomena and fluid instabilities evolving within such plumes.
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Wind‐Enhanced Separation of Large‐Scale River Plumes From Coastal Corners
Abstract Idealized models are analyzed to quantify how large‐scale river plumes interact with coastal corners with and without wind‐driven currents. The configuration has a corner formed by two perpendicular shelves (with constant slope) that are joined with a coastal radius of curvature (rc). The buoyant plume originates from an upstream point source. Thercand wind forcing are varied among runs. Steep‐ and gentle‐slope runs are compared for some situations. Without winds, plumes separate from corners withrcsmaller than two inertial radii (ri); this threshold is twice therc < ritheoretical separation criterion. After separation, no‐wind plumes form an anticyclonic bulge, and reattach farther downstream. Offshore excursion increases asrcdecreases. A downwelling‐favorable wind component along the upstream coast (τsx) favors separation by increasing total plume speed. An upwelling‐favorable wind component along the downstream coast (τsy) also increases offshore excursion. Winds blowing obliquely offshore with both these wind components advect the plume farther offshore. Wind‐driven currents that steer plumes in this situation include a downshelf jet originating on the upstream shelf and continuing around the coastal corner and beyond, offshore and upshelf surface transport downstream of the corner, and surface Ekman transport on the outer shelf. Multiple linear regressions quantify plume position sensitivity torc,τsx, andτsy; results are discussed in a dynamical context. Globally, many river plumes interact with coastal corners under various wind conditions.
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- Award ID(s):
- 2242070
- PAR ID:
- 10640602
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 129
- Issue:
- 7
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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