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1. The Evolution of a Buoyant River Plume in Response to a Pulse of High Discharge from a Small Midlatitude River

   https://doi.org/10.1175/JPO-D-19-0127.1  

   Lemagie, Emily; Lerczak, James (July 2020, Journal of Physical Oceanography)

   null (Ed.)

   Abstract A unique feature of small mountainous rivers is that discharge can be elevated by an order of magnitude during a large rain event. The impact of time-varying discharge on freshwater transport pathways and alongshore propagation rates in the coastal ocean is not well understood. A suite of simulations in an idealized coastal ocean domain using the Regional Ocean Modeling System (ROMS) with varying steady background discharge conditions (25–100 m 3 s −1 ), pulse amplitude (200–800 m 3 s −1 ), pulse duration (1–6 days), and steady downwelling-favorable winds (0–4 m s −1 ) are compared to investigate the downstream freshwater transport along the coast (in the direction of Kelvin wave propagation) following a discharge pulse from the river. The nose of the pulse propagates rapidly alongshore at 0.04–0.32 m s −1 (faster propagation corresponds with larger pulse volume and faster winds) transporting 13%–66% of the discharge. The remainder of the discharge volume initially accumulates in the bulge near the river mouth, with lower retention for longer pulse duration and stronger winds. Following the pulse, the bulge eddy disconnects from the river mouth and is advected downstream at 0–0.1 m s −1 , equal to the depth-averaged wind-driven ambient water velocity. As it transits alongshore, it sheds freshwater volume farther downstream and the alongshore freshwater transport stays elevated between the nose and the transient bulge eddy. The evolution of freshwater transport at a plume cross section can be described by the background discharge, the passage of the pulse nose, and a slow exponential return to background conditions. 

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2. High‐Resolution Observations of Subsurface Fronts and Alongshore Bottom Temperature Variability Over the Inner Shelf

   https://doi.org/10.1029/2018JC014454  

   Connolly, Thomas P.; Kirincich, Anthony R. (January 2019, Journal of Geophysical Research: Oceans)

   Abstract Circulation patterns over the inner continental shelf can be spatially complex and highly variable in time. However, few studies have examined alongshore variability over short scales of kilometers or less. To observe inner‐shelf bottom temperatures with high (5‐m) horizontal resolution, a fiber‐optic distributed temperature sensing system was deployed along a 5‐km‐long portion of the 15‐m isobath within a larger‐scale mooring array south of Martha's Vineyard, MA. Over the span of 4 months, variability at a range of scales was observed along the cable over time periods of less than a day. Notably, rapid cooling events propagated down the cable away from a tidal mixing front, showing that propagating fronts on the inner shelf can be generated locally near shallow bathymetric features in addition to remote offshore locations. Propagation velocities of observed fronts were influenced by background tidal currents in the alongshore component and show a weak correlation with theoretical gravity current speeds in the cross‐shore component. These events provide a source of cold, dense water into the inner shelf. However, differences in the magnitude and frequency of cooling events at sites separated by a few kilometers in the alongshore direction suggest that the characteristics of small‐scale variability can vary dramatically and can result in differential fluxes of water, heat, and other tracers. Thus, under stratified conditions, prolonged subsurface observations with high spatial and temporal resolution are needed to characterize the implications of three‐dimensional circulation patterns on exchange, especially in regions where the coastline and isobaths are not straight. 

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3. The Cross-Shelf Regime of a Wind-Driven Supercritical River Plume

   https://doi.org/10.1175/JPO-D-23-0012.1  

   Yankovsky, Elizabeth; Yankovsky, Alexander (February 2024, Journal of Physical Oceanography)

   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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4. Perspectives on Northern Gulf of Alaska salinity field structure, freshwater pathways, and controlling mechanisms

   https://doi.org/10.1016/j.pocean.2024.103373  

   Reister, Isaac; Danielson, Seth; Aguilar-Islas, Ana (December 2024, Progress in Oceanography)

   The biologically productive Northern Gulf of Alaska (NGA) continental shelf receives large inputs of freshwater from surrounding glaciated and non-glaciated watersheds, and a better characterization of the regional salinity spatiotemporal variability is important for understanding its fate and ecological roles. We here assess synoptic to seasonal distributions of freshwater pathways of the Copper River discharge plume and the greater NGA continental shelf and slope using observations from ship-based and towed undulating conductivity-temperaturedepth (CTD) instruments, satellite imagery, and satellite-tracked drifters. On the NGA continental shelf and slope we find low salinities not only nearshore but also 100–150 km from the coast (i.e. average 0–50 m salinities less than 31.9, 31.3, and 30.8 in spring, summer, and fall respectively) indicating recurring mid-shelf and shelfbreak freshwater pathways. Close to the Copper River, the shelf bathymetry decouples the spreading river plume from the direct effects of seafloor-induced steering and mixing, allowing iron- and silicic acid-rich river outflow to propagate offshore within a surface-trapped plume. Self-organized mapping analysis applied to true color satellite imagery reveals common patterns of the turbid river plume. We show that the Copper River plume is sensitive to local wind forcing and exerts control over water column stratification up to ~100 km from the river mouth. Upwelling-favorable wind stress modifies plume entrainment and density anomalies and plume width. Baroclinic transport of surface waters west of the river mouth closely follow the influence of alongshore wind stress, while baroclinic transport east of the river mouth is additionally modified by a recurring or persistent gyre. Our results provide context for considering the oceanic fate of terrestrial discharges in the Gulf of Alaska. 

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5. Wave erosion, frontal bending, and calving at Ross Ice Shelf

   https://doi.org/10.5194/tc-19-249-2025  

   Sartore, Nicolas B; Wagner, Till J_W; Siegfried, Matthew R; Pujara, Nimish; Zoet, Lucas K (January 2025, The Cryosphere)

   Abstract. Ice shelf calving constitutes roughly half of the total mass loss from the Antarctic ice sheet. Although much attention is paid to calving of giant tabular icebergs, these events are relatively rare. Here, we investigate the role of frontal melting and stresses at the ice shelf front in driving bending and calving on the scale of ∼100 m, perpendicular to the ice edge. We focus in particular on how buoyant underwater “feet” that protrude beyond the above-water ice cliff may cause tensile stresses at the base of the ice. Indirect and anecdotal observations of such feet at the Ross Ice Shelf front suggest that the resulting bending may be widespread and can trigger calving. We consider satellite observations together with an elastic beam model and a parameterization of wave erosion to better understand the dynamics at the ice shelf front. Our results suggest that on average frontal ablation rather consistently accounts for 20±5 m yr−1 of ice loss at Ross Ice Shelf, likely mostly due to wave erosion and smaller-scale, 𝒪(100 m), foot-induced calving. This constitutes only ∼2 % of the total frontal mass loss (since near-front ice velocities are ∼1000 m yr−1). Observational evidence suggests that sporadic larger events can skew this rate (we document one foot-induced calving event of size ∼1 km). Stresses from foot-induced bending are likely not sufficient to initiate crevassing but rather act to propagate existing crevasses. In addition, our results support recent findings by Buck (2024) that additional bending moments, likely due to temperature gradients in the ice, may play a role in driving frontal deflections. The highly variable environment, irregularity of pre-existing crevasse spacing, and complex rheology of the ice continue to pose challenges in better constraining the drivers behind the observed deformations and resulting calving rates. 

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