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1. Surface macro- and micro-nutrients within the Copper River plume region respond to along-shore winds

   https://doi.org/10.1016/j.marchem.2025.104508  

   Ortega, ELS; Reister, I; Danielson, SL; Aguilar-Islas, AM (May 2025, Marine Chemistry)

   T he Copper River is a major source of freshwater to the Northern Gulf of Alaska (NGA) shelf with a seasonal cycle t hat reaches peak discharge in summer. This glacially-fed river also provides a large input of dissolved chemicals t o the NGA, and because of its large particle load, it impacts the distribution of particle-reactive elements. Summertime sampling of shelf water properties was carried out within the Copper River plume region during two y ears: first during a period of upwelling-favorable winds and higher river discharge (4–7 July 2019) and later during lower river discharge and more typical downwelling conditions (11–13 July 2020). Although these wind conditions were observed in separate years, both can occur over the course of a single summer. We found that the e xport of most nutrients to surface shelf waters was enhanced under upwelling-favorable winds accompanied by higher river discharge compared to downwelling conditions and lower discharge. For example, greater cross- shelf plume transport in 2019 resulted in higher mid-shelf surface inventories for nitrate +nitrite (N +N), silicic acid (H4 SiO 4 ), phosphate (PO4 3 − ), dissolved Fe (dFe), and dissolved Cu (dCu) compared to 2020. Entrainment of relatively macronutrient-rich subsurface waters under upwelling conditions may also have contributed t o the enhancement of these mid-shelf nutrient inventories. The observed high N:P ratios in plume waters were likely driven by the scavenging of P within particle-laden plume waters. Similarly, we observed lower than expected [dFe] (1.58 to 6.12 nM) in particle-laden plume waters, likely a result of enhanced scavenging combined with low concentrations of dissolved Fe-binding ligands. Although dNi and dZn have a river source, we observed lower concentrations in surface shelf waters under upwelling conditions, suggesting enhanced dilution b y relatively micronutrient-poor subsurface waters. Results highlight the influence of sub-seasonal variations in atmospheric forcing on nutrient distributions and suggest that this forcing also impacts the location and timing of primary production hotspots during summer, adding to the ecological mosaic of the NGA across a range of temporal and spatial scales. 

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2. 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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3. What Drives the Mean Along‐Shelf Flow in the Northwest Atlantic Coastal Ocean?

   https://doi.org/10.1029/2024JC021079  

   Chen, Ke; Yang, Jiayan (July 2024, Journal of Geophysical Research: Oceans)

   Abstract A long‐standing hypothesis is that the steady along‐shelf circulation in the Northwest Atlantic (NWA) coastal ocean is driven by buoyancy input from continental freshwater runoff. However, the forcing from the freshwater runoff has not been adequately evaluated and compared with other potential driving mechanisms. This study investigates the roles of both wind stress and freshwater runoff in driving the mean along‐shelf flow in the NWA coastal ocean and examines other potential drivers using a newly developed high‐resolution regional model with realistic forcing conditions. The results reveal that wind stress has a larger impact than freshwater runoff on the overall mean circulation and along‐shelf sea‐level gradient on the NWA shelf. While the continental freshwater input consistently contributes to the equatorward along‐shelf flow and higher sea level along the coast, wind stress is more effective for the setup of the broad‐scale circulation pattern by driving the along‐shelf flow on the Labrador Shelf and opposing the flow in the Mid‐Atlantic Bight and on the Scotian Shelf. In addition to the local wind and continental runoff, the sub‐Arctic inflow from higher latitude is an essential part of the NWA shelf circulation system. This remote driver directly contributes to the along‐shelf flow and insulates the shelf flow from the Gulf Stream on the southern shelves. 

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4. Seasonal and Interannual Salinity Variability on the Northeast U.S. Continental Shelf: Insights From Satellite Sea Surface Salinity and Implications for Stratification

   https://doi.org/10.1029/2024JC021534  

   Ryan, Svenja; Ummenhofer, Caroline_C; Gawarkiewicz, Glen_G (November 2024, Journal of Geophysical Research: Oceans)

   Abstract The Northeast U.S. continental shelf (NEUS) is a highly productive and economically important region that has undergone substantial changes in recent years. Warming exceeds the global average and several episodes of anomalously warm, sustained temperatures have had profound impacts on regional fisheries. A majority of recent research studies focused on the analysis of temperature; however, salinity can serve as a valuable tracer as well. With now more than a decade of remote‐sensing sea surface salinity data, we shed new light onto salinity variability in the region with focus on the Mid‐Atlantic Bight and assess its role for modulating stratification on the shelf using historical hydrographic data. Local river discharge drives decreasing salinities not only in spring and summer on the shelf but also in the Slope Sea. In spring, fresher water aids the build‐up of stratification and a low salinity surface layer extends to the shelf break above the pycnocline by the beginning of summer. An observed salinification in the fall is linked to offshore forcing over the slope associated with the presence of Warm Core Rings. Coherent low‐frequency salinity variability is found over the slope and shelf, highlighting that shelf conditions are significantly impacted by offshore variability. Conditions on the NEUS in 2015 were characterized by anomalously high salinities, associated with a northerly position of the Gulf Stream. A freshening between 2015 and 2021, is in agreement with increased river cumulative discharge as well as lower offshore salinities. Overall, salinity serves as a valuable additional tracer of these multi‐variate processes. 

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5. Synchronicity of the Gulf Stream path downstream of Cape Hatteras and the region of maximum wind stress curl

   https://doi.org/10.1038/s41598-024-68461-0  

   Gifford, Ian; Gangopadhyay, Avijit; Andres, Magdalena; Oliver, Hilde; Gawarkiewicz, Glen; Silver, Adrienne (December 2024, Scientific Reports)

   Pan, J (Ed.)

   Abstract The Gulf Stream, a major ocean current in the North Atlantic ocean is a key component in the global redistribution of heat and is important for marine ecosystems. Based on 27 years (1993–2019) of wind reanalysis and satellite altimetry measurements, we present observational evidence that the path of this freely meandering jet after its separation from the continental slope at Cape Hatteras, aligns with the region of maximum cyclonic vorticity of the wind stress field known as the positive vorticity pool. This synchronicity between the wind stress curl maximum region and the Gulf Stream path is observed at multiple time-scales ranging from months to decades, spanning a distance of 1500 km between 70 and 55W. The wind stress curl in the positive vorticity pool is estimated to drive persistent upward vertical velocities ranging from 5 to 17 cm day⁻¹over its ~ 400,000 km²area; this upwelling may supply a steady source of deep nutrients to the Slope Sea region, and can explain as much as a quarter of estimated primary productivity there. 

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