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1. 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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2. Freshwater Composition and Connectivity of the Connecticut River Plume During Ambient Flood Tides

   https://doi.org/10.3389/fmars.2021.747191  

   Whitney, Michael M.; Jia, Yan; Cole, Kelly L.; MacDonald, Daniel G.; Huguenard, Kimberly D. (October 2021, Frontiers in Marine Science)

   The Connecticut River plume interacts with the strong tidal currents of the ambient receiving waters in eastern Long Island Sound. The plume formed during ambient flood tides is studied as an example of tidal river plumes entering into energetic ambient tidal environments in estuaries or continental shelves. Conservative passive freshwater tracers within a high-resolution nested hydrodynamic model are applied to determine how source waters from different parts of the tidal cycle contribute to plume composition and interact with bounding plume fronts. The connection to source waters can be cut off only under low-discharge conditions, when tides reverse surface flow through the mouth after max ambient flood. Upstream plume extent is limited because ambient tidal currents arrest the opposing plume propagation, as the tidal internal Froude number exceeds one. The downstream extent of the tidal plume always is within 20 km from the mouth, which is less than twice the ambient tidal excursion. Freshwaters in the river during the preceding ambient ebb are the oldest found in the new flood plume. Connectivity with source waters and plume fronts exhibits a strong upstream-to-downstream asymmetry. The arrested upstream front has high connectivity, as all freshwaters exiting the mouth immediately interact with this boundary. The downstream plume front has the lowest overall connectivity, as interaction is limited to the oldest waters since younger interior waters do not overtake this front. The offshore front and inshore boundary exhibit a downstream progression from younger to older waters and decreasing overall connectivity with source waters. Plume-averaged freshwater tracer concentrations and variances both exhibit an initial growth period followed by a longer decay period for the remainder of the tidal period. The plume-averaged tracer variance is increased by mouth inputs, decreased by entrainment, and destroyed by internal mixing. Peak entrainment velocities for younger waters are higher than values for older waters, indicating stronger entrainment closer to the mouth. Entrainment and mixing time scales (1–4 h at max ambient flood) are both shorter than half a tidal period, indicating entrainment and mixing are vigorous enough to rapidly diminish tracer variance within the plume. 

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3. COAWST model simulations of warm and cool nearshore rip-current plumes

   https://doi.org/10.5281/zenodo.4272080  

   Moulton, Melissa (January 2020, Zenodo)

   This archive contains COAWST model input, grids and initial conditions, and output used to produce the results in a submitted manuscript. The files are:</p> model_input.zip: input files for simulations presented in this paper   ocean_rip_current.in: ROMS ocean model input file   swan_rip_current.in: SWAN wave model input file (example with Hs=1m)   coupling_rip_current.in: model coupling file   rip_current.h: model header file    model_grids_forcing.zip: bathymetry and initial condition files      hbeach_grid_isbathy_2m.nc: ROMS bathymetry input file      hbeach_grid_isbathy_2m.bot: SWAN bathymetry input file      hbeach_grid_isbathy_2m.grd: SWAN grid input file      hbeach_init_isbathy_14_18_17.nc: Initial temperature, cool surf zone dT=-1C case      hbeach_init_isbathy_14_18_19.nc: Initial temperature, warm surf zone dT=+1C case      hbeach_init_isbathy_14_18_16.nc: Initial temperature, cool surf zone dT=-2C case      hbeach_init_isbathy_14_18_20.nc: Initial temperature, warm surf zone dT=+2C case      hbeach_init_isbathy_14_18_17p5.nc: Initial temperature, cool surf zone dT=-0.5C case      hbeach_init_isbathy_14_18_18p5.nc: Initial temperature, warm surf zone dT=+0.5C case</p> model_output files: model output used to produce the figures      netcdf files, zipped      variables included:           x_rho (cross-shore coordinate, m)           y_rho (alongshore coordinate, m)           z_rho (vertical coordinate, m)           ocean_time (time since initialization, s, output every 5 mins)           h (bathymetry, m)           temp (temperature, Celsius)           dye_02 (surfzone-released dye)           Hwave (wave height, m)           Dissip_break (wave dissipation W/m2)            ubar (cross-shore depth-average velocity, m/s, interpolated to rho-points)      Case_141817.nc: cool surf zone dT=-1C Hs=1m      Case_141819.nc: warm surf zone dT=+1C Hs=1m      Case_141816.nc: cool surf zone dT=-2C Hs=1m      Case_141820.nc: warm surf zone dT=-2C Hs=1m      Case_141817p5.nc: cool surf zone dT=-0.5C Hs=1m      Case_141818p5.nc: warm surf zone dT=+0.5C Hs=1m      Case_141817_Hp5.nc: cool surf zone dT=-1C Hs=0.5m      Case_141819_Hp5.nc: warm surf zone dT=+1C Hs=0.5m      Case_141817_Hp75.nc: cool surf zone dT=-1C Hs=0.75m      Case_141819_Hp75.nc: warm surf zone dT=+1C Hs=0.75m</p> COAWST is an open source code and can be download at https://coawstmodel-trac.sourcerepo.com/coawstmodel_COAWST/. Descriptions of the input and output files can be found in the manual distributed with the model code and in the glossary at the end of the ocean.in file.</p> Corresponding author: Melissa Moulton, mmoulton@uw.edu</p> 

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4. 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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5. Evolving Interior Mixing Regimes in a Tidal River Plume

   https://doi.org/10.1029/2022GL099633  

   Spicer, Preston; Huguenard, Kimberly; Cole, Kelly_L; MacDonald, Daniel_G; Whitney, Michael_M (September 2022, Geophysical Research Letters)

   Abstract Microstructure profiling was utilized to estimate vertical mixing (via vertical turbulent buoyancy flux) during a tidal pulse in the interior Merrimack River plume in calm winds. Multiple stratified shear mixing regimes appear and evolve with time. Initially the plume acts as a nearfield jet, with mixing in the plume (plume layer mixing) and over the plume‐ambient interface (nearfield interfacial mixing). As the plume grows, interfacial mixing is suppressed offshore of the nearfield as currents slow, diminishing turbulent exchange between plume and shelf. At the end of ebb, ambient tidal currents reverse direction below plume, initiating another mode of internal, interfacial mixing (coined here as tidal interfacial mixing), allowing exchange between plume and ambient waters offshore. This work highlights previously unreported tidally modulated mixing within the near and midfield of a river plume. 

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