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  1. 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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  2. Abstract The mixing of river plumes into the coastal ocean influences the fate of river-borne tracers over the inner-shelf, though the relative importance of mixing mechanisms under different environmental conditions is not fully understood. In particular, the contribution to plume mixing from bottom generated shear stresses, referred to as tidal mixing, is rarely considered important relative to frontal and stratified shear (interfacial) mixing in surface advected plumes. The effect of different mixing mechanisms is investigated numerically on an idealized, tidally pulsed river plume with varying river discharge and tidal amplitudes. Frontal, interfacial, and tidal mixing are quantified via a mixing energy budget to compare the relative importance of each to the overall buoyancy flux over one tide. Results indicate that tidal mixing can dominate the energy budget when the tidal mixing power exceeds that of the input buoyancy flux. This occurs when the non-dimensional number, Ri E (the estuarine Richardson number divided by the mouth Rossby number), is generally less than 1. Tidal mixing accounts for between 60% and 90% of the net mixing when Ri E < 1, with the largest contributions during large tides and low discharge. Interfacial mixing varies from 10% to 90% of total mixing and dominates the budget for high discharge events with relatively weaker tides ( Ri E > 1). Frontal mixing is always less than 10% of total mixing and never dominates the budget. This work is the first to show tidal mixing as an important mixing mechanism in surface advected river plumes. 
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  3. 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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  4. Abstract

    Long Island Sound is a large macrotidal estuary. Connecticut River as the primary freshwater source enters near the sound's mouth. The summertime pathways of river water under low discharge and mild wind conditions are studied through both numerical simulations with a passive dye pulse and field surface drifter observations. Within the 19‐day modeling analysis period a third of the river dye pulse remains in the eastern sound; another third of the pulse moves up‐estuary with the near‐bottom dense inflow into the central and western sound with a spring‐neap tidal modulation; and another third leaves the sound with the near‐surface outflow toward the continental shelf through Block Island Sound. The latter pathway is confirmed by field surface drifter tracks. Three scenarios of wind forcing are tested: a WRF‐ROMS Coupled case, a NARR data forcing case, and a No‐Wind case. The results show though that the sound is tidal mixing dominated, mild winds still alter the position and strength of the estuarine exchange flow, and either enhances by the cross‐estuary winds or lateral straining. On the shelf, winds play a more important role on the fresher estuarine water distribution. The sensitivities of circulation, salinity, and numerical drifter tracks to different atmospheric forcings also are studied. The results suggest that the coupled model has better performance to simulate surface drifter tracks.

     
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  5. Abstract

    Recent studies have explored the sensitivity of global ocean model simulations to the treatment of riverine freshwater and the representation of estuarine processes via an estuary box model applied within Community Earth System Model (CESM). This study builds on these efforts by assessing the model skill score relative to a new salinity climatology. The new climatology averages the original observational data of the World Ocean Database directly onto the CESM ocean component tracer grid cells without spatial interpolation, smoothing, or other gap‐filling techniques to mitigate coastal ocean salinity bias present in the World Ocean Atlas. The mean square error for coastal upper ocean salinity relative to climatology is reduced by up to 14%, and the mean square error of near‐surface salinity stratification is reduced by up to 28% near major river mouths in the simulations with improved treatments of river runoff. The improvement in upper ocean bulk salinity is attributed primarily to focusing runoff as point sources thereby avoiding the artificial horizontal spreading of the control run and to applying a locally varying instead of a global constant reference salinity for riverine virtual salt fluxes. The improvements in near‐surface salinity stratification are primarily attributed to adding parameterized estuarine mixing with the estuary box model. Salinity and salinity stratification skill improvements are achieved not just near large rivers but also along the global coast and skill improvements extend far offshore. Despite these improvements, many other sources of model‐climatology mismatch in coastal salinity and stratification remain and merit further attention.

     
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