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1. Generation and Evolution of Internal Solitary Waves in a Coastal Plain Estuary

   https://doi.org/10.1175/JPO-D-23-0151.1  

   Li, Renjian ; Li, Ming ( February 2024 , Journal of Physical Oceanography)

   Large-amplitude internal solitary waves were recently observed in a coastal plain estuary and were hypothesized to evolve from an internal lee wave generated at the channel–shoal interface. To test this mechanism, a 3D nonhydrostatic model with nested domains and adaptive grids was used to investigate the generation of the internal solitary waves and their subsequent nonlinear evolution. A complex sequence of wave propagation and transformation was documented and interpreted using the nonlinear wave theory based on the Korteweg–de Vries equation. During the ebb tide a mode-2 internal lee wave is generated by the interaction between lateral flows and channel–shoal topography. This mode-2 lee wave subsequently propagates onto the shallow shoal and transforms into a mode-1 wave of elevation as strong mixing on the flood tide erases stratification in the bottom boundary layer and the lower branch of the mode-2 wave. The mode-1 wave of elevation evolves into an internal solitary wave due to nonlinear steepening and spatial changes in the wave phase speed. As the solitary wave of elevation continues to propagate over the shoaling bottom, the leading edge moves ahead as a rarefaction wave while the trailing edge steepens and disintegrates into a train of rank-ordered internal solitary waves, due to the combined effects of shoaling and dispersion. Strong turbulence in the bottom boundary layer dissipates wave energy and causes the eventual destruction of the solitary waves. In the meantime, the internal solitary waves can generate elevated shear and dissipation rate in local regions.

   In the coastal ocean nonlinear internal solitary waves are widely recognized to play an important role in generating turbulent mixing, modulating short-term variability of nearshore ecosystem, and transporting sediment and biochemical materials. However, their effects on shallow and stratified estuaries are poorly known and have been rarely studied. The nonhydrostatic model simulations presented in this paper shed new light into the generation, propagation, and transformation of the internal solitary waves in a coastal plain estuary.

    

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2. A Regime Diagram for Internal Lee Waves in Coastal Plain Estuaries

   https://doi.org/10.1175/JPO-D-21-0261.1  

   Li, Renjian ; Li, Ming ( December 2022 , Journal of Physical Oceanography)

   Abstract Using an idealized channel representative of a coastal plain estuary, we conducted numerical simulations to investigate the generation of internal lee waves by lateral circulation. It is shown that the lee waves can be generated across all salinity regimes in an estuary. Since the lateral currents are usually subcritical with respect to the lowest mode, mode-2 lee waves are most prevalent but a hydraulic jump may develop during the transition to subcritical flows in the deep channel, producing high energy dissipation and strong mixing. Unlike flows over a sill, stratified water in the deep channel may become stagnant such that a mode-1 depression wave can form higher up in the water column. With the lee wave Froude number above 1 and the intrinsic wave frequency between the inertial and buoyancy frequency, the lee waves generated in coastal plain estuaries are nonlinear waves with the wave amplitude Δ h scaling approximately with , where V is the maximum lateral flow velocity and is the buoyancy frequency. The model results are summarized using the estuarine classification diagram based on the freshwater Froude number Fr f and the mixing parameter M . The Δ h decreases with increasing Fr f as stronger stratification suppresses waves, and no internal waves are generated at large Fr f . The Δ h initially increases with increasing M as the lateral flows become stronger with stronger tidal currents, but decreases or saturates to a certain amplitude as M further increases. This modeling study suggests that lee waves can be generated over a wide range of estuarine conditions. 

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3. Observations of Tidally Driven Turbulence over Steep, Small-Scale Topography Embedded in the Tasman Slope

   https://doi.org/10.1175/JPO-D-23-0038.1  

   Marques, Olavo B. ; Alford, Matthew H. ; Pinkel, Robert ; MacKinnon, Jennifer A. ; Voet, Gunnar ; Klymak, Jody M. ; Nash, Jonathan D. ( February 2024 , Journal of Physical Oceanography)

   Enhanced diapycnal mixing induced by the near-bottom breaking of internal waves is an essential component of the lower meridional overturning circulation. Despite its crucial role in the ocean circulation, tidally driven internal wave breaking is challenging to observe due to its inherently short spatial and temporal scales. We present detailed moored and shipboard observations that resolve the spatiotemporal variability of the tidal response over a small-scale bump embedded in the continental slope of Tasmania. Cross-shore tidal currents drive a nonlinear trapped response over the steep bottom around the bump. The observations are roughly consistent with two-dimensional high-mode tidal lee-wave theory. However, the alongshore tidal velocities are large, suggesting that the alongshore bathymetric variability modulates the tidal response driven by the cross-shore tidal flow. The semidiurnal tide and energy dissipation rate are correlated at subtidal time scales, but with complex temporal variability. Energy dissipation from a simple scattering model shows that the elevated near-bottom turbulence can be sustained by the impinging mode-1 internal tide, where the dissipation over the bump isO(1%) of the incident depth-integrated energy flux. Despite this small fraction, tidal dissipation is enhanced over the bump due to steep topography at a horizontal scale ofO(1) km and may locally drive significant diapycnal mixing.

   Near-bottom turbulent mixing is a key element of the global abyssal circulation. We present observations of the spatiotemporal variability of tidally driven turbulent processes over a small-scale topographic bump off Tasmania. The semidiurnal tide generates large-amplitude transient lee waves and hydraulic jumps that are unstable and dissipate the tidal energy. These processes are consistent with the scattering of the incident low-mode internal tide on the continental slope of Tasmania. Despite elevated turbulence over the bump, near-bottom energy dissipation is small relative to the incident wave energy flux.

    

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4. Mechanisms of Exchange Flow in an Estuary With a Narrow, Deep Channel and Wide, Shallow Shoals

   https://doi.org/10.1029/2020JC016092  

   Geyer, W. Rockwell ; Ralston, David K. ; Chen, Jia‐Lin ( December 2020 , Journal of Geophysical Research: Oceans)

   Delaware Bay is a large estuary with a deep, relatively narrow channel and wide, shallow banks, providing a clear example of a “channel‐shoal” estuary. This numerical modeling study addresses the exchange flow in this channel‐shoal estuary, specifically to examine how the lateral geometry affects the strength and mechanisms of exchange flow. We find that the exchange flow is exclusively confined to the channel region during spring tides, when stratification is weak, and it broadens laterally over the shoals during the more stratified neap tides but still occupies a small fraction of the total width of the estuary. Exchange flow is relatively weak during spring tides, resulting from oscillatory shear dispersion in the channel augmented by weak Eulerian exchange flow. During neap tides, stratification and shear increase markedly, resulting in a strong Eulerian residual shear flow driven mainly by the along‐estuary density gradient, with a net exchange flow roughly 5 times that of the spring tide. During both spring and neap tides, lateral salinity gradients generated by differential advection at the edge of the channel drive a tidally oscillating cross‐channel flow, which strongly influences the stratification, along‐estuary salt balance, and momentum balance. The lateral flow also causes the phase variation in salinity that results in oscillatory shear dispersion and is an advective momentum source contributing to the residual circulation. Whereas the shoals make a negligible direct contribution to the exchange flow, they have an indirect influence due to the salinity gradients between the channel and the shoal.

    

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5. The Influence of A Cross‐Reef Channel On the Wave‐Driven Setup and Circulation at Ipan, Guam

   https://doi.org/10.1029/2019JC015722  

   Clark, S. Jeanette ; Becker, Janet M. ; Merrifield, Mark A. ; Behrens, James ( July 2020 , Journal of Geophysical Research: Oceans)

   The influence of a deep (30 m), narrow (30 m) cross‐shore channel on the circulation and wave‐induced setup over a shallow (∼0.5 m) and wide (∼400 m) shore‐attached fringing reef is examined using field measurements collected at Ipan, Guam. Mean currents on the reef flat over a 7‐week study period during mid and high tides when the reef is submerged are directed toward the channel with the alongshore component of the current increasing with proximity to the channel. The cross‐shore component of the reef flat current is directed onshore at the sensors in the far‐field of the channel with a weak offshore flow at the current meter located closest to the channel (∼760 m to the north). Low‐frequency fluctuations of the alongshore reef flat current and offshore channel current are significantly correlated and with the incident significant wave height. Mean and low‐frequency fluctuating currents are forced by the spatially variable wave‐driven setup, modulated by tidal elevation, which creates a pressure gradient over the reef flat due to the channel where waves do not break. The dominant alongshore momentum balance on the reef flat is between the pressure gradient and bottom stress, with an inferred drag coefficient ofC_D ∼ 0.01. A simple analytical model is presented that is consistent with the observations and delineates the near‐ and far‐field of the channel as a function of the aspect ratio of the reef. Observations from a longer deployment of channel currents are highly correlated with incident wave height in distinct tidal level bands.

    

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