Tidal Plume Front Research Articles

Three-dimensional observations of tidal plume fronts in estuaries using a synthetic aperture sonar array.

Synthetic aperture sonar (sas) systems are designed to observe stationary scatterers located near the sediment interface. Less commonly, a sas system may be used to observe scattering features located above the sonar in the water column. The Undersea Remote Sensing (USRS) project, sponsored by the Office of Naval Research, was a collaborative Directed Research Initiative (DRI) focused on studying dynamic estuarine water column features. During the USRS DRI, researchers from multiple institutions gathered to observe tidal features at various estuaries along the coast of the United States using both in situ and remote sensing techniques, including sas. The first studied estuary was the mouth of the Connecticut River (CTR). Data captured by a sas system deployed during a tidal event were post-processed to create three-dimensional observations of the structure of the leading edge of the CTR's ebb plume front. From these observations, lobed structures similar in scale to previously reported instabilities are revealed, with the present observations providing additional insight regarding the structure of the bubble distribution behind the front. Velocity estimates of plume features were also determined from sas data and shown to compare favorably with concurrent marine radar estimates.

Observations of Multiple Internal Wave Packets in a Tidal River Plume

AbstractRemotely sensed images document the occurrence of multiple packets of internal solitary waves (ISWs) in the Rhine River plume at the same time. We use a combination of field observations, and non‐hydrostatic and hydrostatic modeling to understand the processes that lead to the generation and retention of multiple ISW packets within the Rhine plume. Previous numerical modeling shows that the tidal plume front is trapped in the mid‐field plume for more than one tidal cycle due to tidal straining and recirculation within the plume, resulting in the presence of multiple fronts in the near‐and mid‐field plume regions. In this work, we show how variations in the strength of these fronts can lead to the release of ISW packets. We conclude that the retention of the fronts in the mid‐field region of the plume and modulation in the strength of the fronts can explain the presence of multiple ISW packets. A frontal Froude number analysis shows that fronts generated during the previous ebb tide can release ISWs in addition to the newly released tidal plume front.

The Evolution of Plume Fronts in the Rhine Region of Freshwater Influence

AbstractThe Rhine region of freshwater influence (ROFI) is strongly stratified, rotational, relatively shallow and has large tides, resulting in a dynamic field of fronts that are formed by multiple processes. We use a 3D numerical model to obtain a conceptual picture of the frontal structure and the processes responsible for generating this multiple front structure in the Rhine ROFI. The horizontal salinity gradient and numerical tracers are used to identify three different types of fronts: outer, inner, tidal plume and relic tidal plume fronts. Tidal plume front (TPF) trajectories together with the tracers demonstrate that TPFs exist for longer than one tidal cycle. A Lagrangian frontogenesis analysis shows that the fronts are strengthened mainly as a result of increased convergence, which is observed to occur at times when tidal straining is large. Additionally the alongshore tidal excursion and the dominance of the tidal currents over the intrinsic frontal propagation speed, trap TPFs within 20 km from the river mouth. Trapping and re‐strengthening maintain several fronts at a time in the mid‐field region, resulting in a multi‐frontal system. The observation of a complex river plume system is expected to be important for cross‐shore exchange, transport and coastal ecology.

River plume source-front connectivity

River plume source-front connectivity, or the relationship between processes at the source, plume body and front, is investigated with numerical simulations of the Merrimack River plume using the Regional Ocean modeling System (ROMS). Source water, i.e. water discharged from the estuary during an ebb pulse, overtakes the front as it propagates offshore, resulting in the strong convergence that exists there. Model results show that much of the water released over an ebb pulse does not interact with the propagating tidal plume front, suggesting that the front decouples from the source and sustains itself as a distinct water mass. When a gate is closed at the estuary mouth during early ebb, the overall plume structure changes; gates closed late in ebb do not significantly effect the along- or cross-shore scale of the plume. The change in plume extent is only evident hours after the early-ebb gate closing, when source water is no longer supplied to the front. These experiments suggest that the growing distance between the highly energetic liftoff region and the radial expansion of the plume front results in a flattening of the surface gradient driving flow in the interior, diminishing source-front connectivity.

The Influence of Tide and Wind on the Propagation of Fronts in a Shallow River Plume

AbstractIn this study we used field data and radar images to investigate the influence of winds and tides on the propagation of tidal plume fronts. The measurements were collected in a shallow shelf region off the Dutch coast, 10 km north of the Rhine River mouth, and they clearly show the passage of distinct freshwater lenses and associated fronts at the surface that propagate all the way to the coastline. These fronts are observed as a sudden drop in near‐surface salinity, accompanied by high cross‐shore shear with onshore velocities at the surface. We determined the arrival time to our measurement site, frontal propagation speed, and structure of the fronts by combining the in situ data and radar images. Frontal Froude numbers show a wide range of values, with an average of 0.44. Our results show that fronts during spring tides are thinner, more mixed, and move faster relative to the ground during calm spring tides when compared to calm neap tides. Downwelling winds during spring tides result in thicker and faster fronts; however, the intrinsic frontal propagation speed indicates that the wind and tide control the frontal propagation mainly due to advection rather than by changing the frontal structure. Strong return currents in the near‐bed layer resulting from fast and thick fronts increase near‐bed turbulence and bed stresses. These high stresses suggest that the passage of fronts in shallow coastal areas can initiate sediment resuspension and contribute to transport processes.

Observations and numerical modeling of the Pearl River plume in summer season

A cruise survey of the Pearl River plume during southeasterly and southwesterly winds, two typical wind patterns in summer in Guangdong coastal waters, is reported and the cruise data are analyzed to unveil the plume dynamics. The Kelvin number is derived from the in situ data, revealing that the Pearl River plume exhibits two different scales in response to the two kinds of the wind forcing. Numerical model simulations based on the regional oceanic modeling system (ROMS) with nesting domains are implemented to explore details of the plume dynamics, validated by cruise observations and tidal gauge sea level data. Modeling results suggest that there is a subtidal, anticyclonic recirculation bulge on the west side out of the estuary mouth under southeasterly winds, showing a recirculating plume in the near-field. When the wind changes to the southwesterly, however, the recirculating plume vanishes. The distinct, supercritical tidal plume front occurs with southeasterly winds prevailing in spring tide due to the strong advection in the tidal plume. The tidal salt deficit flux can reach as high as 12.5% of the mean current flux, and illustrates the combined forcing of tide and wind. Key Points The paper clarifies the Pearl River plume anatomy under two typical winds Spring tide facilitates the supercritical Pearl River tidal plume front Tide determines tidal salt deficit flux strength; wind determines the direction

Marine bird aggregations associated with the tidally-driven plume and plume fronts of the Columbia River

Freshwater discharge from large rivers into the coastal ocean creates tidally-driven frontal systems known to enhance mixing, primary production, and secondary production. Many authors suggest that tidal plume fronts increase energy flow to fish-eating predators by attracting planktivorous fishes to feed on plankton aggregated by the fronts. However, few studies of plume fronts directly examine piscivorous predator response to plume fronts. Our work examined densities of piscivorous seabirds relative to the plume region and plume fronts of the Columbia River, USA. Common murres (Uria aalge) and sooty shearwaters (Puffinus griseus) composed 83% of all birds detected on mesoscale surveys of the Washington and Oregon coasts (June 2003–2006), and 91.3% of all birds detected on fine scale surveys of the plume region less than 40km from the river mouth (May 2003 and 2006). Mesoscale comparisons showed consistently more predators in the central plume area compared to the surrounding marine area (murres: 10.1–21.5 vs. 3.4–8.2birdskm−2; shearwaters: 24.2–75.1 vs. 11.8–25.9birdskm−2). Fine scale comparisons showed that murre density in 2003 and shearwater density in both 2003 and 2006 were significantly elevated in the tidal plume region composed of the most recently discharged river water. Murres tended to be more abundant on the north face of the plume. In May 2003, more murres and shearwaters were found within 3km of the front on any given transect, although maximum bird density was not necessarily found in the same location as the front itself. Predator density on a given transect was not correlated with frontal strength in either year. The high bird densities we observed associated with the tidal plume demonstrate that the turbid Columbia River plume does not necessarily provide fish with refuge from visual predators. Bird predation in the plume region may therefore impact early marine survival of Pacific salmon (Oncorhynchus spp.), which must migrate through the tidal plume and plume front to enter the ocean. Because murres and shearwaters eat primarily planktivorous fish such as the northern anchovy (Engraulis mordax), aggregation of these birds in the plume supports the hypothesis that it is the plume region as a whole, and not just the plume fronts, which enhances trophic transfer to piscivorous predators via planktivorous fishes.

Structure and dynamics of the Columbia River tidal plume front

Time‐dependent buoyant plumes form at the outflow of tidally dominated estuaries. When estuary discharge velocity exceeds plume internal wave speed c, a sharp front forms at the plume's leading edge that expands from the time‐dependent source. Using observations of the Columbia River tidal plume from multiple tidal cycles we characterize time‐evolving plume structure and quantify front speed Uf, plume internal wave speed c, front curvature, and ultimate extent. We identify three distinct stages of propagation: (1) Initially, the plume is strongly influenced by shallow bathymetry near the river mouth. (2) As the front advances offshore the plume detaches from the bottom and expands as a freely propagating gravity current with relatively constant Uf, c and frontal Froude number F = Uf/c. Ambient currents explain intracycle variability in Uf and winds alter front shape. Variability in ambient stratification associated with previous cycles' plume remnants leads to complex fronts and internal waves. (3) Finally, the plume decelerates, adjusts toward geostrophy, and may radiate additional internal waves. Using a simple kinematic model, we suggest that constant frontal propagation speed, Uf = 0.9 ± 0.1 m/s, during stage 2 is primarily controlled by linearly increasing volume flux from the Columbia River mouth. As this discharge rate subsides, the plume expands as a fixed volume with decreasing front speed (stage 3). The plume's final extent is controlled by the Rossby radius, which scales with a length based on the total volume discharged. This provides an integral description of plume front evolution based on the time‐dependent estuary discharge.

Asymmetry of Columbia River tidal plume fronts

Columbia River tidal plume dynamics can be explained in terms of two asymmetries related to plume-front depth and internal wave generation. These asymmetries may be an important factor contributing to the observed greater primary productivity and phytoplankton standing crop on the Washington shelf. The tidal plume (the most recent ebb outflow from the estuary) is initially supercritical with respect to the frontal internal Froude number F R on strong ebbs. It is separated from the rotating plume bulge by a front, whose properties are very different under upwelling vs. downwelling conditions. Under summer upwelling conditions, tidal plume fronts are sharp and narrow (< 20–50 m wide) on their upwind or northern side and mark a transition from supercritical to subcritical flow for up to 12 h after high water. Such sharp fronts are a source of turbulent mixing, despite the strong stratification. Because the tidal plume may overlie newly upwelled waters, these fronts can mix nutrients into the plume. Symmetry would suggest that there should be a sharp front south of the estuary mouth under summer downwelling conditions. Instead, the downwelling tidal plume front is usually diffuse on its upstream side. Mixing is weaker, and the water masses immediately below are low in nutrients. There is also an upwelling–downwelling asymmetry in internal wave generation. During upwelling and weak wind conditions, plume fronts often generate trains of non-linear internal waves as they transition from a supercritical to a subcritical state. Under downwelling conditions, internal wave release is less common and the waves are less energetic. Furthermore, regardless of wind conditions, solition formation almost always begins on the south side of the plume so that the front “unzips” from south to north. This distinction is important, because these internal waves contribute to vertical mixing in the plume bulge and transport low-salinity water across the tidal plume into the plume bulge. F R and plume depth are key parameters in distinguishing the upwelling and downwelling situations, and these two asymmetries can be explained in terms of potential vorticity conservation. The divergence of the tidal outflow after it leaves the estuary embeds relative vorticity in the emerging tidal plume water mass. This vorticity controls the transition of the tidal plume front to a subcritical state and consequently the timing and location of internal wave generation by plume fronts.

Effects of ambient velocity shear on nonlinear internal wave associated mixing at the Columbia River plume front

Large‐amplitude nonlinear internal waves (NLIWs) are frequently observed propagating away from Columbia River tidal plume fronts. They are generated because of the deceleration of the frontal bulge. During the River Influences on Shelf Ecosystems project cruises, the velocity, density structure and acoustic backscatter of the plume fronts and frontal NLIWs were observed using a towed vehicle, vessel‐mounted instrumentation, and a vessel X band radar. These observations indicate that in the presence of strong ambient velocity shear, the NLIWs with maximum amplitudes occur well below the density interface and at a depth deeper than in the absence of shear. This deepening of the NLIW shear is associated with intensified vertical mixing below the interface. This is consistent with a dynamic analysis of NLIWs under the influence of sheared ambient flow, obtained from a high‐order KdV model. There are two mechanisms responsible for the vertical turbulent mixing intensification. The nonlinear interaction between ambient shear and the NLIWs increases total velocity shear and causes the gradient Richardson number Rig to decrease below a critical value in certain depth ranges. Also, because of the presence of the ambient shear, the maximum NLIW velocity shear occurs at a depth below the density interface where there is less stratification.

Observations at the tidal plume front of a high‐volume river outflow

We present shipboard observations of very strong convergence, vertical velocities and mixing, and near‐bed impacts associated with the leading‐edge front of the tidally‐pulsed Columbia River plume. With upwelling‐favorable winds and riverflow of 4900 m3s−1, the plume propagates as a buoyant gravity current with a rotary, bore‐like vertical frontal circulation and downwelling as strong as 0.35 m s−1. In waters as deep as 65 m, near‐bed currents intensify to as much as 1.0 m s−1 after frontal passage, and are often associated with elevated acoustic backscatter. Mixing is locally strong, with an eddy diffusivity of O(0.2 m2s−1) 50 m behind the front, and T‐S diagrams imply plume mixing with 10 m deep ocean water. These observations indicate that the leading‐edge front of a surface‐advected plume can cause exchanges of (a) nutrients between cold subsurface shelf waters and the river plume, and (b) nutrients and sediments across the sediment‐water interface.

A tidal plume front and internal solitons

A tidal plume front (internal tidal bore) propagating from the Apalachicola Bay outflow into the northern Gulf of Mexico is examined in a brief set of observations. Temperature and salinity time series showed a radially spreading “pool” of estuarine water with a character similar to a previously observed “turbulent rotor”, and with a following packet of internal solitons. Features associated with these observations may be pertinent to interpretation of remotely sensed plumes and to predictive modeling.