Impact of Offshore Winds on a Buoyant River Plume System

2013 ◽  
Vol 43 (12) ◽  
pp. 2571-2587 ◽  
Author(s):  
Joseph T. Jurisa ◽  
Robert J. Chant
Keyword(s):  
Steady State ◽  
Froude Number ◽  
Near Field ◽  
Vertical Mixing ◽  
River Plume ◽  
Ocean Modeling ◽  
Salt Flux ◽  
Regional Ocean Modeling System ◽  
Plume Region ◽  
The Cross

Abstract Idealized numerical simulations utilizing the Regional Ocean Modeling System (ROMS) are carried out to examine the response of buoyant river plume systems to offshore-directed wind stresses. It is found that after a few inertial periods of wind forcing the plume becomes detached from the coast and reaches a steady state in terms of the plume’s offshore position, width, and plume-averaged depth, salinity, and velocity. The steady-state offshore position of the plume is a balance between the cross-shore advection driven by the estuarine outflow and the alongshore advection driven by the Ekman velocities, and is described using the ratio of the outflow Froude number and the plume Froude number. The steady-state salinity structure is maintained by a balance between the cross-shore advection of salt creating stratification, the turbulent vertical mixing, and the downstream transport of freshwater continually resetting the system. Plume mixing is also analyzed using a salinity coordinate system to track the changes in freshwater volume in salinity space and time. A dynamical plume region classification is developed with use of a Richardson number–based critical mixing salinity criterion in salinity space. This salinity class–based classification agrees well with a classification based on an alongshore analysis of the salt flux equation. For this classification the near field is dominated by large cross-shore fluxes and the midfield by a diminishing cross-shore salt flux, and in the far field there is a balance between the alongshore salt flux and turbulent mixing.

scholarly journals Ebb-Tide Dynamics and Spreading of a Large River Plume*

2009 ◽  
Vol 39 (11) ◽  
pp. 2839-2856 ◽  
Author(s):  
Ryan M. McCabe ◽  
Parker MacCready ◽  
Barbara M. Hickey
Keyword(s):  
Pressure Gradient ◽  
Near Field ◽  
River Estuary ◽  
Columbia River ◽  
River Plume ◽  
Force Balance ◽  
Salt Flux ◽  
Vertical Shear ◽  
Regional Ocean Modeling System ◽  
Initial Flow

Abstract Momentum balances in the near-field region of a large, tidally pulsed river plume are examined. The authors concentrate on a single ebb tide of the Columbia River plume, using the Regional Ocean Modeling System (ROMS) configured to hindcast flow conditions on the Washington and Oregon shelves and in the Columbia River estuary. During ebb, plume-interior streamwise balances are largely between advection, pressure gradient, and frictional forces. Stream-normal balances in this region reduce to centrifugal, cross-stream pressure gradient, and Coriolis terms (i.e., the “gradient wind” balance commonly assumed in river plume bulge investigations). Temporal derivatives are most important at the plume front and as the ebb progresses. Winds were light and contributed little to the force balance. Midebb stress and vertical salt flux were largest at a midplume depth, where stratification and vertical shear were also high, consistent with shear-induced mixing. Internal stress slows the spreading plume considerably. A kinematic description of the spreading process relates lateral spreading to the momentum dynamics and illustrates that plume spreading is largely a competition between the cross-stream pressure gradient and Coriolis forces. However, the very near-field dome of buoyant water is instrumental in setting initial flow pathways.

scholarly journals The Effects of Rotation and River Discharge on Net Mixing in Small-Mouth Kelvin Number Plumes

2016 ◽  
Vol 46 (5) ◽  
pp. 1421-1436 ◽  
Author(s):  
Kelly L. Cole ◽  
Robert D. Hetland
Keyword(s):  
Near Field ◽  
Vertical Mixing ◽  
Ocean Modeling ◽  
Scaling Analysis ◽  
Far Field ◽  
Lateral Expansion ◽  
Regional Ocean Modeling System ◽  
Salinity Field ◽  
Density Anomaly ◽  
Small Mouth

AbstractSmall-mouth Kelvin number plumes, or plumes with a source width smaller than the deformation radius, are characterized by near-field plume regions of rapid lateral expansion and strong vertical mixing. Net plume mixing, or the dilution of a plume by ocean water between the estuary mouth and the far-field plume, is examined using idealized numerical experiments with the Regional Ocean Modeling System (ROMS). The density anomaly of plume water entering the far field is determined from isohaline analysis of the modeled salinity field. The experiments indicate that when estuarine discharge increases, net plume mixing decreases in a rotating environment but increases in a nonrotating environment. Scaling analysis supports that this opposite trend in behavior is related to rotation turning the plume, limiting the lateral expansion and suppressing shear mixing. The results of this study explain different trends in net plume mixing reported in previous studies and compare favorably to observations of the Fraser River plume.

The Spreading of a Buoyant Plume Beneath a Landfast Ice Cover

2015 ◽  
Vol 45 (2) ◽  
pp. 478-494 ◽  
Author(s):  
Jeremy L. Kasper ◽  
Thomas J. Weingartner
Keyword(s):  
Vertical Mixing ◽  
Ice Cover ◽  
Ocean Modeling ◽  
Freshwater Flux ◽  
Buoyant Plume ◽  
Regional Ocean Modeling System ◽  
Case Subject ◽  
Landfast Ice ◽  
Free Case ◽  
Ice Edge

AbstractIdealized numerical simulations using the Regional Ocean Modeling System demonstrate the effects of an immobile landfast ice cover that is frictionally coupled to an underice buoyant plume established by river discharge. The discharge rapidly increases and decreases over a 30-day period and has a maximum of 6000 m3 s−1. This study examined the response to a landfast ice cover of 26-km width and one that encompasses the entire shelf width. The model setting mimics spring conditions on the Alaskan Beaufort Sea (ABS) shelf. In comparison with the ice-free case subject to the same discharge scenario, underice plumes are broader and deeper, and the downwave freshwater flux is substantially decreased and delayed. Multiple anticyclonic bulges form in the ice-free case, but only a single, large bulge forms when ice is present. These differences are because of the frictional coupling between the ice and plume, which results in an Ekman-like underice boundary layer, a subsurface velocity maximum, and frictional shears that enhance vertical mixing and entrainment. For a partially ice-covered shelf, the plume spreads across the ice edge to form a swift, buoyant, ice-edge jet, whose width accords with the scale of Yankovsky and Chapman for a surface-advected plume. For a fully ice-covered shelf, the buoyant water spreads 60 km offshore over a 30-day period. For a steady discharge of 6000 m3 s−1 and a completely ice-covered shelf, the plume spreads offshore at a rate of ~1.5 km day−1 and extends ~95 km offshore after 60 days.

scholarly journals Buoyancy-driven effects on turbulent diffusivity induced by a river plume in the southern Brazilian shelf

2018 ◽  
Author(s):  
Rafael André Ávila ◽  
Paulo H. R. Calil
Keyword(s):  
Continental Shelf ◽  
Large Scale ◽  
Vertical Mixing ◽  
River Estuary ◽  
River Plume ◽  
La Plata ◽  
Plume Region ◽  
Freshwater Plumes ◽  
Local Water ◽  
Gradient Ratio

Abstract. Freshwater plumes are important flow structures that influence the dynamics and water properties of coastal regions and continental shelves. Turbulence in plume regions is mainly driven by shear instabilities at the interface between plume and oceanic waters, which, in turn, depend on the geometry and outflow of a specific plume region. The Southern Brazilian Shelf presents a highly variable hydrographic distribution modulated by the seasonal wind variation and the freshwater discharge from the La Plata River estuary, which has a significant impact on the continental shelf circulation. This buoyant plume creates strong density gradients and interacts with local water masses resulting in a complex hydrographic pattern. In this study, high resolution hydrography and microstructure measurements were obtained in order to verify the effect of freshwater stratification on vertical mixing in this highly dynamic continental shelf. Results show that the plume is highly stable at southern portions of the shelf, as density displacements, or Thorpe displacements, δT, heat diffusivity, KT, buoyancy flux, Bf, and density gradient ratio, Rp are reduced when compared to the northern areas. Moreover, hydrographic data suggests that the large-scale La Plata River plume has a dynamic mid-field region due to instabilities generated when reaching the shelf break.

scholarly journals Meanders and eddy formation by a buoyant coastal current flowing over a sloping topography

Ocean Science ◽  
2017 ◽  
Vol 13 (6) ◽  
pp. 905-923 ◽  
Author(s):  
Laura Cimoli ◽  
Alexandre Stegner ◽  
Guillaume Roullet
Keyword(s):  
Linear Instability ◽  
Ocean Modeling ◽  
Coastal Current ◽  
Regional Ocean Modeling System ◽  
Non Linear ◽  
Dynamical Regimes ◽  
The Cross ◽  
Linear Instability Analysis ◽  
Channel Configuration ◽  
Sloping Topography

Abstract. This study investigates the linear and non-linear instability of a buoyant coastal current flowing along a sloping topography. In fact, the bathymetry strongly impacts the formation of meanders or eddies and leads to different dynamical regimes that can both enhance or prevent the cross-shore transport. We use the Regional Ocean Modeling System (ROMS) to run simulations in an idealized channel configuration, using a fixed coastal current structure and testing its unstable evolution for various depths and topographic slopes. The experiments are integrated beyond the linear stage of the instability, since our focus is on the non-linear end state, namely the formation of coastal eddies or meanders, to classify the dynamical regimes. We find three non-linear end states, whose properties cannot be deduced solely from the linear instability analysis. They correspond to a quasi-stable coastal current, the propagation of coastal meanders, and the formation of coherent eddies. We show that the topographic parameter Tp, defined as the ratio of the topographic Rossby wave speed over the current speed, plays a key role in controlling the amplitude of the unstable cross-shore perturbations. This result emphasizes the limitations of linear stability analysis to predict the formation of coastal eddies, because it does not account for the non-linear saturation of the cross-shore perturbations, which is predominant for large negative Tp values. We show that a second dimensionless parameter, the vertical aspect ratio γ, controls the transition from meanders to coherent eddies. We suggest the use of the parameter space (Tp, γ) to describe the emergence of coastal eddies or meanders from an unstable buoyant current. By knowing the values of Tp and γ for an observed flow, which can be calculated from hydrological sections, we can identify which non-linear end state characterizes that flow – namely if it is quasi-stable, meanders, or forms eddies.

Relating River Plume Structure to Vertical Mixing

2005 ◽  
Vol 35 (9) ◽  
pp. 1667-1688 ◽  
Author(s):  
Robert D. Hetland
Keyword(s):  
Coordinate System ◽  
Critical Thickness ◽  
High Salinity ◽  
Near Field ◽  
Vertical Mixing ◽  
River Plume ◽  
Plume Structure ◽  
Shear Mixing ◽  
Wind Mixing ◽  
Work Done

Abstract The structure of a river plume is related to the vertical mixing using an isohaline-based coordinate system. Salinity coordinates offer the advantage of translating with the plume as it moves or expanding as the plume grows. This coordinate system is used to compare the relative importance of different dynamical processes acting within the plume and to describe the effect each process has on the structure of the plume. Vertical mixing due to inertial shear in the outflow of a narrow estuary and wind mixing are examined using a numerical model of a wind-forced river plume. Vertical mixing, and the corresponding entrainment of background waters, is greatest near the estuary mouth where inertial shear mixing is large. This region is defined as the near field, with the more saline, far-field plume beyond. Wind mixing increases the mixing throughout the plume but has the greatest effect on plume structure at salinity ranges just beyond the near field. Wind mixing is weaker at high salinity classes that have already been mixed to a critical thickness, a point where turbulent mixing of the upper layer by the wind is reduced, protecting these portions of the plume from further wind mixing. The work done by mixing on the plume is of similar magnitude in both the near and far fields.

scholarly journals A NUMERICAL MODELING STUDY ON UPWELLING MECHANISM IN SOUTHERN MAKASSAR STRAIT

2015 ◽  
Vol 6 (2) ◽  
Author(s):  
Agus S Atmadipoera ◽  
Priska Widyastuti
Keyword(s):  
Vertical Mixing ◽  
Ocean Modeling ◽  
Observation Data ◽  
Physical Processes ◽  
Regional Ocean Modeling System ◽  
Chl A ◽  
Roms Model ◽  
Noaa Avhrr ◽  
Monsoon Winds ◽  
Upwelling Event

ABSTRACT While it has been well documented in the previous studies that upwelling events in the southern Makassar Strait (MAK) during the Southeast Monsoon (SEM) period are associated with low sea surface temperature (SST) and high chlorophyll-a (Chl-a) concentrations in the seawater, the dynamic and physical processes that trigger these upwelling events are still less well understood. In the present study we proposed a mechanism of the upwelling event using a numerical model of the Regional Ocean Modeling System (ROMS).  Model validations showed a high correlation of SST climatology between the model and the NOAA-AVHRR satellite data. Moreover, velocity fields of the Indonesian Throughflow (ITF) Makassar in Libani Channel was well reproduced by proposed model, revealing an intensification of the flow centered near 120 m depth, which is in good agreement with the observation data. The model demonstrated that during the SEM period strong southeasterly winds that blow over southern Sulawesi Island can increase high vertical diffusivity and heat loss through heat flux. Hence, these physical processes lead to increased vertical mixing that, in turn, generates low SST, as a proxy of upwelling event. Furthermore, the upwelling process is enhanced by the ITF Makassar jet that creates large circular eddies flow due to complex topographic within the triangle area of southern Makassar - eastern Java Sea - western Flores Sea. The eddies generate the area of convergence offshore along the ITF pathways and divergence area in the coastal waters close to southern Sulawesi Island.  Model experiment with closing/opening Selayar Strait revealed a change of intensity and area of upwelling, suggesting that the Selayar Island forms a barrier for the outflow from MAK to northern part of Flores Sea. Keywords: Upwelling, ITF Makassar, SE monsoon winds, ROMS-AGRIF, Makassar Strait.

scholarly journals An evaluation of the performance of vertical mixing parameterizations for tidal mixing in the Regional Ocean Modeling System (ROMS)

2019 ◽  
Vol 6 (1) ◽  
Author(s):  
Robin Robertson ◽  
Changming Dong
Keyword(s):  
Water Column ◽  
Ocean Circulation ◽  
Climate Models ◽  
Energy Transport ◽  
Vertical Mixing ◽  
Internal Tides ◽  
Ocean Modeling ◽  
Tidal Mixing ◽  
Ocean Energy ◽  
Regional Ocean Modeling System

AbstractVertical mixing is important in the ocean for maintaining its stratification, redistributing temperature and salinity, distributing nutrients and pollutants, and the energy cascade. It plays a key role in ocean energy transport, climate change, and marine ecosystems. Getting the mixing right in ocean circulation and climate models is critical in reproducing ocean and climate physics. Ocean models, like the Regional Ocean Modeling System (Rutgers ROMS 3.4), provide several options for determining vertical mixing through the vertical mixing parameterization schemes. To evaluate which of these methods best reproduces realistic vertical mixing by internal tides, simulations of baroclinic tides generated by a seamount were performed using seven different vertical mixing parameterizations: Mellor-Yamada 2.5 (MY), Large-McWilliams-Doney’s Kpp (LMD), Nakanishi-Niino’s modification of Mellor-Yamada (NN), and four versions of Generic Length Scale (GLS). The GLS versions in ROMS 3.4 severely overmixed the water column within a day and were not considered realistic. We suspect that a coding error has been introduced for it. We focused on the performance of the MY, LMD, and NN vertical mixing parameterizations. LMD was found to overmix the water column. The performance of MY and NN were nearly equivalent and both well reproduced the observed velocity and diffusivity fields. NN performed slightly better by having a lower rms for M2 and K1, less benthic mixing, more mid-water column mixing, less overmixing, and fewer extremely high diffusivities (> 1 m2 s−1).

scholarly journals Meanders and eddies formation by a buoyant coastal current flowing over a sloping topography

2017 ◽  
Author(s):  
Laura Cimoli ◽  
Alexandre Stegner ◽  
Guillaume Roullet
Keyword(s):  
Linear Instability ◽  
Ocean Modeling ◽  
Coastal Current ◽  
Regional Ocean Modeling System ◽  
Non Linear ◽  
Dynamical Regimes ◽  
The Cross ◽  
Linear Instability Analysis ◽  
Channel Configuration ◽  
Sloping Topography

Abstract. This study investigates the linear and non-linear instability of a buoyant coastal current flowing along a sloping topography. In fact, the bathymetry strongly impacts the formation of meanders or eddies and leads to different dynamical regimes that can both enhance or prevent the cross-shore transport. We use the Regional Ocean Modeling System (ROMS) to run simulations in an idealized channel configuration, using a fixed coastal current structure and testing its unstable evolution for various depths and topographic slopes. The experiments are integrated beyond the linear stage of the instability, since our focus is on the non-linear end state, namely the formation of coastal eddies or meanders, to classify the dynamical regimes. We find three non-linear end states, whose properties cannot be deduced solely from the linear instability analysis. They correspond to a quasi-stable coastal current, the propagation of coastal meanders, and the formation of coherent eddies. We show that the topographic parameter, Tp, defined as the ratio of the topographic Rossby wave speed over the current speed, plays a key role in controlling the amplitude of the unstable cross-shore perturbations. This result emphasizes the limitations of linear stability analysis to predict the formation of coastal eddies, because it does not account for the non-linear saturation of the cross-shore perturbations, which is predominant for large negative Tp values. We show that a second dimensionless parameter, the vertical aspect ratio γ, controls the transition from meanders to coherent eddies. We suggest the use of the parameter space (Tp, γ) to describe the emergence of coastal eddies or meanders from an unstable buoyant current. By knowing the values of Tp and γ for an observed flow, which can be calculated from hydrological sections, we can identify which non-linear end states characterizes that flow, namely if it is quasi-stable, meanders, or forms eddies.

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