Do Eddies Play a Role in the Momentum Balance of the Leeuwin Current?

2005 ◽  
Vol 35 (6) ◽  
pp. 964-975 ◽  
Author(s):  
Ming Feng ◽  
Susan Wijffels ◽  
Stuart Godfrey ◽  
Gary Meyers
Keyword(s):  
Kinetic Energy ◽  
Pressure Gradient ◽  
Reynolds Stress ◽  
Wind Stress ◽  
Mesoscale Eddies ◽  
Eddy Kinetic Energy ◽  
Momentum Balance ◽  
Gradient Force ◽  
Leeuwin Current ◽  
Eastern Boundary

Abstract The Leeuwin Current is a poleward-flowing eastern boundary current off the western Australian coast, and alongshore momentum balance in the current has been hypothesized to comprise a southward pressure gradient force balanced by northward wind and bottom stresses. This alongshore momentum balance is revisited using a high-resolution upper-ocean climatology to determine the alongshore pressure gradient and altimeter and mooring observations to derive an eddy-induced Reynolds stress. Results show that north of the Abrolhos Islands (situated near the shelf break between 28.2° and 29.3°S), the alongshore momentum balance is between the pressure gradient and wind stress. South of the Abrolhos Islands, the Leeuwin Current is highly unstable and strong eddy kinetic energy is observed offshore of the current axis. The alongshore momentum balance on the offshore side of the current reveals an increased alongshore pressure gradient, weakened alongshore wind stress, and a significant Reynolds stress exerted by mesoscale eddies. The eddy Reynolds stress has a −0.5 Sv (Sv ≡ 106 m3 s−1) correction to the Indonesian Throughflow transport estimate from Godfrey’s island rule. The mesoscale eddies draw energy from the mean current through mixed barotropic and baroclinic instability, and the pressure gradient work overcomes the negative wind work to supply energy for the instability process. Hence the anomalous large-scale pressure gradient in the eastern Indian Ocean drives the strongest eddy kinetic energy level among all the midlatitude eastern boundary currents.

scholarly journals Temporal evolution of the momentum balance terms and frictional adjustment observed over the inner shelf during a storm

Ocean Science ◽  
2016 ◽  
Vol 12 (1) ◽  
pp. 137-151 ◽  
Author(s):  
M. Grifoll ◽  
A. L. Aretxabaleta ◽  
J. L. Pelegrí ◽  
M. Espino
Keyword(s):  
Coriolis Force ◽  
Wind Stress ◽  
Water Depth ◽  
Momentum Balance ◽  
Wave Radiation ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Inner Shelf ◽  
Coastal Waves ◽  
Nw Mediterranean

Abstract. We investigate the rapidly changing equilibrium between the momentum sources and sinks during the passage of a single two-peak storm over the Catalan inner shelf (NW Mediterranean Sea). Velocity measurements at 24 m water depth are taken as representative of the inner shelf, and the cross-shelf variability is explored with measurements at 50 m water depth. During both wind pulses, the flow accelerated at 24 m until shortly after the wind maxima, when the bottom stress was able to compensate for the wind stress. Concurrently, the sea level also responded, with the pressure-gradient force opposing the wind stress. Before, during and after the second wind pulse, there were velocity fluctuations with both super- and sub-inertial periods likely associated with transient coastal waves. Throughout the storm, the Coriolis force and wave radiation stresses were relatively unimportant in the along-shelf momentum balance. The frictional adjustment timescale was around 10 h, consistent with the e-folding time obtained from bottom drag parameterizations. The momentum evolution at 50 m showed a larger influence of the Coriolis force at the expense of a decreased frictional relevance, typical in the transition from the inner to the mid-shelf.

Two-equation turbulence modeling of an oscillatory boundary layer under steep pressure gradient

2010 ◽  
Vol 37 (4) ◽  
pp. 648-656 ◽  
Author(s):  
Ahmad Sana ◽  
Hitoshi Tanaka
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Pressure Gradient ◽  
Turbulent Kinetic Energy ◽  
Reynolds Stress ◽  
Turbulence Modeling ◽  
Turbulence Models ◽  
Detailed Comparison ◽  
Stream Velocity ◽  
Oscillatory Boundary Layer

A total of seven versions of two-equation turbulence models (four versions of low Reynolds number k–ε model, one k–ω model and two versions of k–ε / k–ω blended models) are tested against the direct numerical simulation (DNS) data of a one-dimensional oscillatory boundary layer with flat crested free-stream velocity that results from a steep pressure gradient. A detailed comparison has been made for cross-stream velocity, turbulent kinetic energy (TKE), Reynolds stress, and ratio of Reynolds stress and turbulent kinetic energy. It is observed that the newer versions of k–ε model perform very well in predicting the velocity, turbulent kinetic energy, and Reynolds stress. The k–ω model and blended models underestimate the peak value of turbulent kinetic energy that may be explained by the Reynolds stress to TKE ratio in the logarithmic zone. The maximum bottom shear stress is well predicted by the k–ε model proposed by Sana et al. and the original k–ω model.

scholarly journals The Deflection of the China Coastal Current over the Taiwan Bank in Winter

2018 ◽  
Vol 48 (7) ◽  
pp. 1433-1450 ◽  
Author(s):  
Enhui Liao ◽  
Lie Yauw Oey ◽  
Xiao-Hai Yan ◽  
Li Li ◽  
Yuwu Jiang
Keyword(s):  
Pressure Gradient ◽  
Wind Stress ◽  
Large Scale ◽  
Numerical Models ◽  
Gradient Force ◽  
Coastal Current ◽  
Pressure Gradient Force ◽  
Bottom Stress ◽  
In Situ Data ◽  
Offshore Flow

AbstractIn winter, an offshore flow of the coastal current can be inferred from satellite and in situ data over the western Taiwan Bank. The dynamics related to this offshore flow are examined here using observations as well as analytical and numerical models. The currents can be classified into three regimes. The downwind (i.e., southward) cold coastal current remains attached to the coast when the northeasterly wind stress is stronger than a critical value depending on the upwind (i.e., northward) large-scale pressure gradient force. By contrast, an upwind warm current appears over the Taiwan Bank when the wind stress is less than the critical pressure gradient force. The downwind coastal current and upwind current converge and the coastal current deflects offshore onto the bank during a moderate wind. Analysis of the vorticity balance shows that the offshore transport is a result of negative bottom stress curl that is triggered by the positive vorticity of the two opposite flows. The negative bottom stress curl is reinforced by the gentle slope over the bank, which enhances the offshore current. Composite analyses using satellite observations show cool waters with high chlorophyll in the offshore current under moderate wind. The results of composite analyses support the model findings and may explain the high productivity over the western bank in winter.

scholarly journals An Oceanic Current against the Wind: How Does Taiwan Island Steer Warm Water into the East China Sea?

2007 ◽  
Vol 37 (10) ◽  
pp. 2563-2569 ◽  
Author(s):  
Jiayan Yang
Keyword(s):  
Pressure Gradient ◽  
Wind Stress ◽  
Pressure Difference ◽  
East Coast ◽  
Taiwan Strait ◽  
Momentum Balance ◽  
Western Boundary ◽  
Current Layer ◽  
The Taiwan Strait ◽  
The Kuroshio

Abstract Along the Taiwan Strait (<100 m in depth) a northeastward flow persists in all seasons despite the annually averaged wind stress that is strongly southwestward. The forcing mechanism of this countercurrent is examined by using a simple ocean model. The results from a suite of experiments demonstrate that it is the Kuroshio that plays the deciding role for setting the flow direction along the Taiwan Strait. The momentum balance along the strait is mainly between the wind stress, friction, and pressure gradient. Since both wind stress and friction act against the northward flow, it is most likely the pressure gradient that forces the northward flow, as noted in some previous studies. What remains unknown is why there is a considerable pressure difference between the southern and northern strait. The Kuroshio flows along the east coast of Taiwan, and thus the western boundary current layer dynamics applies there. Integrating the momentum equation along Taiwan’s east coast shows that there must be a pressure difference between the southern and the northern tip of Taiwan to counter a considerable friction exerted by the mighty Kuroshio. This same pressure difference is also felt on the other side of the island where it forces the northward flow through Taiwan Strait. The model shows that the local wind stress acts to dampen this northward flow. This mechanism can be illustrated by an integral constraint for flow around an island.

scholarly journals Lagrangian eddy kinetic energy of ocean mesoscale eddies and its application to the Northwestern Pacific

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Mengrong Ding ◽  
Pengfei Lin ◽  
Hailong Liu ◽  
Aixue Hu ◽  
Chuanyu Liu
Keyword(s):  
Kinetic Energy ◽  
Mesoscale Eddies ◽  
Eddy Kinetic Energy ◽  
Northwestern Pacific

scholarly journals Coastal Trapped Waves, Alongshore Pressure Gradients, and the California Undercurrent*

2014 ◽  
Vol 44 (1) ◽  
pp. 319-342 ◽  
Author(s):  
Thomas P. Connolly ◽  
Barbara M. Hickey ◽  
Igor Shulman ◽  
Richard E. Thomson
Keyword(s):  
Pressure Gradient ◽  
Wind Stress ◽  
West Coast ◽  
Current System ◽  
Ocean Model ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Pressure Gradients ◽  
Transport Pathway ◽  
California Current System

Abstract The California Undercurrent (CUC), a poleward-flowing feature over the continental slope, is a key transport pathway along the west coast of North America and an important component of regional upwelling dynamics. This study examines the poleward undercurrent and alongshore pressure gradients in the northern California Current System (CCS), where local wind stress forcing is relatively weak. The dynamics of the undercurrent are compared in the primitive equation Navy Coastal Ocean Model and a linear coastal trapped wave model. Both models are validated using hydrographic data and current-meter observations in the core of the undercurrent in the northern CCS. In the linear model, variability in the predominantly equatorward wind stress along the U.S. West Coast produces episodic reversals to poleward flow over the northern CCS slope during summer. However, reproducing the persistence of the undercurrent during late summer requires additional incoming energy from sea level variability applied south of the region of the strongest wind forcing. The relative importance of the barotropic and baroclinic components of the modeled alongshore pressure gradient changes with latitude. In contrast to the southern and central portions of the CCS, the baroclinic component of the alongshore pressure gradient provides the primary poleward force at CUC depths over the northern CCS slope. At time scales from weeks to months, the alongshore pressure gradient force is primarily balanced by the Coriolis force associated with onshore flow.

An Adjoint Sensitivity Analysis of the Southern California Current Circulation and Ecosystem

2009 ◽  
Vol 39 (3) ◽  
pp. 702-720 ◽  
Author(s):  
Andrew M. Moore ◽  
Hernan G. Arango ◽  
Emanuele Di Lorenzo ◽  
Arthur J. Miller ◽  
Bruce D. Cornuelle
Keyword(s):  
Sensitivity Analysis ◽  
Kinetic Energy ◽  
Wind Stress ◽  
Current System ◽  
Baroclinic Instability ◽  
California Current ◽  
Eddy Kinetic Energy ◽  
Ocean Modeling ◽  
Complex Flow ◽  
California Current System

Abstract Adjoint methods of sensitivity analysis were applied to the California Current using the Regional Ocean Modeling Systems (ROMS) with medium resolution, aimed at diagnosing the circulation sensitivity to variations in surface forcing. The sensitivities of coastal variations in SST, eddy kinetic energy, and baroclinic instability of complex time-evolving flows were quantified. Each aspect of the circulation exhibits significant interannual and seasonal variations in sensitivity controlled by mesoscale circulation features. Central California SST is equally sensitive to wind stress and surface heat flux, but less so to wind stress curl, displaying the greatest sensitivity when upwelling-favorable winds are relaxing and the least sensitivity during the peak of upwelling. SST sensitivity is typically 2–4 times larger during summer than during spring, although larger variations occur during some years. The sensitivity of central coast eddy kinetic energy to surface forcing is constant on average throughout the year. Perturbations in the wind that align with mesoscale eddies to enhance the strength of the circulation by local Ekman pumping yield the greatest sensitivities. The sensitivity of the potential for baroclinic instability is greatest when nearshore horizontal temperature gradients are largest, and it is associated with variations in wind stress concentrated along the core of the California Current. The sensitivity varies by a factor of ∼1.5 throughout the year. A new and important aspect of this work is identification of the complex flow dependence and seasonal dependence of the sensitivity of the ROMS California Current System (CCS) circulation to variations in surface forcing that was hitherto not previously appreciated.

scholarly journals PADRÕES DE VENTO A NÍVEL DE SUPERFÍCIE PARA REGIÃO DA COSTA NORTE DO BRASIL

2016 ◽  
Vol 38 ◽  
pp. 383
Author(s):  
Luiz Eduardo Medeiros ◽  
Gilberto Fisch ◽  
Paulo Iriart ◽  
Felipe Denardin Costa ◽  
Dionnathan Willian Oliveira ◽  
...  
Keyword(s):  
Pressure Gradient ◽  
Surface Pressure ◽  
Large Scale ◽  
Surface Wind ◽  
Hadley Cell ◽  
Radiosonde Data ◽  
Momentum Balance ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Meridional Component

The atmospheric flow near the surface and in the planetary boundary layer (PBL) are investigated for the coastal part of Maranhão state. Near the coast in the PBL the flow is predominantly from the northeast quadrant with its meridional component increasing during the day and being from north-northeast and decreasing during the course of the night to be from east-northeast at early morning. The result of this is a small counterclockwise rotation but with no flow reversals. Through an analysis of extensive radiosonde data it is found that the flow above the PBL is predominantly southeasterly for the region. It is consequence of the outflow from the descending branch of the large-scale circulation of the Hadley cell. For stations further inland the flow is from approximately northeast during period between morning to noon but rotating clockwise to become from southeast-east (SEE) sector at early evening. The clockwise rotation continues in the afternoon and the wind becomes from south, and later southwest when in the evening it quickly becomes from north. The wind rotation during this period is mainly determined by an oscillating surface pressure gradient-force. During the night the local surface wind tendency is not controlled by the gradient-force probably because the air has to go against higher terrain and negative buoyancy becomes an important force of the momentum balance. The oscillating surface pressure-gradient-force is a response to a sea-breeze circulation. In the coast, we speculate that the flow does not reverse its meridional component because the surface pressure-gradient point south there for most of the time.

scholarly journals An invariant representation of mean inertia: theoretical basis for a log law in turbulent boundary layers

2017 ◽  
Vol 813 ◽  
pp. 594-617 ◽  
Author(s):  
Caleb Morrill-Winter ◽  
Jimmy Philip ◽  
Joseph Klewicki
Keyword(s):  
Reynolds Number ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Turbulent Boundary Layers ◽  
Momentum Balance ◽  
Large Reynolds Number ◽  
Gradient Force ◽  
Scaling Analysis ◽  
Pressure Gradient Force ◽  
The Mean

A refined scaling analysis of the two-dimensional mean momentum balance (MMB) for the zero-pressure-gradient turbulent boundary layer (TBL) is presented and experimentally investigated up to high friction Reynolds numbers, $\unicode[STIX]{x1D6FF}^{+}$. For canonical boundary layers, the mean inertia, which is a function of the wall-normal distance, appears instead of the constant mean pressure gradient force in the MMB for pipes and channels. The constancy of the pressure gradient has led to theoretical treatments for pipes/channels, that are more precise than for the TBL. Elements of these analyses include the logarithmic behaviour of the mean velocity, specification of the Reynolds shear stress peak location, the square-root Reynolds number scaling for the log layer onset and a well-defined layer structure based on the balance of terms in the MMB. The present analyses evidence that similarly well-founded results also hold for turbulent boundary layers. This follows from transforming the mean inertia term in the MMB into a form that resembles that in pipes/channels, and is constant across the outer inertial region of the TBL. The physical reasoning is that the mean inertia is primarily a large-scale outer layer contribution, the ‘shape’ of which becomes invariant of $\unicode[STIX]{x1D6FF}^{+}$ with increasing $\unicode[STIX]{x1D6FF}^{+}$, and with a ‘magnitude’ that is inversely proportional to $\unicode[STIX]{x1D6FF}^{+}$. The present analyses are enabled and corroborated using recent high resolution, large Reynolds number hot-wire measurements of all the terms in the TBL MMB.

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