scholarly journals Eddy Shape, Orientation, Propagation, and Mean Flow Feedback in Western Boundary Current Jets

2013 ◽  
Vol 43 (8) ◽  
pp. 1666-1690 ◽  
Author(s):  
Stephanie Waterman ◽  
Brian J. Hoskins
Keyword(s):  
Western Boundary Current ◽  
Upstream Region ◽  
Wave Radiation ◽  
Western Boundary ◽  
Boundary Current ◽  
Mean Flow ◽  
Idealized Model ◽  
Recirculation Gyres ◽  
Flow Interactions ◽  
The Mean

Abstract This manuscript revisits a study of eddy–mean flow interactions in an idealized model of a western boundary current extension jet using properties of the horizontal velocity correlation tensor to diagnose characteristics of average eddy shape, orientation, propagation, and mean flow feedback. These eddy characteristics are then used to provide a new description of the eddy–mean flow interactions observed in terms of different ingredients of the eddy motion. The diagnostics show patterns in average eddy shape, orientation, and propagation that are consistent with the signatures of jet instability in the upstream region and wave radiation in the downstream region. Together they give a feedback onto the mean flow that gives the downstream character of the jet and drives the jet's recirculation gyres. A breakdown of the eddy forcing into contributions from individual terms confirms the expected role of cross-jet gradients in meridional eddy tilt in stabilizing the jet to its barotropic instability; however, it also reveals important roles played by the along-jet evolution of eddy zonal–meridional elongation. It is the mean flow forcing derived from these patterns that acts to strengthen and extend the jet downstream and forces the time-mean recirculation gyres. This understanding of the dependence of mean flow forcing on eddy structural properties suggests that failure to adequately resolve eddy elongation could underlie the weakened jet strength, extent, and changed recirculation structure seen in this idealized model for reduced spatial resolutions. Further, it may suggest new ideas for the parameterization of this forcing.

scholarly journals Eddy-Mean Flow Interactions in the Along-Stream Development of a Western Boundary Current Jet: An Idealized Model Study

2011 ◽  
Vol 41 (4) ◽  
pp. 682-707 ◽  
Author(s):  
Stephanie Waterman ◽  
Steven R. Jayne
Keyword(s):  
Western Boundary Current ◽  
Upstream Region ◽  
Western Boundary ◽  
Driving Mechanism ◽  
Boundary Current ◽  
Mean Flow ◽  
Model Study ◽  
Recirculation Gyres ◽  
Flow Interactions ◽  
Mean Circulation

A theoretical study on the role of eddy-mean flow interactions in the time-mean dynamics of a zonally evolving, unstable, strongly inertial jet in a configuration and parameter regime that is relevant to oceanic western boundary current (WBC) jets is described. Progress is made by diagnosing the eddy effect on the time-mean circulation, examining the mechanism that permits the eddies to drive the time-mean recirculation gyres, and exploring the dependence of the eddy effect on system parameters. It is found that the nature of the eddy-mean flow interactions in this idealized configuration is critically dependent on along-stream position, in particular relative to the along-stream evolving stability properties of the time-mean jet. Just after separation from the western boundary, eddies act to stabilize the jet through downgradient fluxes of potential vorticity (PV). Downstream of where the time-mean jet has (through the effect of the eddies) been stabilized, eddies act to drive the time-mean recirculations through the mechanism of an upgradient PV flux. This upgradient flux is permitted by an eddy enstrophy convergence downstream of jet stabilization, which results from the generation of eddies in the upstream region where the jet is unstable, the advection of that eddy activity along stream by the jet, and the dissipation of the eddies in the region downstream of jet stabilization. It is in this region of eddy decay that eddies drive the time-mean recirculations through the mechanism of nonlinear eddy rectification, resulting from the radiation of waves from a localized region. It is found that similar mechanisms operate in both barotropic and baroclinic configurations, although differences in the background PV gradient on which the eddies act implies that the recirculation-driving mechanism is more effective in the baroclinic case. This study highlights the important roles that eddies play in the idealized WBC jet dynamics considered here of stabilizing the jet and driving the flanking recirculations. In the absence of eddy terms, the magnitude of the upper-ocean jet transport would be significantly less and the abyssal ocean recirculations (and their significant enhancement to the jet transport) would be missing altogether.

scholarly journals Diagnosing the Influence of Mesoscale Eddy Fluxes on the Deep Western Boundary Current in the 1/10° STORM/NCEP Simulation

2019 ◽  
Vol 49 (3) ◽  
pp. 751-764 ◽  
Author(s):  
Veit Lüschow ◽  
Jin-Song von Storch ◽  
Jochem Marotzke
Keyword(s):  
Potential Energy ◽  
Mesoscale Eddy ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Boundary Current ◽  
Mean Flow ◽  
Coriolis Parameter ◽  
Deep Western Boundary Current ◽  
Eddy Fluxes ◽  
The Mean

AbstractUsing a 0.1° ocean model, this paper establishes a consistent picture of the interaction of mesoscale eddy density fluxes with the geostrophic deep western boundary current (DWBC) in the Atlantic between 26°N and 20°S. Above the DWBC core (the level of maximum southward flow, ~2000-m depth), the eddies flatten isopycnals and hence decrease the potential energy of the mean flow, which agrees with their interpretation and parameterization in the Gent–McWilliams framework. Below the core, even though the eddy fluxes have a weaker magnitude, they systematically steepen isopycnals and thus feed potential energy to the mean flow, which contradicts common expectations. These two vertically separated eddy regimes are found through an analysis of the eddy density flux divergence in stream-following coordinates. In addition, pathways of potential energy in terms of the Lorenz energy cycle reveal this regime shift. The twofold eddy effect on density is balanced by an overturning in the plane normal to the DWBC. Its direction is clockwise (with upwelling close to the shore and downwelling further offshore) north of the equator. In agreement with the sign change in the Coriolis parameter, the overturning changes direction to anticlockwise south of the equator. Within the domain covered in this study, except in a narrow band around the equator, this scenario is robust along the DWBC.

scholarly journals Eddy–Mean Flow Interaction in the Kuroshio Extension Region

2011 ◽  
Vol 41 (6) ◽  
pp. 1182-1208 ◽  
Author(s):  
Stephanie Waterman ◽  
Nelson G. Hogg ◽  
Steven R. Jayne
Keyword(s):  
Kuroshio Extension ◽  
Wave Radiation ◽  
Western Boundary ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Flow Interaction ◽  
Flow Interactions ◽  
The Mean ◽  
Jet Structure ◽  
The Kuroshio

Abstract The authors use data collected by a line of tall current meter moorings deployed across the axis of the Kuroshio Extension (KE) jet at the location of maximum time-mean eddy kinetic energy to characterize the mean jet structure, the eddy variability, and the nature of eddy–mean flow interactions observed during the Kuroshio Extension System Study (KESS). A picture of the 2-yr record mean jet structure is presented in both geographical and stream coordinates, revealing important contrasts in jet strength, width, vertical structure, and flanking recirculation structure. Eddy variability observed is discussed in the context of some of its various sources: jet meandering, rings, waves, and jet instability. Finally, various scenarios for eddy–mean flow interaction consistent with the observations are explored. It is shown that the observed cross-jet distributions of Reynolds stresses at the KESS location are consistent with wave radiation away from the jet, with the sense of the eddy feedback effect on the mean consistent with eddy driving of the observed recirculations. The authors consider these results in the context of a broader description of eddy–mean flow interactions in the larger KE region using KESS data in combination with in situ measurements from past programs in the region and satellite altimetry. This demonstrates important consistencies in the along-stream development of time-mean and eddy properties in the KE with features of an idealized model of a western boundary current (WBC) jet used to understand the nature and importance of eddy–mean flow interactions in WBC jet systems.

The Deep Western Boundary Current and Adjacent Interior Circulation at 24°–30°N: Mean Structure and Mesoscale Variability

2020 ◽  
Vol 50 (9) ◽  
pp. 2735-2758
Author(s):  
Tiago Carrilho Biló ◽  
William E. Johns
Keyword(s):  
North Atlantic ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Boundary Current ◽  
Deep Western Boundary Current ◽  
Mean Structure ◽  
The Mean ◽  
Theoretical Evidence ◽  
Anticyclonic Eddies ◽  
Geostrophic Velocities

AbstractThe mean North Atlantic Deep Water (NADW, 1000 < z < 5000 m) circulation and deep western boundary current (DWBC) variability offshore of Abaco, Bahamas, at 26.5°N are investigated from nearly two decades of velocity and hydrographic observations, and outputs from a 30-yr-long eddy-resolving global simulation. Observations at 26.5°N and Argo-derived geostrophic velocities show the presence of a mean Abaco Gyre spanning the NADW layer, consisting of a closed cyclonic circulation between approximately 24° and 30°N and 72° and 77°W. The southward-flowing portion of this gyre (the DWBC) is constrained to within ~150 km of the western boundary with a mean transport of ~30 Sv (1 Sv ≡ 106 m3 s−1). Offshore of the DWBC, the data show a consistent northward recirculation with net transports varying from 6.5 to 16 Sv. Current meter records spanning 2008–17 supported by the numerical simulation indicate that the DWBC transport variability is dominated by two distinct types of fluctuations: 1) periods of 250–280 days that occur regularly throughout the time series and 2) energetic oscillations with periods between 400 and 700 days that occur sporadically every 5–6 years and force the DWBC to meander far offshore for several months. The shorter-period variations are related to DWBC meandering caused by eddies propagating southward along the continental slope at 24°–30°N, while the longer-period oscillations appear to be related to large anticyclonic eddies that slowly propagate northwestward counter to the DWBC flow between ~20° and 26.5°N. Observational and theoretical evidence suggest that these two types of variability might be generated, respectively, by DWBC instability processes and Rossby waves reflecting from the western boundary.

scholarly journals Mixing Nonlocality and Mixing Anisotropy in an Idealized Western Boundary Current Jet

2017 ◽  
Vol 47 (12) ◽  
pp. 3015-3036 ◽  
Author(s):  
Ru Chen ◽  
Stephanie Waterman
Keyword(s):  
Large Scale ◽  
Eddy Diffusivity ◽  
Western Boundary Current ◽  
Wave Radiation ◽  
Western Boundary ◽  
Equilibration Time ◽  
Boundary Current ◽  
Jet Mixing ◽  
Velocity Magnitude ◽  
Axis Length

AbstractMotivated by the key role of western boundary currents in shaping water mass distribution and gyre water exchanges, this study characterizes mixing in an idealized western boundary current jet using a barotropic quasigeostrophic model with numerical particles deployed. Both the nonlocality of mixing, depicted by nonlocality ellipses, and mixing anisotropy, depicted by mixing ellipses, are estimated. Mixing is more nonlocal within the jet compared to the jet flanks. In general, the size of nonlocality ellipses, a metric of the degree of mixing nonlocality, scales with the eddy velocity magnitude and the equilibration time for diffusivity. The tilt and eccentricity of the nonlocality ellipses, a characterization of the anisotropy of mixing nonlocality, agree with those of momentum flux ellipses in the regions where mixing nonlocality is small. Mixing ellipse characteristics are flow regime dependent. In regions dominated by wave radiation, the mixing ellipses align with the contours of the wave streamfunction and are very anisotropic. Inside the recirculations, however, the mixing ellipses are nearly isotropic. Mixing ellipses are zonally elongated in the jet upstream because of the suppression of cross-jet mixing by the jet and the anisotropy of eddy velocity, and they can have negative minor axis length in the jet downstream, indicating negative cross-jet eddy diffusivity, which is consistent with upgradient eddy fluxes there. Thus, despite significant spatial heterogeneity in mixing nonlocality and anisotropy, in this idealized system at least, spatial patterns in these diagnostics tend to be relatively large scale and tied to larger-scale dynamics. The implications of these results to eddy parameterization and jet dynamics are discussed.

scholarly journals On the Steadiness and Instability of the Intermediate Western Boundary Current between 24° and 18°S

2019 ◽  
Vol 49 (12) ◽  
pp. 3127-3143 ◽  
Author(s):  
Dante C. Napolitano ◽  
Ilson C. A. da Silveira ◽  
Cesar B. Rocha ◽  
Glenn R. Flierl ◽  
Paulo H. R. Calil ◽  
...  
Keyword(s):  
Acoustic Doppler Current Profiler ◽  
Western Boundary Current ◽  
Western Boundary ◽  
Intermediate Water ◽  
Horizontal Shear ◽  
Boundary Current ◽  
Mean Flow ◽  
Current Profiler ◽  
Good Proxy ◽  
Seamount Chain

AbstractThe Intermediate Western Boundary Current (IWBC) transports Antarctic Intermediate Water across the Vitória–Trindade Ridge (VTR), a seamount chain at ~20°S off Brazil. Recent studies suggest that the IWBC develops a strong cyclonic recirculation in Tubarão Bight, upstream of the VTR, with weak time dependency. We herein use new quasi-synoptic observations, data from the Argo array, and a regional numerical model to describe the structure and variability of the IWBC and to investigate its dynamics. Both shipboard acoustic Doppler current profiler (ADCP) data and trajectories of Argo floats confirm the existence of the IWBC recirculation, which is also captured by our Regional Oceanic Modeling System (ROMS) simulation. An “intermediate-layer” quasigeostrophic (QG) model indicates that the ROMS time-mean flow is a good proxy for the IWBC steady state, as revealed by largely parallel isolines of streamfunction and potential vorticity ; a scatter diagram also shows that the IWBC is potentially unstable. Further analysis of the ROMS simulation reveals that remotely generated, westward-propagating nonlinear eddies are the main source of variability in the region. These eddies enter the domain through the Tubarão Bight eastern edge and strongly interact with the IWBC. As they are advected downstream and negotiate the local topography, the eddies grow explosively through horizontal shear production.

scholarly journals Energetics of Eddy–Mean Flow Interactions along the Western Boundary Currents in the North Pacific

2019 ◽  
Vol 49 (3) ◽  
pp. 789-810 ◽  
Author(s):  
Xiaomei Yan ◽  
Dujuan Kang ◽  
Enrique N. Curchitser ◽  
Chongguang Pang
Keyword(s):  
Kinetic Energy ◽  
Western Boundary ◽  
Mean Flow ◽  
Boundary Currents ◽  
The North ◽  
Flow Interactions ◽  
The Mean ◽  
Eddy Field ◽  
The Kuroshio ◽  
The North Pacific

AbstractThe energetics of eddy–mean flow interactions along two western boundary currents of the North Pacific, the Kuroshio and Ryukyu Currents, are systematically investigated using 22 years of numerical data from the Ocean General Circulation Model for the Earth Simulator (OFES). For the time-mean and time-varying flow fields, all the energy components and conversions exhibit inhomogeneous spatial distributions. In the two currents, complex cross-stream and along-stream variations are seen in the eddy–mean flow energy conversions. East of Taiwan, the kinetic energy is mainly transferred from the mean flow to the eddy field through barotropic instability, whereas the baroclinic energy conversions form a meridional dipole structure caused by the topographic constraint. In the northern area, particularly, the eddy field drains 2.25 × 108 W of kinetic energy and releases 2.82 × 108 W of available potential energy when interacting with the mean flow, indicating that mesoscale eddies impinging on the Kuroshio decay with baroclinic inverse energy cascades. In the Ryukyu Current, inverse energy conversions from the eddy field to the mean flow also dominate the power transfer in the subsurface layer. The eddy field transfers 0.16 × 108 W of kinetic energy and 1.89 × 108 W of available potential energy to the mean flow, suggesting that meososcale eddies play an important role in maintaining the velocity and hydrographic structure of the current. In other areas, both barotropic and baroclinic instabilities contribute to the generation of eddy kinetic energy with the latter one providing more than 3 times as much power as the former one.

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