scholarly journals Ocean and Atmosphere Storm Tracks: The Role of Eddy Vorticity Forcing

2007 ◽  
Vol 37 (9) ◽  
pp. 2267-2289 ◽  
Author(s):  
Richard G. Williams ◽  
Chris Wilson ◽  
Chris W. Hughes
Keyword(s):  
Storm Track ◽  
Heat Fluxes ◽  
Storm Tracks ◽  
Mean Flow ◽  
Ocean Eddies ◽  
Boundary Currents ◽  
The Mean ◽  
Circumpolar Current ◽  
The Antarctic

Abstract Signatures of eddy variability and vorticity forcing are diagnosed in the atmosphere and ocean from weather center reanalysis and altimetric data broadly covering the same period, 1992–2002. In the atmosphere, there are localized regions of eddy variability referred to as storm tracks. At the entrance of the storm track the eddies grow, providing a downgradient heat flux and accelerating the mean flow eastward. At the exit and downstream of the storm track, the eddies decay and instead provide a westward acceleration. In the ocean, there are similar regions of enhanced eddy variability along the extension of midlatitude boundary currents and the Antarctic Circumpolar Current. Within these regions of high eddy kinetic energy, there are more localized signals of high Eady growth rate and downgradient eddy heat fluxes. As in the atmosphere, there are localized regions in the Southern Ocean where ocean eddies provide statistically significant vorticity forcing, which acts to accelerate the mean flow eastward, provide torques to shift the jet, or decelerate the mean flow. These regions of significant eddy vorticity forcing are often associated with gaps in the topography, suggesting that the ocean jets are being locally steered by topography. The eddy forcing may also act to assist in the separation of boundary currents, although the diagnostics of this study suggest that this contribution is relatively small when compared with the advection of planetary vorticity by the time-mean flow.

scholarly journals The Role of Stationary Eddies in Shaping Midlatitude Storm Tracks

2013 ◽  
Vol 70 (8) ◽  
pp. 2596-2613 ◽  
Author(s):  
Yohai Kaspi ◽  
Tapio Schneider
Keyword(s):  
Energy Transport ◽  
Storm Track ◽  
Static Stability ◽  
Surface Heating ◽  
Storm Tracks ◽  
Mean Flow ◽  
Wavelength Scale ◽  
The Mean ◽  
And Control

Abstract Transient and stationary eddies shape the extratropical climate through their transport of heat, moisture, and momentum. In the zonal mean, the transports by transient eddies dominate over those by stationary eddies, but this is not necessarily the case locally. In particular, in storm-track entrance and exit regions during winter, stationary eddies and their interactions with the mean flow dominate the atmospheric energy transport. Here it is shown that stationary eddies can shape storm tracks and control where they terminate by modifying local baroclinicity. Simulations with an idealized aquaplanet GCM show that zonally localized surface heating alone (e.g., ocean heat flux convergence) gives rise to storm tracks, which have a well-defined length scale that is similar to that of Earth's storm tracks. The storm tracks terminate downstream of the surface heating even in the absence of continents, at a distance controlled by the stationary Rossby wavelength scale. Stationary eddies play a dual role: within about half a Rossby wavelength downstream of the heating region, stationary eddy energy fluxes increase the baroclinicity and therefore contribute to energizing the storm track; farther downstream, enhanced poleward and upward energy transport by stationary eddies reduces the baroclinicity by reducing the meridional temperature gradients and enhancing the static stability. Transports both of sensible and latent heat (water vapor) play important roles in determining where storm tracks terminate.

Drivers of biases in the extratropical storm tracks in CMIP6

2020 ◽  
Author(s):  
Matthew Priestley ◽  
Duncan Ackerley ◽  
Jennifer Catto ◽  
Kevin Hodges ◽  
Ruth McDonald ◽  
...  
Keyword(s):  
Large Scale ◽  
Climate Models ◽  
Storm Track ◽  
Storm Tracks ◽  
Extratropical Cyclones ◽  
Ocean Coupling ◽  
Coupled Climate Models ◽  
The Mean ◽  
The Impact

<p>Extratropical cyclones are the leading driver of the day-to-day weather variability and wintertime losses for Europe. In the latest generation of coupled climate models, CMIP6, it is hoped that with improved modelling capabilities come improvements in the structure of the storm track and the associated cyclones. Using an objective cyclone identification and tracking algorithm the mean state of the storm tracks in the CMIP6 models is assessed as well as the representation of explosively deepening cyclones. Any developments and improvements since the previous generation of models in CMIP5 are discussed, with focus on the impact of model resolution on storm track representation. Furthermore, large-scale drivers of any biases are investigated, with particular focus on the role of atmosphere-ocean coupling via associated AMIP simulations and also the influence of large-scale dynamical and thermodynamical features.</p>

scholarly journals Time-Mean Flow as the Prevailing Contribution to the Poleward Heat Flux across the Southern Flank of the Antarctic Circumpolar Current: A Case Study in the Fawn Trough, Kerguelen Plateau

2013 ◽  
Vol 43 (3) ◽  
pp. 583-601 ◽  
Author(s):  
H. Sekma ◽  
Y.-H. Park ◽  
F. Vivier
Keyword(s):  
Heat Flux ◽  
Southern Ocean ◽  
Antarctic Circumpolar Current ◽  
Mean Flow ◽  
Kerguelen Plateau ◽  
The Mean ◽  
The Cross ◽  
Circumpolar Current ◽  
The Antarctic ◽  
Southern Flank

Abstract The major mechanisms of the oceanic poleward heat flux in the Southern Ocean are still in debate. The long-standing belief stipulates that the poleward heat flux across the Antarctic Circumpolar Current (ACC) is mainly due to mesoscale transient eddies and the cross-stream heat flux by time-mean flow is insignificant. This belief has recently been challenged by several numerical modeling studies, which stress the importance of mean flow for the meridional heat flux in the Southern Ocean. Here, this study analyzes moored current meter data obtained recently in the Fawn Trough, Kerguelen Plateau, to estimate the cross-stream heat flux caused by the time-mean flow and transient eddies. It is shown that the poleward eddy heat flux in this southern part of the ACC is negligible, while that from the mean flow is overwhelming by two orders of magnitude. This is due to the unusual anticlockwise turning of currents with decreasing depth, which is associated with significant bottom upwelling engendered by strong bottom currents flowing over the sloping topography of the trough. The circumpolar implications of these local observations are discussed in terms of the depth-integrated linear vorticity budget, which suggests that the six topographic features along the southern flank of the ACC equivalent to the Fawn Trough case would yield sufficient poleward heat flux to balance the oceanic heat loss in the subpolar region. As eddy activity on the southern flank of the ACC is too weak to transport sufficient heat poleward, the nonequivalent barotropic structure of the mean flow in several topographically constricted passages should accomplish the required task.

scholarly journals Heat fluxes across the Antarctic Circumpolar Current in Drake Passage: Mean flow and eddy contributions

2014 ◽  
Vol 119 (9) ◽  
pp. 6381-6402 ◽  
Author(s):  
Ramiro Ferrari ◽  
Christine Provost ◽  
Young-Hyang Park ◽  
Nathalie Sennéchael ◽  
Zoé Koenig ◽  
...  
Keyword(s):  
Antarctic Circumpolar Current ◽  
Drake Passage ◽  
Heat Fluxes ◽  
Mean Flow ◽  
Circumpolar Current ◽  
The Antarctic

The Antarctic Circumpolar Current in Three Dimensions

2006 ◽  
Vol 36 (4) ◽  
pp. 651-669 ◽  
Author(s):  
Timour Radko ◽  
John Marshall
Keyword(s):  
Antarctic Circumpolar Current ◽  
Three Dimensional ◽  
Perturbation Expansion ◽  
Dimensional Structure ◽  
Residual Circulation ◽  
Mean Flow ◽  
Overturning Circulation ◽  
The Mean ◽  
Circumpolar Current ◽  
The Antarctic

Abstract A simple theory is developed for the large-scale three-dimensional structure of the Antarctic Circumpolar Current and the upper cell of its overturning circulation. The model is based on a perturbation expansion about the zonal-average residual-mean model developed previously by Marshall and Radko. The problem is solved using the method of characteristics for idealized patterns of wind and buoyancy forcing constructed from observations. The equilibrium solutions found represent a balance between the Eulerian meridional overturning, eddy-induced circulation, and downstream advection by the mean flow. Depth and stratification of the model thermocline increase in the Atlantic–Indian Oceans sector where the mean wind stress is large. Residual circulation in the model is characterized by intensification of the overturning circulation in the Atlantic–Indian sector and reduction in strength in the Pacific Ocean region. Predicted three-dimensional patterns of stratification and residual circulation in the interior of the ACC are compared with observations.

scholarly journals Equilibration of the Antarctic Circumpolar Current by Standing Meanders

2014 ◽  
Vol 44 (7) ◽  
pp. 1811-1828 ◽  
Author(s):  
Andrew F. Thompson ◽  
Alberto C. Naveira Garabato
Keyword(s):  
Antarctic Circumpolar Current ◽  
Relative Vorticity ◽  
Alternative Mechanism ◽  
Mean Flow ◽  
Spatially Correlated ◽  
Eddy Characteristics ◽  
Rapid Changes ◽  
The Mean ◽  
Circumpolar Current ◽  
The Antarctic

Abstract The insensitivity of the Antarctic Circumpolar Current (ACC)’s prominent isopycnal slope to changes in wind stress is thought to stem from the action of mesoscale eddies that counterbalance the wind-driven Ekman overturning—a framework verified in zonally symmetric circumpolar flows. Substantial zonal variations in eddy characteristics suggest that local dynamics may modify this balance along the path of the ACC. Analysis of an eddy-resolving ocean GCM shows that the ACC can be broken into broad regions of weak eddy activity, where surface winds steepen isopycnals, and a small number of standing meanders, across which the isopycnals relax. Meanders are coincident with sites of (i) strong eddy-induced modification of the mean flow and its vertical structure as measured by the divergence of the Eliassen–Palm flux and (ii) enhancement of deep eddy kinetic energy by up to two orders of magnitude over surrounding regions. Within meanders, the vorticity budget shows a balance between the advection of relative vorticity and horizontal divergence, providing a mechanism for the generation of strong vertical velocities and rapid changes in stratification. Temporal fluctuations in these diagnostics are correlated with variability in both the Eliassen–Palm flux and bottom speed, implying a link to dissipative processes at the ocean floor. At larger scales, bottom pressure torque is spatially correlated with the barotropic advection of planetary vorticity, which links to variations in meander structure. From these results, it is proposed that the “flexing” of standing meanders provides an alternative mechanism for reducing the sensitivity of the ACC’s baroclinicity to changes in forcing, separate from an ACC-wide change in transient eddy characteristics.

scholarly journals Eddies in Numerical Models of the Antarctic Circumpolar Current and Their Influence on the Mean Flow

1999 ◽  
Vol 29 (3) ◽  
pp. 328-350 ◽  
Author(s):  
S. E. Best ◽  
V. O. Ivchenko ◽  
K. J. Richards ◽  
R. D. Smith ◽  
R. C. Malone
Keyword(s):  
Antarctic Circumpolar Current ◽  
Numerical Models ◽  
Mean Flow ◽  
The Mean ◽  
Circumpolar Current ◽  
The Antarctic

scholarly journals Eulerian and Lagrangian Isopycnal Eddy Diffusivities in the Southern Ocean of an Eddying Model

2014 ◽  
Vol 44 (2) ◽  
pp. 644-661 ◽  
Author(s):  
A. Griesel ◽  
J. L. McClean ◽  
S. T. Gille ◽  
J. Sprintall ◽  
C. Eden
Keyword(s):  
Southern Ocean ◽  
Local Maximum ◽  
Mean Flow ◽  
Eddy Diffusivities ◽  
The North ◽  
Rotational Components ◽  
The Mean ◽  
Circumpolar Current ◽  
Longitudinal Variability ◽  
The Antarctic

Abstract Lagrangian isopycnal diffusivities quantify the along-isopycnal mixing of any tracer with mean gradients along isopycnal surfaces. They are studied in the Southern Ocean of the 1/10° Parallel Ocean Program (POP) model using more than 50 000 float trajectories. Concurrent Eulerian isopycnal diffusivities are estimated directly from the eddy fluxes and mean tracer gradients. Consistency, spatial variation, and relation to mean jets are evaluated. The diffusivities are calculated in bins large enough to reduce contributions from the rotational components that do not lead to net tracer mixing. Because the mean jets are nonzonal and nonparallel, meridional dispersion includes standing eddies and is significantly different from cross-stream dispersion. With the subtraction of the local Eulerian mean, the full Lagrangian diffusivity tensor can be estimated. Along-stream diffusivities are about 6 times larger than cross-stream diffusivities. Along-streamline averages of Eulerian and Lagrangian isopycnal diffusivities are similar in that they are larger north of the Antarctic Circumpolar Current (ACC) and smaller in the ACC in the upper 500 m. Eulerian diffusivities are often twice as large as the Lagrangian diffusivities below 500 m. There is large longitudinal variability in the diffusivities and in their relation to the mean flow. In bins with one prominent jet, diffusivities are reduced at the surface in the jet and increased to the north and south of the jet. There is a local maximum at depths of 500–1000 m. In other bins where mean jets merge and diverge because of topography, there is no consistent relation of the diffusivities with the mean flow. Eulerian fluxes are upgradient in about 15% of the bins.

scholarly journals The Ocean General Circulation near 1000-m Depth

2014 ◽  
Vol 44 (1) ◽  
pp. 384-409 ◽  
Author(s):  
Michel Ollitrault ◽  
Alain Colin de Verdière
Keyword(s):  
Ocean Circulation ◽  
General Circulation ◽  
Neumann Boundary ◽  
Reference Level ◽  
Western Boundary ◽  
Boundary Currents ◽  
Zonal Jets ◽  
The Mean ◽  
Circumpolar Current ◽  
The Antarctic

Abstract The mean ocean circulation near 1000-m depth is estimated with 100-km resolution from the Argo float displacements collected before 1 January 2010. After a thorough validation, the 400 000 or so displacements found in the 950–1150 dbar layer and with parking times between 4 and 17 days allow the currents to be mapped at intermediate depths with unprecedented details. The Antarctic Circumpolar Current (ACC) is the most prominent feature, but western boundary currents (and their recirculations) and alternating zonal jets in the tropical Atlantic and Pacific are also well defined. Eddy kinetic energy (EKE) gives the mesoscale variability (on the order of 10 cm2 s−2 in the interior), which is compared to the surface geostrophic altimetric EKE showing e-folding depths greater than 700 m in the ACC and northern subpolar regions. Assuming planetary geostrophy, the geopotential height of the 1000-dbar isobar is estimated to obtain an absolute and deep reference level worldwide. This is done by solving numerically the Poisson equation that results from taking the divergence of the geostrophic equations on the sphere, assuming Neumann boundary conditions.

scholarly journals A Multiwavenumber Theory for Eddy Diffusivities and Its Application to the Southeast Pacific (DIMES) Region

2015 ◽  
Vol 45 (7) ◽  
pp. 1877-1896 ◽  
Author(s):  
Ru Chen ◽  
Sarah T. Gille ◽  
Julie L. McClean ◽  
Glenn R. Flierl ◽  
Alexa Griesel
Keyword(s):  
Southern Ocean ◽  
Drake Passage ◽  
Mean Flow ◽  
Ocean Eddies ◽  
Eddy Diffusivities ◽  
Wide Range ◽  
The Mean ◽  
Circumpolar Current ◽  
Almost All ◽  
Limiting Case

AbstractA multiwavenumber theory is formulated to represent eddy diffusivities. It expands on earlier single-wavenumber theories and includes the wide range of wavenumbers encompassed in eddy motions. In the limiting case in which ocean eddies are only composed of a single wavenumber, the multiwavenumber theory is equivalent to the single-wavenumber theory and both show mixing suppression by the eddy propagation relative to the mean flow. The multiwavenumber theory was tested in a region of the Southern Ocean (70°–45°S, 110°–20°W) that covers the Drake Passage and includes the tracer/float release locations during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). Cross-stream eddy diffusivities and mixing lengths were estimated in this region from the single-wavenumber theory, from the multiwavenumber theory, and from floats deployed in a global ° Parallel Ocean Program (POP) simulation. Compared to the single-wavenumber theory, the horizontal structures of cross-stream mixing lengths from the multiwavenumber theory agree better with the simulated float-based estimates at almost all depth levels. The multiwavenumber theory better represents the vertical structure of cross-stream mixing lengths both inside and outside the Antarctica Circumpolar Current (ACC). Both the single-wavenumber and multiwavenumber theories represent the horizontal structures of cross-stream diffusivities, which resemble the eddy kinetic energy patterns.

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