Diapositiva 1

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Dynamics of extratropical jet shift
Pablo Zurita Gotor Depto. Física de la Tierra
I Universidad Complutense
CLIVAR SPAIN 2/12/08
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Outline of the talk

1. Zonal index , persistence and eddy-zonal mean
   feedback.
2. Diagnostics of jet position the jet symmetry
   budget.
3. Eddy-driven shift in the two-layer QG model.

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Internal variability of the jet the zonal index

• Leading mode of variability of zonal-mean zonal
  wind
• Represents a meridional shift of the jet
• Has equivalent barotropic structure

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Zonal index and annular modes
The zonal index variability is intimately related
to the annular mode variability
Geostrophic zonal wind
Geopotential height
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Forced annular response
The annular mode response is ubiquitous for many
types of climate perturbations
Projected 21st century stormtrack shift in IPCC
AR4 models (Yin 2005)
Southern Hemisphere geopotential trend, partly
attributed to ozone changes (Thompson and Solomon
2002)
In many circumstances, the annular pattern
dominates the forced response even when the
external forcing has little projection on this
pattern.
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Zonal index persistence

• The zonal index is much more persistent than the
  eddies that drive it
• At long time scales, there might be a role for
  the stratosphere and/or ocean
• At subsynoptic time scales, eddy-zonal mean
  feedback is likely more important
• The eddy forcing is such that it tends to
  reinforce preexisting zonal wind anomalies

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Why is persistence important?
The short answer is predictability There seems
to exist a relation between the persistence of
the mode and the percentage of variance that it
explains (e.g., Yang and Chang 2007)
Likewise, models with more persistent internal
variability show stronger annular trends
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Diagnostics of jet position
Start with zonal-mean zonal momentum equation
This is just a balance between the different
accelerations. However, this tells us nothing
about the position of the jet!
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Diagnostics of jet position
Start with zonal-mean zonal momentum equation
Multiply by latitude
Integrating in y and z
We refer to this equation as the jet symmetry
budget
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Two-layer QG model on the beta plane
Since this model is symmetric about midchannel,
all terms independently vanish We break the
symmetry adding an easterly torque F on one side
only
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Jet sensitivity to torque position
Control run no torque
Remote torque
Jet is symmetric about midchannel
Easterly jet created on the side but the
original jet remains symmetric
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Jet sensitivity to torque position
Control run no torque
Torque within eddy-dominated region
Jet is symmetric about midchannel
When the eddies interact, they make the jet shift
poleward
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Jet sensitivity to torque position
As the torque moves closer to the center of the
channel, the jet shift increases
Eventually, the jet moves back to the center of
the channel and symmetry is recovered
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Jet symmetry budget
Terms contributing to jet shift
Friction
Eddy forcing
Torque forcing

• For a remote torque, its forcing is simply
  balanced by friction
• As the torque approaches the eddy dominated
  region, there is a positive eddy feedback that
  amplifies the torque forcing
• This results in a shift of the jet

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Jet sensitivity to torque strength
At a given position, the jet shift increases with
increasing torque
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Jet sensitivity to torque strength
This can again be explained using the jet
symmetry budget
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The eddies also shift
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Mechanisms of eddy-zonal mean feedback
Thats the big question!!! And a very difficult
question too

• Some proposed mechanisms
• Different eddy lifecycles (LC1 vs LC2) due to
  changes in barotropic shear/refractive index/eddy
  propagation
• Meridional displacement of critical layers due
  to differential acceleration
• Resilience of the jet to stay at its perturbed
  position due to the negative viscosity effect
  (i.e., the eddy generation shifts with the jet)
• But we dont really know

We just submitted a proposal to investigate these
issues
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Conclusions

1. The zonal index (jet shift) is the leading form
   of zonal-mean extratropical variability and a
   fundamental part of the forced response.
2. This mode varies on longer time scales than the
   driving eddies, which might be due to an
   eddy-mean flow feedback
3. Eddy reinforcement may explain why this mode
   dominates the internal variability and why this
   is a preferred response in the forced problem.
4. We have derived an evolution equation for the
   position of the jet, driven by asymmetry. The
   main forcing is the asymmetry in eddy momentum
   flux.
5. In an idealized model, we showed that the eddy
   momentum flux tends to amplify other sources of
   asymmetry, implying a positive feedback.
6. The dynamics of this feedback remains to be
   elucidated.

Thank you for your attention!
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Other variables also shift, including the eddies
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The NAO may be regarded as the regional
manifestation of that annular pattern
Gerber (2005)
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From Lorenz and Hartmann (2001)
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From Polvani and Esler (2008)
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From Gerber et al (2008)