Preferred Modes of Variability and

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Preferred Modes of Variability and Their
Relationship with Climate Change
Seok-Woo Son and Sukyoung Lee
The Pennsylvania State University Department of
Meteorology
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Annular Mode
- Dominant internal variability of the atmosphere
SH
NH

• Leading EOF of SLP

u

• Zonally symmetric

• Quasi-barotropic

• Useful for understanding internal variability

SLP

• Useful for understanding climate change (?)

Thompson et al. 2000
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Thompson et al. 2000
Kushner et al. 2001
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Spatial pattern of annular mode recent trend
in the observed and simulated zonal-mean
circulation
To what extent annular mode is capable of
predicting zonal-mean climate change?
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Purpose and Approaches
Evaluate the predictability of zonal-mean climate
change by annular mode in terms of their spatial
structures.
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Numerical Model

• A dynamic core of GFDL GCM (symmetric boundary
  cond.)

• R30L10 but zonal wave number 15

• Driven by relaxing T toward Te with timescale of
  30 days

• Dissipated by linear friction and 8th order
  hyperdiffusion

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Numerical Model (Cont.)

• Total 49 realizations

C (0.00, 0.17, 0.33, 0.50, 0.67, 0.88, 1.00)
K/day H (0.00, 0.33, 0.67, 1.00, 1.33, 1.67,
2.00) K/day

• Statistics are derived from the last 4500 days of
  each 5000-day integration. Data of both
  hemispheres are used.

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• Strong C weak H ? Double Jet

SJ

• H 1.00K/day ? Single Jet

WJ
DJ
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Internal variability of the jets
One-point correlation of 250-hPa u'
u EOFs
Zonal-index (Jet Meander)
SJ
WJ
Transition
Poleward Propagation
DJ
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Time series of PC1 and PC2
Correlation PC1 vs. PC2
Zonal-index (Jet Meander)
SJ
Transition
WJ
Poleward Propagation
DJ
Poleward Propagation i. Correlation between
PC1 PC2 is very high
ii. Var(EOF2) is comparable to Var(EOF1)
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Annular mode Climate change in the modeI

• Annular mode EOF1 of u

• u is regressed against PC1 time series, unit
  of m/s.

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Predictability of Climate change by Annular mode
Shading correlation 0.8
Predictability is always poor in a poleward
propagation regime.
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• Increase of C ? enhances extratropical
  baroclinicity

• Increase of H ? enhances subtropical
  baroclinicity and
• intensifies Hadley
  circulation

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Summary
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Application to the Southern Hemisphere

• Applied to the SH climate change at equinoctial
  condition
• Global warming at SH ? ENSO-like
  tropical heating enhanced
• extratropical baroclinicity (Son and
  Lee 2005a) ? increase of H and C.

• Structure of the jet
• Wide range of interannual variability
  from single- to double-jet states

• Internal variability
• Both poleward propagation and zonal
  index (e.g., Feldstein 1998
• Hartmann and Lo 1998) with ? 0.5
  and ? 0.3 (Son and Lee 2005b).

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Application to the Southern Hemisphere (Cont.)

• Predictability is marginally good in the SH-like
  parameter regime.

• Annular mode may not be useful for understanding
  paleoclimate change.

Slight climate drift to the poleward propagation
regime ? poor predictability.
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Any comment and suggestion are welcome. Thank you!
Contact information Seok-Woo Son sus141_at_psu.edu
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Dependency of internal variability to the mean
flow

• The meridional radiation of the waves is
  prohibited if the PV gradient of the ambient flow
  is sufficiently sharp (e.g., Hoskins and Ambrizzi
  1993)

• Poleward propagation of westerly anomalies may
  occur only when the PV gradient is relatively
  weak and broad.

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Prediction of Climate-change Direction by
Annular mode?

• Climate change direction (positive or negative
  phase of annular mode) is determined not by the
  annular mode but by the nature of external
  forcing.

• Climate change associated with H increase
  (warming at tropics) ? negative phase of annular
  mode (out of phase).

• Climate change associated with C increase
  (broadening of extratropical baroclinic zone) ?
  positive phase of annular mode (in phase).

-

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Prediction of Climate-change Direction ? (Cont.)
Climate change in SH tropical warming enhanced
extratropical baroclinicity (Son and Lee 2005a) ?
increase of H and C.
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Predictability of Climate change by Annular mode

• measured at both
• subtropics and extratropics

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Shading df 2

• Weak latitudinal dependency of
• duC prediction by annular mode.

• Poor predictability of duH in a
• zonal-index regime is due to
• the mid-latitudes.

• Predictability is generally good
• when ? 0.5 or
• Var(EOF1) 2Var(EOF2)

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Prediction of Climate-change Amplitude by
Annular mode?
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Prediction of Climate-change Amplitude by
Annular mode?