Changes in Poleward Energy Transport and Zonal Mean Atmospheric Circulation in Simulations of 21st C

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Changes in Poleward Energy Transport and Zonal
Mean Atmospheric Circulation in Simulations of
21st Century Climate

• Jeff Yin
• NCAR/ESSL/CGD
• CCWG Meeting, 23 June 2005

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Outline

• IPCC AR4 model experiments
• Poleward shift of the storm tracks (and
  other related variables)
• Changes in poleward energy transport
• Summary

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IPCC AR4 Model Experiments

• SRES A1b 720 ppm CO2 by 2100
• 21st Century climate change Compare years
  2081-2100 from SRES A1b with years 1981-2000 from
  20th Century Expt
• Multi-model ensembles 15 different coupled GCMs,
  one member each

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What Do Storm Tracks Tell Us?

• Defined as regions of large baroclinic wave
  activity, identified as maxima in bandpass eddy
  statistics, such as EKE
• Baroclinic waves produce precipitation and severe
  weather in midlatitudes
• Transport heat, moisture, momentum
• Drive ocean circulation through wind stress,
  momentum flux convergence
• Important for both weather and climate

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Zonal Mean Storm Tracks
2-8 day Eddy Kinetic Energy

• Storm tracks shift poleward and upward
• Storm tracks also tend to strengthen
• Most consistent in seasons with strong storm
  tracks (SH in DJF, JJA NH in DJF)

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Storm Tracks and Baroclinicity

• Baroclinic waves in storm tracks grow in regions
  of large baroclinicity
• Maximum Eady growth rate
• 0.31g N-1T-1 dT/dy
• (Lindzen and Farrell 1980)
• Depends on both meridional temperature gradient
  and static stability

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Zonal Mean Temperature
Temperature

• Notable features in warming
• Maximum in tropical upper troposphere
• Maximum near surface over N. Pole in DJF
• Minimum over Southern Ocean

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Zonal Mean Temp and Baroclinicity
Temperature
Eady growth rate
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Baroclinicity dT/dy vs. static stability
Eady growth rate change due to dT/dy
Eady growth rate change due to static stability
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How Consistent is Poleward Shift?
EKE

• Each line is vertical integral of EKE for 1 GCM,
  (2081-2100) (1981-2000)
• Dot is at latitude of 1981-2000 EKE maximum
• Number of models with poleward shift
• SH 14 in DJF, 12 in JJA
• NH 11 in DJF, 9 in JJA

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Storm Tracks and Ocean Forcing
EKE
dvu/dy
Wind Stress
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Storm Tracks and Precipitation
EKE
Precip
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Poleward Energy Transport

• Current work Place shift of storm tracks in
  context of poleward energy transport
• Total Energy IE PE LE KE
• IE cpT Internal Energy
• PE gz Potential Energy
• LE Lcq Latent Energy
• KE ½(u2v2) Kinetic Energy
• DSE IE PE Dry Static Energy
• Hadley cell transports DSE poleward, Ferrel cell
  transports DSE equatorward

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Energy Transport Divergence
Stat DSE

• Lines are (2081-2100) (1981-2000) change
• Dot at latitude of 1981-2000 max convergence
• Near equator, increased precipitation causes DSE
  divergence
• Hadley cell converges excess DSE poleward of max
  convergence expanded Hadley cell?
• Storm track shift due to Hadley expansion?

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Hadley Cell Expansion Hypothesis

• Start w/increased tropical latent heating
• Hadley cell transports excess heat to subtropics,
  which radiate heat to space
• Latent heating increases faster than radiative
  cooling, so subtropics expand
• Maximum midlatitude temperature gradient is
  pushed poleward
• Storm tracks and associated variables also shift
  poleward

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Energy Transport Divergence
Stat DSE
Trans LE
Trans IE
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Summary

• In 21st century simulations, storm tracks shift
  poleward and upward and intensify
• Related to poleward shift and upward expansion of
  baroclinic regions, dominated by changes in dT/dy
• Related to warming maximum in tropical upper
  troposphere
• Accompanied by poleward shift of wind stress and
  precipitation
• Caused by expansion of Hadley cell?