Interactions between midlatitude storm tracks and tropical convection

1
Interactions between mid-latitude storm tracks
and tropical convection

• Jeff Yin
• NCAR/ESSL/CGD
• TIMES Weather-Climate Interface Workshop
• 28 June 2005

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Outline

• Role of storm tracks in weather climate
• Links between tropical convection storm tracks
• Example from climate change
• Summary
• Future work Storm tracks connect climate
  variability extreme events

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Storm Tracks in Weather Climate

• 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 mid-latitudes
• Transport heat moisture play large role in
  global energy water cycles
• Important for both weather and climate

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Tropical Convection Storm Tracks

• Tropical convection storm tracks are linked to
  tropical mid-latitude weather
• However, they interact on climate time scales
  (months and longer)
• Examples
• ENSO drives changes in Pacific winter storm track
  (California vs. Pacific NW)
• Seamless poleward energy transport between Hadley
  cell and storm tracks

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Energy Transport Divergence
Data from Trenberth and Stepaniak (2003)
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Example from Climate Change

• IPCC AR4 experiments using SRES A1b scenario 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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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 Precipitation
EKE
Precip

• Each line is precip or vertical integral of EKE
  for 1 GCM, (2081-2100) (1981-2000)
• Dot is at latitude of 1981-2000 maximum

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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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Poleward Energy Transport

• Current work Place shift of storm tracks in
  context of poleward energy transport
• How do storm tracks and tropical convection
  respond to changes in energy and water cycles due
  to greenhouse warming?

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Energy Transport Divergence
Stat Energy

• Lines are (2081-2100) (1981-2000) change
• Dot at latitude of 1981-2000 max convergence
• Hadley cell converges excess energy 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 mid-latitude temperature gradient is
  pushed poleward
• Storm tracks also shift poleward
• Or does storm track shift cause Hadley cell
  expansion? (Kim and Lee 2001)

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Energy Transport Divergence
Stat Energy
Trans Energy

• Excess energy delivered to subtropics by Hadley
  cell picked up by storm track transients
• Transients drive stronger Ferrel cell transport

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Summary

• Storm tracks and tropical convection affect
  weather, yet interact on climate time scales
• Climate change example Tropical precip
  increases, Hadley cell expands, and storm tracks
  shift poleward
• Increased latent heating in tropics could be
  driver, but interaction between Hadley cell
  storm track energy transport makes further work
  necessary

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Future Work

• Currently writing proposal to study dynamics of
  relationships between climate variability and
  extreme events
• Storm tracks are key link between patterns of
  climate variability (ENSO, NAO, etc.) and extreme
  weather
• Will explore this in large ensemble of GCM
  climate change experiments
• Suggestions on how to proceed?