Extratropical climate

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Extratropical climate
2
Review of last lecture

• Mean state The two basic regions of SST? Which
  region has stronger rainfall? What is the Walker
  circulation?
• El Nino and La Nina Which region has warm SST
  anomaly during El Nino? 4-year period.
• Global impacts of ENSO
• Effects of El Nino on U.S. winter temperature and
  precipitation Warm temperature around Great
  Lakes, Heavy rainfall in California and Gulf
  coast
• Land-sea temperature contrasts seasonal monsoon

3
Mean state of extratropical climate Westerly
winds and the polar vortex

• The extratropical troposphere is dominated by
  westerly winds and the polar vortex (a persistent
  large-scale cyclone circling the planets
  geographical poles)
• Caused by the temperature contrast between the
  tropics and the polar regions.
• Stronger during winter when the temperature
  contrast is larger

4
Structure of the polar vortex
Warm
5
The boundary of polar vortex is the Polar Jet
Stream

• Caused by steep temperature gradients between
  cold and warm air masses
• Stronger in winter, affect daily weather patterns
  

6
When temperature contrast becomes weaker, polar
vortex slows down becomes wavy
Then cold polar air will enter mid-latitudes
7
Polar Vortex Behind U.S. Big Chill Explained

• https//www.youtube.com/watch?v6KEkSfgHJNk

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Strengthening/Weakening of Polar Vortex(1)
Natural variability

• North Pole
• Arctic Oscillation (AO)
• Previously called North Atlantic Oscillation
  (NAO)
• South Pole
• Antarctic Oscillation (AAO)

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The Arctic Oscillation

• Positive phase Stronger polar vortex, colder
  temperature over Greenland, warm winter in U.S.
• Negative phase Weaker polar vortex, warmer
  temperature over Greenland, cold winter in U.S.

10
Strengthening/Weakening of Polar Vortex(2)
Human-induced global warming
Larger warming in Arctic than in tropics ?
reduces temperature contrast between tropics and
north pole ? weakens polar vortex
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When temperature contrast becomes weaker, polar
vortex slows down becomes wavy
Then cold polar air will enter mid-latitudes more
often
12
Animation Cold US weather and Polar vortex in
the last 30 days

• http//www.esrl.noaa.gov/psd/map/images/fnl/500z_0
  1_30frames.fnl.anim.html

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Impacts of tropical climate on extratropical
climate
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The Pacific/North American Oscillation (PNA)
The way El Nino/La Nina affect U.S.
15
PNA often generate the Pineapple Express
connecting Hawaii and California
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They are also called Atmospheric Rivers
They provide important rainfall, but also create
dangerous flooding for the west coast
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Video Atmospheric Rivers provide important
rainfall, create dangerous flooding

• https//www.youtube.com/watch?vHCe7wVzOABw

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Global climate variabilityCombining the Tropics
and Extratropics
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Atlantic Multi-decadal Oscillation (AMO)

• The Atlantic Multidecadal Oscillation (AMO) is a
  60yr oscillation in water temperatures and is a
  major factor in the increase in Atlantic
  hurricane activity.

20
Global SST anomaly for AMO warm phase
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AMO and thermohaline circulation
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Summary

• Mean state westerly winds, polar vortex
• The natural oscillations associated with
  strengthening/weakening of polar vortex AO, NAO,
  AAO
• The effect of human-induced global warming on
  polar vortex
• Impacts of tropical climate on extratropical
  climate What is the primary way El Nino affect
  extratropics? (PNA)
• Global climate variability the 60-year AMO and
  thermohaline circulation