Extratropical climate
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
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 largescale cyclone circling the planet’s
geographical poles)
• Caused by the temperature contrast
between the tropics and the polar
regions.
• Stronger during winter when the
temperature contrast is larger
Structure of the polar vortex
Warm
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
When temperature contrast becomes weaker,
polar vortex slows down & becomes wavy
Then cold polar air will enter mid-latitudes
Polar Vortex Behind U.S. Big
Chill Explained
• https://www.youtube.com/watch?v=6KEkSf
gHJNk
Strengthening/Weakening of Polar Vortex
(1) Natural variability
• North Pole:
Arctic Oscillation (AO)
Previously called North Atlantic Oscillation (NAO)
• South Pole:
Antarctic Oscillation (AAO)
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.
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
When temperature contrast becomes weaker,
polar vortex slows down & becomes wavy
Then cold polar air will enter mid-latitudes more often
Animation: Cold US weather and
Polar vortex in the last 30 days
• http://www.esrl.noaa.gov/psd/map/images/f
nl/500z_01_30frames.fnl.anim.html
Impacts of tropical climate on
extratropical climate
The Pacific/North American Oscillation (PNA):
The way El Nino/La Nina affect U.S.
PNA often generate the “Pineapple Express”
connecting Hawaii and California
They are also called “Atmospheric
Rivers”
They provide important rainfall, but also create dangerous
flooding for the west coast
Video: Atmospheric Rivers provide important
rainfall, create dangerous flooding
• https://www.youtube.com/watch?v=HCe7w
VzOABw
Global climate variability
Combining the Tropics and Extratropics
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.
Global SST anomaly for AMO warm phase
AMO and thermohaline circulation
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