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
• The extratropical troposphere is dominated by westerly winds and the polar vortex (a persistent large-scale 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
Mean state of extratropical climate: Westerly winds and the polar vortex
• Caused by steep temperature gradients between cold and warm air masses
• Stronger in winter, affect daily weather patterns
The boundary of polar vortex is the Polar Jet Stream
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=6KEkSfgHJNk
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/fnl/500z_01_30frames.fnl.anim.html
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=HCe7wVzOABw
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.
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