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

• 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

Warm

Structure of 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

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=HCe7wVzOABw

Global climate variabilityCombining 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