Monsoon and 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

• T over land > water in summer

Seasonal temperature distributions:

Land-sea Contrast: Seasonal “Monsoon”

• T over land < water in winter

• A seasonal reversal of wind due to seasonal thermal differences between landmasses and large water bodies

• Orographic lifting often enhances precipitation totals

The Seasonal “Monsoon”

World’s major monsoon systems

From Jay Fein 1987

Seasonal variation of monsoon rainfall

Impacts of monsoons:(1) Normal year

Dry season

Wet season

Impacts of monsoons:(2) Failure of monsoon and droughts

Drought affected area in Karnataka, India, 2012, image from Wikipedia

Impacts of monsoons:(3) Excessive monsoon and floods

Image from Doctors without Borders

Polar Vortex Behind U.S. Big Chill Explained

• https://www.youtube.com/watch?v=6KEkSfgHJNk

• 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

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

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

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

• Land-sea temperature contrasts: seasonal monsoon

Extratropical climate

• 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