Extratropical Climate

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Extratropical Climate
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Outline

• Mean state
• Dominant extratropical modes
• Pacific/North American Oscillation
• North Atlantic Oscillation
• Arctic Oscillation
• Antarctic Oscillation
• Polar lows (polar hurricanes)
• Polar sea ice and ice-albedo feedback
• Tropical-extratropical interaction

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Mean State

• Westerly winds and jet streams
• A constant Polar Vortex

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Mean State Antarctic polar vortex

• Transport and mixing of trace gases
• Important for ozone hole

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The Pacific/North American Oscillation (PNA)

• One of the most prominent modes of low-frequency
  variability in the Northern Hemisphere
  extratropics.
• Is associated with a wave train extending from
  the vicinity of Hawaii to north Pacific to
  western Canada to southeastern US.
• The primary way ENSO affects Northern Hemisphere
  extratropical climate.
• The corresponding index shows an upward trend.

Z500mb
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The North Atlantic Oscillation (NAO)

• The NAO is the dominant mode of winter climate
  variability in the North Atlantic region ranging
  from central North America to Europe and much
  into Northern Asia.
• The NAO is a large scale seesaw in atmospheric
  mass between the subtropical high and the polar
  low.
• The corresponding index varies from year to year,
  but also exhibits a tendency to remain in one
  phase for intervals lasting several years.

Z1000mb
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• The two phases of NAO
• The positive phase shows a stronger than usual
  subtropical high pressure center and a deeper
  than normal Icelandic low.The increased pressure
  difference results in more and stronger winter
  storms crossing the Atlantic Ocean on a more
  northerly track.This results in warm and wet
  winters in Europe and in cold and dry winters in
  northern Canada and Greenland. The eastern US
  experiences mild and wet winter conditions.
• The negative phase shows a weak subtropical high
  and a weak Icelandic low.The reduced pressure
  gradient results in fewer and weaker winter
  storms crossing on a more west-east pathway.They
  bring moist air into the Mediterranean and cold
  air to northern Europe. The US east coast
  experiences more cold air outbreaks and hence
  snowy weather conditions.Greenland, however, will
  have milder winter temperatures.

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The Arctic Oscillation (AO)

• The Arctic Oscillation refers to opposing
  atmospheric pressure patterns in northern middle
  and high latitudes.The oscillation exhibits a
  "negative phase" with relatively high pressure
  over the polar region and low pressure at
  midlatitudes (about 45 degrees North), and a
  "positive phase" in which the pattern is
  reversed.
• There are some similarities between AO and NAO.

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• In the positive phase, higher pressure at
  midlatitudes drives ocean storms farther north,
  and changes in the circulation pattern bring
  wetter weather to Alaska, Scotland and
  Scandinavia, as well as drier conditions to the
  western United States and the Mediterranean.
  Frigid winter air does not extend as far into the
  middle of North America as it would during the
  negative phase of the oscillation. This keeps
  much of the United States east of the Rocky
  Mountains warmer than normal, but leaves
  Greenland and Newfoundland colder than usual.
• Weather patterns in the negative phase are in
  general "opposite" to those of the positive phase.

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Time series of the AOPositive phase since 1989
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The Antarctic Oscillation (AAO)

• Similar to the Arctic Oscillation. The two
  together are called the annular modes.
• Time series since 1948.

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Polar Lows (Arctic hurricanes)

• Small cyclones forming over open sea during the
  cold season within polar or arctic air masses
• Typically several hundred kilometers in
  diameter, and last on average only a day or two.
• Tend to form beneath cold upper-level troughs
  or lows when frigid arctic air flows southward
  over a warm body of water.
• They often dissipate just as quickly,
  especially upon making landfall.
• Show spiral or comma shaped patterns of deep
  clouds, sometimes with an inner "eye" similar to
  those seen in tropical cyclones. Convective cloud
  bands occupy the surroundings. May possess warm
  cores. Seldom possess hurricane strength winds.
• Difficult to predict even with current high
  resolution models, because they usually occur in
  remote oceanic regions where data are too sparse
  to define the model initial state.

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Polar Sea Ice

• Sea ice cycle
• Formation, growth, deformation,
  disintegration
• Dynamics
• Wind stress
• Water stress

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Loss of the Arctic sea ice
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The Ice-Albedo FeedbackCan accelerate the
melting
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Tropical-extratropical Interaction

• Zonal mean (3 cells)
• ENSO
• MJO
• The interactions are mutual

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Zonal mean circulation
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Tropical-extratropical interactions associated
with the MJO
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Summary

• Mean state Westerly winds, jet streams, polar
  vortex
• Dominant extratropical modes
• Pacific/North American Oscillation (PNA)
• North Atlantic Oscillation (NAO)
• Arctic Oscillation (AO)
• Antarctic Oscillation (AAO)
• Polar lows (polar hurricanes)
• Polar sea ice and ice-albedo feedback
• Tropical-extratropical interaction