General Circulation of the Atmosphere

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General Circulation of the Atmosphere

• The Westerlies
• Rossby Waves
• Index Cycle
• Blocking Highs and Cut-Off Lows
• Chapters 9.0 - 9.1

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The Westerlies
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• In the extra-tropics, winds aloft flow from west
  to east (westerlies) due to a pressure gradient
  between low and high latitudes

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The Westerlies
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4
The Westerlies
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The Westerlies
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• The westerlies rarely move directly from west to
  east, but rather follow a wavy path in the upper
  atmosphere

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The Westerlies
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The Westerlies
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• Much of our knowledge of these large-scale
  motions is attributed to C. G. Rossby
• The longest wave patterns have wavelengths of
  4000 to 6000 km (Rossby Waves)

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Rossby Waves
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• Definition
• Upper-air waves in the middle and upper
  troposphere with wavelengths of from 4000 to 6000
  kilometers

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Rossby Waves
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Rossby Waves
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• There tends to be three to six Rossby waves on
  the Earth at any given time that move from west
  to east

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Rossby Waves
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• The number of Rossby waves (and the polar front
  and jet stream) varies seasonally
• In winter, the waves tend to be equator-ward and
  stronger (3 waves)
• In summer, the waves tend to shift poleward and
  are weaker (4 waves)

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Rossby Waves
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• The amplitude of the Rossby waves tends to vary
  on a 4 6 week period
• A measure of the changes in amplitude is called
  the index cycle

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Rossby Waves
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Index Cycle
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• The term used to describe the alternation between
  periods of zonal and meridional flow in the
  Rossby waves

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Index Cycle
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• High Index
• Zonal flow
• Low amplitude waves
• Fast moving waves
• Low Index
• Meridional Flow
• High amplitude waves
• Slow moving waves

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Index Cycle
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Index Cycle
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• Once the amplitude is high (low index), it may
  slowly decrease, or a cut-off low and blocking
  high may occur

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Blocking Highs and Cut-Off Lows
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Blocking Highs and Cut-Off Lows
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• Blocking High
• An upper-level area of high pressure that takes
  on a circular flow distinct from the general flow
  around it
• Cut-Off Low
• An upper-level area of low pressure that takes on
  a circular flow distinct from the general flow
  around it

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Blocking Highs and Cut-Off Lows
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Blocking Highs and Cut-Off Lows
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Rossby Waves
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• In addition to the 4 6 week cycle, there are
  interannual variations in the Rossby waves
• We will discuss the interannual variations later
  in the semester when we discuss teleconnections

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