upper-level flow climatology and blocking · upper-level flow climatology and blocking...
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Upper-Level Flow Climatology and Blocking
Atmos 5110/6110Synoptic–Dynamic Meteorology I
Jim SteenburghUniversity of Utah
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Northern Hemisphere Winter• Circumpolar vortex
– Lowest heights over pole
• 3 major troughs/ridges (wave #3)
• Enhanced height gradient over west Pacific and west Atlantic– Pacific and Atlantic
jets/storm track
• Weaker heights over Eurasia
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Northern Hemisphere Winter• 2 major jet cores– West Pacific
• (Pacific Jet)– West Atlantic
• (Atlantic Jet)
• Weaker flow/mean split over western NA and western Europe
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Southern Hemisphere Winter• More maritime
hemisphere, plus– High-elevation continent
at pole– Less extensive mid
latitude topography
• Strong, symmetrical circumpolar vortex
• Lowest heights over Ross Sea
• Weak wave 3 pattern
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Southern Hemisphere Winter• More zonally
continuous jet than in NH winter
• Strongest flow over Australia and western Pacific
• Secondary jet maximum between Africa and Antarctica
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Northern Hemisphere Summer• Weak circumpolar
westerlies– Height gradient further
north than in winter
• Weak, wave #4 with troughs over– NE North America– Bering Sea– Northern Russia– Western Europe
• Subtropical ridging with easterly monsoonal flow near equator
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Northern Hemisphere Summer• Weaker westerlies
with jet core poleward of 40º
• Jet is more zonally continuous than in winter– Weaknesses near
west coasts of Europe and North America
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Southern Hemisphere Summer
• Strong band of westerlies persists– Slight retraction
toward pole compared to winter, but weaker than in NH
• Hint of wave #3
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Southern Hemisphere Summer
• Jet displaced slightly polewardof winter position
• Strong zonal continuity of jet with maximum across southern Oceans
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Blocking• Definition: The large-scale obstruction of the
normal west-to-east progression of cyclones, anticyclones, and upper-level waves at midlatitudes– Can persist in some instances for 10 days or more
• Types– High over low (a.k.a. “Rex” block)– Omega– High amplitude ridge
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Rex Block Example• Most common
over western North America and Europe
• Pattern sometimes referred to as “split flow”
• Topography and/or land-sea contrasts probably play a role in creation and persistence of Rex blocks
HL
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Omega Block Example
• Shaped like Greek letter Ω
• High sandwiched between two lows
HL L
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High Amplitude Ridge Example• Mainly
associated with dry weather (when beneath it) although over western U.S. in winter they can be associated with persistent valley and basin fog
H
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More on Blocks
• Not completely understood
• Can be the result of topographic interactions– Blocks frequently found over or upstream of
major mountain ranges
• Can form or be maintained in response to surface cyclogenesis
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More on Blocks
• High Index Pattern: Characterized by strong zonal flow, progression, and lack of blocking
• Low Index Pattern: Characterized by weak zonal flow and blocking
• Vacillation: Alternating from high to low zonal index flows