The present atmospheric
circulation
Robert Fovell
Atmospheric and Oceanic Sciences
University of California, Los Angeles
rfovell@ucla.edu
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Terms and abbreviations
•
•
•
•
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NH = Northern Hemisphere
PGF = pressure gradient force
CF = Coriolis force
Geostrophic wind = PGF + CF balance
SLP = sea-level pressure
– Average SLP = 1013.25 mb
• Reanalysis = retrospective weather reconstruction
• NCEP = National Centers for Environmental
Prediction
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The sea-breeze circulation
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The sea-breeze circulation
(applied to NH)
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One-cell circulation cell
• Surface wind is cold
to warm & northerly
• “One cell” model
• “Thermally direct”
• Vertical motions
help reduce T
gradient
– Adiabatic
compression &
expansion
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One-cell circulation cell
• Rotunno experiment
provided glimpse
that Earth rotation
complicates matters
• Rotation proxy:
Coriolis force
• Consequence: 1-cell
model fails to
explain global
circulation
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Geostrophic adjustment
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Horizontal pressure gradient
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PGF impels motion
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Coriolis acts to right in NH
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Air starts curving away from
L pressure
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Coriolis keeps tugging
rightward…
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So air curves even more…
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Eventually PGF & Coriolis
come into opposition…
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Eventually PGF & Coriolis
come into opposition…
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Geostrophic adjustment
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Geostrophic balance:
PGF + Coriolis
Note NO flow towards low
Buys-Ballot’s “Law”:
In NH, with wind at back,
low pressure is to your left
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Geostrophic with friction:
some component towards low possible
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Geostrophic equations
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How Earth rotation impacts
the global circulation
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From one cell to…
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Step #1: three cells
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Step #1: three cells
Polar
Ferrel Hadley
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Step #1: three cells
Ferrel cell is
thermally indirect
cool
warm
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Step #1: three cells
storms deserts storms
Consequence of
vertical motion
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http://visibleearth.nasa.gov/view_cat.php?categoryID=1484
A cloud-free Earth
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Zoom in on Africa
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Focus on surface PGFs
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Step #2: geostrophic
adjustment
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Step #2: geostrophic
adjustment
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Step #2: geostrophic
adjustment
Coriolis weak
in tropics…
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Both hemispheres
Coriolis acts
to right…
Coriolis acts
to left…
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Note surface convergence at
Equator
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ITCZ
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Note ITCZ resides a bit north of Equator… more soon
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Theory vs. practice
Atmospheric circulation as seen in
reanalyses
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NCEP global reanalyses
• Reanalyses are retrospective
reconstructions of weather, averaged to
deduce climate
• Not purely observations
– Before satellite era (1979-present) a lot of
guesses
• http://www.esrl.noaa.gov/psd/
cgibin/data/composites/printpage37
Annual average SLP
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Making that plot…
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Annual average SLP
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Expect: surface L at 60˚
Avg. SLP
1013.25 mb
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Expect: surface H at 30˚
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NH winter SLP
Lows prominent over ocean
… but high over inland Asia
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Aleutian & Icelandic Lows
NH summer SLP
Highs dominate, again over
ocean. Low over Asia
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Pacific & Bermuda Highs
Seasonal variation in Asia
winter
summer
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Seasonal variation in Asia
winter
summer
Regional scale sea-breeze:
the monsoon (Arabic for
“season”)
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Pacific High in NH summer
• Dominates SoCal
weather
• Note also large
west-east SLP
gradient, owing to
“regional seabreeze”
• Establishes Walker
circulation
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Walker circulation
Pacific Ocean as “bathtub”
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Walker circulation
Trade winds help pile up water in west Pacific.
Sea level about a half meter higher there
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Downwelling in west Pacific leads to
deep layer of very warm (even hot) water.
The ‘Maritime Continent’ of Indonesia, Philippines
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Warm water supports storms, establishes Walker
circulation, which looks a lot like a sea-breeze.
Subsidence branch reinforces Pacific High
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Periodically, the trade winds relax, weakening Walker
circulation. Storms shift eastward. Drought strikes
Maritime Continent. El Nino.
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Midtropospheric vertical
motion: NH summer
Ascent = purple; descent = yellow/red
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Africa
• Rising motion near
Equator… except:
– Ascent maximum NORTH of
Equator
– Somalia receives
subsidence
• Sinking motion at 30˚N,
30˚S
– Subsidence near Crete
especially large
– Flow over Alps causes
descent
• Note Somalia resides on
western side of Walker
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circulation
Ascent = precipitation
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Midtropospheric vertical
motion: NH summer
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ITCZ
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ITCZ
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ITCZ in East Pacific
(NH summer)
EQ
• Note surface wind
convergence at ITCZ
• Note ITCZ is NORTH of
Equator
• ITCZ position “follows
the sun”
• Mean annual position
resides in NH
– more land in NH
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Midtropospheric
ascent NH summer
To a reasonable
approximation:
where air rises,
precipitation occurs
Surface precipitation
NH summer
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Surface precipitation
NH winter
storm tracks seen
Northern ocean
storm tracks appear
In NH winter;
ITCZ migrates south
Surface precipitation
NH summer
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Theory vs. practice
• Three-cell per hemisphere model is a
reasonable starting point for understanding
atmospheric circulation
• Deficiencies/complications
– Continents cause regional/seasonal-scale sea and
land breezes (monsoons)
– Influences of oceans and storms (Walker
circulation, El Nino)
– Topography also plays an important role
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Reanalysis vs. EdGCM
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Reanalysis vs. EdGCM
EdGCM’s modern vs. modified
climate
Compare & contrast:
•SLP patterns, annual & seasonal
•Precipitation patterns, amounts
•Monsoon and Walker circulations
•ITCZ migrations
•More…
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Reanalysis and EdGCM will
differ…
• Reanalysis is average across decades
with trends; EdGCM control run is
statistically steady (next slide)
• EdGCM cannot represent El Nino and
some other weather phenomena
reanalysis captures
• EdGCM topography very coarse (affects
vertical motion and precipitation)
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Global average
precip rates
(mm/day)
Reanalysis
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EdGCM
Annual average SLP
1018
1005
Changing color tables
can shift perceptions
1000
EdGCM
Equatorial L appears more
prominent
1022
1011
[Color tables differ]
995
Reanalysis
Emphasizes 60˚S and 30˚ Highs
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Annual average
precipitation
Changing color tables
can shift perceptions
EdGCM
[Color tables differ]
Reanalysis
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Surface T difference:
2xCO2 – control runs
Using default options
ALL VALUES ARE POSITIVE
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Surface T difference:
2xCO2 – control runs
Changing color tables
can shift perceptions
Zero-centered color scale
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[end]
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