Wind: Global System
General Circulation of the atmosphere
• Represents "average air flow around the globe
• is created by unequal heating at earth's surface
• Recall:
– On global scale, earth is in radiative equilibrium,
energy in equals energy out
– this is not true, however, at each latitude
– general circulation's function is to transport heat pole
ward
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Single-Cell Model of the General Circulation
• If you assume:
– earth is uniformly covered with water
– sun is directly over equator
– no rotation
• You will end up with a single-cell pattern -->>
– called the Hadley Cell
– warm air rises at the equator, cold air sinks at the poles
Three-Cell Model of the General Circulation
• Three-cells
– Hadley cell
– Ferrel Cell
– Polar Cell
• Surface pressure
– High and low
pressure bands
• Surface winds
– Westerlies
– Trade winds
• ITCZ
• comparison to real
world?
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Satellites provide
a global picture of
winds over the
oceans by
converting sea
roughness into
wind speed and
direction
Cloud movement can tell you global wind movement:
http://www.nrlmry.navy.mil/sat-bin/global.cgi
Observed average surface pressure and
winds during January
• Semi-permanent
pressure areas:
– Bermuda-Azores
High
– Pacific High
– Aleutian Low
• genesis region for
cyclones impacting
the Pacific NW
– Icelandic Low
• Seasonal
pressure areas:
– Siberian High
– Canadian High
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Observed average surface pressure
and winds during July
• Semipermanent
pressure
areas:
– BermudaAzores High
– Pacific High
– Iclandic Low
• Seasonal
pressure
areas:
– Monsoon Low
– Thermal Low
over SW US.
A winter weather map depicting the main features of the general
circulation over North America. Notice that the Canadian high,
polar front, and subpolar lows have all moved southward into the
United States, and that the prevailing westerlies exist south of the
polar front. The arrows on the map illustrate wind direction.
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Global Precipitation Pattern
Produced by the General Circulation
• Zonal distribution of
precipitation around the
globe (roughly
speaking)
– 0°N - low pressure cloudy
– 30°N - high pressure sunny
– 45°N-60°N - low
pressure - cloudy
– polar latitudes - high
pressure - clear
• Global Satellite images
often reveal cloud
patterns consistent with
the above general
circulation
Weather associated with the Pacific and
Bermuda Highs
• Pacific high- moves northward during summer
– Produces strong subsidence inversion on eastern side
– During winter, it moves south allowing polar front to bring precip to SW US
• Bermuda high - transports warm, moist subtropical air to US and southern
Canada
– This air can be unstable
– Weather associated with subtropical highs can vary dramatically depending on
where you are located!
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Weather associated with the Pacific
and Bermuda Highs--proof
Average Wind Flow and Pressure
Patterns Aloft -- January
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Average Wind Flow and Pressure
Patterns Aloft -- July
500 mb
Surface
January
July
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Walker Circulation
http://www.bom.gov.au/info/climate/
The nonuniform distribution of land and sea and
asymmetries in radiative, conductive , and
latent heating that accompany it lead to Walker
Circulations along the equator.
Jet Streams are also part of the
general circulation
Atmospheric jet streams are swiftly flowing air currents
thousands of kilometers long, a few hundred kilometers wide,
and a few kilometers thick.
• Subtropical Jet
– situated above the subtropical highs at about 13 km AGL
– often visible as a plume of moisture extending from the tropics to
the sub-tropical regions
– The jet streams exhibit a "wavy" pattern around the globe....
• Polar Jet
– situated at about 10 km
AGL over the polar
front
• Recall that tropopause
height is proportional to
the mean tropospheric
temperature
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Jet Stream Waves:
• Often have troughs
and ridges
• generally have a jet
maximum (jet streak)
in the base of the
trough
• transport heat pole
ward (cold air south
and warm air north)
• Q: How are the polar
and subtropical jets
form????
a) Position of the polar jet stream and the subtropical jet stream
at the 300-mb level on March 9, 2005. Solid lines are lines of
equal wind speed (isotachs) in knots. Heavy lines show the
position of the jet stream core. (b) Satellite image showing clouds
and positions of the jet streams for the same day.
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Jet Stream Formation - Polar Jet
• Polar Jet:
– boundary between warm
air to the south and cold
air to the north
– location of a large
temperature gradient near
the surface
– Hence, the large
temperature gradient at
the surface across the
polar front creates a large
pressure gradient aloft
Jet Stream Formation - Polar Jet
– recall that the strength of the geostrophic wind is
proportional to the magnitude of the pressure gradient force
– Hence, the large pressure gradient aloft over the polar front
generates a band of strong winds
– this is the jet stream!
– Strong in winter, weak in summer.
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Jet Stream Formation - Subtropical Jet
• Subtropical Jet:
– Forms on pole ward side of Hadley cell
– Hadley cell produce the subtropoical front
– Temperature contract Æ pressure gradiance
– created largely through conservation of
angular momentum
• Other Jet Streams:
– Low-level Jet over Central Plains of US
– Polar Stratospheric Night Jet
Jet Stream and the Conservation of angular
momentum
Angular momentum is defined as the product of the mass (m)
times the velocity (v) times the radial distance (r): Angular
momentum = mvr
The Conservation of angular
momentum:
The angular momentum of the
rotating system does not change
if there are no external twisting
forces.
How ice skater change
rotating speed?
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Atmospheric-Ocean Interactions
• Wind-water interactions
– Surface water draft with wind
– Moving water piles up, creating pressure difference
– The interaction can leads motion several hundred
meter down into the water.
• Ocean currents
– Largely generated by wind- gyres
– Account for about 40% of global heat transport
– Water moves at 20-45 degree angle to wind dir due to
coriolis force
– Move slower than winds - several km/day - km/hr
Wind
Ocean
Current
Red: Warm currents
Blue: Cold currents
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Major
Ocean
Currents:
Ocean Front
• The boundary separating the two masses
of water with contrasting temperatures and
densities
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The Ekman Spiral
1. Winds move the water, and the Coriolis force deflects the water to the
right (Northern Hemisphere).
2. Below the surface each successive layer of water moves more slowly
and is deflected to the right of the layer above.
3. The average transport of surface water in the Ekman layer is at right
angles to the prevailing winds.
Upwelling
• Brings cold water from deeper
regions to surface
• Northerly surface winds create
offshore surface water flow
• Cold, nutrient-rich water rises to
replace the surface water (good
for fishing- Peru)
• Process is dominant when winds
are parallel to coastline
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El Nino (Spanish name for the
male child)
• Initially referred to a weak, warm current
appearing annually around Christmas time along
the coast of Ecuador and Peru (not good for
their fishing industry and quano birds!)
• Can produce significant economic and
atmospheric consequences worldwide
• Occur every 3-7 years, lasting about one year
• Recent major events: 1982-1983 and 1997-1998
• The 97-98 event was the strongest ever
recorded!!
El Nino - Animation of warm pool
• El Nino refers to the eastward
movement of warm water
(shaded red) from the western
equatorial pacific to the eastern
equatorial pacific.
• The animation of sea surface
temperature (SST) anomalies
to the right shows the unusual
warming that occurred during
the 97-98 event.
• Note that an anomaly is a
departure from some "normal"
value. An anomaly can be
either positive (warm) or
negative (cold)
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Trade Winds and Rainfall During A Normal Year
•
•
•
•
•
During a normal year, the easterly trade winds push and pile the water in
the equatorial pacific into the western part of the pacific basin.
The sea level is actually higher in the western pacific
Also note the upward motion and precipitation associated with the warm
water and lower pressure in the western pacific
Also note the cooler water, high pressure and subsidence in the eastern
Pacific.
The large-scale zonal circulation in the equatorial plane is called the
Walker Circulation.
Trade Winds During an El Nino Year
• During an El Nino year, the trade winds weaken
• This allows the water in the western pacific to move to the
eastern pacific in the form of a "kelvin wave"
• The equatorial counter current is also stronger
• Notice the change in the Walker Circulation and associated
precipitation distribution.
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• NASA/JPL - El
Niño/La Niña &
PDO
– http://topexwww.jpl.nasa.
gov/science/e
l-nino.html
• NOAA,SST
animation
– http://www.cd
c.noaa.gov/m
ap/clim/sst_ol
r/sst_anim.sh
tml
La Nina and the
current state of the
tropical pacific
• After the El Nino
phase, La Nina
follows
• La Nina refers to
the unusually cold
water that is found
in the eastern
pacific ocean
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These three images depict the evolution of a warm water Kelvin
wave moving eastward in the equatorial Pacific Ocean during
March and April, 1997. The white areas near the equator
represent ocean levels about 20 cm (8 in.) higher than average,
while the red areas represent ocean levels about 10 cm (4 in.)
higher than average.
El Niño / Southern Oscillation -- ENSO
• The Southern Oscillation: The reversing
surface air pressure at the opposite ends
of the pacific ocean
• So, are we currently in an El Nino, La
Nina, or normal phase?
ENSO index
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El Niño
1. Wet weather in CA and the
southern part of US
2. Warmer than normal
weather over a large part
of North America
La Niño
1. Dry and warmer weather in
CA and the southern part
of US
2. Colder than normal
weather over a large part
of North America
Typical winter weather patterns across North America during
an El Niño warm event (a) and during a La Niña cold event (b).
Regions of climatic abnormalities associated with El
Niño–Southern Oscillation conditions.
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The Pacific Decadal Oscillation (PDO)
Long-term Pacific Ocean temperature Fluctuation
Typical winter sea-surface temperature departure
from normal in °C during the Pacific Decadal
Oscillation’s warm phase (a) and cool phase (b).
The North Atlantic Oscillation (NAO)
Refers to pressure variations associated with the Icelandic low and
subtropical high pressure near the Azores
1. more and stronger storms
crossing the Atlantic on a more
northerly track
2. eastern US will experience mild
and wet winter conditions
Stronger than normal
1. fewer and weaker storms crossing
the Atlantic
2. eastern US experiences more coldair outbreaks and snowy weather
conditions
Weaker than normal
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The Arctic Oscillation
Change in surface atmospheric pressure in polar regions and typical winter weather
patterns associated with the (a) positive (warm) phase and the (b) negative (cold)
phase of the Arctic Oscillation.
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