Download Atmospheric Circulation-II

Atmospheric Circulation-II
GEOL 1350: Introduction To Meteorology
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Overview
Primary Cause of the General Circulation
Three Cell Circulation
Important Large-scale Atmospheric Circulation
Inter-tropical Convergence Zone (ITCZ)
Jet Stream
Walker Circulation
El Nino and Southern Oscillation
North Atlantic Oscillation
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Atmospheric General Circulation (global
circulation) – averaged wind patterns around the
globe (Actual winds at any given place and any
given time may vary considerably from this
average)
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Primary Cause of the General Circulation
•  Unequal heating of the earth’s surface
•  Temperature gradients --- Pressure gradients
--- Winds
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Primary Cause of the General Circulation
This build-up of heat around the equator and depletion in polar
regions is responsible for the poleward flow (transport) of heat
energy to equalize the total energy distribution. That is one of the
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driving forces in the circulation patterns.
Primary Cause of the General Circulation
Jan
Jul
•  The geographical distribution of temperature and its
seasonal variability closely follows the geographical
distribution of solar radiation.
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Primary Cause of the General Circulation
•  Temperature plays a direct role in determining the climate of
every region.
•  Temperature differences are also key in driving the global
atmospheric circulation.
•  Warm air tends to rise because it is light, while cold air tends
to sink because it is dense, this leads to motion in the
atmosphere.
•  The tropical circulation is a good example of this.
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Primary Cause of the General Circulation
•  Surface air converges and rises at the equator, spreads
laterally toward the poles, descends and flows back toward
equator at the surface.
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Three Cells Circulation
•  A thermally direct circulation in the tropics (Hadley Cell),
with rising motion around the equator and sinking motion
at about 30 latitude.
•  A thermally indirect circulation in the middle latitudes
(Ferrel Cell), with rising motion at 60 and sinking motion
along with the Hadley cell at about 30.
•  Another weaker direct circulation in the polar regions
(polar cell) with rising motion at about 60 and sinking
motion over the pole.
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Global Air Circulation
http://www.emc.maricopa.edu/faculty/farabee/biobk/deserts.gif
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Dividing the global into several latitude bands
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Definition of the key terms
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Intertropical convergence zone (ITCZ) – the
boundary where the northeasterly trade winds in
NH converge with the southeasterly trade winds
in the SH
Polar Front – a semi permanent front that
separates tropical air masses from polar air
masses
Subpolar low – a zone of low pressure near the
polar front
Polar Easterlies – shallow layer of northeasterly
flows formed when the southward moving cold
polar air behind the polar front is deflected by the
Coriolis force to the right of its path.
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Weak winds near
equatorial region
and 30o latitude belt
Easterly winds
dominate
from equator to 30o
(Trade winds)
from pole to 60o
(Polar easterly)
Westerly winds
predominate
between 30 to 60o
(mid-latitude)
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Key features of the Three-Cell Model
Polar high
Ferrel cell
Subpolar low
Subtropical high
Tropical low
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Jet Streams –
  very strong winds
concentrated within a narrow
band in the atmosphere
  may be thousands of
kilometers long, a few
hundred meters wide, and a
few kilometers thick
  are usually found at the
tropopause between 10 and
15 km
  the region of the greatest
wind within the jet stream is
called the jet streak
  typically find two jet
streams in the NH – polar
front jet stream and the
subtropical jet stream 17
Jet Streams
•  North polar jet stream meanders with movement of
polar front
–  In winter may extend as far south as 30o N
–  In summer average position is 50o N
–  Because of this migration described as mid-latitude jet
stream
•  Semi-permanent jet stream
forms over tropics in winter
–  Slower than polar jet stream
–  Centered at about 25o N
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rst.gsfc.nasa.gov/Sect14/Sect14_1c.html
What produces Jet Streams?
Rapid changes in pressure in a relative narrow region
sets up a steep pressure gradient that intensifies the
wind speed and causes the jet stream
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Pressure and wind patterns in the real world
•  The presence of continents, mountains, and ice fields
alters the general circulation from the ideal 3-cell model.
•  Semi-permanent high and low pressure systems persist
throughout large periods of the year
–  During winter, highs form over land; lows over oceans. Vice
versa during summer.
•  The major pressure systems, wind belts, ITCZ shift
toward south in January and toward north in July.
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Intertropical Convergence Zone (ITCZ)
Source: NOAA GOES-11 & NASA Earth Observatory
•  Inter-tropical Convergence Zone (ITCZ): The east-west oriented
band of intense convection located over the warmest regions of the
tropics.
•  Inside ITCZ, the deep convection is supported by the release of
latent heat when moisture condenses.
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Intertropical Convergence Zone (ITCZ)
• 
• 
ITCZ moves back and forth across equator following the sun’s zenith
point.
Variation in the ITCZ locations affect rainfall in the tropics.
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Four semi-permanent pressure systems:
Bermudar high and Pacific high (near latitude at
30o),
Icelandic low and Aleutian low (between 40 – 65o)
Other highs and lows that are not permanent in
nature (change significantly during the course of
the year):
Siberian high (becomes thermal low in July),
intense, responsible for Indian monsoons
Canadian high (becomes a thermal low in July),
less intense, responsible for Mexico monsoons
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Walker Circulation
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El Nino
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El Niño
•  Spanish for ‘Christ Child’
•  Originally used by fishermen along the coast of Peru and
Ecuador in referring to a warm ocean current appearing near
Christmas and lasting a few months
•  Fish are less abundant during these warm periods because
there is less ‘upwelling’ of cold nutrient-rich water
•  During some years, however, the water remains especially
warm and the break in the fishing season extends into May
•  The term ‘El Niño’ has become reserved for these extreme
events
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El Niño’s economic impact
•  Economic impact on
Peruvian fishing is
negative
•  Seabirds that feed on
fish also suffer, along
with other parts of the
coastal ecosystem
Rough estimate of damage around the
globe during 1982-83 El Nino.
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El Niño Conditions
•  Once every few years,
the normal pattern
‘breaks down’
•  Pressure decreases in
the eastern high and
pressure increases in
the western low
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The Southern Oscillation Index
•  Mean pressure is lower at Darwin than Tahiti
• The SOI measures the strength of the Pacific Walker circulation
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El Niño Southern Oscillation Index
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El Niño Conditions
•  The easterly trade
winds weaken (or even
reverse)
•  The warm waters of
the western tropical
Pacific come back to
the east because the
trade winds are too
weak to sustain the
tilted sea level
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El Niño Conditions
•  The upwelling or
rising of the cold
nutrient-rich waters
off the South
American coast ceases
•  The upward motion,
clouds and
precipitation moves
eastward into the
central and eastern
Pacific
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Sea-Saw of sea level pressure associated with El Niño and
La Niña is shown. During an El Niño the sea level
pressure in the east Pacific decrease while that
over west Pacific and Indonesia increases.
In a La Niña it is higher pressure in the east and lower
pressure in the west.
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El Nino has global consequences
•  Droughts in Australia
and Indonesia
•  Warm, rainy weather
in the eastern Pacific
and South America
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Droughts in Australia
Flooding in California
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El Nino has global consequences
•  Droughts in Australia
and Indonesia
•  Warm, rainy weather
in the eastern Pacific
and South America
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La Niña
•  Opposite to El Niño
•  Higher than average pressure in the eastern Pacific
and lower than average pressure in the western
Pacific
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La Niña
•  Stronger than normal
easterly trade winds
•  Colder than normal
ocean temperatures off
of South America
•  Flooding in Australia
and Indonesia
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The North Atlantic Oscillation
•  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.
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The North Atlantic Oscillation
•  When the NAO is in its positive phase, low pressure
anomalies over the Icelandic region and throughout the
Arctic combine with high-pressure anomalies across the
subtropical Atlantic to produce stronger-than-average
westerlies across middle latitudes
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The Positive NAO index
•  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
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The negative NAO index
•  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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SUMMARY
1.  The largest pattern of winds that persists around the globe is
called the general circulation.
2.  At the surface in both hemispheres, winds tend to blow from east
in the tropics, from the west in the middle latitudes, and from the
east in polar regions.
3.  Where upper-level westerly winds tend to concentrate into narrow
bands, we find jet streams.
7.  Near the equator, the intertropical convergence zone (ITCZ) is a
boundary where air rises in response to the flowing together of the
northeast trades and the southeast trades.
8.  When the warm water extends over a vast area of the tropical
Pacific, the warming is called a major El Nino event, and the
associated reversal of pressure over Pacific Ocean is called the
Southern Oscillation.
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