Guido Cervone EOS 121

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Chapter 8

• Guido CervoneEOS 121

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Welcome Back
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Chapter 8 Atmospheric Circulation and Pressure
Distributions
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Winds

• We always refer to winds according to their
  direction
• Westerly winds west to east
• Northerly ??
• Zonal winds blowing along a line of latitude
• Meridional winds blowing along a line of
  longitude

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Discovery of the Trade Winds

• It was the Genoese seaman, explorer and
  adventurer Christopher Christopher Columbus
  (1451-1506), who discovered the trade winds
• These winds carried his three modest-size sailing
  vessels all across the Atlantic at its widest, a
  distance of 5400 miles, in 36 days, in 1492.
• Centuries later (in 1970), the Norwegian seaman
  and amateur archeologist Thor Heyerdahl showed
  that the trade winds are capable of blowing a
  sailing vessel built of reeds (made to resemble
  an Egyptian craft) from Morocco to the Caribbean.
  He suggested that the idea of building pyramids
  could have reached Central America by Egyptians
  traveling in this fashion in ancient times.

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Columbus' Voyage
When the trade winds hit the western edge of an
ocean basin, the winds turn first toward the
poles, and then loop back east to become
prevailing westerlies These westerlies, for
example, are what powered Columbus sailing
vessel on the return trip to Europe
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Westerlies and Surfing

• The westerlies are also responsible for the far
  better surfing on the Pacific side of North
  America compared to the Atlantic side.
• On the Pacific side, the westerlies blow in the
  same direction as waves rolling toward shore from
  storms out at sea, building up their height.
• In the Atlantic, the prevailing winds blow
  against the incoming waves, shrinking them down
  to sizes that are less than adequate for surfers.

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Wind Speed and Direction from Quickscat
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Surface Ocean Currents
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George Hadley (1685-1768) proposed a simple
circulation pattern called the single-cell model
to describe the general movement of the
atmosphere. In the single-cell model, air
expands upward, diverges toward the poles,
descends, and flows back toward the equator near
the surface. Winds blowing east-to-west or
west-to-east are referred to as zonal winds
those moving north-to-south or south-to-north
are called meridional.
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The three-cell model divides the circulation of
each hemisphere into three distinct cells the
heat-driven Hadley cell that circulates air
between the Tropics and subtropics, a Ferrel cell
in the middle latitudes, and a polar cell.
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Global Circulation
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Along the equator, strong solar heating causes
air to expand upward and diverge toward the
poles, creating a zone of low pressure at
the equator called the equatorial low or the
Intertropical Convergence Zone. The ITCZ is the
rainiest latitude zone in the world and is
observable as the band of convective clouds and
showers extending from northern South America
into the Pacific on this satellite image.
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At about 20 to 30 latitude, air in the Hadley
cell sinks toward the surface to form the
subtropical highs, large bands of high surface
pressure. Cloud formation is greatly
suppressed and desert conditions are common in
the subtropics. In the Northern Hemisphere, as
the pressure gradient force directs surface air
from the subtropical highs to the ITCZ, the weak
Coriolis force deflects the air slightly to the
right to form the northeast trade winds. In the
Southern Hemisphere, the northward-moving air
from the subtropical high is deflected to the
left to create the southeast trade winds.
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Immediately flanking the Hadley cell in each
hemisphere is the Ferrel cell, which circulates
air between the subtropical highs and the
subpolar lows, or areas of low pressure. On the
equatorial side of the Ferrel cell, air flowing
poleward away from the Northern Hemisphere
subtropical high undergoes a substantial deflectio
n to the right, creating a wind belt called the
westerlies. In the Southern Hemisphere, the
pressure gradient force propels the air
southward, but the Coriolis force deflects it to
the left, producing a zone of westerlies in that
hemisphere as well.
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In the polar cells of the three-cell model,
surface air moves from the polar highs to the
subpolar lows. Compared to the poles, air at
subpolar locations is slightly warmer, resulting
in low surface pressure and rising air. Very
cold conditions at the poles create high surface
pressure and low-level motion toward the equator.
In both hemispheres, the Coriolis force turns the
air to form a zone of polar easterlies in the
lower atmosphere.
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The real world is not covered by a series of
belts that completely encircle the globe.
Instead, we find a number of alternating
semi-permanent cells of high and low pressure.
Among the most prominent features in the Northern
Hemisphere during winter are the Aleutian and
Icelandic Lows over the Pacific and Atlantic
Oceans, and the Siberian High over central Asia.
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In summer, the best-developed semi-permanent
cells are the Hawaiian and Bermuda-Azores Highs
of the Pacific and Atlantic Oceans and the
Tibetan Low of southern Asia.
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The Sahel is a region of Africa bordering the
southern Sahara Desert. During the summer (left),
the ITCZ usually shifts northward and brings rain
to the region. For much of the year, the ITCZ is
located south of the Sahel, and the
region receives little or no precipitation
(right).
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Sahel Region

• region between latitude 10o N and 25o N and
  longitude 16o W and 39o E
• falls within the arid, semi-arid and the dry
  subhumid regions
• southern Sahel region-latitude 10o N), annual
  rainfall is as high as 1100 mm
• extreme north (25o N), the annual rainfall is as
  low as 300mm
• Rainfall duration range
• 5 Months in the south
• 3 months north
• Characterized by
• torrential rains
• Erratic rains and
• flash flooding

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Sahel Region
NOAA 2008
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Average ITCZ Position in Sahel
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Spatial Pattern of Change
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Wind speeds generally increase with height
between the surface and the tropopause largely
because pressure gradient force is typically
stronger at high altitudes. The surfaces
representing the 900, 800, and 700 mb levels all
slant downward to the north, but not by the same
amount. Higher surfaces slope more steeply, which
means that the pressure gradient force is greater.
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The polar front is a strongly sloping boundary
between warm mid-latitude air and cold polar
air. Within the front, the slope of the pressure
surfaces increases greatly because of the abrupt
horizontal change in temperature. With steeply
sloping pressure surfaces there is a strong
pressure gradient force, resulting in the polar
jet stream situated above the polar front near
the tropopause.
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Jet streams can be thought of as meandering
rivers of air, usually 9 to 12 km above sea
level. Their wind speeds average about 180 km/hr
in winter and about half that in summer, though
peak winds can exceed twice these values. Nearer
the equator is the subtropical jet stream,
associated with the Hadley cell. The subtropical
jet stream can bring with it warm, humid
conditions as seen above.
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This hypothetical drawing of the 500 mb surface
reveals that heights decrease from south to north
but also rise and fall through the ridges and
troughs. Vertical changes are highly exaggerated
in the figure. Actual height changes are small
compared to the size of the continent.
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Troughs occur in the middle troposphere where the
500 mb height contours dip equatorward. In the
top map, positions 13 have the 500 mb level at
5610 m. Farther to the south, at positions 46,
the 500 mb level is at 5640 m. In the bottom map,
contour lines are in the same position over the
East and West Coasts as they were up top, but
shift equatorward over the central portion of the
continent. Thus, positions 2 and 5 have lower
pressure than the areas east and west of them.
The zone of lower pressure over the central
part of the continent is a trough.
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Rossby Waves

• A large, slow-moving, planetary-scale wave
  generated in the troposphere by ocean-land
  temperature contrasts and topographic forcing
  (winds flowing over mountains), and affected by
  the Coriolis effect due to the earth's rotation.

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There are anywhere from three to seven Rossby
waves or long waves circling the globe, each with
a particular wavelength and amplitude. Rossby
waves often remain in fixed positions, but also
migrate west to east and are capable of
transporting warm air from subtropical regions to
high latitudes, or cold polar air to low
latitudes.
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Ocean currents are horizontal movements of
surface water that have an impact on the exchange
of energy and moisture between the oceans and the
lower atmosphere. Among these are the North
Equatorial Current and the South Equatorial
Current, which converge, creating the Equatorial
Countercurrent. The warm Gulf Stream flows
northeast to become the North Atlantic Drift and
later the cold Canary Current as it turns
southward. The cold Labrador Current flows south
along the east coast of Canada fed by the East
and West Greenland Drift.
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Ocean currents are driven by winds in the lower
atmosphere that exert a drag on the water at an
angle 45 to the right (Northern Hemisphere) and
continue to shift clockwise as their speed
decreases. At a depth of about 100 m, the current
approaches the opposite direction of the surface
current and begins to die out in a pattern known
as the Ekman spiral.
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Strong offshore winds along a coastal region
sometimes drag the warmer surface waters seaward,
which draws up cooler waters from below to take
their place. This process, called upwelling,
greatly influences sea surface temperatures over
the eastern portions of the major oceans.
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Features such as the Intertropical Convergence
Zone, the westerlies, and large Rossby waves
exist on a global scale. Smaller features, such
as cyclones, anticyclones, troughs, and ridges,
exist at the synoptic scale, covering hundreds
or thousands of square kilometers. Mesoscale
events are on the order of tens of
square kilometers and last for periods as brief
as half an hour. The smallest exchanges of mass
and energy operate at the microscale.
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Monsoon refers to the climatic pattern in which
heavy precipitation alternates with hot, dry
conditions on an annual basis due to the seasonal
reversal in surface winds caused by an
oscillation between high- and low-pressure cells.
During winter (top), dry air flows southward from
the Himalayas. When summer arrives (bottom) moist
air is drawn northward from the equatorial
oceans. Surface heating, convergence, and a
strong orographic effect cause heavy rains over
the southern part of the continent.
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Foehn is the generic name for synoptic scale
winds that flow down mountain slopes, warm by
compression, and introduce hot, dry, and clear
conditions to the adjacent lowlands. Winds
warmed by compression that descend the eastern
slopes of the Rocky Mountains are called
chinooks. The Santa Ana winds of California,
common in the fall and spring, occur when high
pressure develops over the Rocky
Mountains. Katabatic winds originate when air is
locally chilled over a high- elevation plateau
and warmed by compression as it flows down slope.
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Along the coast during the daytime, land surfaces
warm more rapidly than the adjacent water (a),
which causes the air column overlying the land to
expand and rise upward (b). At a height of about
1 km, the rising air spreads outward (c), which
causes an overall reduction in the surface air
pressure. Over the adjacent water less warming
takes place, so the air pressure is greater than
that over land. The air over the water moves
toward the low-pressure area over the land, which
sets up the daytime sea breeze. At night the land
surface cools more rapidly than the water. The
air over the land becomes dense and generates a
land breeze.
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A valley breeze (a) forms when daytime heating
causes the mountain surface to become warmer than
nearby air at the same altitude. The air
expands upward and the air flows from the valley
to replace it. Nocturnal cooling makes the air
dense over the mountain and initiates a mountain
breeze (b).
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El Niño is a recurrent event in the tropical
eastern Pacific in which sea-surface temperatures
are significantly above normal. La Niña is the
inverse event (cold sea-surface
temperatures). The Walker circulation is an
east-west circulation pattern of the Tropics,
characterized by several cells of rising and
sinking air connected by horizontal motions along
parallel lines of latitude. The Southern
Oscillation is the reversal of surface pressure
patterns over the tropical Pacific associated
with El Niño events. The Pacific Decadal
Oscillation is an alternating pattern of sea
surface temperature in the Pacific that reverses
over several decades.
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The next chapter examines air masses and fronts.