Transcript Atm-3 Atm. circulation [text KKC, pp.34

Congratulation to Al Gore
and IPCC for winning
2007 Nobel Peace Prize
An Inconvenient Truth
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Chapter 7: Atmospheric
Circulation and Climate
Objectives:
• Driving force
• Meridional circulation cells
–Hadley, Ferrel & Polar cells
• Surface winds & sea-level pressure
• wind, precipitation and temperature
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• Atmospheric circulation is the large-scale
movement of air.
• The large-scale structure of the atmospheric
circulation varies from year to year, but the basic
structure remains fairly constant.
• Individual weather systems – mid-latitude
weather may occur "randomly“. However, the
average of these systems - the climate - is quite
stable.
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Primary High-Pressure and
Low-Pressure Areas
•
•
•
•
Equatorial low-pressure trough
Subtropical high-pressure cells
Subpolar low-pressure cells
Polar high-pressure cells
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Driving force
• At higher lat., solar
energy flux spreads
over a wider area =>
less solar energy per
unit area.
• Earth emits outgoing
radiation.
•Net radiation = (incoming solar rad.)
- (outgoing rad.) .
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• Net radiation has deficit poleward of 37°, & surplus
equatorward of 37°.
• This means poles should keep cooling while tropics keep
warming. Since this is not happening, other processes
must operate to maintain net energy balance at each lat.
• Atm. & oc. circulation (climate & weather) due to unequal
lat. distr. of energy.
Annual incoming solar rad.
UNBCoutgoing terrestrial rad.
Annual
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Single Convection cell in a
non-rotating Earth
• Imagine the earth as a
non-rotating sphere with
uniform smooth surface
characteristics.
• the sun heats the
equatorial regions much
more than the polar
regions.
In response to this, two
huge convection cells
develop.
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Farrell Cell
polar Cell
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Hardly Cell
• Atmosphere is heated in the equator => Air becomes less dense and rises
=> Rising air creates low pressure at the equator.
• Air cools as it rises because of the lapse rate =>
Water vapor condenses (rains) as the air cools with increasing altitude =>
Creates high rainfall associated with the Intertropical Convergence Zone in
the tropics (ITCZ).
• As air mass cools it increases in
density and descends back to the
surface in the subtropics (30o N
and S), creating high pressure.
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Polar cell and Farrell cell
• In the pole area, the surface is
much cold, especially in winter.
This results in increased air
density near the surface => higher
pressure. The higher density and
pressure lead to divergence =>
surface air moves towards tropic.
The cold air from pole will meet the
warm air from Tropic around to
form “Pole Front Zone.
• For mass conservation, there are
aloft circulations corresponding the
surface circulations, which forms
two cells, called Pole cell and
Farrell cell.
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A idealized pattern of surface wind without
rotation of the earth
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The pattern of surface wind with the rotation of Earth
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• Sfc. winds converg. towards
Eq., deflected by Coriolis =>
Easterlies (NE & SE Trades)
• The sfc. winds converg. at
the ITCZ (Intertropical
Convergence Zone).
– Rising air => clouds.
• Rising air at ITCZ spreads
poleward, sinking at 30°
(high p belts = subtropical
highs).
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• High p at poles => sfc.air flows
equatorward;
Deflected by Coriolis => Polar
Easterlies converg. to the
Subpolar Low (low p at 60°)
• Coriolis deflects air flowing from
subtropical high to subpolar low
=> Westerlies
• Polar cell 60°-90°, Ferrel cell 30°60°
– Where mild air from Ferrel
cell meets cold air from polar
cell => polar front
• Hadley & polar cells are
thermally driven, but Ferrel cell is
a thermally indirect cell.
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– The three-celled model vs. reality: the bottom
line
• Hadley cells are close approximations of real
world
• Ferrel and polar cells do not approximate the
real world
• Model is unrepresentative of flow aloft
• Continents and topographic irregularities
cause model oversimplification
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A) Idealized winds generated by pressure gradient
and Coriolis Force. B) Actual wind patterns
owing to land mass distribution..
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• Horse Latitudes
Around 30°N we see a region of subsiding (sinking)
air. Sinking air is typically dry and free of substantial
precipitation.
Many of the major desert regions of the northern
hemisphere are found near 30° latitude. E.g., Sahara,
Middle East, SW United States.
• Doldrums
Located near the equator, the doldrums are where the
trade winds meet and where the pressure gradient
decreases creating very little winds. That's why sailors
find it difficult to cross the equator and why weather
systems in the one hemisphere rarely cross into the
other hemisphere. The doldrums are also called the
intertropical convergence zone (ITCZ).
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Surface winds & sea-level pressure
(SLP)
January
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Surface winds & sea-level pressure
(SLP)
July
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• N.Hem.: Bermuda (Azores)
High, Pacific High, Icelandic
Low, Aleutian Low.
• July: Bermuda High &
Pacific High stronger &
further north. Icelandic Low
& Aleutian Low weaken &
shift northward.
ITCZ shifts northward
• Jan.: Highs over continents
in N.Hem., lows over contin.
in S. Hem. (monsoon effect).
• July: Lows over continents
in N.Hem., highs over
contin. in S. Hem.
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Meridional cells & precip.
• Northward shift of cells during summer, & southward shift
during winter => precip. changes.
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Shifts in the ITCZ affect the Sahel
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• Hadley, Ferrell, and Polar cells are major players in global
heat transport of the south-north. They are called Latitudinal
circulation, caused by latitudinal difference of incident solar
radiation.
Longitudinal circulation, on the other hand, comes about
because water has a higher specific heat capacity than land
and thereby absorbs and releases heat less readily than land.
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The Walker
circulation is caused
by the pressure
gradient force that
results from a high
pressure system over
the eastern pacific
ocean, and a low
pressure system over
Indonesia. When the
Walker circulation
weakens or reverses,
an El Niño results.
The Southern Oscillation Index (SOI) is calculated from the
monthly or seasonal fluctuations in the air pressure difference
between Tahiti and Darwin.
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• The Walker circulation, which spans almost half the
circumference of Earth, pushes the Pacific Ocean’s trade
winds from east to west, generates massive rains near
Indonesia, and nourishes marine life across the
equatorial Pacific and off the South American coast.
Changes in the circulation, which varies in tandem with
El Niño and La Niña events, can have far-reaching
effects.
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The Polar Front and Jet Streams
•
•
•
•
•
•
Gradual change in temperature with latitude does not always occur
Steep temperature gradients exist between cold and warm air masses
polar front - marks area of contact, steep pressure gradient  polar jet stream
polar jet stream - fast stream of air in upper troposphere above the polar front
stronger in winter, affect daily weather patterns
Low latitudes  subtropical jet stream
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Jet streams
• Jet streams: air
currents thousands of
km long, hundreds of
km wide, a few km
thick (centred near
tropopause).
• Max speed > 200
km/hr.
• Polar jet stream near
polar front, separating
cold air from mild air.
Jet stream turning south
=> cold air moves south.
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• So there is sinking air around 30 degree, which forms
divergence region on surface and convergence region
aloft. The convergence between cold air with warm air
can cause a great temperature gradient in this region,
and further causing a large gradient in pressure =>
speeding the air flow => cause the jet.
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• Subtropical jet stream at ~30°
• Jet streams meander, polar jet may merge
with subtropical jet.
• Polar jet may also branch into 2.
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2 mechanisms for jet streams
1)
Where polar cell meets
Ferrel cell, or Ferrel cell
meets Hadley, airs of
different T meet
=> large T gradient => large
p gradient
=> geostrophic winds.
mv1r1  mv2 r2
2) As air moves from low to
high lat., its circular orbit
shrinks. => orbiting speed incr.
(conservation of angular
momentum; e.g. spinning
skater moves arms
towards body).
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Sea breeze
• Daytime: land warms more than sea
=> rising air & low p on land. Air flows from
sea to land.
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Land breeze
• Night: Land cools more than sea.
=> Sinking air & high p over land. Air flows
from land to sea.
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Valley breeze & mountain breeze
• Daytime: At same elevation, air on mountain slope heated more
than air over valley => low p over mountain slope => air flows
upslope from valley (valley breeze).
• Night: Air on mountain slope cooled more than air over valley =>
mountain breeze.
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Chinook wind
• Chinook: warm, dry wind on eastern slope of Rockies.
• Western slope: condensation => release of latent heat.
Moisture lost from precip.
• Descending wind on eastern slope => warming from
compression.
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Monsoons
• largest synoptic scale winds on Earth
• A seasonal reversal of wind
• Asian monsoon which is characterized by dry (wet), offshore
(onshore) flow conditions during cool (warm) months
• Orographic lifting leads to high precipitation
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Monsoons
• Winter: continents cool more than oc.
=> sinking air & high p over continent
• Summer: continents warm more than oc.
=> rising air & low p over continent
• Most prominent with the massive Asian land mass.
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Winter
monsoon
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Summer
monsoon
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Temperature &
Precipitation
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Mean air temp. at sea level
(Jan.)
• Greater T range over land than over oc.
• Oc. currents affect land T (e.g. Gulf Stream warms
western Europe).
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Mean air temp. at sea level (July)
• Cool Californian Current => cools adjacent land
• Hottest regions at ~20°-30° (not at Eq.)
– High p, subsiding air, clear sky, low humidity => hot deserts.
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Annual T range
• Largest T range over land.
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Temp. records
•High T records:
–World: El Azizia, Libya (32°N) 58°C, in 1922
–Western Hem: Death Valley, CA(36°N) 57°C
–Canada: Midale, Sas.(49°N) 45°C
•Low T record:
–World: Vostok, Antarc. (78°S) -89°C, 1983
–N.Hem.: Verkhoyansk, Russia (67°N) -68°C
–N.America: Snag, Yukon (62°N) -63°C.
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Principal Controls on Temperature
1.
2.
3.
4.
5.
6.
Latitude
Altitude
Atmospheric Circulation
Land-Water Contrasts
Ocean Currents
Local Effects
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Marine v.
Continental
Climates
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Principal Controls on Temperature
1.
2.
3.
4.
5.
6.
Latitude
Altitude
Atmospheric Circulation
Land-Water Contrasts
Ocean Currents
Local Effects
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Ocean
Circulation
• Primary driving force – wind (generated by pressure differences)
– Links atmosphere and ocean together
• East coast of continents  northward moving currents
– Transfer energy poleward
• West coast of continents  southerly currents
• Energy transferred to atmosphere overlying oceans
– affects coastal areas
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Hydrological cycle
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Mean annual precip.
•Driest regions near 30° and poles: high p, subsiding air.
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