Download Unit 17

How can the general wind circulation around
the globe be described?
■ How is this circulation produced, and how
does it govern the climates of the earth?
■ What is a jet stream, and how is it important
to aviation?
■ How does the global circulation vary from
season to season?
■
Unit 17
General circulation and
Jet streams
The general circulation
General circulation
L
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L
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Coole
r
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Hadley cell - equator to 30°
Ferrel cell - 30° to 60°
Polar cell
- 60° to pole
See Fig. 7-4 p. 7-5
H
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Warm
Single cell breaks down into three cells
ler
Coo
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H
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SE trades
In NH, air moving north deflected
to the right Þ upper westerlies
In SH, air moving south deflected
to the left Þ upper westerlies
Surface
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In NH, air moving south deflected
to right Þ NE trade winds
In SH, air moving north deflected
to left Þ SE trade winds
Average surface circulation
Air, initially moving from high to low
pressure, is deflected to the right in the
NH, and to the left in the SH. Thus:
Equatorward surface flow turns west
(eastery trades)
Polward upper flow turns east (upper-level
westerlies)
■
S
Aloft
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■
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■
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These circulation cells
are called Hadley cells
Effect of rotation
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Westerlies
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1020
mb
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L
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Equatorial heating Þ low pressure, rising motion
Excess heat energy transported poleward aloft
Cooling in polar regions Þ high pressure, sinking
Cool surface polar air flows equatorward
But, these flows are not observed. Why?
NE trades
Eq
1005 mb
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Earth
Surface
N
mb
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N Equator 0°S
30°
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1020
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H
Driven by unequal heating between tropical
and polar regions
■ Leads to pressure differences
■ Review: Single-cell model (Lecture unit 6;
Fig. 7-3)
300 mb
Westerlies
Near equator,low-level convergence and rising
motion in warm air
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–
■
■
■
■
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Called intertropical convergence zone (ITCZ)
Light winds, lots of rain
At about 30° latitude, subtropical high with sinking
motion and clear weather
Between equator and about 30° latitude, trade
winds (easterlies)
At about 60° latitude, polar front, with rising motion
and low pressure
Between subtropical high and polar front, midlatitude westerlies
In polar region, polar high and sinking motion
Between polar front and polar high - polar easterlies
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Average 300 mb circulation
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High pressure near equator
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–
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Warmest air has high pressure aloft and low
pressure near surface
Equatorial high at 300 mb, ITCZ at surface
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Coldest air has low pressure aloft and high
pressure near surface
Polar vortex at 300 mb, polar high at surface
Westerlies in between
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–
Strongest in winter hemisphere because pole
colder in winter (means lower pressure aloft over
pole in winter)
Weaker in summer hemisphere
Global circulation aloft
■
Upper westerlies have waves in them
–
■
On average, there is a trough over the
eastern parts of the NH continents
Also, there are zones of strong upper-level
flow, called jet streams
–
–
–
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Polar jet stream near 55° lat above polar front
Substropical jet stream near 25° lat
Most marked in winter
Horizontal temp change across them
Jet stream velocity
■
Surface
During winter, the temperature contrast between the tropics and
the poles is greatest, resulting in a strong jet stream. PFJ
winds weaken in summer when temperature contrast are less.
Shades of blue
indicate winds less
than 60 kts, while
winds greater than
120 kts are shown
in shades of red.
Jet streaks are
regions of very
fast winds embedded in the jet
stream: 160 kts +.
300 mb
90°
H
L
L
H
H
H
30°
L
L
L
0°
H
H
H
-30°
L
L
H
-90°
L
Low pressure near poles
–
■
Average surface and 300 mb
circulations
L
90°
L
L
60°
60°
30°
H
-60°
H
0°
H
-30°
L
L
-90°
-60°
Jet streams
The jet stream is a current of fast moving air found in the
upper levels of the atmosphere. This rapid current is typically
thousands of kilometers long, a few hundred kilometers wide,
and only a few kilometers thick. Jet streams are usually found
somewhere between 10-15 km (6-9 miles) above the earth's
surface. The position of this upper-level jet stream denotes the
location of the strongest surface temperature contrast
polar
front jet
(PFJ)
The atmosphere in cross-section
↑ ITCZ
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Monsoon
Modifications by continents
■
Heating and cooling patterns affect mean
surface pressure and flow patterns
–
–
■
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In summer, continents hotter than sea
–
–
■
A warm air column has low pressure near the
surface and high pressure aloft
A cold column has high pressure near the
surface and low pressure aloft
Any large-scale wind pattern that undergoes
a seasonal reversal in direction (Fig. 7-6).
■ Driven by land/sea temperature differences
■ Asia landmass is hot in summer
■
Heating Þ low surface pressure near over
Asia (monsoon trough), N. Africa and SW US
Surface highs over the cooler ocean water
How do the continents modify the surface
pressure pattern in winter?
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–
■
Low-level flow is cyclonic (direction of flow
around a low), convergent and onshore
Upper-level flow is anticyclonic & divergent
Rising motion gives lots of rain
Winter
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–
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Low-level flow anticyclonic, divergent & offshore
Upper flow cyclonic and convergent
Sinking motion means little rain
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