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Winds and Global Circulation
• Atmospheric Pressure
• Winds
• Global Wind and Pressure Patterns
• Oceans and Ocean Currents
• El Niňo
Precipitation in Montana
How is Energy Transported
to its “escape zones?”
• Both atmospheric and ocean transport are crucial
• Buoyancy-driven convection drives vertical transport
• Latent heat is at least as important as sensible heat
Atmospheric Circulation
in a nutshell
• Hot air rises (rains a lot) in the tropics
• Air cools and sinks in the subtropics
(deserts)
• Poleward-flow is deflected by the Coriolis
force into westerly jet streams in the
temperate zone
• Jet streams are unstable to small
perturbations, leading to huge eddies
(storms and fronts) that finish the job
Atmospheric Circulation
in a nutshell
• Winds are initially generated by differences
in heating at the Earth’s surface
• Geostrophic winds result in rotational
movement around high and low pressure
centers.
Ocean Circulation
in a nutshell
• Surface winds cause large, clockwise rotating
gyres in the northern hemisphere and
counterclockwise gyres in the southern
hemisphere
• Salinity and temperature differences in water
cause sinking of water (deep water formation) in
the North Atlantic and Southern Ocean
(Antarctic)
• EL Nino is a quasi periodic rocking of the
Ocean-Atmosphere system in the tropical Pacific
Atmospheric Pressure
As the atmosphere is held
down by gravity, it exerts a
force upon every surface
(pressure = force per unit
area)
At sea level the force is the
weight of 1 kg of air that
lies above each square
centimeter of the surface
(around 15 lbs per in2)
atmospheric
pressure decreases
rapidly with altitude
near the surface
Therefore a small
change in elevation
will often produce a
significant change in
air pressure
How winds
are made
Two columns of air–
same temperature
same distribution of mass
500 mb level
1000 mb 1000 mb
Cool the left column;
warm the right column
The heated column
expands
The cooled
column
contracts
500 mb
original 500 mb level
500 mb
1000 mb 1000 mb
The level of the 500 mb surface changes;
the surface pressure remains unchanged
The level corresponding to
500 mb is displaced
downward in the cooler
column
original 500 mb level
new 500 mb
level in cold
air
The 500 mb surface is
displaced upward in the
warmer column
new 500 mb
level in warm
air
The surface pressure
remains the same since
both columns still contain
the same mass of air.
1000 mb 1000 mb
A pressure difference in the horizontal
direction develops above the surface
The 500 mb surface is
displaced upward in the
warmer column
The 500 mb surface is
displaced downward in
the cooler column
original 500 mb level
Low
High
new 500 mb
level in warm air
new 500 mb
level in cold air
1000 mb 1000 mb
The surface pressure
remains the same since
both columns still contain
the same mass of air.
Air moves from high to low pressure in
the middle of the column,
causing the surface pressure to change.
original 500 mb level
Low
wind
High
1003 mb 997 mb
Air moves from high to
low pressure at the surface…
Where would we
have rising motion?
original 500 mb level
Low
High
Low
High
1003 mb 997 mb
What have we just observed?
• Starting with a uniform atmosphere at rest, we
introduced differential heating
• The differential heating caused different rates of
expansion in the fluid
• The differing rates of expansion resulted in pressure
differences along a horizontal surface.
• The pressure differences induced horizontal flow
(wind) in the fluid
• This is how the atmosphere converts heating into
motion
• Winds are the result of differential heating
Surface Pressure Maps
• Altitude-adjusted surface station pressures are used to
construct sea level pressure contours
Differences in air pressure = a pressure gradient
The pressure gradient forces acts at right
angles to the isobars (90 degrees)
820
830
820
840
850
830
860
870
840
880
890
850
860
weak pressure
gradient
strong pressure
gradient
Global
Circulation
90oN
Cold
High
Pressure
60oN
But heat is
transported from
the Equator to the
Poles - how?
30oN
0o
30oS
60oS
90oN
Earth
Warm
Low Pressure
SUN
Fig. 7-6, p. 151
How is Energy Transported
to its “escape zones?”
• Both atmospheric and ocean transport are crucial
• Buoyancy-driven convection drives vertical transport
• Latent heat is at least as important as sensible heat
What a single cell convection model would look like for a
non-rotating earth
• Thermal convection
leads to formation
of convection cell
in each hemisphere
• Energy transported
from equator
toward poles
• What would
prevailing wind
direction be over N.
America with this
flow pattern on a
rotating earth?
What’s wrong
with the 1-cell model?
• Neglects effect of rotation
- with rotation, winds would cause earth to spin down
- with rotation, the upper level winds would accelerate to
unphysical speeds near the pole. You would be funneling all
the air from the Equator down at the Poles
- It is not a stable solution for wind circulation
Fig. 8-1, p. 172
Coriolis Force acts to the right
in the Northern Hemisphere
Physics
Coriolis Effect
The Coriolis Effect deflects moving objects to the right in the
northern hemisphere and to the left in the southern.
General Circulation of the Earth’s
Atmosphere
90oN
Deflection
60oN
30oN
0o
No Deflection
30oS
60oS
90oN
Deflection
Deflection is
least at the
equator and
greatest at the
poles
Wind patterns on a rotating Earth
3 circulation cells in each hemisphere
Fig. 7-12, p. 154
Wind patterns on a rotating Earth
3 circulation cells in each hemisphere
warm air rises at the
equator producing low
pressure (Intertropical
Convergence Zone,
ITCZ) and flows
towards the poles
90oN
60oN
30oN
0o
30oS
60oS
90oN
L
90oN
60oN
30oN
0o
30oS
60oS
90oN
H
L
H
Cold air sinks at 30o
N and S latitude
Creating high pressure
(subtropical high
pressure, STH)
90oN
60oN
30oN
0o
30oS
60oS
90oN
Northeasterly and southeasterly
surface winds flow from the
subtropical high pressure belts
(30o N and S) to the low
pressure belt (ITCZ) at
H
the equator (calm winds:
doldrums)
L
H
westerly surface winds
flow from the subtropical
high pressure belts towards
higher latitudes
90oN
60oN
30oN
westerly surface winds are forced
to rise around 60o N and S latitude
when they encounter cold polar
easterly winds from the poles
L
resulting in Subpolar Low
pressure (SPL) belts
H
L
0o
H
30oS
60oS
90oN
L
H
90oN
60oN
L
H
30oN
L
0o
H
30oS
60oS
90oN
H
L
cold air sinks at the
poles producing
polar high (PH)
pressure regions
Figure 5.17, p. 163
H
polar jet stream
Jet streams are
streams
of fast
moving
air aloft
subtropical
that occur
jet streams
where
atmospheric
temperature
gradients
are strong
90oN
60oN
L
H
30oN
L
0o
H
30oS
60oS
L
90oN
H
polar jet stream
Key features of three cell model
• Hadley cell (thermally direct cell)
- driven by meridional gradient in heating
- air rises near equator and descends near 30 degrees
- explains deserts; trade winds; ITCZ
• Ferrel Cell (indirect thermal cell)
- driven by heat transports of eddies
- air rises near 60 degrees and descends near 30 degrees
- explains surface westerlies from 30-60
• Weak winds found near
– Equator (doldrums)
– 30 degrees (horse latitudes)
• Boundary between cold polar air and mid-latitude warmer air
is the polar front
Geostrophic
Winds
Coriolis Force acts to the right
in the Northern Hemisphere
Physics
Coriolis Effect
The Coriolis Effect deflects moving objects to the right in the
northern hemisphere and to the left in the southern.
low pressure
Coriolis Force
pressure
geostrophic
gradient force
992
996
1000
1004
1008
1012
1016
1020
high pressure
winds
Gradient Wind
“Geostrophic Wind”
• The Geostrophic wind is flow in a straight line in
which the pressure gradient force balances the
Coriolis force.
Lower Pressure
994 mb
996 mb
998 mb
Higher Pressure
Note: Geostrophic flow is often a good approximation high in the atmosphere (>500 meters)
High pressure (anticyclone)
Side View
From above
H
L
surrounding air is
relatively low
L
H
air descends
L
Low pressure (depressions, cyclone)
Side View
From above
L
H
surrounding air is
relatively high
H
L
air ascends
H
Friction forces
Near the surface, friction reduces the speed of the wind,
This reduces the Coriolis Force,
Which changes the direction of the geostrophic wind,
The pressure gradient force over powers the Coriolis effect,
As a result wind flow across the isobars.
H
anticyclone
H
L
Northern
Hemisphere
cyclone
Southern
Hemisphere
L
Global Temperature
patterns and
weather
Temperature Patterns
• Stronger
seasonal
heating and
cooling on land
produces
asymmetry
• Poleward
distortion of
isotherms over
northern high
latitude oceans
• Equatorward
distortion over
subtropics
Seasonal Migration of ITCZ
• Mean position is somewhat north of Equator
• Strong departures from zonal mean position driven by
seasonal heating over land
(Especially over Asia, S. America, Africa)
Monsoons
In July the position of the ITCZ moves North
• low pressure over land causes winds to flow off the ocean
• this brings heavy rainfall
Figure 5.20, p. 167
Monsoons
In January high pressure over the land produces dry winds
Air is flowing towards the ITCZ
Figure 5.20, p. 167
Elevation of the 500 mb isobar
Polar Front Jet Stream
• Polar front jet stream
forms along polar front
where strong thermal
gradient causes a strong
pressure gradient
• Strong pressure gradient
force and coriolis force
produce strong west wind
parallel to contour lines
• Polar jet sometimes splits
into north and south
branches
• Fast air currents, 1000’s of
km’s long, a few hundred km
wide, a few km thick
• Typically find two jet streams
(subtropical and polar front) at
tropopause in NH
• When would you expect the
jets to be strongest?
Jet Streams
Rossby Waves
Smooth westward
flow of upper air
westerlies
Develop at the polar
front, and form
convoluted waves
eventually pinch off
Primary mechanism
for poleward heat
transfere
Pools of cool air
create areas of low
pressure
The “dishpan” experiment
• A tank of water with a hot equator and a cold pole is
rotated
– Troughs, ridges and eddies are produced, similar to
patterns observed in earth’s general circulation
movies
http://jrscience.wcp.muohio.edu/coriolis/satmovies.html#anchor1386282
The Earth’s Oceans
Ocean currents produced by:
1) winds
2) density differences in sea water
3) Coriolis force
4) shape of ocean basins
5) astronomical factors (TIDES)
Ocean Currents
driven mostly by wind
blowing over the surface
however, currents move
slowly
lag behind wind speed so
often called drifts
wind
Ocean currents
• large continuously moving loops (gyres)
• produced by winds, Coriolis force and land masses
Figure 5.32, p. 175
Fig. 8-2, p. 172
Each hemisphere contains a tropical and subtropical gyre
N. Subtropical Gyre
North Tropical Gyre
EQUATOR
South Tropical Gyre
S. Subtropical Gyre
Surface Currents
redistribute heat
Upwelling
where cold water rises from deep ocean areas and where
the Coriolis forces prompts ocean currents to diverge
from coastlines
Figure 5.37, p. 180
Deep-sea currents
• driven by differences in temperature and salinity
• much slower than surface currents
Figure 5.32, p. 180
The Ocean Conveyor Belt
Deep-sea currents
• driven by differences in temperature and salinity
• much bigger and slower than surface currents
El Niño Southern Oscillation (ENSO)
•
Trade winds promote cold water upwelling in eastern tropical Pacific
– Cool, deep water is nutrient rich and supports rich ecosystem
(plankton, fish, birds,…)
•
Weaker trades lead to weaker upwelling. Warm nutrient-poor tropical water replaces the
cold, nutrient-rich water .
–
•
called El Niño (boy child)
Every few years this El Niño (surface warming) persists and is widespread
– Huge ecosystem and economic losses
– Alters weather patterns over much of the world
82-83
+3
86-87
72-73
+2
57-58
65-66
97-98
76-77
El Niño
+1
0
La Niña
-1
-2
1950
1955
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
Sea Surface Temperatures
(oC)
El Niño
Normal
La Niña
La Niña: cold surface water
moves over central and
eastern Pacific.
El Niño
Sea Surface
Temperature
Anomalies oC
Normal
La Niña
Normal conditions – equatorial Pacific
ENSO (El Nino - Southern Oscillation)
conditions
Animations on the web
Idealized ENSO wave
http://www.cdc.noaa.gov/people/joseph.barsugli/anim.html
For animation of most recent anomaly asee
http://www.cdc.noaa.gov/map/clim/sst_olr/sst_anim.shtml
Why do we care about ENSO?
• Global impacts on weather.
• Long timescale (months) yields improved
seasonal prediction.
• Provides insight into coupled behavior of
oceans and atmosphere … may lead to
better overall understanding of climate
Weather Variation: ENSO cycle
winter
summer
Impacts of El Niño
• Droughts
– Fires
– Agricultural productivity
– Water supply
• Extreme Precipitation
–
–
–
–
Floods
Erosion
Disease
Transportation
• Impacts through marine food chain
– Natural ecological responses
– Economic