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global atmospheric circulation - reasons
In low latitudes the
absorption of solar
radiation is much greater
than the emission
(energy surplus)
In polar regions the
emission is greater than
the absorption (energy
deficit)
The nonradiative
transport of energy is
necessary. The direction
of this transport is from
low towards high
latitudes.
This model was described
by British meteorologist
George Hadley (1685-1768)
in 1735.
single-cell model of
circulation by Hadley
Assumptions:
- non rotating Earth
- uniform surface
- equator in the surface of ecliptic
Three-cell model of atmopheric circulation
described by American meteorologist William Ferrel in 1856
Assumptions:
- rotating Earth
- uniform surface
- equator in the surface of ecliptic
http://www.physicalgeography.net/fundamentals/7p.html
Trade winds – north-eastern winds on
the northern hemisphere and southeastern wind on the southern
hemisphere
Intertropical convergence zone – the
line of convergence of trade winds
subtropical highs – zone of quasiconstant highs located near tropics
Westerlies – zone with winds with
strong western component
Polar front – front between tropical
and polar air masses
Eastern winds zone – zone of
prevailing eastern wind near the poles
Three cell model
Coriolis Effect
Global Atmospheric Circulation
Jet Streams
The Jet Stream and Rossby
Waves
Seasonal Variations in Latitude - Monsoons
Seasonal variations are less
likely to affect the equatorial
region and the poles
Seasonal Pressure and Precipitation Patterns
Air masses
and fronts
Air mass
An air mass is an extremely large body of air whose
properties of temperature and moisture are fairly
similar in any horizontal direction and in vertical
direction
Regions where air masses originate are known as
source regions. The longer the air remains stagnant
over its source region, the more likely it will acquire
properties of the surface below.
Ideal source region:
 high pressure
 flat uniform surface
 huge area
Air masses
Air mass classification:
Arctic and antarctic air A
Polar air P
Tropical air T
Equatorial air E
continental and maritime air
mA, cA
mP, cP
mT, cT
E – always humid air
Air mass transformation
After leaving the source
region air masses
transform. The process of
transformation depends on
the temperature of surface
it encounters.
Warm air over the colder
surface cools in the lower
layer. Warm air above
cooler air produces a
stable lapse rate with little
vertical mixing. In moist
air stratiform clouds form
accompanied by drizzle
or/and fog.
A stable lapse rate slows
the mixing of the air and
air mass transformation
Air mass transformation
When the air mass is
colder than the
underlying surface it is
warmed from
below,which results in
steeper lapse rate and
instability in low level.
In this case convection
can appear.
The instability makes the
transformation faster,
because of strong mixing.
Fronts
A front is a transition
zone between two air
masses of different
properties
(temperature,
humidity, density)
If they have different
densities
(temperatures) one of
them is called warm the
other cold
If they have different
humidities one of them is
called wet the other dry
The air masses have both horizontal and vertical extent; the upward
extension is referred to as a frontal surface or frontal zone.
Warm front
The warmer, less-dense air rides up and over the colder, moredense surface air. This rising of warm air over cold, called
overrunning, produces clouds and precipitation well in advance
of the front's surface boundary. warm air overriding the cold air
creates a stable atmosphere. A temperature inversion - called a
frontal inversion – exists in the region of the upper-level front at
the boundary where the warm air overrides the cold air.
Under the Nimbostratus rain (or snow) falls. It can also
accompany As cloud.
Cold front
A mass of cold, dense air wedges under the warm air, forcing it
upward. As the moist, unstable air rises, it condenses into a
series of cumuliform clouds. At the front itself, a relatively
narrow band of thunderstorms produces heavy showers with
gusty winds
occluded
fronts
cold-type
occluded
front
warm-type
occluded
front
Oceanic circulation
The movement of water around the oceans has two parts
which are strongly linked:
a density driven circulation which is driven by the
differences in the density of seawater at different
locations. The density of seawater depends on its
temperature and how salty it is. As a result, this
movement is known as the thermohaline circulation
(Greek: thermo = heat, háls = salt).
a wind driven circulation which results in huge surface
currents like the Gulf Stream.
Wind driven circulation
The Gulf Stream
The Gulf Stream is one of the most important wind
driven currents. It transports very warm tropical
water from the Caribbean Sea and the Gulf of Mexico
across the North Atlantic to northern Europe. The
warmth of the water heats the air above and the
movement of this warm air is a very important way
by which heat is transported northwards. As a result
of this heat transport, northern Europe is very much
warmer than corresponding latitudes in North
America and countries around the Pacific Ocean.
For example, the yearly average temperature at Iqaluit (64oN,
068oW) in the Northwest Territories of Canada is -9.1 oC. This
compares with an average for Trondheim (63oN, 010oE)
in Norway of +4.8 oC. Long term records suggest that, as a result
of the Gulf Stream, average temperatures in Northern Europe are
9 oC higher than the average temperatures for the same latitude
elsewhere.
The Gulf Stream is an example of a western boundary current, a
current which flows along the western side of a major ocean
basin. The corresponding current in the Pacific Ocean is the
Kuroshio Current, and in the Indian Ocean, the Aghulas
Current. They result from an interaction between the shape of
the ocean basin, the general direction of the wind and the
rotation of the earth. They all have a high velocity (the Gulf
Stream has an average velocity of 1 m s-1, thats 3.6 km h-1) they
are all quite narrow (between 100 and 200 km wide) and all have
a very important influence on the climate of the region. Eastern
boundary currents also occur; these transport cold surface
waters from the poles to the equator. They tend to be weaker
than their western counterparts.
Pidwirny, M. (2006). "Surface and Subsurface Ocean Currents: Ocean
Current Map". Fundamentals of Physical Geography, 2nd Edition. Date
Viewed. http://www.physicalgeography.net/fundamentals/8q_1.html
Normal conditions:
Australia/
Indonesia
El Niño conditions:
Australia/
Indonesia
Typical
walker
circulation
South
America
Weakened
Walker
circulation
South
America
http://www.enn.com/specialreports/elnino/what.asp
Sir Gilbert Walker
Strong
El Niňo
Jacob Bjerkness
El Niňo
La Niňa
La Niňa
El Niňo
El Niňo
http://www.oc.nps.edu/webmodule
s/ENSO/effects.html
La Niňa