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Winds and the Global Circulation
y
System
Chapter
5
Winds and the Global Circulation
System
Winds are primarily caused by unequal heating of the
Earth’s atmosphere, which results in differences in
pressure and causes air at the surface to move toward the
warmer location
location.
Atmospheric Pressure
Atmospheric pressure occurs because air molecules have mass
and are constantly being pulled toward the Earth by gravity.
The standard unit of pressure is the pascal (Pa), which is equal
to one newton p
per square
q
metre ((N m-2)).
One newton is the force required to move an object with a mass
of 1 kg so that it accelerates by 1 metre per second (i.e., 1 N = 1
kg m-1 s-2).
For meteorological purposes, Environment Canada measures
pressure in kilopascals (kPa).
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Atmospheric Pressure
At sea level, the average pressure of the atmosphere is
101.3 kilopascals.
Note: 101.3 kPa equals 1013 millibars (mb).
Atmospheric Pressure
Measuring Atmospheric Pressure
Atmospheric pressure is measured by a barometer.
Another type of barometer (most common) is the aneroid barometer
which
hi h uses a thi
thin-walled,
ll d sealed
l d canister
i t ffrom which
hi h some air
i h
has
been removed to establish a partial vacuum.
The canister flexes as air pressure changes, and through a mechanical
linkage a needle moves across the scale.
The same principle is used in a barograph, which records changes in
pressure over a period of time by tracing a line on a paper chart.
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Atmospheric Pressure
Air Pressure Changes with Altitude
Air is compressible, so the mass and density of the atmosphere
decrease progressively with altitude above the Earth’s surface.
As a result, about 95 percent of Earth’s atmosphere
by mass is found below an altitude of 25 km.
About 50 percent of the mass of the atmosphere lies below
about 5.5 km altitude, and 99 percent below 30 km.
Atmospheric Pressure
Air Pressure Changes with Altitude
The relationship between air pressure, air temperature, and the
density of the air is expressed by the ideal gas law as:
PV = nRT
P = pressure (in atmospheres; 1 atmosphere = 101.3 kPa)
V = volume (litres)
T = temperature (K)
R = universal gas constant (8.314 J K-1 mol-1)
n = number of moles (a measure of the amount of
substance)
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Atmospheric Pressure
Air Pressure Changes with Altitude
Meteorological measurements in the troposphere are
routinely associated with various constant pressure
surfaces.
The standard altitudes used by Environment Canada are
850, 700, 500, and 250 hPa (hectopascal = mb).
Wind
Wind is air motion across the Earth’s surface and is
predominantly horizontal (vertical air motion is known by
other terms, such as updrafts or downdrafts).
Wind is characterized by direction (determined by a wind
vane)) and
d velocity
l it ((measured
db
by an anemometer;
t ms-11).
)
Wind direction is always given as the direction from which
the wind (i.e. a west wind is one that comes from the west
and moves to the east.
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Wind
Winds and Pressure Gradients
Wind is caused by differences in atmospheric pressure
(pressure gradient) from place to place with air moving
from high to low pressure (pressure gradient force).
Pressure conditions are shown on a map using isobars —
lines connecting places of equal pressure.
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Wind
The Coriolis Effect and Winds
The pressure gradient force moves air from high pressure to low
pressure; however, on a global scale, the direction of air motion is also
affected by Earth’s rotation.
This is termed the Coriolis effect, and causes air currents to appear to
follow gently curving paths as they blow across the Earth’s surface
(deflection is to the right in the northern hemisphere and to the left in
the southern hemisphere).
Deflection is strongest near the poles and decreases to zero at the
equator.
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Wind
Surface Winds on an Idealized Earth
Since the surface and atmosphere is heated more intensely at
the equator a zone of surface low pressure forms what is known
as the
e equa
equatorial
o a trough.
oug
The heated air rises at the equator, which creates
two large-scale convection loops (in each hemisphere) called
Hadley cells (where air rises over the equator and is drawn
toward the poles).
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Wind
Surface Winds on an Idealized Earth
Where the Hadley cell circulation descends, surface pressures
are high (subtropical high-pressure belts), centred at about lat.
30° in each hemisphere.
Winds around the subtropical high-pressure centres spiral
outward and move toward the equator (strong northeast and
southeast trade winds) and also toward the midlatitudes.
These winds move toward the equatorial trough where the air
converges and rises at the intertropical convergence
zone (ITCZ).
Wind
Surface Winds on an Idealized Earth
Air spiralling outward from the subtropical high-pressure centres
toward the poles produces the zone of vigorous southwesterly
winds in the northern hemisphere and northwesterly winds in the
southern
th
hemisphere.
h i h
Between about lat. 30° and 60°, the pressure and wind pattern
become more complex.
A conflict zone, delineated by the polar front, occurs where
colder polar air meets the warmer subtropical air.
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Wind
A Simple Convective Wind system
The principle that heating of the air produces a pressure
gradient that causes air to move is important and is
associated with the establishment of surface thermal highand low-pressure zones (low surface pressure is
associated with warm air, and high surface pressure with
cool air).
At the global scale, the equatorial trough is a region of
persistent low pressure, whereas polar regions with
permanent ice and snow are characterized by high
pressure.
Global Wind and Pressure Patterns
The general circulation of the atmosphere is also affected by the
different configuration of landmasses and ocean basins in the
northern (two large continental masses separated by oceans and
an ocean at the pole) and southern (a large ocean with a cold,
ice-covered continent at the centre)) hemispheres.
p
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Global Wind and Pressure Patterns
Subtropical High Pressure Belts
Southern Hemisphere - three large high-pressure cells that
persist year round over the oceans (fourth, weaker high-pressure
cell forms over Australia in July
July, as the continent cools during the
southern hemisphere winter).
Northern hemisphere - two large high-pressure cells occur over
the subtropical oceans — the Hawaiian High in the Pacific and
the Azores High (commonly referred to as the Azores-Bermuda
High) in the Atlantic.
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Global Wind and Pressure Patterns
The ITCZ and the Monsoon Circulation
Insolation is most intense when the sun is directly overhead. Over
the course of the year, the subsolar point migrates north and south
of the equator so that it lies at the Tropic of Cancer on the June
solstice and at the Tropic of Capricorn on the December solstice.
solstice
Since Hadley cell circulation (and the position of the ITCZ) is driven by
energy input from the sun, the general global pressure and wind belts
also shift with the seasons.
This seasonal change in pressure and the latitudinal movement of the
ITCZ create a reversing wind pattern that underlies a monsoon climate.
Global Wind and Pressure Patterns
The ITCZ and the Monsoon Circulation
In Asia for instance, the winter monsoon is marked by a strong
outflow of dry, continental air from the north across China,
Southeast Asia, India, and the Middle East.
In the summer, warm, humid air from the Indian Ocean and the
southwestern Pacific moves northward and northwestward into
Asia, passing over India, Indochina, and China.
This airflow is known as the summer monsoon and is
accompanied by heavy rainfall in southeastern Asia.
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Global Wind and Pressure Patterns
Cyclones and Anticyclones
Low- and high-pressure centres are familiar features on daily
weather maps.
Unlike the semi-permanent highs and lows displayed on the
January and July world maps, the daily maps show the pressure
systems that move through a region and bring about changing
weather conditions.
Low-pressure centres, or cyclones, are often associated with
cloudy or rainy weather, and high-pressure centres, or
anticyclones, usually bring fair weather.
Global Wind and Pressure Patterns
Cyclones and Anticyclones
In a low-pressure system the pressure gradient is inward (Coriolis
effect and friction with the surface cause the surface air to move at an
angle across the pressure gradient - creates a convergent, inward
spiralling motion)
motion).
In the northern hemisphere, the cyclonic spiral is counter-clockwise
because the Coriolis effect acts to the right (opposite in the southern
hemisphere)
For anticyclones, the pressure gradient is out from the centre; this
creates a divergent, outward spiralling motion (spiral clockwise in the
northern hemisphere, counter-clockwise in the southern hemisphere).
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Global Wind and Pressure Patterns
Cyclones and Anticyclones
Cyclones and anticyclones are large features of the lower
troposphere — often 1,000 km or more in diameter.
The horizontal motion of air in adjacent low- and high-pressure
centres is generally connected in a convection loop.
Global Wind and Pressure Patterns
Local Winds
Local winds can be divided into two groups:
1) those that develop only in one area because a local topographic
feature or body of water affects air movements (sea and land
breezes).
breezes)
2) winds that an area’s inhabitants consider to be distinctive, despite
the fact that they may originate many hundreds of kilometres away.
Examples:
sea and land breezes, mountain and valley winds, drainage or kata
batic winds, mistral, Santa Ana, Bora, Chinook.
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Winds in the Upper Atmosphere
Pressure decreases less rapidly with height in warmer air than in
colder air.
Insolation is greatest near the equator and least near the poles;
this creates a latitudinal temperature gradient.
Because of this permanently maintained temperature difference,
the isobaric surfaces slope downward from the warmer low
latitudes towards the cooler poles, which creates a pressure
gradient force.
Winds in the Upper Atmosphere
The Geostrophic Wind
The pressure gradient force generated by the temperature
differences between high and low latitudes is termed the thermal
wind.
As the air moves, it is influenced by the Coriolis effect
(deflected right in the northern hemisphere, left in the southern
hemisphere).
Thus, airflow toward the poles in both hemispheres will be
subject to a progressively stronger force that is directed toward
the east.
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Winds in the Upper Atmosphere
The Geostrophic Wind
Winds in the upper troposphere are not affected by friction.
At these higher altitudes, the horizontal air pressure gradient
causes air to accelerate across the isobars from areas of high
pressure toward areas of low pressure.
Winds in the Upper Atmosphere
The Geostrophic Wind
The Coriolis effect then deflects the airflow (right in the northern
hemisphere).
As the wind gains speed, the Coriolis effect increases in
magnitude until it balances the pressure gradient force.
At that point, the sum of forces on the moving air is zero, so its
speed and direction remain constant.
Consequently, the winds blow parallel to isobars (geostrophic
wind).
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Winds in the Upper Atmosphere
Rossby Waves, Jet Streams and the Polar Front
The path of the upper-air westerlies periodically experiences
undulations, called Rossby waves (number varies between 3 and 7)
Contact zone between cold polar air and warm tropical air known as the
polar front.
Rossby wave circulation brings warm air toward the poles and cold air
toward the equator and is a primary mechanism of heat transport.
It is also the reason weather in the midlatitudes is often so variable, as
pools of warm, moist air and cold, dry air alternately pass over
midlatitude land masses.
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Winds in the Upper Atmosphere
Rossby Waves, Jet Streams and the Polar Front
Airflow has a pronounced north–south — or meridional —
component when the Rossby waves follow strongly meandering
p
paths.
Prior to their meridional flow airflow is predominately from west
to east — or zonal.
Such variation in flow paths influences the time it takes
for surface weather systems to pass through a region and will
affect how quickly weather changes at midlatitudes.
Winds in the Upper Atmosphere
Rossby Waves, Jet Streams and the Polar Front
Jet streams are narrow currents of fast-moving (upper) air that extend
for thousands of kilometres but are only a few hundred kilometres in
width and of limited vertical depth.
Jet streams occur at the approximate altitude of the tropopause (10 to
12 km), where atmospheric pressure gradients are strong.
This accounts for their development along the polar front where
pressure changes rapidly in response to the strong temperature
gradient between polar and subtropical air masses.
The polar jet streams essentially mark the boundary of the
Rossby waves in both the northern and southern hemispheres.
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Winds in the Upper Atmosphere
Rossby Waves, Jet Streams and the Polar Front
There are two smaller subtropical jet streams closer to the
equator.
These occupy a position at the tropopause above the subtropical
high-pressure cells at about lats. 20° to 40° in the northern and
southern hemispheres (the poleward limit of the Hadley cells).
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Temperature Layers of the Ocean
Sea surface temperatures vary with latitude and season.
In tropical regions, mean annual sea surface temperatures
typically range from 25–28°C, decreasing to 15–22°C through
the midlatitudes and falling to as low as -3°C in the polar regions.
Between 30° N and 30° S, the western sides of the ocean basins
tend to be warmer due to westward drift imposed by the trade
winds.
This pattern reverses from about lat. 40° toward the poles; in
these regions, the eastern sides of the ocean basins are
generally warmer due to the effect of currents such as the North
Atlantic Drift.
Temperature Layers of the Ocean
Water temperature is generally highest at the surface and
decreases with depth because the principal heat sources are
solar insolation and heat supplied by the overlying atmosphere.
Below this warm layer, temperatures drop rapidly in a zone
known as the thermocline.
Very cold water extends from the thermocline to the deep ocean
floor, where temperatures range from 0°C to
5°C.
About 90 percent of the total volume of the oceans is contained
in the deep water below the thermocline.
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Ocean Currents
An ocean current is any persistent, dominantly horizontal
flow of ocean water, which can occur at the surface or at
any depth.
Surface currents are driven by prevailing winds.
Deep currents are powered by changes in temperature and
density occurring in surface waters, causing them to sink.
Ocean Currents
Surface Currents
The pattern of surface ocean currents is strongly related to
prevailing (energy is transferred from wind to water by the friction
of the air blowing over the water surface).
Because of the Coriolis effect, the actual direction of
water drift is deflected several degrees from the direction of the
driving wind.
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Ocean Currents
Surface Currents
Ocean currents move warm waters toward the poles and cold
waters toward the equator, and consequently, they are important
regulators of air temperatures.
The ocean current system includes large circular movements,
called gyres, which are centred approximately at lat. 30°.
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Ocean Currents
Surface Currents
One of the most dramatic phenomena associated with ocean
surface currents is El Niño.
g an El Niño event, Pacific surface currents shift into an
During
unusual pattern.
Pacific upwelling along the Peruvian coast ceases and a weak
equatorial eastward current develops in response to a reduction
in the strength of the trade winds.
Ocean Currents
Surface Currents
Global patterns of precipitation also change during El Niño
events, bringing floods to some regions and droughts to others.
In contrast to El Niño is La Niña, in which normal Peruvian
coastal upwelling is enhanced, trade winds strengthen, and cool
water is carried far westward in an equatorial plume.
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Ocean Currents
Deep Currents and Thermohaline Circulation
Deep currents move water in a slow circuit across the floors of
the world’s oceans.
They are generated when surface waters become denser (colder
and more saline)) and slowly sink downward.
This slow flow pattern, which links all of the world’s oceans, is
commonly referred to as the great ocean conveyor.
It is a thermohaline circulation process that depends on the
change in density of sea water caused by temperature and
salinity differences in North Atlantic Ocean waters.
A Look Ahead
The energy flows associated with atmospheric pressure
and winds are closely linked to global precipitation patterns.
Chapter 6 discusses the connection between atmospheric
circulation and precipitation processes.
processes
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