Climate Change … Global Winds Distribution of Pollutants … Arctic Impacts
R. E. Ruthenberg 8/7/07
Introduction
Recent discussions of jet aircraft emissions and their potential climate change ("global
warming") impacts have highlighted a need for a tutorial focus on global wind patterns,
as they are fundamental to understanding pollutant distribution paths. Pollutant
transport into the arctic region is of particular interest in that high rates of arctic warming
are being experienced in recent years/decades, which raises questions of jet aircraft
emissions contributions to this warming effect.
These emissions are both ground or low altitude based (i.e. within the troposphere) as
well as high altitude based in the tropopause and lower stratosphere during "cruising".
Also, many international flights are by way of "over the top" routes, which places
emissions directly into the arctic region.
This tutorial's objective is to provide an improved layman understanding of primary
global wind circulation patterns, in order to improve communication on these important
issues, between various technically separated parties, including meteorologists, climate
change physicists and environmentalists. It is largely a compendium of existing
information, along with contextual comments and discussion.
Global Wind Systems
[From: www.physics.isu.edu/weather/kmdbbd/notesc11.pdf ]
{It will be helpful to the reader's understanding to also refer to the first two graphics below.}
General Circulation of the Atmosphere
Although the global scale circulation of the earth’s atmosphere is influenced
by several factors, the most fundamental process driving general circulation is
uneven heating of the earth's surface by solar radiation. Although the energy
received by the earth as a whole is balanced by energy radiated back into space,
this is not true for every specific location. Some locations receive more heat
energy than they emit, some receive less. The tropics, for instance, receive much
more heat from the sun than is radiated back into space. The Polar Regions receive
much less energy than they emit. Global circulation patterns are a way of
redistributing energy from areas of the earth with an energy surfeit to areas with
an energy deficit, through the atmosphere. The oceans have very similar patterns
that exist for the same reasons. In an attempt to better understand general
circulation, scientists have developed computer models of global-scale atmospheric
circulation. When these models are combined with ocean current models they are
referred to as coupled models.
The single-cell model is the most basic model of general atmospheric
circulation. The single-cell model assumes a non-rotating earth covered entirely by
water (such a uniform surface would insure even heating for each point at a given
latitude) and that the sun is always at equinox. This model predicts an enormous
pattern of circulation from the equator to the poles. This circulation is called a
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Hadley Cell (after British meteorologist George Hadley). In a Hadley cell air rises
from the equatorial regions, spreads pole ward at the tropopause, converges and
sinks near the Polar Regions, then moves back toward the equator at the earth’s
surface. Although this simple model has some utility, it fails to describe observed
large-scale circulation accurately, largely because it fails to take into account the
rotation of the earth.
The three-cell model is a variant of the single-cell model that assumes a
rotating earth. The predictions of the three-cell model correspond much more
closely with observation than those of the one-cell model.
In the three-cell model, the atmosphere above latitudes from the equator to
300 N is dominated by air circulating in the first of the three cells. In this region
the air over the equator is warm and horizontal pressure gradients are weak. Little
wind exists in this region known as the doldrums. This region has an unstable
atmosphere resulting in towering cumulus clouds known as hot towers. Rising air
reaches the tropopause and is deflected to the north and south. The Coriolis Force
begins to deflect the northward moving air to the right producing westerly winds
aloft. Air converges as it moves northward and cools (due to increasing latitude).
At latitudes of about 30 degrees north, cooling and convergence produce a
large, heavy mass of air piled above the earth’s surface. As this large, heavy mass
of air sinks it produces a widespread, semi-permanent high-pressure system known
as a subtropical high. While sinking the air warms by compression producing clear
skies and warm, dry climates. This region, known as the Horse Latitudes, is
characterized by weak surface winds because of the weak horizontal pressure
gradients. At the surface some of the descending air moves back toward the
equator and is deflected to the right as it moves. The trade winds are
northeasterly winds in this region of the world.
Near the equator, northeasterly winds from the Northern Hemisphere
converge with southeasterly winds from the Southern Hemisphere. This is known as
the intertropical convergence zone or ITCZ. The convergence further aids the
ascent of air in the equatorial regions resulting in a widespread area of low
pressure known as the equatorial low. This cell, characterized by ascent at the
equator, northward movement along the tropopause, convergence and descent at 30
degrees north, and southward movement toward the equator along the surface is
known as the Hadley Cell in the three-cell model.
At 30 degrees north, the air descending to the surface that does not move
south toward the equator begins to move north and is deflected right by the
Coriolis force. This results in a SW or WSW prevailing surface wind in much of the
Northern Hemisphere known as Westerlies. Mild air moving north and east along
the earth’s surface encounters much cooler air moving south and west (north or
polar easterlies) from the Polar Regions. These air masses of greatly differing
temperature and density do not readily mix, resulting in the formation of the polar
front along the boundary where the air masses meet. The polar front is a zone of
strong convergence and rapidly rising air that results in a widespread surface low
pressure system known as the subpolar low. This region of rapid, widespread ascent
produces conditions ripe for storm formation. Some of the rising air, upon reaching
the tropopause, moves back southward towards the region of convergence above
300 N. This circulation, northward along the surface, upward at the polar front,
back to the south along the tropopause, and sinking toward the surface at the
Horse Latitudes is known as a Ferrel Cell. Polar air, being cooler and denser than
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the air moving up from the south, can push the location of the polar front further
south, especially in the wintertime, producing a cold polar outbreak over the
continental U.S. This is known in the “Weather Channel” vernacular as “an outbreak
of arctic air.”
The portion of the air aloft that does not move south above the polar front
moves north and east toward the poles where it converges and sinks. This
circulation, which results in high pressure at the pole (the polar high), is known as
the polar cell and is the weakest of the three cells.
The three-cell model, while not perfect, does accurately describe most of
the large-scale patterns of circulation observed in the atmosphere, especially those
patterns associated with surface flow.
Jet Streams
Jet streams are narrow, meandering bands of high-speed wind that form
along boundaries between warm and cold air, usually near the tropopause. Jet
Streams are many thousands of kilometers long and may be up to a few hundred
kilometers wide but only a few kilometers thick. Wind speeds range from 100 to
200 knots. Over North America the polar jet is found in the vicinity of the polar
front and the subtropical jet forms further south near the horse latitudes.
****
The global wind pattern is also known as the "general circulation" and the surface winds of each
hemisphere are divided into three wind belts:
Polar Easterlies: From 60-90 degrees latitude.
Prevailing Westerlies: From 30-60 degrees latitude (aka Westerlies).
Tropical Easterlies: From 0-30 degrees latitude (aka Trade Winds).
How the Coriolis Force Affec t s Global Winds
The wind rises from the equator and moves north and south in the higher layers of the
atmosphere. Around 30° latitude in both hemispheres the Coriolis force prevents the air from
moving much farther. At this latitude there is a high pressure area, as the air begins sinking
down again.
As the wind rises from the equator there will be a low pressure area close to ground level
attracting winds from the North and South.
At the Poles, there will be high pressure due to the cooling of the air.
Keeping in mind the bending force of the Coriolis force, we thus have the following general
results for the prevailing wind direction:
Prevailing Wind Direc tions
Latitude
Direction
90-
60-
60°N
30°N
NE
SW
30-0°N
0-30°S
NE
SE
30-
60-
60°S
90°S
NW
SE
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The size of the atmosphere is grossly exaggerated in the picture below (which was made on a
photograph from the NASA GOES-8 satellite). In reality the atmosphere is only about 10 km
thick, i.e. 1/1200 of the diameter of the globe. That part of the atmosphere is more accurately
known as the troposphere. This is where a majority of our "weather" occurs.
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A good winds animation can be found at:
http://www.eas.slu.edu/People/RWPasken/GlobalWind.swf
The reader will note there the two jet streams that exist and, in particular, the polar-front jet
stream. The animation is incomplete in that it doesn't show the return flows resulting from the
atmospheric upwelling in the polar-front convergence zones. The northern zone should show
flows moving both north and south from the top of this zone i.e. the top of the troposphere,
similar to the animation at the equator and as shown in the graphic diagram above.
This is critical in that one can see that a significant portion of the world's developed countries
emissions are near this zone (see the world map between about 30-60 degrees north latitude),
resulting in ground level winds transporting emissions into this zone, thence rising up and (a
portion) transferring to the arctic region by high level north-bound winds (the "return" of the
cold, ground level Polar Easterlies winds). Additionally, jet aircraft cruise at the top of the
troposphere, often in jet streams paralleling the polar-front; these emissions experience
propagation into the arctic by both the high altitude north-bound return winds and the westerly,
undulating, jet streams (which often undulates and moves from the N.E. U.S. across Greenland).
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It's also important to remember that the location of the polar-front lies generally along the
Canadian border in the summer but slides south, as low as 30 degrees latitude in the winter. This
means that much of the U.S. ground-level emissions travel generally northeast toward the polarfront on the prevailing winds in the summer, while traveling generally southwest toward the
front in the winter. "Generally" recognizes that moving high and low pressure areas modify
these prevailing directions on a local/regional basis, for periods of hours/days.
Of course, in the case of jet aircraft traveling the world, the location of the of their emissions
depositions into the atmosphere will obviously be important to where the varying wind patterns
will preferentially carry them.
This first reminds us that aircraft tend to fly "great-circle" routes rather than straight-line map
routes, as portrayed in the above world map, in order to minimize travel distances. [Lay a
straight paper edge on a globe from point-to-point.] This can be seen in the example graphic
below, showing great-circle routes and distances from Seattle to various locations. [Citation:
www.pmel.noaa.gov/ ~kessler/figures.html ]. Here it is seen, for instance, that the great-circle
route to Paris does not follow the 48th parallel but, amazingly, travels much farther north,
crossing over southern Greenland around 70 degrees north latitude, which is north of the Arctic
Circle (65 degrees)!
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This can be seen in a global sense in the below graphic, which characterizes estimated densities
of aircraft emission densities, noting for instance the upward (great-circle) portion in the high
traffic routes between the U.S. and Europe. There again, it's important to see that this hightraffic route (see also flight patterns in Appendix) places a large amount of emissions up toward
Greenland, along or north of the polar-front, allowing easy transport of the emissions into
Greenland and the arctic region via the high altitude north flowing polar-front return winds.
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Note also the critical information about the vertical location of aircraft emissions depositions.
The altitude bins with the highest fuel burn and emissions are between 9 and 12 km (or
approximately 29,500 ft and 39,400 ft). This corresponds to the frequent use of these altitudes
for en route travel. The relatively high levels of HC and CO in the 0 to 1 km band is due to the
higher emissions characteristics for those pollutants at lower power settings (e.g., during taxiing
and idle conditions). [CO2, an important green-house gas, is not shown but would be
proportional to "fuel burn".
Reference, for additional detail:
http://www.faa.gov/about/office_org/headquarters_offices/aep/models/sage/media/FAA-EE2005-02__SAGE-Inventory_Report-Text.pdf
Then there is finally the further impact of the important Arctic Oscillation winds.
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The Arctic Oscillation
The Arctic Oscillation refers to opposing atmospheric pressure patterns in northern middle and
high latitudes.
The oscillation exhibits a "negative phase" with relatively high pressure over the polar region
and low pressure at midlatitudes (about 45 degrees North), and a "positive phase" in which the
pattern is reversed. In the positive phase, higher pressure at midlatitudes drives ocean storms
farther north, and changes in the circulation pattern bring wetter weather to Alaska, Scotland
and Scandinavia, as well as drier conditions to the western United States and the
Mediterranean. In the positive phase, frigid winter air does not extend as far into the middle of
North America as it would during the negative phase of the oscillation. This keeps much of the
United States east of the Rocky Mountains warmer than normal, but leaves Greenland and
Newfoundland colder than usual. Weather patterns in the negative phase are in general
"opposite" to those of the positive phase, as illustrated below.
Over most of the past century, the Arctic Oscillation alternated between its positive and negative
phases. Starting in the 1970s, however, the oscillation has tended to stay in the positive phase,
causing lower than normal arctic air pressure and higher than normal temperatures in much of
the United States and northern Eurasia.
Effects of the Positive Phase
Oscillation
|
Effects of the Negative Phase
of the Arctic
of the Arctic Oscillation
(Figures courtesy of J. Wallace, University of Washington)
The polar front and its associated jet stream have a major influence on the weather conditions
surrounding it. Many storms form along the polar front in the vicinity of the jet stream's maximum
winds. Because of the association of the jet stream with the polar front, media weather maps
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often include the jet stream's position as a rough indicator of continental divisions between
warm and cold air masses. Here is why.
From a climatological viewpoint, the position of the polar front forms a more-or-less even band
around the globe at mid-latitudes in both hemispheres. The average polar-front position slips
north and south with the seasons in response to the annual hemispheric heating cycle, shrinking
poleward in the summer months and then expanding toward the subtropics during the winter.
From a meteorological viewpoint, however, the polar front is not an evenly encircling latitude belt
but an undulating zone around the globe that is ever-changing as masses of warm and cold air
push away from their regions of origin.
Of particular interest to meteorologists and weather forecasters is the ever-changing pattern of
long-waves that form around the polar front. These long-waves, very visible on polar projection
maps, undulate around the hemisphere with three to six wave cycles. (Long waves are also
known as Rossby waves in honour of Swedish meteorologist C.G. Rossby who gave us many
early insights into the impacts of upper atmosphere features on weather.)
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At times, that global belt fits tight, having three or four, small-amplitude undulations (little northsouth latitude variation) around the hemisphere. At other times, it has as many as six largeamplitude loops (great north-south variation). How those wave loops sit over the hemisphere, or
portion thereof, determines what temperature regimes are experienced on the surface below.
When a long section of the polar front, as seen on continental weather maps, is smooth like the
surface of a calm sea, the upper-level winds, including associated jet streams, run generally
parallel to the latitude lines, a condition called zonal flow. Under zonal-flow, north-south
undulations of the frontal boundary are small, and the surface temperatures across the
continent, as seen in the isotherm (lines of equal temperature) pattern on the weather map,
layer in zonal (east-west) bands with warm air to the south and cold air to the north.
****
Discussion
It's clear that global wind patterns, which vary with location and altitude, play a
determining role in the global transport of both ground-level and high altitude emissions
of gasses, aerosols and particulates that can impact global warming effects.
Ground-level emissions experience not only the global winds general circulation
patterns but also the turbulence associated with the Planetary Boundary Layer of the
troposphere, caused both by ground obstruction friction and solarization heating
convection currents. Additionally, localized, moving high/low pressure centers effects
cause vertical mixing and transport toward the tropopause. All or a combination of
these effects can result in pollutants rising up to the tropopause before being carried
there by a major front e.g. the polar-frontal zone.
The above graphics demonstrate strikingly how much of the U.S. to Europe air traffic
travels north of the typical polar-front, with the majority of emissions deposited at high
altitudes, to be carried to the north pole on south winds (sinking in the region of the
pole).
Additional resources
• For definition and discussion of the Planetary Boundary Layer and convection
processes within the troposphere:
http://en.wikipedia.org/wiki/Planetary_boundary_layer
• For discussion on the troposphere: http://en.wikipedia.org/wiki/Troposphere
• For more on atmospheric circulation:
http://en.wikipedia.org/wiki/Atmospheric_circulation
•
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APPENDIX
The figure maps commercial flight tracks for one 24-hour day (8/2/2002). [Source:
AERO2K_Global_Aviation_Emissions_Inventories_for_2002_and_2025 ]
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