Download Atmospheric Circulation

General Circulation of the Atmosphere
General Circulation of the Atmosphere extended definition
Winds circulate around the globe because of the rotation of the earth and the energy from the sun.
Why do I care? The general direction of the winds varies around the globe depending on factors like
latitude and proximity to oceans. The direction of the wind at various levels in the atmosphere
determines the local climate and steers around weather systems and severe weather.
The circulation of wind in the atmosphere is driven by the rotation of the earth and the incoming
energy from the sun. Wind circulates in each hemisphere in three distinct cells which help transport
energy and heat from the equator to the poles. The winds are driven by the energy from the sun at the
surface as warm air rises and colder air sinks.
Figure A
http://sparce.evac.ou.edu/q_and_a/air_circulation.htm, SPaRCE
The circulation cell closest to the equator is called the Hadley cell. Winds are light at the equator
because of the weak horizontal pressure gradients located there. The warm surface conditions result in
locally low pressure. The warm air rises at the equator producing clouds and causing instability in the
atmosphere. This instability causes thunderstorms to develop and release large amounts of latent
heat. Latent heat is just energy released by the storms due to changes from water vapor to liquid water
droplets as the vapor condenses in the clouds, causing the surrounding air to become more warm and
moist, which essentially provides the energy to drive the Hadley cell.
The Hadley Cell encompasses latitudes from the equator to about 30°. At this latitude surface high
pressure causes the air near the ground to diverge. This forces air to come down from aloft to "fill in"
for the air that is diverging away from the surface high pressure. The air flowing northward from the
equator high up in the atmosphere is warm and moist compared to the air nearer the poles. This causes
a strong temperature gradient between the two different air masses and a jet stream results. At the 30°
latitudes, this jet is known as the subtropical jet stream which flows from west to east in both the
Northern and Southern Hemispheres. Clear skies generally prevail throughout the surface high pressure,
which is where many of the deserts are located in the world.
Figure B: General Wind Directions
Image from NASA
From 30° latitude, some of the air that sinks to the surface returns to the equator to complete the
Hadley Cell. This produces the northeast trade winds in the Northern Hemisphere and the southeast
trades in the Southern Hemisphere. The Coriolis force impacts the direction of the wind flow. In the
Northern Hemisphere, the Coriolis force turns the winds to the right. In the Southern Hemisphere, the
Coriolis force turns the winds to the left.
From 30° latitude to 60° latitude, a new cell takes over known as the Ferrel Cell. This cell produces
prevailing westerly winds at the surface within these latitudes. This is because some of the air sinking at
30° latitude continues traveling northward toward the poles and the Coriolis force bends it to the right
(in the Northern Hemisphere). This air is still warm and at roughly 60° latitude approaches cold air
moving down from the poles. With the converging air masses at the surface, the low surface pressure at
60° latitude causes air to rise and form clouds. Some of the rising warm air returns to 30° latitude to
complete the Ferrel Cell.
The two air masses at 60° latitude do not mix well and form the polar front which separates the warm
air from the cold air. Thus the polar front is the boundary between warm tropical air masses and the
colder polar air moving from the north. (The use of the word "front" is from military terminology; it is
where opposing armies clash in battle.) The polar jet stream aloft is located above the polar front and
flows generally from west to east. The polar jet is strongest in the winter because of the greater
temperature contrasts than during the summer. Waves along this front can pull the boundary north or
south, resulting in local warm and cold fronts which affect the weather at particular locations.
Above 60° latitude, the polar cell circulates cold, polar air equatorward. The air from the poles rises at
60° latitude where the polar cell and Ferrel cell meet, and some of this air returns to the poles
completing the polar cell. Because the wind flows from high to low pressure and taking into account the
effects of the Coriolis force, the winds above 60° latitude are prevailing easterlies.
Walker Circulation
Figure C
http://www.southwestclimatechange.org/climate/global/dynamics
In contrast to the Hadley, Ferrel and polar circulations that run along north-south lines, the Walker
circulation is an east-west circulation. Over the eastern Pacific Ocean, surface high pressure off the west
coast of South America enhances the strength of the easterly trade winds found near the equator. The
winds blow away from the high pressure toward lower pressure near Indonesia. Upwelling, the rising of
colder water from the deep ocean to the surface, occurs in the eastern Pacific along South America near
Ecuador and Peru. This cold water is especially nutrient-rich and is stocked with an abundance of large
fish populations. By contrast the water in the western Pacific, near Indonesia, is relatively warm. The air
over Indonesia rises because of the surface low pressure located there and forms clouds. This causes
heavy precipitation to fall over the western tropical Pacific throughout the year. The air then circulates
back aloft towards the region above the surface high pressure near Ecuador and this becomes the
Walker circulation. The air sinks at this surface high pressure and is picked up by the strong trade winds
to continue the cycle.
Figure D
http://www.taiga.net/youYukon/col067.html
On some occasions, the Walker circulation and the trade winds weaken, allowing warmer water to
"slosh back" towards the eastern tropical Pacific near South America. You can think of this as blowing a
fan over a bathtub full of water. If the fan blows steadily, the water at the side farthest from the fan will
tend to pile up downwind. If you suddenly slow the fan down, some of the water that was built up will
surge back towards the fan. The warmer water will cover the areas of upwelling, cutting off the flow of
nutrients to the fish and animals that live in the eastern Pacific Ocean. This warming of the eastern
Pacific Ocean is known as El Niño. The warmer water will also serve as a source for warm, moist air
which can aid in the development of heavy thunderstorms over the mass of warm water.
How does this relate to agriculture?
Changes in the Hadley cell and Walker circulation can result in dramatic climate variations for many
regions. In an El Niño winter, for example, the presence of the warm water in the eastern Pacific shifts
the position of the subtropical jet, leading to heavy rainfall in Florida and southern Georgia. You can
learn more about how El Niño and its opposite, La Niña, affects the weather in the Southeast at
www.agroclimate.org, which allows you to look at differences in climate in different years depending on
the El Niño phase.
In a warming climate, the Hadley cell could increase in length and alter the climate of regions around
30°. For example, many deserts in the northern hemisphere are located around the 30° latitude, and if
the Hadley cell were to increase in length, that could cause dry conditions to move north of 30°.
Ultimately, this would alter the precipitation patterns of many regions, including the Southeast.
Jet Streams
Jet Streams extended definition
Jets are fast moving ribbons of air high up in the atmosphere. They are responsible for transporting
highs and lows. They affect precipitation and temperatures, and they mark boundaries between massive
air masses.
Why do I care? Jet streams are important to the transport of highs and lows that affect our weather and
lives on a daily basis.
Jet streams are the major means of
transport for weather systems. A jet stream
is an area of strong winds ranging from 120250 mph that can be thousands of miles
long, a couple of hundred miles across and a
few miles deep. Jet streams usually sit at the
boundary between the troposphere and the
stratosphere at a level called the
tropopause. This means most jet streams
are about 6-9 miles off the ground. Figure A Figure A: Jet Stream Wind Speeds
is a cross section of a jet stream.
Image from NOAA
Figure B: Location of Jet Streams
Image from NOAA
The dynamics of jet streams are actually quite complicated, so this is a very simplified version of what
creates jets. The basic idea that drives jet formation is this: a strong horizontal temperature contrast,
like the one between the North Pole and the equator, causes a dramatic increase in horizontal wind
speed with height. Therefore, a jet stream forms directly over the center of the strongest area of
horizontal temperature difference, or the front. As a general rule, a strong front has a jet stream
directly above it that is parallel to it. Figure B shows that jet streams are positioned just below the
tropopause (the red lines) and above the fronts, in this case, the boundaries between two circulation
cells carrying air of different temperatures.
Figure C: Typical Locations of Jet Streams Across North America
Image from NASA
The two jet streams that directly affect our weather in the continental US are the polar jet and the
subtropical jet. They are responsible for transporting the weather systems that affect us. The polar
front is the boundary between the cold North Pole air and the warm equatorial air. The polar jet sits at
roughly 60°N latitude because this is generally where the polar front sits. The subtropical jet is at
roughly 30°N latitude. The subtropical jet is located at 30°N because of the temperature differences
between air at mid-latitudes and the warmer equatorial air. The polar and subtropical jets are both
westerly, meaning they come from the west and blow toward the east. Both jets move north and south
with the seasons as the horizontal temperature fields across the globe shift with the areas of strongest
sunlight.
Figure D: Typical Locations of Polar Jet Stream Throughout Summer and Winter
http://www.meted.ucar.edu/afwa/climo/intro/media/graphics/polar_front_summer_winter.jpg
In the winter the polar jet moves south and becomes stronger because the North Pole gets colder but
the equator stays about the same temperature. This increases the temperature contrast and moves the
strengthened polar front jet farther south. As you probably have noticed, jet streams are not just
straight across, but have a wavy pattern. The jets follow the contours of low and high pressure areas
(troughs and ridges, respectively), which move like waves in the atmosphere across the earth. In winter
when the polar jet dips into the US, the troughs and ridges affect what kind of weather an area will
have. Figure D is a comparison of the strength and position of the polar jet in summer versus winter. If
a trough is sitting over you, it is generally very cold and snowy or rainy. If a ridge is sitting over you, it is
generally warm and dry. So if it’s winter and the jet stream looks like it does in the picture, Utah would
be warmer than average and Tennessee would be colder than average, possibly experiencing some
snow.
The subtropical jet also moves and evolves over time. The strongest effect of the subtropical jet in the
southeastern US is in winters during years when El Niño is occurring in the eastern Pacific Ocean. The
subtropical jet is then positioned in such a way that southern Georgia and Florida are right underneath
the main jet, and experience colder and wetter conditions than years when there is no El Niño.
El Niño
El Nino extended definition
El Niño is a climate pattern representing an unusual warming of the eastern tropical Pacific Ocean which
is linked to impacts on weather and climate patterns around the world.
Why do I care? El Niño affects areas all over the world through the warming of the Pacific Ocean. El Niño
impacts include drought conditions, warmer or cooler than normal temperatures for different areas,
precipitation pattern changes, and many other impacts which could all pose a threat for agriculture,
animal and human health, and plant life.
El Niño was first observed by Western scientists as an unusual ocean warming along the coasts of Peru
and Ecuador in South America. The timing of this warming often coincided with the Christmas season,
so it was called El Niño for the coming of the Christ Child. We now know that this pool of unusually
warm water can shift atmospheric circulation patterns around the globe, leading to changes in
temperature and precipitation in areas far away from the warm pool of water. Each El Niño event is a
little different from the next, with variations in the timing of the warming and the position of the
warmest water. ENSO, El Niño-Southern Oscillation, refers to the fluctuations in the temperature and
pressure of the surface waters between the eastern and western tropical Pacific as part of the Walker
circulation.
Figure A: Normal vs. El Niño Conditions
http://www.gma.org/surfing/weather/elnino.html
El Niño events typically occur every three to seven years and start when the trade winds blowing from
the northeast, near the equator in the Northern Hemisphere, weaken. In a strong event, the winds can
actually reverse and flow from the west. The trade winds weaken because the air pressure gradient
between the eastern and western tropical Pacific decreases. As the winds die down, warm water from
the western equatorial Pacific sloshes back towards South America, thickening the layer of warm surface
water and cutting off the upwelling of nutrients from the deep ocean near Peru and Ecuador. This leads
to the death of fish and the birds that feed on the nutrients, leading to economic impacts on commercial
fishing and guano gathering for fertilizer from the nesting areas of the birds.
An area of thunderstorms and intense rainfall often accompanies the pool of warmest water, leading to
changes in precipitation amounts along the South American coast. This causes flooding in western South
America where it is usually dry and makes it excessively dry in the western Pacific where precipitation
normally falls.
Figure B
El Nino Climate Changes Felt Around the World, Image from www.ncdc.noaa.gov
All of these changes in the Pacific Ocean change the global atmospheric circulation, which in turn affects
the weather and climate patterns all over the globe. Since the air flows unimpeded around the earth a
shift in the atmospheric patterns at one location (similar to placing a large rock in a river) can lead to
shifts in the flow of air and atmospheric patterns downstream. Figure B shows some of the known
variability in climate associated with the occurrence of an El Niño episode.
In addition, the pool of warm water in the eastern Pacific ocean aids in the transfer of heat from this
location. This also affects weather patterns all over the world. Areas usually receiving great amounts of
rainfall could potentially dry out during an El Niño event and vice versa for other areas that are usually
dry. Over the central Pacific Ocean, typhoons usually increase during strong El Niño events due to the
ample supply of warm water at the surface. Since El Niño usually reaches its maximum strength in
winter, El Niño has the strongest impacts on the weather in December, January, and February in the
Southeast, although it can continue to impact weather regionally for several months.
Figure C: Moderate El Niño Winter Conditions
Image from NOAA, Moderate El Nino Winter Conditions
The effects of El Niño also impact the location of the jet streams. The subtropical jet stream moves up
into the southern United States during the winter months of an El Niño year. This jet brings warm, moist
air into southern California. During the wintertime, southern California would then be mostly wet.
South-central Texas and the southeast US will be mostly cool and wet because the cold, moist air from
the Pacific Ocean entering the southern states enhances clouds and rainfall and cools temperatures due
to lack of direct sunlight. In turn, much of the northern United States and Canada will be warm during an
El Niño event because the polar jet stream swings farther east over the northeastern United States. For
much of the southeastern United States during the summer of an El Niño event, the climate is usually
warmer and drier. The strong subtropical jet stream also tends to disrupt the development of
hurricanes in the Atlantic Ocean and Gulf of Mexico. Because of that, fewer hurricanes are likely during
an El Niño year in the tropical Atlantic.