Chapter 7 Notes
Circulation of the Atmosphere
Satellite Image of Swirling Wind Pattern Lee of an Island
Global atmospheric circulation can be thought of as a series of deep rivers
of air that encircle the planet. Embedded in the main currents are vortices
of various sizes including hurricanes, tornados, and mid-latitude cyclones.
These rotating wind systems develop and die out with somewhat
predictable regularity. The smallest eddies, like dust devils, last only a few
minutes, while larger and more complex systems, such as hurricanes, may
survive for several days.
Winds are generated by pressure differences caused by unequal heating
of Earth’s surface. Because the tropics receive more solar radiation than
Earth’s polar regions, Earth’s winds blow in an unending attempt to
balance inequalities in surface temperatures.
Macroscale, Mesoscale, and Microscale Circulation
Scales of Atmospheric Motion
Macroscale winds – include large planetary-scale flow that blow consistently for
weeks or longer as well as smaller feature like hurricanes. Examples of
macroscale winds include the prevailing westerlies that dominate the airflow in
the United States, and the trade winds
Mesoscale winds – circulation associated with tornadoes, thunderstorms, other
local winds that generally last from minutes to hours. Mesoscale winds are
usually less than 100 kilometers across, and some—thunderstorms and
tornadoes—also have a strong vertical component.
Microscale winds – have short life spans, usually a few seconds to minutes,
and include dust devils, gusts, and general atmospheric turbulence.
Local Winds – Mesoscale Winds
Winds are named for the direction from which
they blow, example: a north wind blows from
north to south.
Land and Sea Breezes – A sea breeze
develops as cooler air over water moves
inland, while a land breeze is the result of
cooler air over land moves out to the water.
Sea breezes tend to moderate coastal
temperatures.
Mountain and Valley Breezes – During the day,
air along mountain slopes is heated more
intensely that air at the same elevation over the
valley floor. This warmer air glides up along the
mountain slope and generates a valley breeze.
The reverse happens after sunset. Rapid
cooling along the mountain slopes cools the
air, which drains into the valley below and a
mountain breeze.
Chinook (Foehn) Winds - These winds are
created when a strong pressure gradient
develops in a mountainous region. As the air
descends the leeward slopes of the mountains,
it is heated adiabatically, and can result in rapid
warming. Chinooks winds usually occur in the
winter and early spring.
Katabatic (Fall) Winds – Winds that originate
when cold air, situated over a highland area is
set in motion. Under the influence of gravity,
the slides over the highland rim and descends
into the lowlands.
Tornado, Mesoscale Circulation
Country Breezes – A wind associated with
urban areas. This circulation pattern is
characterized by a light wind blowing into the
city from the surrounding country-side. The
breeze is initiated as warm, less-dense air in
the city rises and is replaced by cooler air from
outside the city.
Global Circulation
Knowledge of global winds comes from two sources: the patterns of
pressure and winds observed worldwide, and theoretical studies of fluid
motion.
Idealized Three-Cell Circulation Model
Classical Circulation Models
1) Single-Cell Model
Models global circulation on a non-rotating Earth. It is a simple convection
system produced by unequal heating of the atmosphere. Hot, less dense air
rises at the equator until it reaches the top of the troposphere where it then
begins to spread toward the poles. At the poles, the now dense air sinks and
moves toward the equator.
2) Three-cell Circulation Model (pictured)
In the zones between the equator and roughly 30O latitude north and south a
convection system similar to the single-cell model exists. Circulation between
30O and 60O latitude is more complicated. The net surface flow is pole-ward,
but because of the Coriolis force the winds have a strong westerly component.
At the poles, air subsidence produces a surface flow that moves toward the
equator and is deflected into polar easterlies.
Idealized Pressure Belts; Belts Broken Up by Real Pressure Cells
Observed Distribution of Pressure and Winds
1) Idealized Zonal Pressure Belts
If Earth had a uniform surface, two latitudinally oriented belt of high and two of
low pressure would exist. Near the equator, the warm rising air is associated
with the pressure zone know as the equatorial low. In the belts about 20O to
35O where the westerlies and trade winds originate are the pressure zones
known as the subtropical highs. Another low pressure region is situated at
about 50O to 60O latitude where the polar easterlies and prevailing westerlies
clash to form a convergence zone known as the subpolar low. Finally, at
Earth’s poles are the polar highs, from which the polar easterlies originate.
Average Surface Pressure and Global Circulation in January and July
Observed Distribution of Pressure and Winds
2) Semipermanent Pressure systems: The Real World
Because the Earth’s surface is not uniform, the only true zonal distribution
of pressure exists along the subpolar low in the Southern Hemisphere. In
reality, Earth’s pressure patterns vary in strength or location during the
course of the year. Factors that influence the location and strength of
pressure patterns include amount of solar heating and the proportion of land
compared to ocean
Monsoons
The greatest seasonal change in
Earth’s global circulation is the
monsoon. Monsoon refers to a wind
system that exhibits a pronounced
season reversal in direction. In
general, winter is associated with
winds that blow predominantly off the
continents and that produce a dry
winter monsoon. In the summer warm
moisture-laden air blows for the sea
toward the land. As a result, the
summer monsoon is usually
associated with abundant moisture.
Monsoon Circulation
Prevailing Westerly Winds Aloft
Prevailing Westerlies
After World War II, one of the most important meteorological discoveries
was the existence of the westerlies, a circulation of air aloft in the middle
latitudes that has a strong west-to-east component.
Two forces interact to produce the westerlies. First, there is a strong
pressure gradient from high pressure at the equator to low pressure at
the poles. The pressure gradient results in air flowing south to north (in
the Northern Hemisphere). Once air aloft starts moving toward the north
the Coriolis force acts upon it and bends the motion to the right. The
balance between the pressure gradient force and the Coriolis force
generates a wind with a dominant west-to-east component.
Jet Streams
Embedded within the westerly flow aloft are narrow ribbons of high-speed
winds that travel for thousands of kilometers. These ribbons of winds are
called jet streams, and they have velocities between 200 km/hr to 400
km/hr.
Jet streams are located in regions of the atmosphere where large
horizontal temperature differences occur over short distances. These large
temperature contrasts occur along linear zones called fronts.
1) The polar jet stream
The polar jet stream occurs along a major frontal zone called the polar
front. The polar jet occurs mainly in the middle latitudes and migrates
with the seasons. During the summertime the polar jet moves north
while the opposite happens in the winter.
2) The subtropical jet stream
A semipermanent jet exists over the subtropics and occurs mainly in
the wintertime
Ocean Currents
Global Winds and Ocean Currents
Where the atmosphere and ocean are in contact, energy is passed from moving
air to the water through friction. The drag exerted by winds blowing steadily
across the ocean causes the surface layer of water to move. The movement of
water like this is called ocean currents., and a relationship exists between the
oceanic circulation and the general atmospheric circulation.
Ocean currents have an important effect on climate. Poleward-moving warm
currents moderate the climate on land near (relatively) the poles. Also, cold
ocean currents impact conditions on the land by reducing the amount of rainfall
in coastal regions alongside the cold current.
Ocean currents also lay a role in maintaining Earth’s heat balance. They transfer
heat from the tropics to the polar regions.
Southern Oscillation and El Nino
El Niño and La Niña
Typically, atmospheric circulation in the tropical Pacific is dominated by strong
trade winds which generate a strong equatorial current that flows westward from
South America toward Australia and the Philippines. In addition, a cold oceanic
current flows equatorward along the coast of Ecuador and Peru. Near the end of
each a warm countercurrent, flowing eastward along the equator develops, and
warm water begins to accumulate along the coasts of Ecuador and Peru.
Usually the warm countercurrent lasts for a few weeks, but occasionally—every
three to seven years—the countercurrent is unusually strong. The result is that
the normally cold offshore waters are replaced with warm equatorial waters.
These episodes are called El Niño.
La Niña is the opposite of El Niño, that is the surface temperatures of the Pacific
are colder than normal. Like El Niño, La Niña has distinctive weather patterns.
Major El Niño events are related to the large-scale atmospheric circulation. Each
time an El Niño occurs, the barometric pressure drops over large portions of the
southeastern Pacific , while in the western Pacific the pressure rises. When the El
Niño ends, the pressure difference in the two regions swings back in the opposite
direction. This see-saw pattern of atmospheric pressure is called the Southern
Oscillation.
The effects of El Niño are somewhat variable depending in part on the
temperatures and size of warm pools. During most El Niños, warmer-than-normal
winters occur in the northern United States and Canada, and colder-than-normal
winters are experienced in the Southwest and Southeast. Also, the eastern
United States experiences wetter-than-normal conditions.
Global Distribution of Precipitation
In general, regions influenced by high
pressure, with its associated
subsidence and divergent winds,
experience dry conditions. Conversely,
regions under the influence of low
pressure and its converging winds and
ascending air receive ample
precipitation.
However, air pressure is not the only
factor that determines the presence or
absence of precipitation. Because cold
air has a low capacity for moisture
compared with warm air, there should
be a latitudinal variation in
precipitation with low latitudes (near
the equator) receiving the greatest
amounts of precipitation and high
latitudes receiving the least.
Also, the distribution of land and water
complicates the precipitation pattern.
Large landmasses in the middle
latitudes commonly experience
decreased precipitation toward their
interiors. Additionally, the effects of
mountain barriers alter the idealized
precipitation models. Windward
mountain slopes receive abundant
precipitation while the leeward slopes
and adjacent lowlands are usually
deficient in moisture.
Global Distributions of Precipitation;
Annually, in July, and in January