Transcript 09_AtmosphericCirculation

NAS 125: Meteorology
Atmosphere’s Planetary Circulation
El Niño, part 1
• Global weather went wild in 1982-1983.
– Crippling droughts struck Australia, India, Indonesia, the
Philippines, Mexico, Central America, and southern Africa.
• Huge drought-related wildfires raged across Australia and Borneo.
• Australia’s worst drought in 200 years caused $2 billion in crop
damage.
– Floods devastated parts of western and southeastern United
States, Cuba, and northwestern South America.
– Destructive tropical cyclones lashed French Polynesia and
Hawai’i.
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El Niño, part 2
• Weather gone wild (continued):
– Abnormally warm waters over a 13,000-km stretch of the
equatorial Pacific caused massive dieoffs of fish, seabirds,
marine mammals, and corals.
• Collapse of plankton population led to a collapse of the anchovy
population, which in turn led to a collapse of other fish populations,
which in turn led to a collapse of seabird and marine mammal
populations.
• Adult seabirds abandoned their young on Kiribati.
– In all, the weather-related events, called “the most
disastrous in recorded history,” cost 2000 human lives,
about $13 billion in damage, and vast ecological havoc).
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El Niño, part 3
• The cause of the disaster was an unusual warming of
surface waters in the eastern equatorial Pacific off the
coast of Ecuador and Peru.
• Residents of the area had known about the weather
pattern caused by the warming for some time, and
called it El Niño.
– The rest of the world knew about it after 1982-1983.
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General circulation, part 1
• According to an idealized circulation pattern (on a
non-rotating earth with a uniform surface), unequal
heating would create a two-cell circulation pattern.
– The cells, one in each hemisphere, would have rising air at
the equator and descending air at the poles.
– Winds at the surface would blow from the poles toward the
equator, and winds aloft would blow from the equator
toward the poles.
• The rotation of the Earth and its variable surfaces
create a complex atmospheric circulation pattern.
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General circulation, part 2
• Only the tropical regions have complete vertical
circulation cells, called Hadley cells.
– In a Hadley cell, heating at the equator warms the air
above, causing it to rise to elevations of about 15 km,
where it cools, moves poleward, then subsides. The air
descends at roughly 30° north or south latitude.
– There are two Hadley cells.
• Outside the tropical and subtropical latitudes, vertical
cells do not exist or are weakly and sporadically
developed.
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General circulation, part 3
• Global pressure patterns drive global wind patterns.
• The patterns observed migrate north and south with
the patterns of solar heating produced as the Earth
orbits the sun.
– The north-south migration of climate patterns is enhanced
over the continents and reduced over the oceans.
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General circulation, part 4
• The general circulation of the atmosphere has seven
surface components:
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Subtropical highs
Intertropical convergence zone
Polar highs
Subpolar lows
Trade winds
Midlatitide westerlies
Polar easterlies
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General circulation, part 5
• Subtropical highs
– The subtropical latitudes serve as the “source” of the major
surface winds of the planet.
– The subtropical highs (STHs) are large semipermanent
anticyclones centered at about 30° latitude over the oceans
– Their average diameter is about 3,200 kilometers.
– They develop from the descending limbs of Hadley cells.
– The location of the subtropical highs are coincident with
most of the world’s major desert belts.
– Migration of the anticyclones affects weather of
midlatitudes.
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General circulation, part 6
• Subtropical highs (continued)
– The Horse latitudes are areas in the subtropical highs
characterized by warm, tropical sunshine and an absence of
wind.
• They exist because the weather within a subtropical high is nearly
always clear, warm, and calm.
– The subtropical highs serve as source for two of the world’s
three major surface systems:
• Trade winds
• Westerlies
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General circulation, part 7
• Intertropical convergence zone
– The intertropical convergence zone (ITCZ) is a belt of calm
air where the northeast trades and southeast trades
converge, generally in the vicinity of the equator (or at least
the heat equator).
• The zone is also called the equatorial front, the intertropical front,
and the doldrums.
• Intertropical convergence zone thunderstorms provide the updrafts
where all the rising air in of the tropics ascends.
• The zone often appears as a narrow band of clouds over oceans, but
it is less distinct over continents.
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General circulation, part 8
• Polar highs
– The polar highs are anticyclones centered over the polar
regions.
• The Antarctic high is quite different from the Arctic high because it
forms over an extensive, high-elevation, and very cold continent,
while the Artic high forms primarily over sea ice.
• The Antarctic high is strong, persistent, and almost permanent,
while the Arctic high is much less pronounced and more ephemeral.
• The polar highs are the source of the polar easterlies, which blow
toward the equator and toward (not from) the west.
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General circulation, part 9
• Subpolar lows
– The subpolar lows are a zone of low pressure situated at
about 50° to 60° of latitude in both the Northern and
Southern hemispheres.
• They often contain the polar front.
• The characteristics vary in either hemisphere because the continents
modify circulation in the Northern hemisphere, while circulation in
the Southern hemisphere is over a virtually continuous expanse of
ocean, the Southern Ocean.
• The polar front is the meeting ground of the cold polar easterlies
and the warm midlatitude westerlies, and is the site of genesis of
many midlatitude weatehr systems.
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General circulation, part 10
• Trade winds
– The trade winds are the major wind system of the tropics,
originating from the equatorward sides of the subtropical
highs and blowing toward the west as well toward the
equator.
– The trades are the most reliable of all winds in terms of
both direction and speed.
– They are named for the direction they blow from.
• In the Northern Hemisphere, they blow from the northeast, so are
called the northeast trades.
• In the Southern Hemisphere, they blow from the southeast, so are
called the southeast trades.
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General circulation, part 11
• Trade winds (continued)
– The trades are warming, drying winds, but are capable of
holding enormous amounts of moisture.
• They generally do not release moisture unless forced by a
topographic barrier or a pressure disturbance.
– The winds typically pass over low-lying islands, drying them of
moisture and turning them into desert islands.
– On the other hand, windward slopes exposed to the trades, as in the
mountains of Hawai’i, are some of the wettest places on Earth.
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General circulation, part 12
• Midlatitude westerlies
– The westerlies are the great wind system of the
midlatitudes, flowing from west to east around the world in
a latitudinal zone between about 30° and 60˚ both north and
south of the equator.
– They originate from the poleward side of the subtropical
highs, blowing toward the poles and toward the east.
– There are two cores of high-speed winds at high altitudes in
the westerlies:
• Polar front jet stream
• Subtropical front jet stream
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General circulation, part 13
• Midlatitude westerlies (continued)
– A major feature of the midlatitude westerlies are the
Rossby waves, sweeping north-south undulations that
frequently develop aloft.
• The undulating motion of the Rossby waves, coupled with the
migratory pressure systems and storms associated with the
westerlies, give the middle latitudes more short-run weather
variability than any other place on Earth.
• Anticyclonic circulation at the surface is associated with ridges in
the waves, while cyclonic circulation is associated with troughs in
the waves.
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General circulation, part 14
• Polar easterlies
– The polar easterlies are a global wind system that occupies
most of the area between the polar highs and about 60° of
latitude.
• The winds move generally from east to west and are typically cold
and dry.
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General circulation, part 15
• Vertical patterns of the general circulation
– Winds in the upper elevations of troposphere differ from
surface winds.
• The most dramatic difference occurs between surface trade winds
and the upper-elevation antitrade winds.
• Antitrade winds are tropical upper air winds that blow toward the
northeast in the Northern Hemisphere and toward the southeast in
the Southern Hemisphere.
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General circulation, part 16
• Vertical patterns (continued):
– Trade wind inversion
• Forms over tropical and subtropical oceans
• The air just over the surface forms the marine boundary layer.
• Air subsiding in the descending limb of the Hadley circulation
undergoes compressional warming.
• Where the descending air meets the marine boundary layer, a
temperature inversion, in which the upper air is warmer than the
lower air; this is the trade wind inversion.
• The air in the trade wind inversion is very stable, as the inversion
acts as a cap over the vertical movement of air, thus the convective
development of clouds and precipitation.
– It also limits orographic precipitation.
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Monsoons, part 1
• A monsoon is a seasonal reversal of winds; a general
onshore movement in summer and a general offshore
flow in winter, with a very distinctive seasonal
precipitation regime.
• Monsoons are the most significant disturbance to the
pattern of general circulation.
– Offshore flow is wind movement from land to water.
– Onshore flow is wind movement from water to land.
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Monsoons, part 2
• Monsoons control the climates of regions with more
than half of the world’s population.
• The origin of monsoons is still not understood,
though there is increasing evidence that it is
associated with upper-air phenomena, particularly jet
stream behavior.
• Monsoons have an essential effect: Their failure or
even the late arrival of monsoonal moisture inevitably
causes widespread starvation and economic disaster.
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Rossby waves, part 1
• The Rossby waves were first described by Carl
Gustav Rossby in the 1930s.
• In the waves, winds exhibit anticyclonic flow in the
ridges, and cyclonic flow in the troughs.
• Typically, there are two to five waves in a hemisphere
at any given time.
• Wave characteristics:
– Wavelength
– Amplitude
– Wave number
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Rossby waves, part 2
• Types of flow in Rossby waves:
– There is a north-south, or meridional, component; and an
east-west, or zonal component.
– Zonal flow occurs when there is a minimal north-south
component (minimal amplitude) in the Rossby waves.
– Meridional flow occurs when there is a considerable northsouth component (considerable amplitude) in the Rossby
waves.
– In split flow, the wave configuration westerlies to the north
differs from the configuration of those to the south.
– There is no predictable flow cycle.
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Rossby waves, part 3
• Blocking systems:
– When Rossby circulation over North America is strongly
meridional, whirling masses of air separate from the main
westerly flow.
– Such cutoff air may prevent the usual east-west flow of the
westerlies, thus setting up blocking systems.
• These blocking systems are often responsible for extreme weather
patterns in the United States.
– 1986 drought in Southeast
– 1988 drought in the Midwest and Great Plains
– 1993 floods in the Midwest, with drought in the Southeast
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Rossby waves, part 4
• Short waves:
– Short waves are ripples superimposed on the Rossby
waves.
• Short waves travel much more quickly than Rossby waves.
• Short waves are more numerous than Rossby waves.
– Short waves and Rossby waves both contribute to cyclone
development in the midlatitudes.
• Westerly winds strengthen in ridges and weaken in troughs, leading
to divergence in the middle and upper troposphere east of a trough
(and west of a ridge).
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Jet streams, part 1
• Jet streams are narrow corridors of very strong winds.
• The most prominent in the Northern Hemisphere is
the polar front jet stream, above the polar front in the
upper troposphere.
– Westbound flights avoid it because it is a headwind, which
makes the trip take more time; while eastbound flights seek
it because it is a tailwind, which makes the trip take less
time.
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Jet streams, part 2
• Jet streams are created as a result of the relationship
between air temperature and density.
– Cold air in the troposphere is denser than warm air, so
density decreases more rapidly in a column air than in one
of warm air.
• At higher altitudes, pressure is greater in warm air than in cold air.
– Thus, the difference in density between cold and warm air
becomes greater with increasing elevation.
• Thus, horizontal pressure gradients become greater with increasing
elevation.
• The speed of pressure gradient winds in turn increase with
increasing elevation.
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Jet streams, part 3
• Jet stream creation (continued):
– Because of the lack of friction, the interaction between the
pressure gradient force and the Coriolis effect results in
geostrophic winds, with cold air to the left of the direction
of motion and warm air to the right.
• Temperature gradients are reversed in the
stratosphere, so the pressure and speed gradients are
reversed as well.
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Jet streams, part 4
• Jet streaks
– The polar front jet stream is not uniformly well defined
around the globe.
– Wind speeds are greatest where the jet is best defined.
– In those areas, wind speeds may increase as much as 100
km, thus forming jet streaks.
– The strongest jet streaks are formed in eastern North
America and Asia where there is a strong contrast in land
and water temperatures.
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Jet streams, part 4
• Jet streams help in the genesis and maintenance of
synoptic-scale cyclones by contributing to divergence
aloft.
– Isotachs are lines of equal wind speed.
– Jet streaks can be subdivided into four quadrants: upper
right, lower right, upper left, and lower left.
– Air accelerates as it enters a jet streak, and decelerates as it
exits one.
– The change in air speed generates divergence in the leftfront and right-rear quadrants, while convergence occurs in
the right-front and left-rear quadrants.
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Jet streams, part 5
• Jet streaks and cyclogenesis (continued):
– In general, the strongest horizontal divergence is in the leftfront quadrant for a straight jet streak or a cyclonically
curved one.
• This is where cyclones are more likely to form.
– For an anticyclonically curved jet streak, the strongest
horizontal divergence is in the right-rear quadrant.
– Jet streaks typically migrate west to east more quickly than
westerly troughs and ridges; the strongest divergence aloft
is where a jet streak is on the east side of a trough.
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Jet streams, part 6
• The position of the polar jet stream shift seasonally,
migrating south during the winter and north during
the summer.
– This plays a role in the typical location of severe weather,
such as tornado location.
• Other jet streams include the subtropical jet, the
tropical easterly jet, and a low-level jet that helps
trigger nighttime thunderstorms in the Mississippi
Valley.
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El Niño, part 1
• El Niño is an anomalous oceanographic-weather
phenomenon of the eastern equatorial Pacific,
particularly along coast of South America.
– It occurs when southeastern trades abnormally slacken or
reverse direction, which triggers a warm surface flow,
which displace the cold, nutrient-rich upwelling that
usually prevails on the surface.
– Upwelling is driven by Ekman transport, in which
prevailing winds parallel to the coast trigger transport of
water from the depths to the surface; transport is in a spiral
motion with depth referred to as an Ekman spiral.
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El Niño, part 2
• El Niño (continued):
– El Niño was once believed to be a local phenomenon, but is
now understood to be associated with changes in global
pressure, wind, and precipitation.
– It occurs every few years around Christmas time (El Niño
is Spanish for “the Christ child”).
– Even though archeological and paleoclimatological records
have indicated past El Niño phenomenon, it was not until
1982–1983 that great attention was drawn to it.
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El Niño, part 3
• Normal pattern: In order to understand El Niño
phenomenon, it is critical to understand normal
pressure, wind, and ocean current patterns in the
Pacific.
– Dominance of the subtropical high in the eastern Pacific
causes westward movement of the trade winds toward low
pressure cell in the western Pacific.
• Trade winds create frictional drag on the Pacific Ocean and create
westward moving warm equatorial current.
• The removal of surface water near the western coast of South
America allows cold water to upwell.
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El Niño, part 4
• Normal pattern (continued):
– This phenomenon is known as the Southern Oscillation, a
large-scale fluctuation in sea-level atmospheric pressure
that occasionally occurs in the eastern and western tropical
Pacific; caused by differences in water temperature.
• When El Niño and Southern Oscillation coincide (called ENSO),
unusual atmospheric and oceanic conditions are more frequent and
more intense than when either event occurs alone.
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El Niño, part 5
• El Niño pattern: Every few years the normal pressure
pattern changes.
– High pressure develops over northern Australia, and low
pressure develops to the east over Tahiti.
– The pressure reversal causes the trade winds to reverse
direction, and this allows warm water from the western
Pacific to “backwash” toward the eastern Pacific.
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El Niño, part 6
• El Niño pattern (continued):
– For many months before the onset of El Niño, the trade
winds pile up warm water in the western Pacific, and then a
bulge of warm equatorial water about 25 cm high moves
eastward in a series of bulges known as Kelvin waves.
• These waves can take 2-3 months to arrive off the coast of South
America.
• This causes the sea level to rise off the coast of South America as
the warm water pools.
• This impedes the upwelling of cold water off of the coast, and thus
causes temperatures off the coast of South America to rise.
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El Niño, part 7
• El Niño pattern (continued):
– The shift in the normal pressure pattern in the Pacific can
cause increased precipitation in the deserts of Peru,
droughts in northern Australia and Indonesia, decreased
monsoon activity in South Asia, and more powerful winter
storms in the southwestern United States.
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El Niño, part 8
• La Niña: Unusually cold temperatures in the eastern
Equatorial Pacific.
– Both El Niño and La Niña are extreme cases of a naturally
occurring climate cycle that involves large-scale changes in
sea-surface temperatures across the eastern tropical Pacific.
– La Niña is not as predictable as El Niño.
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El Niño, part 9
• Causes of ENSO
– There is no clear “trigger” of ENSO. It is not clear whether
the changes in the ocean temperature or the changes in the
pressure and wind occur first.
– The effects of ENSO are also not completely predictable.
• Some generalizations, however, can be made (i.e., floods are more
likely to occur in California during El Niño years).
– There is increasing recognition of El Niño connections with
atmospheric and oceanic conditions outside of the Pacific.
• These connections are known as teleconnections.
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El Niño, part 10
• Causes of ENSO (continued):
– ENSO may be influenced by other ocean-atmosphere
cycles such as the North Atlantic Oscillation and the Pacific
Decadal Oscillation.
– Deployed instrument buoys (the Tropical
Atmosphere/Ocean Array) are now used to monitor El Niño
and aid in making climatic predictions.
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El Niño, part 11
• El Niños are becoming more frequent and
progressively warmer in recent years.
– The 1997-98 El Niño was the strongest in history, and it
developed more rapidly than any other in the last 40 years.
• The triggering mechanisms are still unclear.
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