Atmosphere

1
Weather Studies Introduction to Atmospheric
ScienceAmerican Meteorological Society

• Chapter 9
• Atmospheres Planetary Circulation

Credit This presentation was prepared for AMS by
Michael Leach, Professor of Geography at New
Mexico State University - Grants
2
Case-in-Point

• 1983 was a year of wild weather
• There were 2000 weatherrelated deaths
• 13 billion in weather-related property damage
• There were floods, droughts, and fires worldwide
• Just prior to these weather extremes, the ocean
  circulation off the northwest coast of South
  America changed drastically
• Off the coast of Ecuador, the quantity of
  plankton decreased 20-fold
• This reduced the number of anchovy
• Fish that fed on anchovy declined markedly
• Commercial fisheries off the coast of Ecuador and
  Peru collapsed
• On Christmas Island, where 17 million seabirds
  normally nest, most adult birds abandoned their
  nesting grounds for sites with more food, leaving
  their young to starve
• All of this was driven by large-scale
  atmosphere/ocean circulation changes attributed
  to El Niño

3
Driving Question

• What are the principal features of the
  planetary-scale atmospheric circulation, and how
  does the circulation affect weather and climate?
• This chapter describes planetary-scale
• Pressure systems
• Wind belts
• Circulation patterns of middle latitude
  westerlies
• Anomalous variations in the circulation regime
  that result in El Niño and La Niña

4
Idealized Circulation Pattern

• To start with, assume a non-rotating Earth
• Also assume a uniform solid surface
• Sun heats the equatorial regions more intensely
  than the poles a temperature gradient develops
• Convection cell forms when cold, dense air sinks
  at the poles and flows at the surface toward the
  equator, where it forces warm, less dense air to
  rise. Aloft, equatorial air flows toward the
  poles.

5
Idealized Circulation Pattern

• If the idealized planet starts to rotate from
  west to east, the Coriolis Effect comes into play
• Northern Hemisphere surface winds are diverted to
  the right and blow toward the southwest
• Southern Hemisphere surface winds are diverted to
  the left and blow toward the northwest
• Winds blow counter to planets direction of
  rotation

6
Idealized Circulation Pattern

• Circulation is maintained in the atmosphere of
  our idealized Earth because the planetary-scale
  winds split into 3 belts in each hemisphere
• 3 belts are
• 0 to 30
• 30 to 60
• 60 to 90
• Now some winds blow with and some blow against
  the planets rotation

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Idealized Circulation Pattern

• Surface winds converge along the equator and
  along 60 latitude circles
• Convergence leads to rising air, expansional
  cooling, cloud development and precipitation
• Convergence zones are belts of relatively low
  surface air pressure
• Surface winds diverge at the poles and along the
  30 latitude circles
• Air descends, is compressed and warms, and
  weather is generally fair
• Divergence zones are belts of relatively high
  surface air pressure

8
Features of the Planetary-Scale Circulation

• Adding continents and ocean basins complicates
  the picture
• Some pressure belts break into separate cells
• Important pressure contrasts develop over land
  versus sea
• Maps show mean sea-level air pressure during
  January (top) and July (bottom) these are
  semi-permanent features

9
Features of the Planetary-Scale Circulation

• Schematic representation of the planetary-scale
  surface circulation of the atmosphere

10
Features of the Planetary-Scale Circulation

• Pressure Systems and Wind Belts
• Subtropical anticyclones
• Form near 30 N and S latitude over oceans
• Extend from the ocean surface to the tropopause
  and exert a major influence on weather and
  climate over vast areas of the ocean and
  continents
• Subsiding air extends outward from the eastern
  sides
• Compressional warming, low relative humidity, and
  fair skies are common
• Major deserts are located on eastern flanks
• The far western portions are characterized by
  less subsidence, less stable air, and frequent
  episodes of cloudy, stormy weather

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Features of the Planetary-Scale Circulation

• Pressure Systems and Wind Belts, cont.
• Subtropical anticyclones
• Common is a weak horizontal pressure gradient
  over a large area
• Winds are weak and
• Horse latitudes (30 to 35 degrees N and S) result
• Surface winds poleward of the horse latitudes are
  the midlatitude westerlies
• Winds blowing out of the high pressure cells
  toward the equatorial lows are called the trade
  winds
• Trade winds from the two hemispheres converge
  into a broad east-west equatorial belt of light
  and variable winds called the doldrums. In that
  belt, ascending air induces cloudiness and
  rainfall and the most active weather develops
  along the intertropical convergence zone (ITCZ).

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Features of the Planetary-Scale Circulation

• Pressure Systems and Wind Belts, cont.
• Poleward of subtropical anticyclones
• Surface westerlies flow into regions of low
  pressure
• These are the Aleutian low and the Icelandic low
  in the Northern Hemisphere
• In the Southern Hemisphere, this is a nearly
  continuous belt of low pressure surrounding
  Antarctica
• Surface westerlies meet and override the polar
  easterlies along the polar front
• In places where the polar front is well-defined,
  it is a potential site for development of
  extra-tropical cyclones
• Polar highs are shallow, cold anticyclones that
  develop at high latitudes

13
Features of the Planetary-Scale Circulation

• Winds Aloft
• Aloft, winds in the middle and upper troposphere
  blow away from the ITCZ
• These feed into the subtropical highs
• Resulting convection cells are called Hadley cells

14
Features of the Planetary-Scale Circulation

• Winds Aloft, continued
• Aloft in middle latitudes, winds blow from west
  to east in a wavelike pattern of ridges and
  troughs
• These winds are responsible for the movement of
  the synoptic-scale weather systems
• Their north/south components contribute to
  poleward heat transport

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Vertical Cross Section of Prevailing Winds in the
Troposphere
The altitude of the tropopause is directly
related to the mean air temperature of the
troposphere. The tropopause is found in three
segments, occurring at highest altitudes in the
tropics and lowest altitudes in the polar regions.
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Features of the Planetary-Scale Circulation

• Trade Wind Inversion
• A persistent and climatically significant feature
  of the planetary-scale circulation over the
  eastern portions of tropical ocean basins
• Key to formation is the descending branch of the
  Hadley cell
• Descending air is warmed by compression and its
  relative humidity decreases
• This air encounters the marine air layer
  overlying the ocean surface
• Where SST are low, layer is cool, humid, and
  stable
• Where SST are high, layer is warm, more humid,
  less stable, and well mixed by convection
• The trade wind inversion is formed at the
  altitude where air subsiding from above meets the
  top of the marine layer, trapping the cool marine
  layer near the surface
• Develops to the east and southeast of the center
  of a subtropical high
• Acts as a cap on the vertical development of
  clouds and rain

17
Features of the Planetary-Scale Circulation

• Seasonal Shifts
• Pressure systems, the polar front, the planetary
  wind belts, and the ITCZ follow the sun, shifting
  toward the poles in spring and toward the equator
  in autumn
• Planetary-scale systems in both hemispheres move
  north and south in tandem
• Subtropical anticyclones exert higher surface
  pressure in summer
• Icelandic low deepens in winter and weakens in
  summer
• The Aleutian low disappears in summer
• Seasonal reversals of pressure occur over the
  continents due to the contrast in solar heating
  of sea versus land
• Continents at middle and high latitudes are
  dominated by relatively high pressure in winter
  and low pressure in summer
• Northward migration of the ITCZ triggers summer
  monsoon rains in Central America, North Africa,
  India, and Southeast Asia

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Features of the Planetary-Scale Circulation

• ITCZ follows the sun
• It reaches farthest north in July
• It retreats to its most southerly latitudes in
  January

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Features of the Planetary-Scale Circulation

• Same latitude, different climates
• In summer, San Diego is under the eastern edge of
  the Hawaiian subtropical high and has a distinct
  dry season
• In summer, Charleston is on the humid, unstable
  side of the Bermuda high

20
Features of the Planetary-Scale Circulation
Long-term average pattern of wind-driven
ocean-surface currents. Gyres in the ocean basin
are driven by the planetary-scale atmospheric
circulation.
21
Monsoon Circulation

• Seasonal reversal of prevailing winds
• Results in wet summers and relatively dry winters
• Vigorous monsoon over portions of Africa and
  Asia, where rains are essential for drinking
  water and agriculture
• Over much of India, monsoon rains account for
  over 80 of the annual precipitation
• Depends on seasonal shifting of global
  circulation patterns
• Also depend on seasonal contrasts in heating and
  cooling of water and land
• Ocean has greater thermal inertia than the land

22
Monsoon Circulation

• In spring, there is relatively cool air over the
  ocean and relatively warm air over land
• Horizontal pressure gradient is directed from sea
  to land
• Produces an onshore flow of humid air
• Over land, intense solar heating generates
  convection
• Expansional cooling causes condensation, clouds,
  and rain
• Release of latent heat increases buoyancy and
  contributes to uplift and instability, triggering
  even more cloud development and rainfall
• Aloft, the air spreads seaward and subsides over
  the relatively cool ocean surface

23
Monsoon Circulation

• In Autumn, radiational cooling chills the land
  more than the adjacent ocean surface
• Horizontal pressure gradient is directed from
  land to sea
• Produces an offshore flow of air
• Over land, air subsides, and dry surface winds
  sweep seaward
• Air rises over the relatively warm ocean surface,
  completing the monsoon circulation

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Monsoon Circulation

• Topography complicates the monsoon circulation
  and the geographical distribution of rainfall
• Monsoon rainfall is neither uniform nor continual
• Rainy season consists of active and dormant
  phases
• The planetary circulation (e.g., ITCZ shifts) and
  the strength and distribution of convective
  activity vary from one year to the next
• Variation affects the intensity and duration of
  monsoon rains
• The southwest monsoon affects the American
  Southwest and brings a dramatic increase in
  rainfall during July and August

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Waves in the Westerlies

• Between 2 and 5 waves generally encircle the
  hemisphere at any one time
• These long waves are called Rossby waves, and
  characterize the westerlies above the 500-mb
  level
• They are measured by
• Wavelength
• Distance between successive troughs or ridges
• Amplitude
• North-south extent
• Number of waves
• In winter, waves strengthen
• Fewer waves
• Longer wavelength
• Greater amplitude
• Seasonal changes stem from variations in the
  north-south air pressure gradient, which is
  steeper in winter because of the greater
  temperature gradient

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Waves in the Westerlies

• Zonal and Meridional Flow Patterns
• Westerlies have 2 components
• North-south airflow is the meridional component
• West-to-east airflow is the zonal component
• If north-south component is weak, the result is a
  zonal flow pattern
• North-south exchange of air masses is minimal
• If flow is in a pattern of deep troughs and sharp
  ridges, the result is a meridional flow pattern
• Greater temperature contrasts develop across the
  U.S. and southern Canada
• Stage is set for development of extra-tropical
  cyclones
• If northern westerlies have a wave configuration
  differing from the southern westerlies, a
  complicated split flow pattern may exist

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Waves in the Westerlies

• Zonal and Meridional Flow
• These two images illustrate extremes of zonal and
  meridional flow
• Westerlies generally shift back and forth between
  zonal and meridional flow
• There is no regularity to this shift
• The affects long-range weather forecasting
  accuracy

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Waves in the Westerlies

• Blocking Systems and Weather Extremes
• North American weather is more dramatic when the
  westerlies are strongly meridional
• Undulations may become so dramatic that masses of
  air separate from the flow
• Cutoff highs and lows result
• These prevent normal west-to-east circulation,
  and are called blocking systems
• Extremes in weather such as drought or flooding
  rains or excessive heat or cold can result

29
Waves in the Westerlies

• Blocking Systems and Weather Extremes, cont.
• (A) Prevailing circulation pattern in the mid to
  upper troposphere during the summer of 1988. The
  blocking warm anticyclone over the central U.S.
  contributed to severe drought
• (B) The long-term average circulation pattern

30
Waves in the Westerlies

• Blocking Systems and Weather Extremes, cont.
• The map illustrates principal features of the
  prevailing atmospheric circulation pattern during
  the summer of 1993
• A blocking circulation pattern was responsible
  for record flooding in the Midwest and drought
  over the Southeast

31
Waves in the Westerlies

• Blocking Systems and Weather Extremes, cont.
• The top map illustrates the total rainfall for
  the period June-August 1993, expressed as a
  percentage of the long-term average
• The bottom picture shows a sign marking the crest
  of floodwaters at Missouri City, MO
• Heavy rains falling on the drainage basins of the
  Missouri and Upper Mississippi River valleys
  saturated soils and triggered excessive runoff,
  all-time record river crests, and severe
  flooding. Property damage totaled about 26.7
  billion (2002 dollars).

32
Waves in the Westerlies

• Short Waves
• Ripples superimposed on Rossby long waves
• Propogate rapidly through the Rossby waves
• 12 or more waves are generally found in a
  hemisphere at any one time
• Westerlies accelerate in ridges and slow in
  troughs, inducing horizontal speed convergence
  aloft ahead of ridges and horizontal speed
  divergence aloft ahead of troughs

33
Waves in the Westerlies

• Short Waves, cont.
• The figure is a schematic representation of the
  relationship between waves in the westerlies and
  a surface high and low

34
Jet Streams

• Narrow corridors of very strong wind
• In middle latitudes, the most prominent jet
  stream (polar front jet stream) is located above
  the polar front in the upper troposphere between
  the midlatitude tropopause and the polar
  tropopause
• Follows the path of the planetary westerly waves
• Winds may top 160 km per hr (100 mph)
• Eastbound aircraft seek it as a tail wind

35
Jet Streams

• The polar front is a narrow transition zone
  between relatively cold and warm air masses
• Cold air is denser than warm air, so pressure
  drops more rapidly in a column of cold air than
  warm air
• Even if air pressure is the same at the surface,
  there is a horizontal pressure gradient aloft
  (directed from warm to cold air) that increases
  with increasing altitude

36
Jet Streams

• Coriolis Effect balances the horizontal pressure
  gradient force
• Winds blow parallel to the polar front with cold
  air to the left of the direction of motion in the
  Northern Hemisphere
• Due to a strengthening horizontal pressure
  gradient with increasing altitude, wind speed
  increases with altitude in the troposphere and is
  highest near the tropopause
• Where the polar front is well defined, jet stream
  winds are stronger and jet streaks may develop
• Strongest jet streaks develop in winter along the
  east coasts of North America and Asia
• Great temperature contrast between snow-covered
  land and ice-free sea surface
• Jet streaks may have wind speeds as high as 350
  km per hr (217 mph)

37
Jet Streams

• The figure below is a map view of a jet streak
  with associated regions of horizontal divergence
  and convergence aloft. The contour lines are
  isotachs, lines of equal wind speed (in km per
  hr). In a straight jet streak, the strongest
  horizontal divergence is in the left-front
  quadrant, supplying upper-air support for
  extra-tropical cyclone development.

38
Jet Streams

• Undergoes important seasonal shifts
• Strengthens in winter and weakens in summer
• Map shows long-term average seasonal locations
• When polar front jet stream is south of your
  location, weather is relatively cold. When it is
  north, the weather is relatively warm

39
Jet Streams

• Subtropical Jet stream
• Found on the poleward side of Hadley cells near
  the break in the tropopause between tropical and
  middle latitudes
• Strongest in winter
• Less variable with latitude than the polar front
  jet stream
• Other jet streams
• Tropical easterly jet
• Feature of the summer circulation at about 15
  degrees N over North Africa and south of India
  and Southeast Asia
• Low-level jet stream
• Several hundred meters above Earths surface
• Surges up Mississippi River valley
• Contributes to the development of nocturnal
  thunderstorms

40
El Niño and La Niña

• In El Niño,
• Trade winds weaken
• SST rise well above long-term averages over the
  central and eastern tropical Pacific
• Areas of heavy rainfall shift from the western
  into the central tropical Pacific
• La Niña is a period of exceptionally strong trade
  winds across the tropical Pacific with lower than
  usual SST in the central and eastern tropical
  Pacific
• El Niño is the warm phase and La Niña is the cold
  phase of the tropical atmosphere/ocean
  interaction
• El Niño and La Niña influence the prevailing
  circulation of the atmosphere in middle
  latitudes, especially in winter
• Weather extremes that may accompany El Niño are
  opposite those associated with La Niña

41
El Niño and La Niña

• Historical Perspective
• Originally was the name given by fisherman to the
  seasonal occurrence of an unusually warm
  southward flowing ocean current and poor fishing
  off the coast of Peru and Ecuador during the
  Christmas season
• Warm weather episodes are relatively brief (1-2
  months) and then SST and fisheries return to
  normal levels
• Now, scientists reserve the term El Niño for
  long-lasting atmosphere/ocean anomalies
• Occurs every 3-7 years
• Persists for 12-18 months or longer
• Accompanied by significant Pacific SST changes,
  major changes in atmosphere and ocean circulation
  patterns, and collapse of important South
  American fisheries

42
El Niño and La Niña

• Historical Perspective
• An important step in understanding El Niño was
  the discovery of the southern oscillation in 1924
  by Sir Gilbert Walker
• Seesaw variation in air pressure across the
  tropical Indian and Pacific Oceans
• Influences monsoon rains in India
• In 1966, Jacob Bjerknes demonstrated a
  relationship between El Niño and the southern
  oscillation (ENSO)
• An El Niño episode begins when the air pressure
  gradient across the tropical Pacific begins to
  weaken, heralding the slackening of the trade
  winds
• Intense El Niño of 1982-83 brought attention to
  weather impacts worldwide

43
El Niño and La Niña

• Historical Perspective, cont.
• Today, the southern oscillation index is computed
  by subtracting the Darwin pressure from the
  Tahiti pressure divided by the standard deviation
  of that quantity
• Strong positive values indicate La Niña
  conditions
• Strong negative values indicate El Niño conditions

44
El Niño and La Niña

• Neutral Conditions in the Tropical Pacific
• If Earth did not rotate, frictional coupling
  between the wind and the ocean surface would push
  a thin layer of water in the same direction of
  the wind, and the surface layer would drag the
  layer beneath it
• Because Earth rotates, the shallow layer of
  surface water set in motion by the wind is
  deflected to the right of the wind direction in
  the Northern Hemisphere and to the left in the
  Southern Hemisphere
• Except at the equator, each layer of water put
  into motion by the layer above shifts direction
  because of the Coriolis Effect
• The model to plot the direction and speed of
  water layers is known as the Ekman spiral
• The net water movement of 90 to the wind
  direction due to the coupling between wind and
  surface water is known as Ekman transport

45
Ekman Spiral and Ekman Transport
In the Northern Hemisphere, the surface layer of
water moves at 45 degrees to the right of the
wind direction. The net transport of water
through the wind driven column is 90 degrees to
the right of the wind.
46
El Niño and La Niña

• Typically, southerly or southwesterly winds
  blowing along the west coast of South America
  drive warm surface waters to the left (westward)
  via Ekman transport, away from the coast
• In the process known as upwelling,
  cold-nutrient-rich waters move upward from depths
  of 200 to 1000 m (650 to 3300 ft)
• Abundance of nutrients brought to sunlit surface
  waters spurs an explosive growth of
  phytoplankton, which supports a highly productive
  fishery

47
El Niño and La Niña

• Neutral Conditions in the Tropical Pacific, cont.
• Walker Circulation (large convective-type
  circulation)
• High SST in the western tropical Pacific lowers
  surface air pressure and low SST in the eastern
  tropical Pacific raises air pressure
• During neutral conditions, the east-west SST
  gradient reinforces the trade winds by
  strengthening the east-west pressure gradient
• Trades become warmer and more humid as they flow
  over the ocean surface
• In the western tropical Pacific warm humid air
  rises, expands, and cools, leading to
  thunderstorm formation
• Aloft, air flows eastward and sinks over the
  cooler water of the eastern tropical Pacific

48
Neutral Conditions Compared with El Niño
Conditions

• (A) Neutral (long-term average) conditions
  featuring relatively strong easterlies
• (B) El Nino conditions with weaker easterlies

49
El Niño and La Niña

• The Warm Phase El Niño
• Air pressure falls over the eastern tropical
  Pacific and rises over the western Pacific
• Pressure gradient weakens winds slacken and may
  reverse direction west of 180 longitude
• A thick layer of warm surface water drifts slowly
  eastward over the tropical Pacific
• In the western tropical Pacific, SST drops,
  sea-level falls, thermocline rises
• In the eastern Pacific, SST rises, sea-level
  rises, thermocline deepens, upwelling is blocked,
  fish harvest plummets, coral bleeching occurs
• El Niño has a ripple effect on the weather and
  climate throughout the world. A linkage between
  atmospheric circulation changes in widely
  separated regions of the globe is known as a
  teleconnection.
• The 1997-98 El Niño rivaled its 1982-83
  predecessor as the most intense of the 20th
  century

50
El Niño and La Niña
Principal features of the winter atmospheric
circulation over North America during a typical
El Niño.
51
El Niño and La Niña

• The Cold Phase - La Niña
• A period of unusually strong trade winds and
  exceptionally vigorous upwelling in the eastern
  tropical Pacific
• SST anomalies opposite those of El Niño, although
  the magnitude of anomalies are not as great
  (typically 2 to 3 Celsius degrees below the
  long-term average)
• Brings opposite weather extremes than El Niño

52
El Niño and La Niña

• Prediction and Monitoring
• Some empirical and dynamical numerical models
  have been developed
• Overall results have been mixed
• Dynamical models performed well in detecting the
  onset of the 1997-98 El Niño due to the
  increasing amount of ocean/atmosphere
  observational data from the tropical Pacific
• ENSO Observing System
• Consists of an array of moored and drifting
  buoys, island and coastal tide gauges, ship-based
  measurements, and satellites
• One component is the TAO/TRITON instrument array
  consisting of 70 deep-sea moorings
• TOPEX/Poseiden and TRMM satellites also essential
  to monitoring conditions

53
Components of ENSO Observing System
54
Evolution of the 1997-98 El Niño
Evolution of the 1997-98 El Nino derived from
changes in ocean surface height (compared to
long-term averages) as measured by the
TOPEX/Poseiden satellite.
55
Evolution of the 1997-98 El Niño
56
Evolution of the 1997-98 El Niño
57
El Niño and La Niña
Average rainfall in millimeters per day across
the tropics for the period January 1998
December 2003. Data was obtained from the TRMM
Microwave Imager and IR sensors supplemented by
conventional rain gauge measurements.
58
El Niño and La Niña

• Frequency of El Niño and La Niña
• By April 2005, 26 nations adopted a common index
  for defining El Niño and La Niña in the region
  bounded by 120 W and 170 W and 5 N and 5 S
• El Niño positive SST departure from normal
  0.5 Celsius degree, averaged over 3 consecutive
  months
• La Niña negative SST departure from normal 0.5
  Celsius degree, averaged over 3 consecutive months