Transcript AMS Weather Studies

AMS Weather Studies
Introduction to Atmospheric Science, 4th Edition
Chapter 9
Atmosphere’s Planetary
Circulation
© AMS
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Case-in-Point
 In the winter of 1997-98, the San Francisco Bay area
experienced considerable flooding due to an unusually
high sea level and wind-driven waves
 On one of the worst days of flooding, 3 February 1998,
waters from the Bay inundated U.S. Highway 101 to a
depth of up to 1.2 m (4 ft) north of the Golden Gate Bridge
 Several factors control sea level in San Francisco Bay
– The most important are the regular astronomical tides
 Two high tides and two low tides per day
– Second factor is the slow long-term trend of rising sea level due to
global warming
 Causes expansion of sea water and melting of land-based glaciers
– El Niño is also a major factor
 The 1997-98 El Niño, one of the most intense of the 20th century,
winter winds blew strongly from the south along the California coast,
driving surface water toward the shore and raising sea level
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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
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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.
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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
planet’s direction of rotation
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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
planet’s 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
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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 semipermanent features
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Features of the Planetary-Scale Circulation
 Schematic representation of the planetary-scale
surface circulation of the atmosphere
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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
© AMS
 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 eastwest 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
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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
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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 synopticscale 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
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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
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Features of the Planetary-Scale
Circulation
 Ocean Surface Currents
– Surface ocean currents are wind-driven
 Horizontal movement of ocean surface waters to a large extent
mirrors the long-term average planetary scale atmospheric
circulation
– Gyres, large-scale circulation regimes, appear in each
of the major ocean basins and are associated with the
prevailing winds that help set up the currents
 Trades winds and the westerlies associated with the semipermanent subtropical highs drive the subtropical gyres
 The counterclockwise surface winds in the Aleutian and
Icelandic sub-polar low pressure systems drive the sub-polar
gyres
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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.
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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
© AMS
 Ocean has greater thermal inertia than the land
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Surface wind
patterns over Asia
during
A.Wet (summer)
monsoon
B.Dry (winter)
monson
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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
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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
© AMS 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 northsouth 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
© AMS
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
© AMS
 Cutoff highs and lows result
 These prevent normal west-toeast circulation, and are called
blocking systems
 Extremes in weather such as
drought or flooding rains or
excessive heat or cold can
result
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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
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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
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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).
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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
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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
© AMS
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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
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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
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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)
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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.
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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
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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 Earth’s surface
 Surges up Mississippi River valley
 Contributes to the development of nocturnal thunderstorms
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El Niño, La Niña, and the Southern
Oscillation
 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
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El Niño, La Niña, and the Southern
Oscillation
 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
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El Niño, La Niña, and the Southern
Oscillation
 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
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El Niño, La Niña, and the Southern
Oscillation
 Historical Perspective
– 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
© AMS
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El Niño, La Niña, and the Southern
Oscillation
 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
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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
©
AMS column is 90 degrees to the right of the wind.
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El Niño, La Niña, and the Southern
Oscillation
 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, coldnutrient-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
© AMS
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Average rainfall per day across the tropical Pacific from 1998-2007
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El Niño and La Niña
 Neutral Conditions in the Tropical Pacific, cont.
– Walker Circulation (large convective-type circulation)
© AMS
 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
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El Niño, La Niña, and the Southern
Oscillation
© AMS
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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
© AMS
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El Niño, La Niña, and the Southern
Oscillation
© AMS
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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
© AMS
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El Niño, La Niña, and the Southern
Oscillation
© AMS
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El Niño and La Niña
 Frequency of El Niño and La Niña
– NOAA’s Climate Prediction Center launched its new ENSO Alert
System in February 2009
 El Niño or La Niña watch: issued when conditions in the equatorial
Pacific are favorable for the development of El Niño or La Niña
conditions within the next three months
 El Niño or La Niña advisory: issued when El Niño or La Niña conditions
have developed and are expected to continue
© AMS
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El Niño, La Niña, and the Southern
Oscillation
© AMS
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El Niño, La Niña, and the Southern
Oscillation
 Historical Episodes
– In 1982-83, the weather seemed to go wild in
many parts of the world as a result of the newly
discovered phenomenon El Niño
 Spurred further research on atmosphere-ocean
circulation changes
– The 1997-98 El Niño rivaled the 1982-83 El
Niño as the most intense of the 20th century
© AMS
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Evolution of the 1997-98 El Niño
© AMS
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El Niño, La Niña, and the Southern
Oscillation
 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
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© AMS
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El Niño, La Niña, and the Southern
Oscillation
Sea level record at a location along
the equator in the eastern
tropical Pacific derived from
measurements made by the TOPEX/
Poseidon satellite
© AMS
TAO buoy
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