Chapter 8: Atmospheric Circulation and Pressure Distributions

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Chapter 8: Atmospheric Circulation and
Pressure Distributions
JS
Single-Cell Model:
Explains Why There are Tropical Easterlies
JP
With Earth Rotation
Without Earth Rotation
Hadley Cell
Ferrel Cell
Polar Cell
(driven by eddies)
L
H
L
H
Coriolis Force
‰ General Circulation in the Atmosphere
‰ General Circulation in Oceans
‰ Air-Sea Interaction: El Nino
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Breakdown of the Single Cell Î Three-Cell Model
‰ Absolute angular momentum at Equator = Absolute angular momentum at 60°N
(Figures from Understanding Weather & Climate and The Earth System)
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Atmospheric Circulation: Zonal-mean Views
Single-Cell Model
Three-Cell Model
‰ The observed zonal velocity at the equatoru is ueq = -5 m/sec.
Therefore, the total velocity at the equator is U=rotational velocity (U0 + uEq)
‰ The zonal wind velocity at 60°N (u60N) can be determined by the following:
(U0 + uEq) * a * Cos(0°) = (U60N + u60N) * a * Cos(60°)
(Ω*a*Cos0° - 5) * a * Cos0° = (Ω*a*Cos60° + u60N) * a * Cos(60°)
u60N = 687 m/sec !!!!
(Figures from Understanding Weather & Climate and The Earth System)
This high wind speed is not
observed!
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1
Properties of the Three Cells
The Three Cells
thermally indirect circulation
thermally direct circulation
ITCZ
JS
Hadley Cell
JP
Ferrel Cell
Polar Cell
(driven by eddies)
L
Equator
(warmer)
E
H
30°
(warm)
W
L
E
H
60°
(cold)
Pole
(colder)
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Precipitation Climatology
Subtropical
High
(Figures from Understanding Weather & Climate and The Earth System)
midlatitude
Weather system
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Thermally Direct/Indirect Cells
(from IRI)
‰ Thermally Direct Cells (Hadley and Polar Cells)
Both cells have their rising branches over warm
temperature zones and sinking braches over the cold
temperature zone. Both cells directly convert thermal
energy to kinetic energy.
‰ Thermally Indirect Cell (Ferrel Cell)
This cell rises over cold temperature zone and sinks over
warm temperature zone. The cell is not driven by thermal
forcing but driven by eddy (weather systems) forcing.
ITCZ
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2
Is the Three-Cell Model Realistic?
Upper Tropospheric Circulation
‰ Yes and No!
(Due to sea-land contrast and topography)
‰ Only the Hadley Cell can be
identified in the lower latitude
part of the circulation.
‰ Circulation in most other
latitudes are dominated by
westerlies with wave patterns.
Yes: the three-cell model explains reasonably well the
surface wind distribution in the atmosphere.
‰ Dominated by large-scale
waver patterns (wave number 3
in the Northern hemisphere).
No: the three-cell model can not explain the circulation
pattern in the upper troposphere. (planetary wave motions
are important here.)
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Bottom Line
(from Weather & Climate)
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Semi-Permanent Pressure Cells
• Pressure and winds associated with Hadley cells are close
approximations of real world conditions
• Ferrel and Polar cells do not approximate the real world as well
• Surface winds poleward of about 30o do not show the persistence of
the trade winds, however, long-term averages do show a prevalence
indicative of the westerlies and polar easterlies
• For upper air motions, the three-cell model is unrepresentative
• The Ferrel cell implies easterlies in the upper atmosphere where
westerlies dominate
• Overturning implied by the model is false
‰ The Aleutian, Icelandic, and Tibetan lows
– The oceanic (continental) lows achieve maximum strength during
winter (summer) months
– The summertime Tibetan low is important to the east-Asia
monsoon
‰ Siberian, Hawaiian, and Bermuda-Azores highs
– The oceanic (continental) highs achieve maximum strength during
summer (winter) months
• The model does give a good, simplistic approximation of an earth
system devoid of continents and topographic irregularities
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3
January
July
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Global Distribution of Deserts
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Sinking Branches and Deserts
(from Weather & Climate)
(from Global Physical Climatology)
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4
Thermal Wind Relation
Thermal Wind Equation
∂U/∂z ∝ ∂T/∂y
‰ The vertical shear of zonal wind is related
to the latitudinal gradient of temperature.
‰ Jet streams usually are formed above
baroclinic zone (such as the polar front).
(from Weather & Climate)
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Subtropical and Polar Jet Streams
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Jet Streams Near the Western US
Pineapple Express
‰ Subtropical
Jet
Located at the higher-latitude end of the
Hadley Cell. The jet obtain its
maximum wind speed (westerly) due the
conservation of angular momentum.
‰ Polar
Jet
‰ Both the polar and subtropical jet
streams can affect weather and climate
in the western US (such as California).
Located at the thermal boundary
between the tropical warm air and the
polar cold air. The jet obtain its
maximum wind speed (westerly) due the
latitudinal thermal gradient (thermal
wind relation).
‰ El Nino can affect western US
climate by changing the locations and
strengths of these two jet streams.
(from Riehl (1962), Palmen and Newton (1969))
(from Atmospheric Circulation Systems)
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Scales of Motions in the Atmosphere
Cold and Warm Fronts
co
l
d
fr
on
t
Mid-Latitude Cyclone
wa
rm
fro
n
t
(From Weather & Climate)
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They Are the Same Things…
Tropical Hurricane
‰ The hurricane is
characterized by a
strong thermally direct
circulation with the
rising of warm air near
the center of the storm
and the sinking of
cooler air outside.
(from Weather & Climate)
‰ Hurricanes: extreme tropical storms over Atlantic and eastern
Pacific Oceans.
‰ Typhoons: extreme tropical storms over western Pacific Ocean.
(from Understanding Weather & Climate)
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‰ Cyclones: extreme tropical storms over Indian Ocean and
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Australia.
Prof. Jin-Yi Yu
6
Monsoon: Another Sea/Land-Related
Circulation of the Atmosphere
How Many Monsoons Worldwide?
North America Monsoon
Winter
Asian Monsoon
‰ Monsoon is a climate feature that is characterized by
the seasonal reversal in surface winds.
‰ The very different heat capacity of land and ocean
surface is the key mechanism that produces monsoons.
Summer
‰ During summer seasons, land surface heats up faster
than the ocean. Low pressure center is established over
land while high pressure center is established over
oceans. Winds blow from ocean to land and bring large
amounts of water vapor to produce heavy precipitation
over land: A rainy season.
Australian
Monsoon
‰ During winters, land surface cools down fast and sets
up a high pressure center. Winds blow from land to
ocean: a dry season.
South America Monsoon
(figures from Weather & Climate)
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(figure from Weather & Climate)
East Africa Monsoon
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Valley and Mountain Breeze
Sea/Land Breeze
‰ Sea/land breeze is also produced by the different heat capacity of
land and ocean surface, similar to the monsoon phenomenon.
‰ However, sea/land breeze has much shorter timescale (day and
night) and space scale (a costal phenomenon) than monsoon (a
seasonal and continental-scale phenomenon).
(figure from The Earth System)
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8
Basic Ocean Current Systems
Basic Ocean Structures
Warm up by sunlight!
Upper Ocean
surface
circulation
‰ Upper Ocean (~100 m)
Shallow, warm upper layer where light is
abundant and where most marine life can be found.
‰ Deep Ocean
Deep Ocean
Cold, dark, deep ocean where plenty supplies of
nutrients and carbon exist.
deep ocean circulation
No sunlight!
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Six Great Current Circuits in the World Ocean
‰ 5 of them are geostrophic gyres:
North Pacific Gyre
South Pacific Gyre
North Atlantic Gyre
South Atlantic Gyre
Indian Ocean Gyre
‰ The 6th and the largest current:
Antarctic Circumpolr Current
(also called West Wind Drift)
(Figure from Oceanography by Tom Garrison)
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(from “Is The Temperature Rising?”)
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Characteristics of the Gyres
(Figure from Oceanography by Tom Garrison)
‰ Currents are in geostropic balance
‰ Each gyre includes 4 current components:
two boundary currents: western and eastern
two transverse currents: easteward and westward
Western boundary current (jet stream of ocean)
the fast, deep, and narrow current moves warm
water polarward (transport ~50 Sv or greater)
Eastern boundary current
the slow, shallow, and broad current moves cold
water equatorward (transport ~ 10-15 Sv)
Trade wind-driven current
the moderately shallow and broad westward
current (transport ~ 30 Sv)
Westerly-driven current
the wider and slower (than the trade wind-driven
current) eastward current
Volume transport unit:
1 sv = 1 Sverdrup = 1 million m3/sec
(the Amazon river has a transport of ~0.17 Sv)
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Major Current Names
‰ Western Boundary Current
Step 1: Surface Winds
‰ Trade Wind-Driven Current
Gulf Stream (in the North Atlantic)
North Equatorial Current
Kuroshio Current (in the North Pacific)
Brazil Current (in the South Atlantic)
Eastern Australian Current (in the South Pacific)
Agulhas Current (in the Indian Ocean)
South Equatorial Current
‰ Eastern Boundary Current
Canary Current (in the North Atlantic)
California Current (in the North Pacific)
Benguela Current (in the South Atlantic)
Peru Current (in the South Pacific)
Western Australian Current (in the Indian Ocean)
‰ Westerly-Driven Current
North Atlantic Current (in the North Atlantic)
North Pacific Current (in the North Pacific)
(Figure from Oceanography by Tom Garrison)
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Winds and Surface Currents
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Step 2: Ekman Layer
(frictional force + Coriolis Force)
Polar Cell
Ferrel Cell
Hadley Cell
(Figure from The Earth System)
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(Figure from Oceanography by Tom Garrison)
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10
Ekman Spiral – A Result of Coriolis Force
Ekman Transport
(Figure from The Earth System)
(Figure from The Earth System)
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Step 3: Geostrophic Current
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Thermohaline Conveyor Belt
(Pressure Gradient Force + Corioils Foce)
‰ Typical speed for deep ocean current:
0.03-0.06 km/hour.
NASA-TOPEX
Observations of
Sea-Level Hight
‰ Antarctic Bottom Water takes some 2501000 years to travel to North Atlantic and
Pacific.
(Figure from Oceanography by Tom Garrison)
(from Oceanography by Tom Garrison)
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11
Thermohaline Circulation
Global Warming and Thermohaline Circulation
‰ If the warming is slow
‰ Thermo Î temperature
‰ Haline Î salinity
The salinity is high enough to still produce a thermohaline
circulation
ÎThe circulation will transfer the heat to deep ocean
ÎThe warming in the atmosphere will be deferred.
Density-Driven Circulation
Cold and salty waters go down
Warm and fresh waters go up
‰ If the warming is fast
Surface ocean becomes so warm (low water density)
ÎNo more thermohalione circulation
ÎThe rate of global warming in the atmosphere will increase.
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Mid-Deglacial Cooling: The Younger Dryas
‰ The mid-deglacial pause in
ice melting was accompanied
by a brief climate osscilation
in records near the subpolar
North Atlantic Ocean.
‰ Temperature in this region
has warmed part of the way
toward interglacial levels, but
this reversal brought back
almost full glacial cold.
‰ Because an Arctic plant
called “Dryas” arrived during
this episode, this middeglacial cooling is called
“the Younger Dryas” event.
(from Earth’s Climate: Past and Future)
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Interactions Within Climate System
‰ This hypothesis argues that millennial
oscillations were produced by the internal
interactions among various components of
the climate system.
‰ One most likely internal interaction is the
one associated with the deep-water
formation in the North Atlantic.
‰ Millennial oscillations can be produced from
changes in northward flow of warm, salty
surface water along the conveyor belt.
‰ Stronger conveyor flow releases heat that
melts ice and lowers the salinity of the North
Atlantic, eventually slowing or stopping the
formation of deep water.
‰ Weaker flow then causes salinity to rise,
completing the cycle.
(from Earth’s Climate: Past and Future)
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Precipitation Climatology
East-West Circulation
(from IRI)
(from Flohn (1971))
‰ The east-west circulation in the atmosphere is related to
the sea/land distribution on the Earth.
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Walker Circulation and Ocean Temperature
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Walker Circulation and Ocean
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El Nino
03/1983
La Nina
09/1955
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El Nino and Southern Oscillation
‰ Jacob Bjerknes was the first one to
recognizes that El Nino is not just an
oceanic phenomenon (in his 1969
paper).
‰ In stead, he hypothesized that the
warm waters of El Nino and the
pressure seasaw of Walker’s Southern
Oscillation are part and parcel of the
same phenomenon: the ENSO.
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‰ Bjerknes’s hypothesis of coupled
atmosphere-ocean instability laid the
foundation for ENSO research.
Jacob Bjerknes
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Polar Front Theory
Coupled Atmosphere-Ocean System
‰ Bjerknes, the founder of the
Normal Condition
El Nino Condition
Bergen school of meteorology,
developed polar front theory
during WWI to describe the
formation, growth, and
dissipation of mid-latitude
cyclones.
(from NOAA)
Vilhelm Bjerknes (1862-1951)
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a birds-eye view of 2 of the largest
El Niño events of last century:
15
Delayed Oscillator: Wind Forcing
Atmospheric Wind Forcing
(Figures from IRI)
Oceanic Wave Response
Rossby Wave
Kevin Wave
Wave Propagation and Reflection
‰ It takes Kevin wave
(phase speed = 2.9 m/s)
about 70 days to cross the
Pacific basin (17,760km).
‰ The delayed oscillator
suggested that oceanic Rossby
and Kevin waves forced by
atmospheric wind stress in the
central Pacific provide the
phase-transition mechanism
(I.e. memory) for the ENSO
cycle.
‰ It takes Rossby wave
about 200 days (phase
speed = 0.93 m/s) to cross
the Pacific basin.
‰ The propagation and reflection
of waves, together with local
air-sea coupling, determine the
period of the cycle.
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Why Only Pacific Has ENSO?
(Figures from IRI)
North Atlantic Oscillation
‰ The NAO is the dominant
mode of winter climate
variability in the North
Atlantic region ranging from
central North America to
Europe and much into
Northern Asia.
‰ Based on the delayed oscillator theory of ENSO, the ocean
basin has to be big enough to produce the “delayed” from
ocean wave propagation and reflection.
‰ It can be shown that only the Pacific Ocean is “big” (wide)
enough to produce such delayed for the ENSO cycle.
‰ The NAO is a large scale
seesaw in atmospheric mass
between the subtropical high
and the polar low.
‰ It is generally believed that the Atlantic Ocean may
produce ENSO-like oscillation if external forcing are
applied to the Atlantic Ocean.
‰ The Indian Ocean is considered too small to produce
ENSO.
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(from http://www.ldeo.columbia.edu/res/pi/NAO/)
‰ The corresponding index
varies from year to year, but
also exhibits a tendency to
remain in one phase for
intervals lasting several
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Positive and Negative Phases of NAO
Positive Phase
Positive Phase
Negative Phase
Negative Phase
‰ A stronger and more northward
storm track.
‰ A weaker and more zonal storm
track.
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Positive NAO Index
• Stronger subtropical high and a deeper than normal
Icelandic low.
• More and stronger winter storms crossing the Atlantic
Ocean on a more northerly track.
• Warm and wet winters in Europe and in cold and dry
winters in northern Canada and Greenland
• The eastern US experiences mild and wet winter
conditions
Negative NAO Index
• Weak subtropical high and weak Icelandic low.
• Fewer and weaker winter storms crossing on a more
west-east zonal pathway.
• Moist air into the Mediterranean and cold air to
northern Europe
• US east coast experiences more cold air outbreaks
and hence snowy weather conditions.
• Greenland, however, will have milder winter
temperatures
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North Atlantic Oscillation
= Arctic Oscillation
= Annular Mode
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17

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