Chapter 11: Atmospheric/Oceanic
Circulation
Objectives:
• Driving force
• Meridional circulation cells
–Hadley, Ferrel & Polar cells
• Surface winds & sea-level pressure
• wind, precipitation and temperature
• Surface and deep oceanic circulation
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• Atmospheric circulation is the large-scale
movement of air.
• The large-scale structure of the atmospheric
circulation varies from year to year, but the basic
structure remains fairly constant.
• Individual weather systems – mid-latitude
weather may occur "randomly“. However, the
average of these systems - the climate - is quite
stable.
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Primary High-Pressure and
Low-Pressure Areas
•
•
•
•
Equatorial low-pressure trough
Subtropical high-pressure cells
Subpolar low-pressure cells
Polar high-pressure cells
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Driving force
• At higher lat., solar
energy flux spreads
over a wider area =>
less solar energy per
unit area.
• Earth emits outgoing
radiation.
•Net radiation = (incoming solar rad.)
- (outgoing rad.) .
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• Net radiation has deficit poleward of 37°, & surplus
equatorward of 37°.
• This means poles should keep cooling while tropics keep
warming. Since this is not happening, other processes
must operate to maintain net energy balance at each lat.
• Atm. & oc. circulation (climate & weather) due to unequal
lat. distr. of energy.
Annual incoming solar rad.
UNBCoutgoing terrestrial rad.
Annual
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Single Convection cell in a
non-rotating Earth
• Imagine the earth as a
non-rotating sphere with
uniform smooth surface
characteristics.
• the sun heats the
equatorial regions much
more than the polar
regions.
In response to this, two
huge convection cells
develop.
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Farrell Cell
polar Cell
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Hardly Cell
• Atmosphere is heated in the equator => Air becomes less dense and rises
=> Rising air creates low pressure at the equator.
• Air cools as it rises because of the lapse rate =>
Water vapor condenses (rains) as the air cools with increasing altitude =>
Creates high rainfall associated with the Intertropical Convergence Zone in
the tropics (ITCZ).
• As air mass cools it increases in
density and descends back to the
surface in the subtropics (30o N
and S), creating high pressure.
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Polar cell and Farrell cell
• In the pole area, the surface is
much cold, especially in winter.
This results in increased air
density near the surface => higher
pressure. The higher density and
pressure lead to divergence =>
surface air moves towards tropic.
The cold air from pole will meet the
warm air from Tropic around to
form “Pole Front Zone.
• For mass conservation, there are
aloft circulations corresponding the
surface circulations, which forms
two cells, called Pole cell and
Farrell cell.
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A idealized pattern of surface wind without
rotation of the earth
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The pattern of surface wind with the rotation of Earth
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– The three-celled model vs. reality: the bottom
line
• Hadley cells are close approximations of real
world
• Ferrel and polar cells do not approximate the
real world
• Model is unrepresentative of flow aloft
• Continents and topographic irregularities
cause model oversimplification
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A) Idealized winds generated by pressure gradient
and Coriolis Force. B) Actual wind patterns
owing to land mass distribution..
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• Sfc. winds converg. towards Eq.,
deflected by Coriolis => Easterlies
(NE & SE Trades)
• The sfc. winds converg. at the
ITCZ (Intertropical Convergence
Zone).
– Rising air => clouds.
– region called “doldrums” by
sailors.
• Rising air at ITCZ spreads
poleward, sinking at 30° (high p
belts = subtropical highs).
– region called “horse latitudes”
(horses thrown overboard or
eaten by sailors).
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• High p at poles => sfc.air flows
equatorward;
Deflected by Coriolis => Polar
Easterlies converg. to the
Subpolar Low (low p at 60°)
• Coriolis deflects air flowing from
subtropical high to subpolar low
=> Westerlies
• Polar cell 60°-90°, Ferrel cell 30°60°
– Where mild air from Ferrel
cell meets cold air from polar
cell => polar front
• Hadley & polar cells are
thermally driven, but Ferrel cell is
a thermally indirect cell.
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• Horse Latitudes
Around 30°N we see a region of subsiding (sinking)
air. Sinking air is typically dry and free of substantial
precipitation.
Many of the major desert regions of the northern
hemisphere are found near 30° latitude. E.g., Sahara,
Middle East, SW United States.
• Doldrums
Located near the equator, the doldrums are where the
trade winds meet and where the pressure gradient
decreases creating very little winds. That's why sailors
find it difficult to cross the equator and why weather
systems in the one hemisphere rarely cross into the
other hemisphere. The doldrums are also called the
intertropical convergence zone (ITCZ).
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Surface winds & sea-level pressure
(SLP)
January
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Surface winds & sea-level pressure
(SLP)
July
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• N.Hem.: Bermuda (Azores)
High, Pacific High, Icelandic
Low, Aleutian Low.
• July: Bermuda High &
Pacific High stronger &
further north. Icelandic Low
& Aleutian Low weaken &
shift northward.
ITCZ shifts northward
• Jan.: Highs over continents
in N.Hem., lows over contin.
in S. Hem. (monsoon effect).
• July: Lows over continents
in N.Hem., highs over
contin. in S. Hem.
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The Sahel
reflects seasonal
migration
of the ITCZ
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Shifts in the ITCZ affect the Sahel
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• Hadley, Ferrell, and Polar cells are major players in global
heat transport of the south-north. They are called Latitudinal
circulation, caused by latitudinal difference of incident solar
radiation.
Longitudinal circulation, on the other hand, comes about
because water has a higher specific heat capacity than land
and thereby absorbs and releases heat less readily than land
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The Upper Troposphere
• Two important points:
– Pressure decreases more rapidly in cold air
– Temperature in lower troposphere decreases poleward
• Creates upper level pressure gradients
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• 500 mb heights decrease poleward
• pressure gradient stronger during the winter
• 500 mb heights are higher in the summer than in the winter
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Westerly Winds in the Upper Atmosphere
• the existence of the upper level pressure gradient  air is being
pushed toward poles
• Coriolis effect deflects upper air  Westerlies dominate upper troposphere
• Strongest during winter
• explains why storms move eastward,
flight times
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The Polar Front and Jet Streams
•
•
•
•
•
•
Gradual change in temperature with latitude does not always occur
Steep temperature gradients exist between cold and warm air masses
polar front - marks area of contact, steep pressure gradient  polar jet stream
polar jet stream - fast stream of air in upper troposphere above the polar front
stronger in winter, affect daily weather patterns
Low latitudes  subtropical jet stream
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Rossby Waves
• Rossby waves - largest of these planetary wave patterns
• seasonal change – fewer, more well-developed waves in winter, with
stronger winds
• instrumental in meridional transport of energy and storm development
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The Walker
circulation is caused
by the pressure
gradient force that
results from a high
pressure system over
the eastern pacific
ocean, and a low
pressure system over
Indonesia. When the
Walker circulation
weakens or reverses,
an El Niño results.
The Southern Oscillation Index (SOI) is calculated from the
monthly or seasonal fluctuations in the air pressure difference
between Tahiti and Darwin.
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• The Walker circulation, which spans almost half the
circumference of Earth, pushes the Pacific Ocean’s trade
winds from east to west, generates massive rains near
Indonesia, and nourishes marine life across the
equatorial Pacific and off the South American coast.
Changes in the circulation, which varies in tandem with
El Niño and La Niña events, can have far-reaching
effects.
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Jet streams
• Jet streams: air
currents thousands of
km long, hundreds of
km wide, a few km
thick (centred near
tropopause).
• Max speed > 200
km/hr.
• Polar jet stream near
polar front, separating
cold air from mild air.
Jet stream turning south
=> cold air moves south.
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• So there is sinking air around 30 degree, which forms
divergence region on surface and convergence region
aloft. The convergence between cold air with warm air
can cause a great temperature gradient in this region,
and further causing a large gradient in pressure =>
speeding the air flow => cause the jet.
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• Subtropical jet stream at ~30°
• Jet streams meander, polar jet may merge
with subtropical jet.
• Polar jet may also branch into 2.
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2 mechanisms for jet streams
1)
Where polar cell meets
Ferrel cell, or Ferrel cell
meets Hadley, airs of
different T meet
=> large T gradient => large
p gradient
=> geostrophic winds.
mv 1r1  mv 2 r2
2) As air moves from low to
high lat., its circular orbit
shrinks. => orbiting speed incr.
(conservation of angular
momentum; e.g. spinning
skater moves arms
towards body).
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Sea breeze
• Daytime: land warms more than sea
=> rising air & low p on land. Air flows from
sea to land.
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Land breeze
• Night: Land cools more than sea.
=> Sinking air & high p over land. Air flows
from land to sea.
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Valley breeze & mountain breeze
• Daytime: At same elevation, air on mountain slope heated more
than air over valley => low p over mountain slope => air flows
upslope from valley (valley breeze).
• Night: Air on mountain slope cooled more than air over valley =>
mountain breeze.
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Chinook wind
• Chinook: warm, dry wind on eastern slope of Rockies.
• Western slope: condensation => release of latent heat.
Moisture lost from precip.
• Descending wind on eastern slope => warming from
compression.
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Monsoons
• largest synoptic scale winds on Earth
• A seasonal reversal of wind
• Asian monsoon which is characterized by dry (wet), offshore
(onshore) flow conditions during cool (warm) months
• Orographic lifting leads to high precipitation totals
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Monsoons
• Winter: continents cool more than oc.
=> sinking air & high p over continent
• Summer: continents warm more than oc.
=> rising air & low p over continent
• Most prominent with the massive Asian land mass.
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Winter
monsoon
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Summer
monsoon
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Meridional cells & precip.
• Northward shift of cells during summer, & southward shift
during winter => precip. changes.
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Oceanic Circulation
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Temperature T
• Vertical profile:
– Solar radiation absorbed within
100m of sea surface.
– Wind => surface mixed layer of 50200m, (T is nearly uniform).
– Thermocline occurs between 2001000m depth: T decr. rapidly with
depth.
– Below thermocline, T decr. very
slowly to 0-3oC at oc. bottom.
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Mixing layer
thermocline->
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• Thermocline: The thermocline is the
transition layer between the mixed layer at
the surface and the deep water layer. In
the thermocline, the temperature
decreases rapidly from the mixed layer
temperature to the much colder deep
water temperature.
• The mixed layer and the deep water layer
are relatively uniform in temperature, while
the thermocline represents the transition
zone between the two.
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Vertical temperature section in Atlantic
South
North
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Surface currents
• Gyres: Large horizontal
circulation cells.
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• The trade wind brings water flowing from the
east to west. After the water arrives at the west
boundary, the water is deflected northward. The
water then come under the influence of westerly
wind, which cause the water to flow eastward.
When the water arrives at the eastern boundary,
some of water goes to polar region, and some
flows to equator. The water that flows to the
equator come back under the influence of the
trade wind, and are blown westward again. This
forms a large circulation in subtropical region =>
subtropical gyre (clockwise in NH).
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Ocean’s role in global heat
transport
• Oc. transports almost as much heat poleward
as atm.:
Oc. dominates at low lat., atm. dominates at
mid-high lat.
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• Heat capacity: amount of energy needed to
raise temp. of a unit mass by 1°C.
• Water has a high heat capacity:
– Temp. range over land many times that over oc.,
as heat cap. of water much larger than that of
soils/rocks.
– Oc. heat capacity ~1600 times of atm.
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• Oc. has strong moderating effect on
climate, e.g. coastal regions milder
than inland.
• Large heat capacity => difficult to
change oc. => oc. has long "memory"
& major role in climate time scale,
where atm. becomes "slave" to oc.
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• Wind stress:
Often we are much more interested in the force of the
wind, or the work done by the wind. The horizontal force
of the wind on the sea surface is called the surface wind
stress. The force per unit area that wind exerts on the
surface of the ocean.
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•
•
•
Coriolis Force = Wind stress
Wind stress = tangential force on a unit area of oc.
surface
When the surface water moves, it drags along the water just
below it, making the water just below it moving.
The current has a speed of V0 to the northeast. In general,
the surface current is 45° to the right of the wind when looking
downwind in the northern hemisphere. The current is 45° to
the left of the wind in the southern hemisphere. Below the
surface, the velocity decays exponentially with depth:
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• Nansen (1890s)
observ. iceberg
moving 20-40o to
right of wind.
• Ekman (1905) sol’n.
has surface current
at 45o to right of wind
in N.Hem. (to the left
in S.Hem.) (Coriolis
effect).
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• On surface, the moving is at just 45 degree to the
right of wind; at subsurface, a thin layer below
surface, the moving is at an angle which is larger
than 45 degree to the right; With the increase of
depth, the angle become lager and lager until the
current moves just opposite to surface current at
some depth (around 100m). This is called Ekman
Spiral.
• Ekman layer: from surface to some depth where the
current moves at the direction opposite to the surface
current.
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Ekman Mass Transports
• Flow in the Ekman layer carries mass. For many
reasons we may want to know the total mass
transported in the layer, called as Ekman mass
transport. The transport is perpendicular to the wind
stress, and to the right of the wind in the northern
hemisphere.
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Application of Ekman Theory
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Upwelling & downwelling
• Wind blowing
alongshore can
generate offshore
Ekman transp.
=> upwelling
Onshore Ekman
transp. =>
downwelling
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• Along Equator, Easterlies => Ekman transport away
from Eq. => strong upwelling along Eq.
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• In N.Hem., surface current spirals to the right
with incr. depth. Observ. wind driven layer
(Ekman layer) is ~10-100m
• The depth-integrated mass tranport (Ekman
transport) is at 90o to right of wind in N.Hem.
i.e. wind balances Coriolis.
Wind
Coriolis
Ekman transport
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Geostrophic currents
 Tilt in sea level (SL) => pressure gradient =>
pressure (p) force. When p force is balanced by the
Coriolis force => geostrophic current.
SL
Coriolis
current
Low p
p force
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High p
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Gradual buildup of a geostrophic current:
current
Low p
High p
p force
Coriolis force
Coriolis force
Low p
High p
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N.Hem.: low lat. easterlies, mid lat. westerlies
=> converging Ekman transport & high sea level (SL) at
~30°N
=> geostrophic currents.

Ekman
transp.
45°N
Coriolis
force
p force
geostrosphic
current
H
30°N
15°N
High SL
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• Pressure gradient from SL tilt
disappears by ~1000m depth =>
geostrophic current only in top
1000m.
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3 forces in upper ocean:
• wind stress, pressure gradient, Coriolis
• In Ekman layer (top 100m) mainly
Coriolis balancing wind stress.
• 100-1000m: mainly Coriolis balancing
pressure gradient => geostrophic current.
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Deep Water Circulation
Thermohaline circulation
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• Originally the deep water is formed in North Atlantic, near Greenland,
iceland and Norwegian sea (NADW). The NADW sinks into bottom and
then further moves southward. The NAWD will move to Antarctic region
and merge with ABW (Antarctic bottom water), and move northward to
arrive at the North Pacific. Meanwhile, the surface current near the western
Pacific ocean moves southward in the form of gyre, and further cross
Indian ocean and back to Atlantic ocean to replace water there sinking into
bottom.
• So, the thermohaline circulation includes a deep ocean circulation from the
North Atlantic Ocean to the North Pacific to bring deep water (salty and
cold) into Pacific Ocean;
and a surface current
from the North Pacific to
North Atlantic ocean. Both
circulations act to make
the water mass
conservation.
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• The effect of Thermohaline circulation on
climate
(1) THC transports heat from the south to
North to warm the North Atlantic and
Europe.
(2) adjust the low latitude climate too by
transporting surplus heat
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Change in annual temperature 30 years after a collapse of the
thermohaline circulation
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What are the consequences of an El Nino?
* increased rainfall in the southern US and the west coast of
South America
* drought in Western Pacific and India
* collapse of fisheries off western coast of South America
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El Nino – Southern Oscillation (ENSO)
What is El Nino?
Warming of sea surface temperatures (SSTs) in the central and
eastern tropical Pacific
What is the Southern Oscillation?
Changes in the pressure pattern and trade wind strength (direction)
in the tropical Pacific
Together El Nino (EN) and the Southern Oscillation (SO) = ENSO
Act as a major disruption of the ocean-atmosphere system in the
tropical Pacific
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SSTs anomalies during an ENSO event
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NASA, 2003
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