The El Nino-Southern Oscillation

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The El Nino-Southern Oscillation
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Outline

• 1. General Circulation of the Atmosphere
• 2. The Walker Circulation
• 3. The Coriolis Force
• 4. Air-sea interactions
• 5. Thermocline and ocean wave dynamics
• 6. Societal and ecological impacts

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Heat balance of the atmosphere

• More solar energy is absorbed In the tropics
• per second
• than is emitted back to
• space per second.
• However
• the tropics are not warming up!
• Energy must be transported away from the tropics
• What transports away the thermal energy (heat)?

Energy per time per area
1 W unit for power 1 Nm/s 1 J/s 200 W
1 human climbing stairs Power is the rate at
which energy is used
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Heat transport is accomplished by atmospheric and
oceanic motion

• To get rid of the excess heat in the tropics,
  something must carry the heat
• air, water vapor, water can carry heat
• 1 PW
• 1,000,000,000,000,000 W
• 10,000,000,000,000
• Light bulbs a 100W

PW
atmosphere
ocean
Atmosphere and ocean have to be in motion. A
dynamic balance.
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The meridional atmospheric circulation trade
winds, jet streams, westerlies, Hadley Circulation
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The Hadley Circulation
Warm moist air rises in the tropical
Intertropical Convergence Zone (ITCZ) while it
ascends it cools and precipitation forms cold
and dry air flows poleward and sinks in the
subtropics The supply of air in the tropics is
maintained by the trade winds This meridional
circulation cell, which exists in the northern
and southern hemisphere is called Hadley
Cell after George Hadley (1685-1768)
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Surface Atmospheric Winds and rainfall
Annual Means (Jan-Dec)
Aleutian Low
Westerlies
Trade wind
Trade wind
Westerlies
Trade winds converge in the Intertropical
Convergence Zone (ITCZ)
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Sea Surface Temperature and rainfall What drives
the Hadley circulation?
Warmest waters gt largest convection gt largest
rainfall Rainfall means condensated water vapor
gt condensation energy warms air gt rising motion
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The zonal tropical Walker Circulation
Vertical velocity at equator
Height hPa
West Pacific Warm ocean Rising air Lots of
precipitation
East Pacific Cold ocean descending air dry
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The zonal tropical Walker Circulation
Westward (easterly) surface trade winds on the
equator Low sea level pressure in western
Pacific, High sea level pressure in eastern
Pacific
Walker
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Variations of the Walker Circulation

• How does a change in western/eastern Pacific sea
  surface temperature affect the strength of the
  Walker circulation and the strength of the
  equatorial trade winds?
• How does a change in sea level pressure (SLP) in
  the western Pacific affect the sea level pressure
  in the eastern Pacific?
• Walker correlated the SLP variations in the
  western Pacific with SLP variations elsewhere
• The result led to an astonishing discovery.

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A tropical pressure seesaw
Surface Air Pressure Pattern
Walker
There is an east-west pressure seesaw - called
the Southern Oscillation. If SLP is anomalously
high (low) in the Western tropical Pacific, it
will be anomalously low (high) in the Eastern
Tropical Pacific, this varying pressure
difference drives changes in the equatorial trade
winds
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The tropical Pacific pressure seesaw
Sea level pressure in west Pacific and eastern
tropical Pacific are anti-correlated The strength
of this pressure dipole characterizes the
strength of the equatorial trade winds
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The Southern Oscillation index (SOI)
Walker could not explain the 3-4 yr periodicity
of the SOI, nor could he explain the spatial
pattern. Hence the SOI met with skepticism that
it was a real phenomenon, rather than a
statistical artifact.
Darwin
Tahiti
The strength of the equatorial trade winds is
proportional to the SOI Trade wind changes want
to move ocean waters. But how?
Southern Oscillation Index (SOI) Tahiti - Darwin
surface air pressure
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The Coriolis Force an apparent force
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The Coriolis Force a real-life experience
Counter-clockwise
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The Coriolis Force

Freely moving objects on the surface of the Earth
experience the Coriolis force, And appear to
veer to the right in the Northern Hemisphere and
to the left in the Southern Hemisphere. Rather
than flowing directly from high pressure to low
pressure their path is bent for the rotating
observer. The Coriolis Force can be computed
from Fc 2 x mass x velocity x 2 x p / (rotation
period) sin (latitude) Coriolis force is zero
on equator or for a non-moving object (for
rotating observer)
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Equatorial Easterlies drive ocean upwelling an
application of Coriolis
Equatorial trade winds proportional to SOI
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Ekman divergence
Depth
Meridional Velocity
Cold water is upwelled in eastern equatorial
Pacific because of easterly winds Easterly winds
are there because of zonal temperature difference
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Tropical air-sea coupling

• Higher SST in east Pacific
• weakening of pressure gradient
• Weakening of Walker Circulation
• Weakening of trade winds
• Weakening of upwelling of cold waters
• Higher SST in east Pacific

• Lower SST in east Pacific
• strengthening of pressure gradient
• Strengthening of Walker Circulation
• Strengthening of trade winds
• Strengething of upwelling of cold waters
• Lower SST in east Pacific

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ENSO merging the Southern Oscillation and
eastern equatorial Pacific ocean warming

• El Niño arises from coupled interactions between
  the ocean and the atmosphere
• Involves the entire tropical Pacific, not just
  coastal SA
• Causes shifts in tropical rainfall patterns that
  can affect mid-latitude climate through
  atmospheric teleconnections
• Might be predictable

Strong pressure difference strong trade winds
Weak pressure difference weak trade winds
El Niño
La Niña
J. Bjerknes, 1966, 1969, 1972
ENSO El Nino-Southern Oscillation
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Definition of El Nino
El Niño A phenomenon in the equatorial Pacific
Ocean characterized by a positive sea surface
temperature departure from normal (for the
1971-2000 base period) in the Niño 3.4 region
greater than or equal in magnitude to 0.5 degrees
C (0.9 degrees Fahrenheit), averaged over three
consecutive months. La Niña A phenomenon in the
equatorial Pacific Ocean characterized by a
negative sea surface temperature departure from
normal (for the 1971-2000 base period) in the
Niño 3.4 region greater than or equal in
magnitude to 0.5 degrees C (0.9 degrees
Fahrenheit), averaged over three consecutive
months.
Nino 3.4 box
El Nino is the warm phase, La Nina the cold phase
of ENSO
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What is happening below the surface?
Sea level 30cm higher
Thermocline shallow In EEP during
normal conditions
Sea Surface Height
Temperature front In upper ocean thermocline
Temperature
Temperature section on equator
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What is happening below the surface?
Weakening of equatorial trade winds
Thermocline deep In EEP during El NIno
Temperature
Temperature section on equator
Temperature section on equator
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