Transcript El Niño, La Niña and the Southern Oscillation

El Niño, La Niña and the
Southern Oscillation
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Some History
• In the early 20th Century, Sir Gilbert
Walker discovered a recurrent pattern in
sea level pressure data from the Pacific
region.
• He called this pattern “The Southern
Oscillation.”
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The Southern Oscillation
Sir Gilbert Walker
(1868-1958)
Darwin
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Tahiti
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The Oceanic Connection
Jacob Bjerknes
(1897-1975)
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Ekman Transport
• Balance between
surface wind stress
and Coriolis force
• In the Northern
Hemisphere, the
Ekman transport is
directed to the right
of the flow. (To the
left in the Southern
Hemisphere)
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Equatorial Upwelling
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Coastal Upwelling
• Motion of surface
waters away from
coast requires
upwelling of water
from below to
satisfy continuity
of mass.
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Andes Mts.
S. Pacific Ocean
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Observed SST Distribution
Winds have a
major influence
on tropical SST
pattern.
Equatorial Upwelling
Coastal Upwelling
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SST and Atmospheric Circulation
Rising air;
clouds and
precipitation
Sinking air;
very little
precipitation
Walker Circulation
Warm
Cold
Western Equatorial Pacific
Eastern Equatorial Pacific
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In The Tropics…
• Sea surface temperatures are strongly
influenced by surface winds.
• Atmospheric circulation is strongly
influenced by the sea surface
temperatures.
• Therefore…
Strong air-sea interactions are possible
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La Niña conditions:
Strong cold tongue
El Niño conditions:
Cold tongue absent
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Recent History of El Niño and La Niña
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Recent History of El Niño and La Niña
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El Niño is Quasiperiodic
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Current SST Anomalies
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El Niño’s Life Cycle
Initiation
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Peak
Source: IRI
Decay
Temporal Evolution
of El Niño/La Niña
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ENSO Events Can Evolve Differently
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Thermocline
Temperatures
and
Anomalies
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Thermocline Fluctuations
Cross-section of temperatures and currents along Equator
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Subsurface Structure
Source: NASA Goddard Space Flight Center
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The delayed oscillator
The leading theoretical model is the delayed oscillator [see
Battisti and Hirst, 1989]:
¶Ts(t)
= bTs(t) - cTs(t - t )
¶t
Here, Ts is the temperature in the East Pacific, b and c are positive
constants, and  is a time-lag determined by equatorial oceanic
adjustment.
• The first term on the RHS can be thought of a representing a positive
feedback associated with the atmosphere, e.g., the large-scale DarwinTahiti pressure difference (the SOI).
• The second term represents a negative feedback associated with
thermocline adjustment via equatorial waves.
• The time delay is the time required for Rossby waves to propagate
westward, reflect at the boundary, and return to the region of origin.
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Equatorial Kelvin & Rossby Waves
2-layer oceanic SWE model
Surface currents (l) and thermocline
displacements (r) for a Gaussian
perturbation
c g [Kelvin] = g¢H ; g¢ = g(r2 / r1 -1)
Kelvin wave: Non-dispersive,
eastward propagating (~2 m/s for H
= 150 m)
c g [Rossby] = g¢H /(2l +1) ; l =1,2,...
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Rossby waves: Dispersive,
westward propagating (fastest is 1/3
of Kelvin wave group velocity)
Propagating equatorial waves
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http://iri.columbia.edu/climate/ENSO/enso.html
ENSO and Global Climate
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ENSO Teleconnections
ENSO 500mb Geopotential Pattern
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PNA 300mb Geopotential Pattern
ENSO and Global Climate
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ENSO and U.S. Climate
(Winter Season)
El Niño
La Niña
Temperature
Precipitation
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More Information
• The International Research Institute for
Climate Prediction at Columbia
University’s Lamont-Doherty Earth
Observatory has an excellent web site with
information on El Niño.
• http://iri.columbia.edu/climate/ENSO/
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