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Mechanisms of Past Climate Change (16:375:553) Spring 2013 El Niño, La Niña and the Southern Oscillation
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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)
DarwinTahiti
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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.
Andes Mts.
S. Pacific Ocean
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Observed SST Distribution
Equatorial Upwelling
Coastal Upwelling
Winds have amajor influenceon tropical SSTpattern.
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SST and Atmospheric Circulation
Warm Cold
Western Equatorial Pacific Eastern Equatorial Pacific
Rising air;clouds andprecipitation
Sinking air;very littleprecipitation
Walker Circulation
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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 Peak Decay
Source: IRI
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Temporal Evolutionof El Niño/La Niña
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ENSO Events Can Evolve Differently
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ThermoclineTemperatures
andAnomalies
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Thermocline Fluctuations
Cross-section of temperatures and currents along Equator
Mechanisms of Past Climate Change (16:375:553)
Subsurface Structure
Source: NASA Goddard Space Flight Center
Mechanisms of Past Climate Change (16:375:553)
The delayed oscillatorThe leading theoretical model is the delayed oscillator [see Battisti and Hirst, 1989]:
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 Darwin-Tahiti 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.
Mechanisms of Past Climate Change (16:375:553)
Equatorial Kelvin & Rossby Waves2-layer oceanic SWE model
Surface currents (l) and thermocline displacements (r) for a Gaussian perturbation
Kelvin wave: Non-dispersive, eastward propagating (~2 m/s for H = 150 m)
Rossby waves: Dispersive, westward propagating (fastest is 1/3 of Kelvin wave group velocity)
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Propagating equatorial waves
http://iri.columbia.edu/climate/ENSO/enso.html
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ENSO and Global Climate
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ENSO TeleconnectionsENSO 500mb Geopotential Pattern PNA 300mb Geopotential Pattern
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ENSO and Global Climate
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ENSO and U.S. Climate(Winter Season)
Temperature
Precipitation
El Niño La Niña
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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/
