Mechanisms of Past Climate Change (16:375:553) Spring 2013 El Niño, La Niña and the Southern...

Mechanisms of Past Climate Change (16:375:553)

Spring 2013

El Niño, La Niña and the Southern Oscillation


Spring 2013

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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El Niño is Quasiperiodic


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Current SST Anomalies


El Niño’s Life Cycle

Initiation Peak Decay

Source: IRI


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Temporal Evolutionof El Niño/La Niña


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


Subsurface Structure

Source: NASA Goddard Space Flight Center


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.


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)


Propagating equatorial waves

http://iri.columbia.edu/climate/ENSO/enso.html


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ENSO and Global Climate


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/

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