1. El Niño, La Niña and the Southern Oscillation
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2. 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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3. The Southern Oscillation
Sir Gilbert Walker
( )
Tahiti
Darwin
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4. The Oceanic Connection
Jacob Bjerknes ( )
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5. 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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6. Equatorial Upwelling
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7. 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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8. Observed SST Distribution
Winds have a major influence on tropical SST pattern.
Equatorial Upwelling
Coastal Upwelling
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9. 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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10. 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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15. La Niña conditions: Strong cold tongue
El Niño conditions: Cold tongue absent
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16. Recent History of El Niño and La Niña
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17. Recent History of El Niño and La Niña
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18. El Niño is Quasiperiodic
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19. Current SST Anomalies
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20. El Niño’s Life Cycle
Initiation
Peak
Decay
Source: IRI

21. Temporal Evolution of El Niño/La Niña
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22. ENSO Events Can Evolve Differently
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23. Thermocline Temperatures and Anomalies
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24. Thermocline Fluctuations
Cross-section of temperatures and currents along Equator
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25. Subsurface Structure
Source: NASA Goddard Space Flight Center

26. The delayed oscillator
The 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.

27. Equatorial Kelvin & Rossby Waves
Surface currents (l) and thermocline displacements (r) for a Gaussian perturbation
2-layer oceanic SWE model
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)

28. Propagating equatorial waves

29. ENSO and Global Climate
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30. ENSO Teleconnections ENSO 500mb Geopotential Pattern
PNA 300mb Geopotential Pattern

31. ENSO and Global Climate
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32. ENSO and U.S. Climate (Winter Season)
El Niño
La Niña
Temperature
Precipitation
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33. 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.
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