El Ni ñ o, La Ni ñ a and the Southern Oscillation

1. El Niño, La Niña and the Southern Oscillation

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.”

3. The Southern Oscillation Sir Gilbert Walker (1868-1958) Tahiti Darwin

4. The Oceanic Connection Jacob Bjerknes(1897-1975)

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)

6. Equatorial Upwelling

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

8. Observed SST Distribution Winds have amajor influenceon tropical SSTpattern. Equatorial Upwelling Coastal Upwelling

9. SST and Atmospheric Circulation Sinking air;very littleprecipitation Rising air;clouds andprecipitation Walker Circulation Warm Cold Eastern Equatorial Pacific Western Equatorial Pacific

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

15. La Niña conditions:Strong cold tongue El Niño conditions:Cold tongue absent

16. Recent History of El Niño and La Niña

17. Recent History of El Niño and La Niña

18. El Niño is Quasiperiodic

19. Current SST Anomalies

20. Initiation Peak Decay El Niño’s Life Cycle Source: IRI

21. Temporal Evolutionof El Niño/La Niña

22. ENSO Events Can Evolve Differently Spring 2013

23. ThermoclineTemperaturesandAnomalies

24. Thermocline Fluctuations Cross-section of temperatures and currents along Equator

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

29. ENSO and Global Climate

30. ENSO Teleconnections ENSO 500mb Geopotential Pattern PNA 300mb Geopotential Pattern

31. ENSO and Global Climate

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

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. • http://iri.columbia.edu/climate/ENSO/