1. El Niño - Southern Oscillation
Time series of SSTs in the East Pacific (region Nino3.4)
ENSO dominant mode of global climate variability
on interannual time scales

2. Mean State of the tropical Pacific coupled Ocean-Atmosphere System
Warmest SSTs in west, “cold tongue” in east
Precipitation associated with warmest SSTs
Easterly trade winds advect equatorial surface waters westward

3. The tropical Pacific thermocline
Below surface conditions
Surface conditions

4. The Walker Circulation
Mean ascent, and low surface pressure, over warmest SST associated with deep convection
Subsidence, and high surface pressure, in non-convecting regions
Equatorial trades blow from high to low pressure (balanced by friction since Coriolis force =>0)
Low slp
High slp

5. El Nino During El Nino trade winds slacken
E-W tilt of thermocline & upwelling of cold water are reduced.
SST rises in central/eastern equatorial Pacific
Changes Walker Circulation

6. SST anomalies during El Nino
Dec 1982
Sept 1987

7. The 1997/98 El Nino
Jan 1997
Colors indicate absolute temperatures from the warmest in red
(about 28-30deg C) to the coldest in blue (about 12-18deg C)

8. The 1997/98 El Nino
Jun 1997
Nov 1997

9. The 1997/98 El Nino
Nov 1997

10. The 1997/98 El Nino
Mar 1998

11. The 1997/98 El Nino
Jan 1997
Mar 1998
Colors indicate temperature anomalies relative to the long-term mean from the warmest in red to coldest in blue.

12. The 1997/98 El Nino
Jun 1997

13. The 1997/98 El Nino
Nov 1997

14. The 1997/98 El Nino
Mar 1998

15. What about La Nina?
In La Nina conditions SST in the central and eastern equatorial Pacific is unusually cold & easterly trade winds are unusually strong
Dec 1982
Nov 1988
La Nina conditions sometimes occur in the year following an El Nino event (e.g followed 1987 El Nino)

16. Walker Circulation
British mathematician, director general of observations for India (formed after monsoon failure of worst famine in Indian history)
Arrived in 1904, shortly after huge famine caused by drought
Goal to predict Indian Monsoon
Found that many global climate variations, including Monsoon rains in India, were correlated with the Southern Oscillation
Fig. 9.

17. The Southern Oscillation Index
Darwin
Tahiti
Mean pressure is lower at Darwin than Tahiti
The term “Southern Oscillation” was also coined by Gilbert Walker
The SOI measures the strength of the Pacific Walker circulation

18. Fig. 10

19. ENSO
Bjerknes recognised that the “El Nino” warming of the ocean was related to variations in the SOI.
During El Nino:
SOI is low
Trades are weak
Precipitation is enhanced over central equatorial Pacific (indicated by low OLR)

20. The “Nino3” SST index
Note that El Nino events do not occur regularly

21. Why is El Nino important?
Major climate and economic impacts on countries around the tropical Pacific, and further afield.
Droughts in some regions, floods in others
Collapse of coastal fishery in Peru (largest average annual catch of marine fish in the world)

22. Observing El Nino

27. TAO Data TAO on Youtube: https://www.youtube.com/watch?v=nzBAWirHMvA
Live TAO data:

28. Satellite Observations of Sea Surface Height

29. 3-D animation of the tropical Pacific as it cycles through El Nino then La Nina. The surface shown is sea level (cm) and the surface is colored according to the SST anomalies associated with each event.

30. Figure from S.-P. Xie (U Hawaii)

31. Chronology of Events in the History of Understanding El Niño and La Niña
late 1800s Fishermen coin the name El Niño to refer to the periodic warm waters that appear off the coasts of Peru and Ecuador around Christmas.
1928 Gilbert Walker describes the Southern Oscillation, the seesaw pattern of atmospheric pressure readings on the eastern and western sides of the Pacific Ocean.

32. 1957 Large El Niño occurs and is tracked by scientists participating in the International Geophysical Year. Results reveal that El Niño affects not just the coasts of Peru and Ecuador but the entire Pacific Ocean.
1969 Jacob Bjerknes, of the University of California, Los Angeles, publishes a seminal paper that links the Southern Oscillation to El Niño.

33. 1975 Klaus Wyrtki, of the University of Hawaii, tracks sea levels across the Pacific and establishes that an eastward flow of warm surface waters from the western Pacific causes sea surface temperatures to rise in the eastern Pacific.
1976 Researchers use an idealized computer model of the ocean to demonstrate that winds over the far western equatorial Pacific can cause sea surface temperature changes off Peru.

34. 1982 A severe El Niño develops in an unexpected manner, but its evolution is recorded in detail with newly developed ocean buoys.
1985 Several nations launch the Tropical Ocean-Global Atmosphere (TOGA) program, a 10-year study of tropical oceans and the global atmosphere.
1986 Researchers design the first coupled model of ocean and atmosphere that accurately predicts an El Niño event in 1986.

35. 1988 Researchers explain how the "memory" of the ocean--the lag between a change in the winds and the response of the ocean--influences terminations of El Niño and the onset of La Niña.
The array of instruments monitoring the Pacific, plus coupled ocean-atmosphere models, enable scientists to warn the public of an impending El Niño event.