1. El Niño- Southern Oscillation
ENSO
El Niño- Southern Oscillation

2. El Niño- Southern Oscillation (ENSO)
Interactions, inter-relationships between:
Atmospheric circulation
Ocean circulation
Climate
El Niño- oceanic component
Southern Oscillation- atmospheric component

3. ENSO El Niño Named for anomalous warm current off Peru
Tends to occur around Christmas time
Intensity and duration - variable

4. ENSO Southern Oscillation
Oscillation is the distribution and intensity of high and low pressure systems across the equatorial Pacific
Affects wind patterns
Affects surface ocean circulation patterns
Affects climate

5. Western Pacific Warm Pool

6. January
East Pacific High
Indonesian Low
KKC 5-19

7. ENSO El Nino-Southern Oscillation
Name given to a series of conditions that focus on processes in the Equatorial Pacific, but have global consequences

8. Atmospheric Circulation - Eq Pac
Walker Cell
East to west atmospheric circulation cell
Superimposed on the Hadley cell
Similar to Hadley cell, but follows lines of latitude instead of lines of longitude

9. Atmospheric Circulation - Eq Pac
Walker cell
Normally initiated at the Indonesian Low
Indonesian Low
Low pressure system near Indonesia/Australia
Overlying the western Pacific warm pool

11. January
Indonesian Low
East Pacific High
KKC 5-19

12. Walker Cell
Darwin
Indonesian Low
Tahiti
East Pac High
KKC 15-12

13. Walker Cell Air rises over Indonesian Low
Transported to the east aloft
Sinks in eastern equatorial Pacific (at high pressure)
Transported to west at ground level = Trade winds
Discovered by British mathematician in 1900’s studying failure of the Indian monsoon

14. Normal Conditions
= Walker cell
From NOAA website (see KKC 15-13a)

15. Normal Conditions Low Pressure over Indonesia
High Pressure over eastern equatorial Pacific
Strong Walker cell
Strong Trade winds
Weak Equatorial Counter Current
Upwelling near Peru
Western Pac ~8 warmer than eastern Pac
Sea surface about 0.5 m higher near Indonesia
Rain in western Pac/dry in eastern Pac
Thermocline ~100m in western Pac, m in eastern Pac

16. Normal Conditions
Garrison’s Oceanography text, 8-15a

17. Normal Conditions Green = cold Red = warm Sea Surface Temperature
(SST)
Garrison 8-15b

18. Normal Conditions
Sea Surface Temperature (SST) Anomalies
NOAA site

19. El Niño Conditions
Decreased pressure gradient across the equatorial Pac
Weakening of Walker cell
Weakening of Trade winds
Stronger Equatorial Counter Current
Migration of warm pool to the east
Reduced upwelling and deepening of the thermocline in the east
Shift in rainfall to the east
Eastward displacement of atmospheric heat source (and associated global climate changes)

20. El Niño Conditions
From NOAA website (see KKC 15-13b)

21. El Niño Conditions Garrison 8-15c Weakening of Walker
Decreased pressure gradient
Weaken Trade winds
Stronger Eq Counter Current
Migration of warm pool
Reduced upwelling (east)
Shallower thermocline (west)
Shift in rainfall to the east
Garrison 8-15c

22. El Niño Conditions
Garrison 8-15d

23. Comparison
Normal
El Nino
Garrison 8-15

24. El Niño Conditions
Sea Surface Temperature (SST) Anomalies
NOAA site

25. Comparison- SST Anomaly
Normal
El Nino
NOAA site

26. Development of El Niño Red and white = sea level above normal Garrison
White = sea level 30 cm above normal
~3 C above normal
Garrison

27. ENSO Pattern Tend to develop during April – June
Maximum Strength in Dec. – Feb.
Persist 9-12 months (up to 2 yrs)
Recur every 2-7 yrs

28. Simulations

29. Effects on Upwelling Reduced upwelling = fewer nutrients
= less food at bottom of the food chain

30. Effects of Upwelling
Collapse of Peruvian anchovy industry

31. La Niña Reverse of ENSO conditions
Stronger trades
Stronger Walker cell
More convection over Indonesia
Shallower thermocline to the east
Increased upwelling near Peru
Cooler eastern Pacific
Typically follow El Ninos, but less common

32. La Niña
El Niño

33. El Nino Animations Comparisons of El Ninos (and La Ninas
Current conditions

34. Southern Oscillation Index (SOI)
Oscillation in atmospheric pressure across the Pacific
Measure of intensity of ENSO
Contrast
Western Pac vs. eastern Pac
Indonesian Low vs. So. Pac high
Darwin, Australia vs. Tahiti

35. Southern Oscillation Index (SOI)
0 = normal conditions
Negative
Darwin higher than normal
Tahiti lower than normal
= ENSO
Positive
Darwin lower than normal/Tahiti higher
= La Niña

36. Southern Oscillation Index (SOI)
Similar to
KKC 15-17

37. Record of ENSO El Nino La Nina Multivariable index: Sea level pressure
Zonal and meridional surface winds
SST
Surface air temperature
Total cloudiness

38. ENSO Frequency Quasi Periodic Every 2-7 years Coral records
~1 event every 4 years and 1 strong event per decade for past century
Duration and extent varies- each ENSO is unique
= prolonged ENSO conditions

39. Coral Records of ENSO
La Niña
El Niño

40. * * * * * * *
Seasonal cycle
Variations between events

41. ENSO Comparison

42. Causes?

43. Cause(s)? Probably internal to climate system
Timing  ocean/atmosphere dynamics
~surface ocean response times
Driving energy = Pacific warm pool
Random/chaotic aspects- noise in the climate system

44. ENSO Teleconnections Global Effects (based on statistical correlation)
Large scale disturbance of convergence zones in the Tropics (impacts Hadley cells)
Impact on the Indian monsoon- reduced moisture source
Drought in Australia and Africa
Heavy rain in Peru/Calif

45. ENSO Teleconnections Global Effects
Shift in tropospheric subtropical jet stream- driven by equator to pole gradient (ENSO = steeper gradient)
Florida- cool, wet winters
NW and NE N. America- warmer (snowier)
More Pacific tropical storms
Fewer Atlantic and Gulf tropical storms

46. Teleconnections
NOAA website,
see KKC 15-18

47. Teleconnections La Nina El Nino
COAPS Website - Center for Ocean - Atm Prediction Studies

48. ENSO and Hurricanes
Tang, B. H., and J. D. Neelin, 2004: ENSO Influence on Atlantic hurricanes via tropospheric warming. Geophys. Res. Lett.: Vol 31, L24204.

49. ENSO and Hurricanes
ENSO conditions tend to suppress development of tropical storms/hurricanes in the Atlantic (inc. number in Pacific)
Wind shear
Drier conditions
during hurricane season
(Gh)/guides/mtr/hurr/enso.rxml
University of Illinois- World weather 2010 project

50. La Nina Teleconnections
More rain Indonesia/Australia (Phillipines)
Droughts US south and southwest
More rain US northwest
Increased hurricane activity
Florida- warm, dry winter

51. La Nina
Teleconnections
NOAA website,
see KKC 15-18

53. End of ENSO Events “Seeds of own destruction” Rossby Wave Kelvin Wave
slower, weaker reflection of Kelvin Wave
thermocline depths
= shallowing of thermocline in east
 re-est. of pressure gradient
Strengthen Trades
 re-est. western warm pool…

54. History of ENSO Events Evidence as far back as 5000 yrs
Sediments in Peruvian lakes
Difficult to detect such short term events far back in geologic record
Coral records- appear to be becoming more frequent over past few decades
Natural variability?
Global warming?

55. 1998- strong ENSO, then La Nina
2000 – Fall “La Nada”; ENSO neutral
Fall 2002 – El Nino
2005- Weak ENSO
2006- Weak La Nina
Early Weak ENSO
July 07- Jan 09- Maturing/weakening La Nina
moderate ENSO (‘72-73; ‘82-83; ’86-87, ‘02-03)
El Nino
La Nina