1. El Niño-Southern Oscillation
Current Weather
ENSO Terminology
Neutral, Warm-Phase, and Cold-Phase
ENSO Impacts
For Next Class: Read Moran Ch. 8 (pp )
Reminder: Exam II is on March 2. Review sheet is posted!

8. ENSO What is ENSO and why is it important?
What are the three major phases of ENSO?

9. El Niño-Southern Oscillation

10. ENSO
ENSO is a coupled phenomenon, depending on feedbacks between the ocean and the atmosphere.
Changes in the oceanic sea surface temperatures (SSTs) drive changes in atmospheric circulation which feedback and further alter the ocean.

11. El Niño-Southern Oscillation
Coupling of the ocean and atmosphere in the tropical Pacific Ocean
El Niño (warm-phase of ENSO): above average sea-surface temperatures
La Niña (cold-phase of ENSO): below average sea-surface temperatures
La Nada (ENSO-neutral): average sea-surface temperatures

12. ENSO Neutral Conditions in the Tropical Pacific
Contrast between relatively high air pressure over the central and eastern tropical Pacific and relatively low air pressure over the western tropical Pacific ultimately drives the trade winds
Walker Circulation: east-west oriented atmospheric circulation across the equatorial Pacific
© AMS

13. ACCION 2013: Climate variability
Coastal upwelling
Andes
Cold water
Warm water
Thermocline
Temperature
Depth
Trade winds
Ekman transport
Figure: University of Washington

15. ENSO
Warm Phase
With the onset of El Niño, air pressure falls over the eastern tropical Pacific and rises over the western tropical Pacific
Trade winds slacken in the western and central equatorial Pacific
Warm surface waters in the eastern tropical Pacific reduces upwelling
© AMS

16. El Niño (Warm Phase ENSO)

18. Piura - Perú

19. The rainfall in Piura and El Niño
1992
2002
1972
1987

20. Rainfall occurs when sea surface temperature (SST) exceeds a threshold
Woodman, 1999

21. 1997 e-mail from Anton Seimon in Peru

22. El Niño, La Niña, and the Southern Oscillation
Cold Phase
During La Niña, surface waters are colder than usual over the central and eastern tropical Pacific and somewhat warmer than usual over the western tropical Pacific
Lower than usual SST in the east topical Pacific inhibit rainfall while higher than usual SST in the west enhance rainfall
© AMS

23. Positive feedback between ocean and atmosphere (Bjerknes 1969)
ACCION 2013: Climate variability
Positive feedback between ocean and atmosphere (Bjerknes 1969) 1
1. Warming 2
2. Displacement of convection 3
3. Weakening of easterly winds 4
4. Eastward currents and deepening of thermocline
Peru
Australia
Thermocline
Figura: NOAA PMEL

25. El Niño, La Niña, and the Southern Oscillation
Frequency
Multivariate ENSO Index forms the basis for operational definitions of El Niño and La Niña
Based on six variables measured in the tropical Pacific
sea-level air pressure
zonal component of surface wind
meridional component of surface wind
surface air temperature
sky cloud cover
sea-surface temperature
ENSO Alert System launched in February 2009
© AMS

26. How do different phases of ENSO typically impact the southeastern U.S.?

28.
El Niño
El Niño
El Niño
La Niña

31. Chacaltaya Glacier 2003 Disappeared in 2009 2016
Source:

32. Francou et al. 2004

33. ENSO Impacts in Central Andes
El Niño (warm-phase ENSO) is typically associated with warm and more variable precipitation, leading to glacier retreat
low albedo bare ice is exposed to intense tropical sun at higher temperatures
La Niña (cold-phase ENSO) is typically associated with cool and sometimes wetter conditions, leading to glacier advance
high albedo fresh snow covers most glacier surfaces, cloud cover is increased, and temperatures are lower

34. ENSO Links