1. El Nino Southern Oscillation (ENSO)
AOS 101 Weather and Climate
Lisha M. Roubert
University of Wisconsin-Madison
Department of Atmospheric & Oceanic Sciences

2. El Niño
El Niño is an oscillation of the ocean-atmosphere system in the tropical Pacific having important consequences for weather around the globe.
El Niño is characterized by unusually warm ocean temperatures in the Equatorial Pacific, as opposed to La Niña, which is the opposite phase of El Niño.
ENSO events occur irregularly, every 2-7 years.

3. How did ENSO get its name?
El Niño was originally recognized by fisherman off the coast of South America as the appearance of unusually warm water in the Pacific ocean, occurring near the beginning of the year. El Niño means The Little Boy or Christ child in Spanish. This name was used for the tendency of the phenomenon to arrive around Christmas.
La Niña means The Little Girl. La Niña is sometimes called anti-El Niño.

4. What causes ENSO?
El Niño results from interaction between the surface layers of the ocean and the overlying atmosphere in tropical Pacific.
The internal dynamics of the coupled ocean- atmosphere system determine the onset and termination of El Niño events. The physical processes are complicated, but they involve unstable interaction between the air and planetary scale oceanic waves.

5. Teleconnections: Linking ENSO to other locations
Boreal Winter (Winter for NH)
Teleconnections mean that weather changes at one location might be related to weather changes at other remote locations.
The relationship between El Niño events and climatic variations in the Equatorial Pacific region is extremely strong . The relationship outside this area is harder to prove.  Weather anomalies occur all over the globe every year, but some tend to recur with most or all El Niño events and are referred to as teleconnections.
El Niño impacts are strongest and most widespread when water temperatures reach the annual maximum (boreal winter).
Above normal warmth of the water, el Niño redistributes convection, which causes changes in the jet stream.
During the austral winter, El Niño impacts are weaker and confined mostly to the Southern Hemisphere.
Austral Winter (Winter for SH)

6. Impacts of ENSO on North America and the Atlantic Basin
Drier than normal conditions over the North American monsoonal region.
The Pacific Northwest experiences drier than normal conditions during Fall and Winter.
Atlantic hurricane activity is suppressed due to cooling.

7. The Atmosphere during non-ENSO conditions
In normal, non-El Niño condition, the trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador.
Sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels.
Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry. 

8. During ENSO
During El Niño the trade winds relax in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west.
The result was a rise in sea surface temperature.
 Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific.

9. ENSO Animation

10. How do we recognize ENSO?
ENSO can be seen in measurements of Sea Surface Temperatures.
SST and winds are near normal with warm water in western Pacific and the “cold tongue” (cold water) in the eastern Pacific.
Winds in the western Pacific are weak and winds in the eastern Pacific blow towards the west.
Warm water has spread from the western Pacific towards the east , the "cold tongue" has weakened.
The winds in the western Pacific are blowing strongly towards the east, pushing the warm water eastward. 

11. ENSO associated with change in Sea Level
The images show sea surface topography from NASA's TOPEX satellite, sea surface temperature's from NOAA's AVHRR satellite sensor and sea temperature below the surface as measured by NOAA's network of TAO moored buoys.
Normal
ENSO

12. Developing La Niña
La Niña

13. So many measurements to monitor ENSO, but where does all this data come from?
There are many resources to obtain data. We call this the ENSO Observing System.
There is even a system of moored buoys located in the Atlantic that are specifically designed to monitor ENSO.

14. How do we monitor ENSO?
A variety of indices are used to characterize ENSO because it affects so many elements of the atmosphere-ocean climate system. The main ones are:
the Southern Oscillation Index (SOI): it is given by the difference in sea-level pressure between Tahiti and Darwin, Australia
the Niño 3 index: it refers to the anomalous SST within the region bounded by 5N-5S and 150W-90W.
Red=El Niño Events
Blue=La Niña Events

15. What is happening now?
La Niña is present across the equatorial Pacific.
Negative sea surface temperature anomalies persist across much of the Pacific Ocean.
La Niña is expected to last into the Northern Hemisphere spring 2011.

16. Homework: Study for quiz #2. The material covered in the quiz will be:
-Lab 3: Temperature Trends
-Lab 4: Extreme Temperature Events
-Lab 5: Precipitation