1. El Nino Southern Oscillation
Benedetto Schiraldi

2. Outline Over view How Does ENSO Influence US? El Nino SST Images
Normal Walker Circulation
El Nino Event Circulation
La Nina Event Circulation
How Does ENSO Influence US?
El Nino SST Images
Mechanism for El Nino

3. Walker Circulation
Atmospheric circulation sets up a low pressure system along the western Equatorial South Pacific. This low pressure center in the western equatorial Pacific is balanced by a high pressure center along the eastern equatorial Pacific. Under normal conditions atmospheric winds persist east to west resulting in the build up of warm equatorial water to the west and thus greater sea surface height in the western Pacific and a deeper thermocline then the eastern Pacific. Waters to the east are advected northward from the coast of South America.

4. El Nino Event
When winds begin to relax Walker Circulation breaks down. Without the wind maintaining higher sea surface heights in the west is not possible. The pressure gradient resulting from this imbalance in sea surfaces height causes an Equatorial Kelvin wave to propagate east and a Rossby wave to propagate westward. Along with this Kelvin wave warm water also begins to propagate east resulting in a deepening of the normally shallow thermocline to the east. The two waves combined have a tendency to deepen the overall mixed layer of the equatorial Pacific. This eastward propagation of warm water causes a positive feedback with the trade winds forcing climate in such a way known as an El Nino Event.

5. La Nina Event
When the system is thrown back into Walker Circulation trade winds rapidly intensify. This causes a very strong could tongue in the eastern Pacific pushing further west then normal Walker Circulation. This intense cold tongue results in a shallow equatorial thermocline to the east. Strong trade winds force warmer Equatorial water west resulting in a much deeper thermocline. This phase of southern oscillation is known as La Nina.

6. El Nino Winter
Florida State University Center For Ocean-Atmosphere Prediction Studies

7. La Nina Winter
Florida State University Center For Ocean-Atmosphere Prediction Studies

9. Sea surface temperature anomaly during the peak of the El Niño, the strongest Central Pacific El Niño observed to date. High Resolution (Source: NASA JPL).

10. La Nina 2010
Image from NASA/JPL Ocean Surface Topography team oceanography satellite 

12. Deviations from normal sea surface temperatures (left) and sea surface heights (right) at the peak of the central Pacific El Niño, as measured by NOAA polar orbiting satellites and NASA's Jason-1 spacecraft, respectively. The warmest temperatures and highest sea levels were located in the central equatorial Pacific. Image credit: NASA/JPL-NOAA 

13. Possible factor which Influences El Nino
Madden Julian Oscillation-
30-90 day cycle of eastward propagating deep convection
Strong MJO have been known to cause intense breakdown of Walker circulation resulting in a Kelvin wave across the equator causing the rapid onset of an El Nino event
(Ie El Nino)

14. References Introduction to Physical Oceanography, Robert Stewart, 2002
California Institute of Technology, NASA Jet Propulsions Laboratory
Florida State University, Center For Ocean Atmosphere Predictions Studies
National Oceanic and Atmospheric Administration Southern Head Quarters