El Nino Southern Oscillation Benedetto Schiraldi

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

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. http://www.srh.noaa.gov/images/fwd/climate/enso/pacific-el.GIF

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. http://www.srh.noaa.gov/images/fwd/climate/enso/pacific-el.GIF

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.

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

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

Sea surface temperature anomaly during the peak of the 2009-10 El Niño, the strongest Central Pacific El Niño observed to date. High Resolution (Source: NASA JPL).

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

Deviations from normal sea surface temperatures (left) and sea surface heights (right) at the peak of the 2009-2010 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 

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. 1982 El Nino)

References Introduction to Physical Oceanography, Robert Stewart, 2002 California Institute of Technology, NASA Jet Propulsions Laboratory http://www.jpl.nasa.gov/ Florida State University, Center For Ocean Atmosphere Predictions Studies http://www.coaps.fsu.edu/ National Oceanic and Atmospheric Administration Southern Head Quarters http://www.srh.noaa.gov/images/fwd/climate/enso/pacific-el.GIF