Allie Marquardt

Outline Overview of El Niño A Change in the Atmosphere Resulting Changes in the Ocean Sea Surface Temperature Thermocline Rossby Waves and Dissipation

What is El Niño? El Niño is characterized by warm sea surface temperatures (SST) in the Pacific Ocean around the equator, but is associated with a large scale change in the ocean-atmosphere system The Climate Prediction Center will declare the onset of El Niño once the change in SST has been at least 0.5° for 3 months Source:

A Change in the Atmosphere… After a rise in surface pressure over the Indian Ocean and fall in surface pressure over the central and eastern Pacific, the easterly trade winds in the south Pacific are weakened H L L H Normal Conditions El Niño A decrease in the pressure gradient will result in a decrease in the magnitude of the wind

…Results In a Change in the Ocean WARM COLD During normal conditions, the trade winds push surface water towards the west, replenishing the surface with cool water from underneath During El Niño, the weaker trade winds cut off the upwelling

The Thermocline Warm water collects in the Western Pacific Ocean, causing a large slope in the themocline due to thermal expansion Since warm water extends to the Eastern Pacific, the thermocline is found at a deeper level, causing the slope to be not a steep

Rossby Waves Although the heat moves eastward, there are very slow freely propagating equatorial waves that move westward Since the are slow, they have a time lag on the order of months Rossby wave help determine the thermocline structure, once they reach the western boundary Since the propagate in the opposite direction, they cancel out the effects of the eastward Kelvin waves, but at a slower pace, dissipating the El Niño conditions

References National Academy of Science: El Niño and La Niña ( NOAA El Nino Theme Page ( story.html) Schneider, Edwin K., Bohua Huang, and J. Shukla (1995),Ocean wave dynamics and El Niño, Journal of Climate,