What is El Niño?

It’s an El Niño year. The news is blowing up about it—Indonesia is on fire, Texas is under water, and Californians invoke the holy “El Niño” in hopes that above average rainfall will end the Drought in addition to causing flooding and mudslides (sadly, it will not). What are these people talking about? How does this mysterious ocean oscillation work?

El Niño Sea Surface Height Anomaly. image courtesy of the National Oceanic and Atmospheric Administration

El Niño conditions occur about every 3-7 years when the standard Pacific Ocean climate equilibrium unravels slightly. Odd weather patterns associated with this anomaly are caused by changes in ocean surface temperatures.

The world ocean has 2 layers: the surface and the deep. These layers don’t mix because they differ in density. Like oil on vinegar, the warmer, less saline surface layer sits on top of the cold, very saline deep water. The gradient between the two layers is called the thermocline.

image courtesy of The Training Spectrum Scuba Instructing

Normal Conditions

In the Pacific, the Trade Winds (prevailing winds of yore) blow west. In normal conditions, the force of the trade winds against the surface layer cause surface height to be greater in the West Pacific than in the East Pacific.

Normal Pacific Ocean Conditions. image courtesy of University of Sydney Physics

The force of the Trade Winds piles the less dense surface water up on the western side of the ocean. This deep layer of warm water is called the “Western Warm Pool.” It creates low pressure systems over the ocean in the West Pacific, which cause monsoons and other intense storms in this region (Australia, South Asia, etcetera). In the East Pacific, the displaced surface water is replaced by cold deep water, which upwells from under the surface layer. The fishing industries of many South American countries like Peru are dependent on upwelling, which attracts all manner of ocean life due to its high nutrient content.

El Niño Conditions

During an El Niño year, this balance is disrupted. El Niño conditions form when the wind bursts opposing the Trade Winds strengthen enough to negate some of their force. Without the Trade Winds forcing surface water toward Asia, the difference in surface height between the West and East Pacific decreases; that is, the thermocline shifts up in the West and down in the East, sending warm water flowing across the Pacific toward North and South America.

El Niño Pacific Ocean Conditions. image courtesy of University of Sydney Physics

As the sea surface height of the ocean levels out and surface temperatures in the East Pacific rise, cold-water upwelling all but ceases because of the increased depth of the warm surface layer. Low pressure systems develop due to the higher ocean surface temperatures, which causes precipitation to increase in areas like California.

Further Reading:
Indonesia Forest Fires
Massive El Niño… likely to drench key California drought zone

Special thanks to the professors, guest lecturers, and teaching assistants of Earth Systems 10 (Autumn Quarter 2015) for providing the foundation for this article.


  1. Rheme, M., Scheitlin, T. (December 17, 2014). El Niño and La Niña: Sea Surface Temperature Anomalies, 1997-1999 [Graphic.] University Corporation for Atmospheric Research. Data Obtained from National Oceanic and Atmospheric Administration. Retrieved from https://www.youtube.com/
  2. Herring, D. (1999, April 27). What is El Niño? Retrieved November 22, 2015, from http://earthobservatory.nasa.gov/
  3. National Oceanic and Atmospheric Administration. Sea surface height anomaly. [Digital image]. Retrieved from http://sos.noaa.gov/
  4. The Training Spectrum. (2009). Thermocline [Digital image]. Retrieved fromhttp://www.scuba-tutor.com/
  5. University of Sydney Physics. What effect do strong trade winds have on sea temperatures and the thermocline? [Digital image]. Retrieved fromhttp://www.physics.usyd.edu.au/
  6. University of Sydney Physics. What effect do the weaker trade winds have on sea temperatures and the thermocline? [Digital image]. Retrieved fromhttp://www.physics.usyd.edu.au/