The first type of deviation from the usual Pacific circulation is El Niño, or an El Niño/Southern Oscillation (ENSO) event, characterized by a low Southern Oscillation Index (SOI). El Niño was named for the Christ Child because the Peruvian and Ecuadorian fishermen noted that it brought warm waters to their coast starting around Christmas time. ENSOs occur once every three to eight years. They strengthen the jet stream over the Pacific and change the paths of many tropical storms, causing shifts in weather patterns all over the globe. This causes weather abnormalities all over the globe, and not just of the cold and wet type that we experience here in California. Heat spells, droughts, floods, fires, and monsoons are some of the most notable effects.

 

• How do things change in the Pacific during El Niño?

  OCEAN: One of the first signs that El Niño is forthcoming is a decrease in the difference between the temperatures of the West and East Pacific oceans. The North Equatorial Current slows and water does not move West at nearly the same rate, which causes build up of warm water on the Eastern Pacific Coast. As a result, the thermocline sinks down in the East, preventing the upwelling of nutrient-rich cold water on the Peruvian coast. This slows the current even further. Meanwhile, in the West relatively cold water accumulates.

• In this picture of surface temperatures in the tropical Pacific, you can see how warm water accumulates in the East Pacific during an El Niño year. Compare with normal and La Nina years.

  ATMOSPHERE: Another major sign that El Niño is on its way is a reduction in the Easterlies, equatorial winds that usually blow from East to West. Because of this slow down, the Walker Circulation changes: The region of low pressure and heavy rainfall never makes it all the way to the West Pacific as under normal circumstances; instead it remains over the warm East and Central Pacific. (see appendix for a picture of the changes in the Walker Circulation during an El Nino year) As a result, the Western Pacific sees much less rain than usual and there may be serious droughts. The Eastern Pacific sees much more rain and flooding.

   

• Normal Conditions

  

  Note the slant of the thermocline, the collection of warm water in the West Pacific, and the low pressure, heavy rainfall area over the West Pacific.

• El Niño Conditions

  

  Compare the relatively even Pacific Thermocline, Eastward redistribution of warm water, and central location of the low pressure, heavy rainfall area. Compare with La Niña conditions.

   

  SUMMARY: During El Niño, the West Pacific coast experiences cooler water, higher pressure, and less rain, while the East Pacific coast has warmer water, lower pressure, and more rain than normal.

The second type of deviation is La Niña, which occurs when the SOI is very high. La Niña tends to have the opposite effects on global weather of El Niño.

 

• How do things change in the Pacific during La Niña?

  OCEAN: The North Equatorial Current strengthens, so that the thermocline sinks even more in the West Pacific and remains shallow in the East. The Eastern upwelling is particularly strong, and its cold waters move West with the strong currents, so water temperatures in the central Pacific are much colder than usual.

  La Niña: Surface Temperatures

  

  In this picture of surface temperatures in the tropical Pacific, you can see how warm water accumulates in the West Pacific during a La Niña year. Compare with normal and El Nino years.

   

  ATMOSPHERE: During a La Niña episode, the normal trade winds, especially the Easterlies along the equator, strengthen and drive the stronger ocean currents. Due to the colder water in the central Pacific, the air does not begin to rise until it is much closer to the Western edge, delaying storm formation and causing storms to focus in the Western Pacific more than usual.

• La Niña Conditions

 Note the relative slant of the Pacific Thermocline, the accumulation of warm water in the West Pacific, and the concentration of low pressure and heavy rainfall over the West Pacific. Compare with normal and El Nino conditins.

SUMMARY: During a La Niña event, the Western Pacific experiences more rain and lower pressure than usual. The Central Pacific is much colder than usual. The Eastern Pacific is colder, experiences higher pressure, and has less rain than usual.