El Nino and La Nina

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If you are an avid weather enthusiast, you’ve probably heard a lot about El Nino in recent years. It seems to get blamed for any kind of unusual or bad weather, especially along the West Coast. But up until February 2005, El Nino and its counterpart La Nina have been loosely defined across North America.

Even with the lack of a concise definition, it has been know for nearly a century that sea surface temperatures in the equatorial Pacific have a drastic effects on weather patterns. Over time, scientists have learned sea surface temperatures affect tropical rainfall. That starts a domino affect that alters the jet stream and much of the weather globally, enough to have social effects certain parts of the world.

The National Oceanic Atmospheric Administration, Meteorological Service of Canada and the National Meteorological Service of Mexico came together to formally define El Nino as “a phenomenon in the Pacific Ocean characterized by a positive sea surface temperature departure from normal…greater than or equal 0.5 degrees Celsius in magnitude.” These events are restricted within 120 to 170 W longitude, and within 5 degrees of the equator. La Nina was defined in a similar manner with negative sea surface temperature departures of a half a degree Celsius or more. To officially qualify as an El Nino or La Nina, the temperature anomaly needs to be sustained for three months or more.

El Nino was named after the Christ Child by fisherman off the coast of South America as it most often appears in late December. El Nino events produce rains over the coastal desert brings periods of growth called, anos de abundancia (years of abundance). The establishment of an El Nino pattern creates higher pressure and drought conditions in Darwin, Australia, while lower pressure along the international dateline results in heavy rainfall.

La Nina, El Nino’s counterpart, was named for abnormal cooling of the equatorial waters for the first time in 1986. During La Nina phases, trade winds are stronger and the water brought to the surface off the coast of South America is initially cooler that normal. In the western Pacific, waters are warmer than normal causing greater amounts of evaporation leading to heavy rainfall events.

The formation of these phenomena begins as water along the South American coast is drawn up from below the surface. This water, which initially starts out around 68 degrees Fahrenheit gradually warms to around 80 degrees as it flows west. The layer which warms due to solar radiation is fairly shallow, usually less than 100 meters (330 feet) deep. Extremely warm temperatures are tough to obtain because of the process of evaporation. As the water continues to warm, more water is lost due to evaporation, thus resulting in a net cooling of the surface. This process makes it difficult to obtain sea surface temperatures greater than 30 degrees Celsius (86 degrees Fahrenheit).

However, when above normal water temperatures develop; drastic effects are felt on the fishing industry. The warm water is not as nutrient packed as the cooler water, and the amount of plankton decreases under such an event. This decrease in available food causes a significant decrease in the number of available anchovies, thus resulting in a net loss of fish available for fisherman and birds. These events often lead to widespread starvation which may result in death.

El Nino is only part of a weather pattern that brings drastic changes in pressure and winds to large parts of the world. With the impact that these two phenomena have on the global circulation, it is important to forecast strength and duration of an El Nino or La Nina event. El Nino and La Nina are cyclic but not periodic phenomena, meaning on average they occur once every four to five years but can vary from every other year to 10 years between episodes. This makes studying them very difficult because an episode may not be realized until they have formed. Unfortunately, because these events have been discovered in the recent past and are not periodic in occurrence, the cause of these events has not been determined. Scientists thus far have only determined characteristics that are common with each phenomenon. Hypotheses on the cause of El Nino have ranged from bursts of magma along the ocean floor to snowfall amounts over Asia, but have yet to be proven. Until then, atmospheric scientists will continue to determine the role and the drastic effects both El Nino and La Nina play in the global earth system.