Before 1982, few Americans had ever heard of the term El Niño. Much media and public attention was focused upon this large scale anomalous atmospheric and oceanic condition during the 1997-98 winter when numerous Pacific storms battered the West Coast and Southeast, while the northern tier of states remained exception ally mild. Briefly, El Niño, named after "the Christ child" by Peruvians some 200 years ago, is associated with a noticeable warming of the equatorial Pacific Ocean waters along the South American Coast. This phenomenon that occurs every three to five years produces disastrous effects upon the local economy because the warming causes a reduction in the upwelling of cold water that reduces the fish population and hence the income from the fishing industry.
Then in 1998, the opposite condition, called La Niña appeared. The term La Niña was proposed about 1988 to identify a circulation regime associated with anomalously cold ocean waters in the eastern Pacific. Moderate La Niña conditions reappeared over portions of the eastern and central Pacific at the end of 2000 and into February 2001, then disappeared by late March, with a return to slightly negative sea-surface temperature (SST) anomalies across the eastern Pacific during late 2001. For some scientists, the situation that continued into early 2002 was what they termed an "ENSO-neutral" period. (ENSO is a commonly used contraction for El Niño and Southern Oscillation.) Beginning in late Spring 2002, another a "warm-phase ENSO" or El Niño episode developed, which was weaker than the most recent major El Niño of 1997-98, as warmer waters developed across the equatorial Pacific, with the largest temperature anomalies on the order of 2 to 3 Celsius degrees above normal sea-surface temperatures found just east of the International Dateline, with temperatures for the region. With some slight interruptions, these weak El Niño conditions continued through mid 2005, when another La Niña event unfolded as the eastern Pacific had below average temperatures, while the central Pacific experienced above average temperatures. This La Niña event was short-lived, as warming of the eastern Pacific during 2006, provided early signs that an El Niño was forming. However, in early 2007 the temperature pattern changed, leading to a change from El Niño to La Niña conditions. The La Niña event that developed during 2007 continued through much of 2008 and into early 2009. At the end of 2008, sea surface temperatures were more than one Celsius degree below the long-term average across sections of the east-central and central equatorial Pacific from off the coast of South America to west of the Dateline. However, by mid 2009, sea surface temperatures had increased across much of the equatorial Pacific. As of this writing, "positive SST anomalies" (observed surface temperatures above the long-term averages) were found across the central and east-central equatorial Pacific, with some areas having a nearly one Celsius degree above the long term average. The eastern equatorial Pacific had smaller positive SST anomalies. Therefore, NOAA's Climate Prediction Center has issued an El Niño advisory as part of its newly instituted ENSO Alert System. Scientists report that El Niño conditions are present across the equatorial Pacific and that it should continue through the upcoming Northern Hemisphere 2009-2010 winter (December through February).
The reason for the attention paid to these events in the equatorial Pacific is that within the last several decades atmospheric and oceanic scientists have suggested a relationship between unusual weather conditions in many other areas of the world and the El Niño events in the eastern Pacific. Research is currently being conducted to understand these "teleconnections" and to explain the causes in an effort to predict future El Niño and La Niña events. In fact, the terms El Niño and La Niña now refer to the larger scale anomalous atmospheric and oceanic patterns seen across the globe. Following the significant 1982-83 El Niño event, a major international effort called TOGA (Tropical Oceans, Global Atmosphere) was launched. Between 1985 and 1994, the weather and near-surface ocean conditions in the tropical Pacific were closely monitored by investigators using moored ocean buoys, drifting buoys, ship measurements, and from satellites in both geosynchronous and polar orbits. Infrared radiation (IR) sensors onboard these satellites provide a continuous worldwide estimate of the sea surface temperatures (SST). The buoy network and the satellite surveillance, continued under the administration of NOAA, provided the scientific community with the first indications of the major 1997-98 El Niño event. By July 1997, a major warming of the tropical surface waters in the eastern and central Pacific Ocean was detected, as the SST in some regions reached 4.5 Celsius degrees above the long term climatological mean (called a "positive SST anomaly") extending over an area greater than that of the continental United States. This pattern persisted for the next eight months. By late 1998, negative SST anomalies developed in the Pacific, and as noted above, La Niña, or the cold phase had developed.
Several Web sites focusing on the El Niño and La Niña provide up-to-date information concerning the SST and other El Niño indicators. You can use these sources to monitor the present conditions across the tropical Pacific, compare the recent El Niño event with other historic predecessors, and learn how these events may affect the weather and climate elsewhere on the planet. A special El Niño theme page produced by the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory (PMEL) in Seattle, WA contains background information and various types of current information sources. One such source is the set of real time plots of sea surface temperature and wind observations provided by an array of moored ocean buoys in the Pacific Ocean. Other links from this page provide El Niño forecasts and information in a question and answer format. A NOAA La Niña theme page is also available.
Another instructive site is the TOPEX/Poseidon and El Niño homepage. The TOPEX/Poseidon satellite, a part of a partnership between U.S. and French scientists, accurately measured global sea level every 10 days. From this altimeter information, current El Niño and La Niña conditions were monitored and displayed. However, in January 2006, this mission finally ended after 13 years and 5 months of service when the spacecraft lost its ability to maneuver. The Jason satellite, which had been launched in December 2001, now makes measurements of global sea level.
EFFECTS OF El NIÑO and LA NIÑA
The Climate Prediction Center has a website entitled ENSO Temperature and Precipitation Composites, where a suite of maps can be viewed that show how the temperature, precipitation and snow patterns over three-month "seasons" across the nation are affected by El Niño and La Niña conditions. The data were collected from 1950 through the present and plotted as departures from 1971-2000 mean values. Thirteen El Niño cases (as listed below) and 16 La Niña cases were considered since 1950. Consider only the top row identified as "composite." The left panel represents the average temperature, precipitation or snow departures from normal for a given three-month season, while the right panel shows the frequency of occurrence of above or below average conditions as a percentage for the set of all individual El Niño or La Niña years. For example, take the case of temperature for the late winter and early spring (JFM). For an El Niño situation, the Southern States extending from Arizona eastward to the Carolinas would have below average temperatures, with Florida experiencing temperature departures that would be at least one Celsius degree below average. The frequency of occurrence of below average temperatures would be in excess of 60 percent, or approximately eight years out of the 13 El Niño cases. On the other hand, the nation's northern tier of states would have above average JFM temperatures in an El Niño case. Such a positive temperature anomaly situation across the northern Rockies and adjacent high Plains occurred more than 70 percent of all El Niño years.
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Prepared by Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2009, The American Meteorological Society.