Before 1982, few Americans had ever heard of the term El Niņo, when exceptionally stormy weather conditions were reported along the West Coast. Much media and public attention was focused upon this large-scale anomalous atmospheric and oceanic condition again during the 1997-98 winter when numerous Pacific storms battered the West Coast and Southeast, while the northern tier of states remained exceptionally mild. Briefly, El Niņo, 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 in December. This phenomenon that occurs every three to five years had been long known to produce disastrous effects upon the local South American economy. Warming of the coastal Pacific waters causes a reduction in the upwelling of nutrient-rich cold water reducing the fish population and hence the income from the fishing industry. Studying these El Niņo events, some researchers had found a correlation between these episodes and the Southern Oscillation, a quasi-periodic variation in the atmospheric pressure across the tropical Pacific Ocean. The term ENSO, a contraction for El Niņo and Southern Oscillation, has been used to describe the combined effects of these two phenomena.
Notably in 1998, the opposite condition, called La Niņa appeared. The term La Niņa had been proposed about 1988 to identify an event associated with anomalously cold ocean waters in the eastern Pacific. The reason attention has been paid to these warming and cooling events in the equatorial Pacific is that atmospheric and oceanic scientists have seen a relationship between El Niņo/La Niņa events in the eastern Pacific and unusual weather conditions in many other areas of the world. Research continues to predict future El Niņo and La Niņa events and to explain the causes in an effort to understand these "teleconnections." In fact, the terms El Niņo and La Niņa now refer to the larger scale anomalous atmospheric and oceanic patterns seen across the globe.
Following the significant 1982-83 El Niņo 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 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 Niņo or "warm-phase ENSO" and the 1998 "cold phase" La Niņa events.
WIDESPREAD EFFECTS OF El NIŅO and LA NIŅA
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 coterminous United States are affected by El Niņo and La Niņa conditions. The data were collected from 1950 through the present and are plotted as departures from 1981-2010 mean values, grouped on six separate pages (e.g., El Niņo temperature, El Niņo precipitation, etc.) accessed from separate tabs. Fourteen El Niņo cases (listed below the maps) and 16 La Niņa cases were considered. Considering the top row ("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 Niņo or La Niņa years. For example, take the case of temperature for the late meteorological winter and early meteorological spring (JFM). For an El Niņo 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 14 El Niņo cases. On the other hand, the nation's northern tier of states would have above average JFM temperatures in an El Niņo case. Such a positive temperature anomaly situation across the northern Rockies and adjacent high Plains occurred little more than 40 percent of all El Niņo years. For the same set of months (JFM) in a La Niņa year, the Northwest, the northern Rockies and the northern Plains would most likely experience below average temperature conditions.
KEEPING TRACK OF CURRENT El NIŅO/ LA NIŅA INDICATORS
Several Web sites focusing on the El Niņo and La Niņa provide up-to-date information concerning the SST and other El Niņo indicators. You can use these sources to monitor the present conditions across the tropical Pacific, compare a recent El Niņo event with other historic predecessors, and learn how these events may affect the weather and climate elsewhere on the planet. A special El Niņo 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 Niņo forecasts and information in a question and answer format. A NOAA La Niņa theme page is also available that describes La Niņa episodes.
Another site, the Ocean Surface Topography from Space homepage, provides global sea level data obtained from NASA's Jason satellite. From this altimeter information, current El Niņo and La Niņa conditions can be monitored and displayed.
As of this writing, "negative SST anomalies" (observed surface temperatures below the long-term averages) or La Niņa conditions remained across the central equatorial Pacific, with some areas having values between 0.5 and 1.0 Celsius degrees below the long-term average, with the largest negative anomalies just west of the International Dateline. However, waters have been warming, as evident by positive sea-surface temperature anomalies greater than one Celsius degree over the eastern Pacific (near South America) and across the far western Pacific. Scientists report that based upon these current observations together with dynamical model forecasts, the La Niņa conditions have weaken with some models indicating a return to ENSO-neutral conditions by the end of April 2012.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
Š Copyright, 2012, The American Meteorological Society.