16-20 October 2017


Before 1982, few Americans had ever heard of the term El Niño, 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 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 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 Niño 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 Niño and Southern Oscillation, has been used to describe the combined effects of these two phenomena.

Notably in 1998, the opposite condition, called La Niña appeared. The term La Niña 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 Niño/La Niña events in the eastern Pacific and unusual weather conditions in many other areas of the world. Research continues to predict future El Niño and La Niña events and to explain the causes in an effort to understand these "teleconnections." 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 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 or "warm-phase ENSO" and the 1998 "cold phase" La Niña events. A major El Niño event developed during the second half of 2015 that had SST values across the central regions that were as large as the record high SST values during the very strong El Niño in 1997-1998 (in the region identified as identified as Niño 3.4). However, this El Niño abated during boreal spring of 2016, becoming a weak La Niña event during the last three months of 2016. By the beginning of February La Niña conditions had essentially disappeared, leaving an ENSO-neutral environment, indicative of neither El Niño or La Niña conditions. This ENSO-neutral situation continued through seven months.


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 Niño and La Niña 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 Niño temperature, El Niño precipitation, etc.) accessed from separate tabs. Nineteen El Niño cases (listed below the maps) and 18 La Niña 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 Niño or La Niña years. For example, take the case of temperature for the late meteorological winter and early meteorological 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 eleven years out of the 19 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 between 50 and 60 percent of all El Niño years. For the same set of months (JFM) in a La Niña year, the Northwest, the northern Rockies and the northern Plains would most likely experience below average temperature conditions.


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 a 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 that describes both El Niño and La Niña episodes as well as ENSO conditions.

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 Niño and La Niña conditions can be monitored and displayed.

The National Weather Service's Climate Prediction Center (CPC) issues an El Niño or La Niña Watch when conditions are favorable for the development of El Niño or La Niña conditions within the next six months.

As of this writing (mid-October 2017), ENSO-neutral conditions were observed to continue through September, as cooler than average surface waters were detected across the eastern and central equatorial Pacific Ocean. In addition, subsurface waters were also cooling. Therefore, forecasters at CPC recently issued a La Niña watch that indicates a 55 to 65 percent chance that a La Niña event could develop during this Northern Hemisphere fall and winter of 2017-18.

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Prepared by Edward J. Hopkins, Ph.D., email
© Copyright, 2017, The American Meteorological Society.