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 2017, 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 before a weak La Niña returned during the last three months of 2017 and continued into early 2018. By summer 2018, ENSO-neutral reemerged and continued into the fall.
WIDESPREAD EFFECTS OF El NIÑO and LA NIÑA
NOAA's 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 weather data were collected from 1950 through the present and maps were plotted as departures from corresponding 1981-2010 mean values, separated into 21 strong El Niño cases (listed below the maps) and 19 strong La Niña cases. The sets of maps are grouped on six separate pages accessed from separate tabs found above the maps: El Niño temperature, El Niño precipitation, and El Niño snowfall; La Niña temperature, La Niña precipitation, and La Niña snowfall. The twelve tabs to the left of the maps are three-month seasons, starting with Jan-Feb-Mar and running through Dec-Jan-Feb.
Consider the set of maps appearing on this page, which shows temperature anomalies for the late meteorological winter and early meteorological spring (JFM) for an El Niño situation. The left panels represent the average temperature departures from normal for a given three-month season, while the right panels show 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). The first row, marked "composite" , represent the departure of temperature, precipitation or snowfall during the an El Niño or La Niña event during the given 3-month period in the left panel and the frequency of occurrence out of all El Niño or La Niña years in the right panel. The second row marked "trend" represents the departures of the averages the recent ten years for temperature and fifteen years for precipitation from the 1981-2010 normals. Finally, the third row is marked "composite + trend," which incorporates the influence of recent trends on the ENSO composites.
Focus your attention on the bottom row ("composite + trend") for an El Niño situation for the first three months of the year. The Southeastern States would have below average temperatures, with the Carolinas and Georgia experiencing temperature departures that would be nearly one Celsius degree below average. The frequency of occurrence of below average temperatures (right panel) in this region would be 50 to 70 percent, or over twelve years out of the 21 El Niño cases would have had below average temperatures. On the other hand, the nation's northern tier of states would have above average JFM temperatures in an El Niño case, especially in the interior Northwest. Such a positive temperature anomaly situation across the northern Rockies and adjacent high Plains occurred between 70 and 90 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, while the southern tier of states would have a reasonably high likely of above average temperatures. Similar displays are available for precipitation or snow for both El Niño or La Niña years.
KEEPING TRACK OF CURRENT El NIÑO/ LA NIÑA INDICATORS
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. PMEL also has a page that focuses upon La Niña. 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, identified as the Global Tropical Moored Buoy Array Program. Other links from the PMEL's El Niño Theme Page provide El Niño forecasts and information in a question and answer format. A NOAA Climate.gov page is also available that describes both El Niño and La Niña episodes as well as ENSO conditions.
Another site from NASA is the Ocean Surface Topography from Space homepage that provides global sea level data obtained from NASA's TOPEX/Poseidon and Jason satellites. 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. CPC will issue an El Niño or La Niña Advisory when El Niño or La Niña conditions are currently observed and are expected to continue. Forecasters at the International Research Institute for Climate and Society (IRI) also are involved with issuing advisories and watches. Additional information is available for this alert system involving these watches and advisories.
As of this writing (mid-October 2018), ENSO-neutral conditions were observed to continue through September, as near to above average sea surface temperatures of the waters were detected across most of the equatorial Pacific Ocean. However, the surface waters across the equatorial Pacific Ocean where beginning to warm, as indicated by an increase in the area of above-average sea surface temperatures. Furthermore, subsurface waters were also warming in the waters of the equatorial Pacific to the east of the Dateline. Additionally, the state of the tropical atmosphere through September also indicated ENSO conditions, but recent trends would suggest a developing El Niño. Based upon their assessment of the various computer forecast models that they use, the CPC and IRI forecasters have continued a El Niño watch that indicates a 70 to 75 percent chance that El Niño conditions will develop during the remainder of this Northern Hemisphere fall and continue through the boreal winter of 2018-19.
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
© Copyright, 2018, The American Meteorological Society.