From: Subject: Wednesday Weather Topic in Greater Depth Date: Thu, 5 Nov 2009 10:54:54 -0800 MIME-Version: 1.0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/dstreme/learn/w_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Wednesday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Wednesday, 4 = November=20 2009

MONITORING EL NI=D1O and LA NI=D1A

Before 1982, few Americans had ever heard of the term El = Ni=F1o. Much=20 media and public attention was focused upon this large scale anomalous=20 atmospheric and oceanic condition during the 1997-98 winter when = numerous=20 Pacific storms battered the West Coast and Southeast, while the northern = tier of=20 states remained exceptionally mild. Briefly, El Ni=F1o, named after "the = Christ=20 child" by Peruvians some 200 years ago, is associated with a noticeable = warming=20 of the equatorial Pacific Ocean waters along the South American Coast. = This=20 phenomenon that occurs every three to five years produces disastrous = effects=20 upon the local economy because the warming causes a reduction in the = upwelling=20 of cold water that reduces the fish population and hence the income from = the=20 fishing industry.

Then in 1998, the opposite condition, called La Ni=F1a = appeared. The=20 term La Ni=F1a was proposed about 1988 to identify a circulation regime = associated=20 with anomalously cold ocean waters in the eastern Pacific. Moderate La = Ni=F1a=20 conditions reappeared over portions of the eastern and central Pacific = at the=20 end of 2000 and into February 2001, then disappeared by late March, with = a=20 return to slightly negative sea-surface temperature (SST) anomalies = across the=20 eastern Pacific during late 2001. For some scientists, the situation = that=20 continued into early 2002 was what they termed an "ENSO-neutral" period. = (ENSO=20 is a commonly used contraction for El Ni=F1o = and=20 Southern Oscillation.) Beginning in late = Spring=20 2002, another a "warm-phase ENSO" or El Ni=F1o episode developed, which = was weaker=20 than the most recent major El Ni=F1o of 1997-98, as warmer waters = developed across=20 the equatorial Pacific, with the largest temperature anomalies on the = order of 2=20 to 3 Celsius degrees above normal sea-surface temperatures found just = east of=20 the International Dateline, with temperatures for the region. With some = slight=20 interruptions, these weak El Ni=F1o conditions continued through mid = 2005, when=20 another La Ni=F1a event unfolded as the eastern Pacific had below = average=20 temperatures, while the central Pacific experienced above average = temperatures.=20 This La Ni=F1a event was short-lived, as warming of the eastern Pacific = during=20 2006, provided early signs that an El Ni=F1o was forming. However, in = early 2007=20 the temperature pattern changed, leading to a change from El Ni=F1o to = La Ni=F1a=20 conditions. The La Ni=F1a event that developed during 2007 continued = through much=20 of 2008 and into early 2009. At the end of 2008, sea surface = temperatures were=20 more than one Celsius degree below the long-term average across sections = of the=20 east-central and central equatorial Pacific from off the coast of South = America=20 to west of the Dateline. However, by mid 2009, sea surface temperatures = had=20 increased across much of the equatorial Pacific. As of this writing, = "positive=20 SST anomalies" (observed surface temperatures above the long-term = averages) were=20 found across the central and east-central equatorial Pacific, with some = areas=20 having at least one Celsius degree above the long term average. The = eastern=20 equatorial Pacific had smaller positive SST anomalies. Therefore, NOAA's = Climate=20 Prediction Center has issued an El Ni=F1o advisory as part of its newly = instituted=20 ENSO Alert System. Scientists report that El Ni=F1o conditions are = present across=20 the equatorial Pacific and that it should continue through the upcoming = Northern=20 Hemisphere 2009-2010 winter (December through February).

The reason for the attention paid to these events in the equatorial = Pacific=20 is that within the last several decades atmospheric and oceanic = scientists have=20 suggested a relationship between unusual weather conditions in many = other areas=20 of the world and the El Ni=F1o events in the eastern Pacific. Research = is=20 currently being conducted to understand these "teleconnections" and to = explain=20 the causes in an effort to predict future El Ni=F1o and La Ni=F1a = events. In fact,=20 the terms El Ni=F1o and La Ni=F1a now refer to the larger scale = anomalous=20 atmospheric and oceanic patterns seen across the globe. Following the=20 significant 1982-83 El Ni=F1o event, a major international effort called = TOGA (Tropical Oceans, Global Atmosphere) was launched. Between = 1985 and=20 1994, the weather and near-surface ocean conditions in the tropical = Pacific were=20 closely monitored by investigators using moored ocean buoys, drifting = buoys,=20 ship measurements, and from satellites in both geosynchronous and polar = orbits.=20 Infrared radiation (IR) sensors onboard these satellites provide a = continuous=20 worldwide estimate of the sea surface temperatures (SST). The buoy = network and=20 the satellite surveillance, continued under the administration of NOAA, = provided=20 the scientific community with the first indications of the major 1997-98 = El Ni=F1o=20 event. By July 1997, a major warming of the tropical surface waters in = the=20 eastern and central Pacific Ocean was detected, as the SST in some = regions=20 reached 4.5 Celsius degrees above the long term climatological mean = (called a=20 "positive SST anomaly") extending over an area greater than that of the=20 continental United States. This pattern persisted for the next eight = months. By=20 late 1998, negative SST anomalies developed in the Pacific, and as noted = above,=20 La Ni=F1a, or the cold phase had developed.

Several Web sites focusing on the El Ni=F1o and La Ni=F1a provide = up-to-date=20 information concerning the SST and other El Ni=F1o indicators. You can = use these=20 sources to monitor the present conditions across the tropical Pacific, = compare=20 the recent El Ni=F1o event with other historic predecessors, and learn = how these=20 events may affect the weather and climate elsewhere on the planet. A = special El = Ni=F1o theme=20 page produced by the National Oceanic and Atmospheric = Administration's=20 Pacific Marine Environmental Laboratory (PMEL) in Seattle, WA contains=20 background information and various types of current information sources. = One=20 such source is the set of real time plots of sea surface temperature and = wind=20 observations provided by an array of moored ocean buoys in the Pacific = Ocean.=20 Other links from this page provide El Ni=F1o forecasts and information = in a=20 question and answer format. A NOAA La Ni=F1a theme = page is=20 also available.

Another instructive site is the TOPEX/Poseidon and El = Ni=F1o=20 homepage. The TOPEX/Poseidon satellite, a part of a partnership = between U.S.=20 and French scientists, accurately measured global sea level every 10 = days. From=20 this altimeter information, current El Ni=F1o and La Ni=F1a conditions = were=20 monitored and displayed. However, in January 2006, this mission finally = ended=20 after 13 years and 5 months of service when the spacecraft lost its = ability to=20 maneuver. The Jason satellite, which had been launched in December 2001, = now=20 makes measurements of global sea level.

EFFECTS OF El NI=D1O and LA NI=D1A

The Climate Prediction Center has a website entitled = ENSO=20 Temperature and Precipitation Composites, where a suite of maps can = be=20 viewed that show how the temperature, precipitation and snow patterns = over=20 three-month "seasons" across the nation are affected by El Ni=F1o and La = Ni=F1a=20 conditions. The data were collected from 1950 through the present and = plotted as=20 departures from 1971-2000 mean values. Thirteen El Ni=F1o cases (as = listed below)=20 and 16 La Ni=F1a cases were considered since 1950. Consider only the top = row=20 identified as "composite." The left panel represents the average = temperature,=20 precipitation or snow departures from normal for a given three-month = season,=20 while the right panel shows the frequency of occurrence of above or = below=20 average conditions as a percentage for the set of all individual El = Ni=F1o or La=20 Ni=F1a years. For example, take the case of temperature for the late = winter and=20 early spring (JFM). For an El Ni=F1o situation, the Southern States = extending from=20 Arizona eastward to the Carolinas would have below average temperatures, = with=20 Florida experiencing temperature departures that would be at least one = Celsius=20 degree below average. The frequency of occurrence of below average = temperatures=20 would be in excess of 60 percent, or approximately eight years out of = the 13 El=20 Ni=F1o cases. On the other hand, the nation's northern tier of states = would have=20 above average JFM temperatures in an El Ni=F1o case. Such a positive = temperature=20 anomaly situation across the northern Rockies and adjacent high Plains = occurred=20 more than 70 percent of all El Ni=F1o years.

Return to the Wednesday= Daily=20 Summary

Return to DataStreme = Atmosphere=20 website

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu =
=A9=20 Copyright, 2009, The American Meteorological Society. =