Initial newspaper, television, and radio reports in the summer of 1997 on the issue of El Niño and its impact on rainfall in the West in many cases were misleading. For example, early predictions of a spectacularly wet winter for extreme southern California (by meteorologists and oceanographers in the southern portion of the state) were reported widely as applying to the whole state, and other quotes, such as Californians should build an ark, garnered much attention.
The impact of such statements on the public was astonishing. By late summer, it was evident that over-reaction were spreading across the West, particularly in California, with overstatement and alarm evident in the tone of many newspaper articles. All of this led to poorly drawn conclusions.
The term El Niño refers to a rapid warming of sea-surface temperature (SST) in the eastern tropical Pacific, chiefly along the north-central coast of South America and westward. El Niño events are characterized by a rather dramatic onset of large positive temperature deviations (on the order of 2o C, or about 4o F, or greater), usually peaking in December (hence the name, which refers to Christmas, El Niño, or the Christ Child).
Warmings occur rather frequently (on the order of once every four years or so) as part of the Southern Oscillation (see Figure1), a cyclical change in the pressure and temperature distribution along the equator. In the current El Niño Southern Oscillation (ENSO) event, ocean temperatures are over 5o C (almost 11o F) warmer than the long-term normal in that area.Click here for Figure 1.
It is important to note that the term El Niño does not refer to a series of catastrophic, flood-producing weather events in California or elsewhere, a hurricane or series of hurricanes, or even a period of drought in the West, although any of these can and do occur as an impact of El Niño conditions along the South American coastline.
The present El Niño is a so-called Type 1 El Niño (Fu et. al, 1986). Type 1 events involve the strongest SST anomaly (>2.0o C) and extend from approximately 160o E to 80o W. From 1949 to 1993, there were eight Type 1 ENSO events; however, the present event is dissimilar to other Type 1 events in that it grew and matured much more rapidly than previous events.
This unprecedented early development raised speculation that the present El Niño would peak and dissipate too soon to impact the winter precipitation systems that affect the West. As of early December, this did not seem likely. A recent series of forecasts of SST anomalies by three-month period through next spring from the National Centers for Environmental Prediction (NCEP) shows that, although the temperature anomalies associated with El Niño are forecast to decrease slightly by March 1998, apparently they will persist through the spring (see Figure 2).Click here for Figure 2.
The unusually warm pattern in the eastern tropical and subtropical Pacific is expected to create a southern branch of the storm track, as depicted schematically in Figure 3. This southern branch, often referred to as a subtropical storm track, should extend from the latitude of Hawaii (approximately 20o N) to the southern or central coast of California, and then along the southern tier of states.Click here for Figure 3.
Storms moving along this path carry warm and very moist subtropical air to the West Coast and are associated with heavy precipitation and high snowlines in California. Warm temperatures at high elevations often exacerbate the situation by creating massive snowmelt, adding to runoff and contributing substantially to local flooding along stream margins. The position of the subtropical branch determines which portion of California and/or the West receives the heaviest precipitation.
Further east, in regions where upper tropospheric ridging is more typical in a normal winter, the surface cyclones moving in the southern branch are associated with greater cloudiness than usual and, north of the surface lows, anomalously strong surface northerlies. Thus, in these regions, cooler and wetter than normal conditions are typical with Type 1 El Niño events.
Type 1 events have a very strong signal in the precipitation record of California (Schonher and Nicholson, 1989) and the West. Figure 4 shows the composite precipitation anomalies (in inches) by climatic division across the coterminous United States for eight Type 1 events since 1950. Generally speaking, the southwestern portion of the United States, Gulf Coast, and the Mississippi River Valley have experienced wetter than normal conditions, with the strongest positive departures in California, and negative departures in coastal and Cascade Range portions of Washington State.Click here for Figure 4.
The absolute values of the anomalies do not tell the whole story. Figures 5a, 5b, and 5c, show the precipitation history in California, Colorado, and Washington for the eight Type 1 El Niño events listed above. The information plotted on these maps was obtained by the authors by dividing the anomalies shown in figure 2 by the mean rainfall for each climatic division.Click here for Figure 5a. Click here for Figure 5b. Click here for Figure 5c.
In coastal California, the greatest departures from normal have occurred from San Francisco south and in the desert southeastern portions of the state. The western and southwestern regions of Colorado have experienced between 110% and 120% of normal precipitation, whereas coastal Washington incurred only 90-95% of the long-term normal precipitation.
Given these historical anomalies, the question is How remarkable or unremarkable has the precipitation been that has historically occurred with Type 1 events? Figure 6 shows the precipitation anomalies shown in Figure 2 as a fraction of the standard deviation of precipitation. Generally speaking, a departure of one standard deviation or more can be considered significant, since it means that the given precipitation is greater than about 70% of the totals in the period of record.Click here for Figure 6.
Note that the information shown is consistent with the observation that there is enough data to support the concern for above-normal precipitation in the southwestern portions of the United States, with departure between +1.0 and +1.5 standard deviations for large areas of California and Nevada and small portions of Utah and Arizona. However, nothing in Figures 2, 3, or 4 suggests that the early concerns that hugely above normal precipitation, on the order of 300-400% (two to four standard deviations) should be expected in any area of the West this upcoming winter.
It is also important to keep in mind that while both predictions based upon computerized models of the linked physics of the oceans and the atmosphere (not shown here) and statistical analyses of the past rainfall record (summarized above) suggest wetter than normal conditions for most of the Southwest, two of the eight recent Type 1 El Niños were associated with below average precipitation in certain areas, including north-central California. The greatest positive anomalies occurred for the El Niños of 1982-83 (180-200% in California) and 1957-58 (170-190% in California).
Not all cool season flooding events in the West occur during El Niño years, and not all El Niño years produce widespread flooding. It is true that more frequent storms and heavier precipitation are more typical in the Southwest, particularly in California, during Type 1 El Niños. However, the potential for flooding or mudslides also strongly relates to the phasing of precipitation events (even a day or two break between weather systems can make a large difference in the flood potential), as well as to the saturation of soils, and, for coastal flooding, the state of the tides.
The present El Niño is one of the strongest (if not the strongest) of the 20th century. There is no reason, however, for panic, nor is there any evidence to support the overreactions stimulated by early news reports this past summer. On the other hand, there is evidence to support a reasonable concern for a wetter-than-normal winter for most of the Southwest and even a much wetter-than-normal cool season in southern California.
John Monteverdi, Professor of Meteorology, Department of Geosciences, San Francisco State University (SFSU)
Jan Null, Adjunct Professor of Meteorology, SFSU, and Lead Forecaster, National Weather Service
Fu, C., H.F. Diaz, and J.O. Fletcher, 1986, Characteristics of the Response of Sea Surface Temperature in the Central Pacific Associated with Warm Episodes of the Southern Oscillation. Monthly Weather Review 114: pp. 1716-1738.
Schonher, T. and S.E. Nicholson, 1989, The Relationship between California Rainfall and ENSO Events, Journal of Climate 2: pp. 1258-1269.
A more detailed explanation of El Niño, also written by the authors, can be found on their
departmental Web site: http: //tornado.sfsu.edu/geosciences/elninhttp://www.colorado.edu/hazards/o/.
The authors can be contacted at the Department of Geosciences, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132; (415) 338-2061; fax: (415) 338-7705; e-mail: firstname.lastname@example.org and email@example.com.
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