Link to USGS home

In Reply Refer To:
Mail Stop 415


August 8, 1997

To: District Chiefs and Data Chiefs

From: Thomas H. Yorke, Chief, Office of Surface Water

Subject: El Nino Advisory

As you may be aware, NOAA's Climate Prediction Center issued an "El Nino/Southern Oscillation (ENSO) Diagnostic Advisory" on July 15, 1997. The advisory states that "model predictions indicate that warm episode conditions will continue throughout the remainder of the year and into 1998." Harry Lins, the Global Change Hydrology Program Coordinator in the Office of Surface Water, prepared a briefing statement on ENSO and its possible implications for the Water Resources Division (WRD) operations that is worthwhile reading for all WRD field personnel. As Harry points out, El Nino events tend to produce specific hydrologic responses in some regions of the country. The enhanced "predictability" associated with ENSO may provide WRD offices the opportunity to make use of long-lead forecast information in planning some of their winter and spring season operations. At the very least, it's a good opportunity for us to evaluate the utility of such forecast information for our stream gaging and related activities. I encourage each of you to carefully read the attached statement. I also encourage you to contact Harry directly should you have any questions.



Prepared by
Harry F. Lins, Office of Surface Water

Since early spring of this year, atmospheric and oceanic conditions in the tropical Pacific Ocean have increasingly indicated the onset of a major ENSO (El Nino-Southern Oscillation) warm episode. ENSO is a coupled ocean-atmosphere condition spanning the tropical Pacific basin that is the primary determinant of interannual climate variability worldwide. It is characterized by both warm (El Nino) or cold (La Nina) phases, and each produces quasi-predictable (i.e., preferred) effects on the U.S. This brief is designed to increase your awareness of the potential for anomalous and/or extreme hydrologic conditions that may accompany or follow this ENSO episode, as well as to familiarize you with some of the documented regional effects of El Nino.

Although ENSO is now known to have been occurring aperiodically for millenia, most of what we know about it has been acquired over the past 20 years. Indeed, it was not until the major El Nino of 1982-83 (considered to be the most intense El Nino of the 20th century) that enough instrumentation was in place to provide an adequate picture of the atmospheric and oceanic dynamics characteristic of ENSO. Today, our understanding of the phenonmenon is such that a number of groups around the world are making regular predictions of its various aspects. Currently these forecasts have only modest skill, but enough so that NOAA's National Centers for Environmental Prediction has made ENSO forecasting one of its primary long-lead experimental forecast products.

From research conducted as part of the USGS Global Change Hydrology Program, as well as from NSF- and NOAA-sponsored research in academia, we have learned that streamflow has "preferred" patterns of variation in some regions of the United States both during and after ENSO events. This is not to say that every time there's an El Nino or La Nina flows will increase in one place and/or decrease in another but, rather, to indicate a general tendency toward above- or below-normal flows. For example, streamflow is enhanced along the Gulf Coast and in the Southwest and North Central U.S., and is reduced in the Pacific Northwest and Northeast U.S. During the 20th century, the three largest droughts in the Pacific Northwest occurred in the year after an El Nino event. There is also some evidence for increased streamflow along the Alaskan coast during El Nino. However, areas such as California and British Columbia are not consistently connected to ENSO variations, although they have been connected in certain extreme cases. Notably, the strongest ENSO precipitation signal over North America affects the Gulf Coast region of the U.S. (including Texas) and parts of northern Mexico, where wetter than normal conditions are found during winter. Increased winter streamflow in Texas has also been connected to El Nino. For more information on the known teleconnections between ENSO and hydroclimatic conditions in the U.S., please refer to the papers listed at the end of this message.

Regarding the current event, sea surface temperatures in June and July were more than 2 degrees C above normal across the entire Pacific east of the 160 W longitude and more than 3 degrees C above normal along the west coast of South America. This event has developed so rapidly and intensely that it could possibly exceed the 1982-83 event in magnitude. Atmospheric circulation patterns consistent with those generally experienced during El Nino events were also evident in June and July. This resulted in drier than normal conditions over Indonesia and wetter than normal conditions over the islands of the central tropical Pacific. Details of these conditions are available in the online version of NOAA's Climate Diagnostics Bulletin at

The National Centers for Environmental Prediction statistical and dynamical model predictions indicate that warm episode (El Nino) conditions will continue throughout 1997 and into the first five months of 1998. Details of predicted temperature and precipitation conditions for the U.S. and Canada corresponding to the continuing warm episode, are also available at after clicking on "predictions" and then on "Experimental Long-Lead."

Given the growth in our knowledge base regarding ENSO and its effects, the current event poses an opportunity for all of us in the hydrology business to evaluate hydrologic variability and patterns of persistence over the next 6 to 12 months in a new light. In particular, Districts in the Pacific Northwest (Washington, Oregon, Idaho, and parts of Montana, Utah and Nevada) and the Northeast (Pennsylvania, New Jersey, Maryland, Delaware and parts of Virginia) should be sensitive to a potential for drier than normal conditions, and possibly severe drought during the upcoming winter. Districts in the Southwest, North Central, and Gulf Coast regions (including all or parts of California, Arizona, New Mexico, Texas, Louisiana, Alabama, Georgia, Florida, North Dakota, South Dakota, Minnesota, and Iowa) should be anticipating a potential for above normal streamflow conditions and possible flooding during the coming winter and spring months. All District personnel should be encouraged to become more familiar with ENSO and its hydrologic effects. They should also feel free to contact Harry Lins, Office of Surface Water (hlins) for additional information.

Selected References:

Cayan, D.R., and Webb, R.H., 1992, El Nino/Southern Oscillation and streamflow in the western United States: in, Diaz, H.F., and Markgraf, V., eds., El Nino: Historical and Paleoclimatic Aspects of the Southern Oscillation. Cambridge University Press, Cambridge, 476 p.

Kahya, E., and Dracup, J.A., 1993, U.S. streamflow patterns in relation to the El Nino/Southern Oscillation: Water Resources Research, v. 29, pp. 2491-2503.

Piechota, T.C., and Dracup, J.A., 1996, Drought and regional hydrologic variation in the United States: Associations with the El Nino-South Oscillation: Water Resources Research, v. 32, pp. 1359-1373.

[an error occurred while processing this directive]