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.
Attachment
INCIPIENT EL NINO AND POSSIBLE IMPLICATIONS FOR WRD OPERATIONS
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 http://www.cpc.ncep.noaa.gov/
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 http://nic.fb4.noaa.gov 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.
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