UCAR News Release

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2002-3

FOR IMMEDIATE RELEASE: March 18, 2002

Tip Sheet: Drought Experts, Background,
and Web Sites

BOULDER—As a dry winter turns toward spring, lack of precipitation and persistent water shortages threaten to produce a record-setting drought across the northeastern United States and other parts of the country. If drought persists into the summer months, the risk for wildfires will also increase.

Persistent large weather patterns this winter, perhaps related to the developing El Niño in the Pacific, have produced wetter than normal conditions in northern California and much drier, warmer conditions in southern California, Arizona, and states east of the Rockies. Forecasters will be keeping an eye on how weather patterns influenced by El Niño set up this spring, a critical period for the drought-stricken East Coast from Maine to Florida.

This tip sheet lists experts who can comment on the current situation and place it in global or historical perspective. Answers to key questions and an annotated list of Web sites suggesting where to get current drought maps and additional data follow.

The researchers listed are scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, or one of the 66 universities that make up the University Corporation for Atmospheric Research (UCAR). NCAR is managed by UCAR under sponsorship of the National Science Foundation.

Drought experts

Heidi Cullen 303-497-8132 hcullen@ucar.edu
Scientist, NCAR
Specialty: Possible links between La Niña and the multiyear drought in Central-Southwest Asia, including Afghanistan; ways to provide better information to national and international organizations responding to drought.

Michael Glantz 303-497-8119 glantz@ucar.edu
Senior scientist, NCAR
Specialty: Societal effects of weather and climate around the globe, including drought across Africa, Afghanistan, and other parts of Central Asia; relationship between drought and agricultural practices; drought and flood impacts of El Niño worldwide.

Robert Harriss 303-497-8106 harriss@ucar.edu
Director, Environmental and Societal Impacts Group, NCAR
Specialty: The cascading impacts of drought on urban environments; linkages among drought's effects on people, water infrastructure, and nature; ability of agricultural systems to adapt to drought.

Eugenia Kalnay 301-405-5370 ekalnay@atmos.umd.edu
Professor and chair, Department of Meteorology, University of Maryland
Specialty: Improving the ability of computer models to forecast the weather; using these numerical weather models to predict regional climate; examining large weather patterns and regional feedbacks to understand causes of climate anomalies.

Gerald Meehl 303-497-1331 meehl@ucar.edu
Senior scientist, NCAR
Specialty: Climate variability and projections of future climate, including projections of increased likelihood of floods and drought; relationship of El Niño to drought in the Pacific region.

Kevin Trenberth 303-497-1318 trenbert@ucar.edu
Head, Climate Analysis Section, NCAR
Specialty: Analysis of El Niño and other global climate regimes, including persistent patterns that can lead to drought or floods; the global water cycle; relationship between climate change (including global warming) and extremes of weather and climate, such as floods and drought.

Olga Wilhelmi 303-497-8126 olgaw@ucar.edu
Visiting Scientist, NCAR
Specialty: Drought impacts and ways of assessing and planning for them; development of drought vulnerability assessment techniques using remote sensing and Geographic Information Systems; delivery of risk information to multiple users, including farmers and water resource planners.

Donald Wilhite 402-472-4270 dwilhite1@unl.edu
Director, National Drought Mitigation Center/International Drought Information Center, University of Nebraska-Lincoln
Specialty: Drought policy, including governmental responses to drought, planning methodologies, and identifying appropriate policy instruments.

Connie Woodhouse 303-497-6297 woodhous@ngdc.noaa.gov
Scientist, University of Colorado and NOAA Paleoclimatology Program
Specialty: Analysis of tree ring data and other proxy records of paleoclimate to understand natural climate variability in the past several thousand years; reconstruction of drought and streamflow in the Great Plains and central Rocky Mountains.

Some frequently asked questions about drought

How is drought measured?

Drought, which is challenging to define, can be assessed in a number of ways. Scientists often analyze rainfall relative to the average for a given season, because a given amount of rain may be sufficient or deficient depending on the location and time of year. From a meteorological standpoint drought is characterized by a prolonged lack of rainfall over a period ranging from weeks to years. From a hydrological standpoint it is assessed in terms of a decline in river runoff; depletion of lakes, reservoirs, and aquifers; and other measures of water supply. From an agricultural standpoint it is measured in terms of soil moisture. These components are not independent, and their relationship varies according to the intensity and frequency of precipitation, not just the amount. Clouds play a role, too, by influencing evaporation rates.

It may take months for a meteorological drought to produce hydrological or agricultural effects. Some crops can get through a short-term meteorological drought if the long-term subsoil moisture is not yet depleted, and plants may survive a long-term hydrological drought if rains return at the right time. Another key effect of drought is the heightened increase in wildfires.

How do forecasters predict the beginning and end of drought?

Drought occurs somewhere in the United States every summer. Small, local droughts happen by chance when rainstorms simply miss an area. At the regional level, drought is usually associated with unusual weather patterns and changes in storm tracks, some of which are at least partly predictable. Regional droughts begin quietly. Often it takes months for a rainfall deficit to become large enough to affect water supplies. Once a drought occurs, it’s difficult to predict when it will end. In July and August, the amount of evaporation usually exceeds precipitation over the continental United States, so a drought established in springtime is unlikely to end in summer, unless a tropical storm drenches the area.

Normally, some of the energy that reaches the Earth’s surface from the Sun goes into evaporating moisture. If the ground is parched, more energy is available for heating the ground, and this extra heating (especially in the summer) can help generate heat waves. The absence of surface moisture can also help create weather patterns that in turn favor less rainfall.

What do El Niño or La Niña have to do with drought?

Both El Niño and La Niña set up persistent, unusual weather patterns and create regional droughts around the world. El Niño–caused dryness in Southern Asia, Indonesia, Australia, and Northeast Brazil is replaced by increased rains when La Niña is in effect. And while La Niña brings dry conditions to Ecuador, Peru, and parts of Africa, those same regions are at risk for floods during El Niño events. Researchers now suspect the drought since 1998 in Central-Southwest Asia may be related to La Niña conditions.

In the United States during winter, El Niño tends to produce above-average rainfall across the southern tier of states from California to Florida, with below-average precipitation across the Pacific Northwest and Northern Plains. In spring and summer, northern parts of the United States have often been wetter during El Niño events, while other areas have been drier. For example, the floods in the upper Mississippi basin during the spring and summer of 1993 were influenced by El Niño. There's some tendency for these patterns to be reversed during La Niña.

It's important to note, however, that both short- and long-term drought can occur independently of these phenomena.

Could global warming lead to more droughts in the future?

As the atmosphere warms, the warmer air can hold more water. In addition, some of the heating associated with global warming goes into evaporating moisture from the Earth's surface. That evaporative drying increases the risk of drought in general, according to NCAR's Kevin Trenberth. Additionally, because there's more moisture in the atmosphere, weather systems such as thunderstorms or snowstorms can gather in more moisture before releasing it. The result may be stronger storms with more rain or snow where these occur. "With global warming we expect greater extremes in the weather, with more risk of both floods and droughts," Trenberth says.

Web sites

National Drought Mitigation Center
The NDMC is one of the nation’s chief sources of current detail and background materials on drought and its consequences. The site offers definitions, indices, and many links to other drought-related Web sites. The “Drought Watch” section links to the multi-institutional Drought Monitor. Experts synthesize a number of drought indices to produce the monitor’s map of current U.S. drought impacts, issued weekly on Thursdays.

USGS Water Watch
Streamflow levels at hundreds of U.S. Geological Survey gauges across the nation are depicted as a percentage of normal for the time of year. Users can plot the average streamflow across a region (such as the mid-Atlantic) over a year’s time or more.

CPC Drought Severity Index
Here NOAA’s Climate Prediction Center provides a weekly analysis of U.S. drought as measured through the long-term Palmer Index. The index is sensitive to groundwater trends over many months, so it may not immediately reflect the strength of a short, intense dry spell.

NOAA/CPC Current Monthly and Seasonal Outlooks
Also from CPC, this site provides outlooks of above- or below-normal precipitation across the United States for the next month and for overlapping three-month intervals that extend more than a year out. The forecasts are updated near the middle of each month.

IRI Southwest Asia Drought Forecast
The International Research Institute for Climate Prediction worked with a multi-institutional team to prepare this forecast for winter 2001–02 and spring 2002 for Afghanistan and the surrounding region. The site includes a discussion of the possible influence of La Niña on the Southwest Asian drought beginning in 1998.

NOAA's Drought Information Center
A roundup of links to NOAA and other sites, this page provides drought information sites for 12 states, as well as links to current data on U.S. soil moisture and the weekly crop moisture index.

North American Drought: A Paleo Perspective
This site, from NOAA's Paleoclimatology Program, explains how data from tree rings, fossilized pollen, and other proxy data provide information about past droughts and the natural variability of drought in North America on time scales from decades to millennia. Includes discussion of the 1930s Dust Bowl and what's needed to better predict future droughts.