Tip Sheet: La Niña

BOULDER -- With surprising speed, the past year's El Niño has evolved into its oceanic counterpart, La Niña. This cooling of the central and eastern tropical Pacific may already have influenced this fall's string of powerful Atlantic hurricanes, from Bonnie to Mitch. This winter, La Niña could help trigger dramatic temperature swings across the central and eastern United States and increase the likelihood of drought across the Southeast and Southwest. What do the world's leading La Niña scientists expect?

A new World Wide Web site brings together the latest scientific consensus on La Niña and its impacts. The site emerged from the world's first summit devoted to El Niño's less-studied counterpart. "Review of the Causes and Consequences of Cold Events: A La Niña Summit," hosted last July by the National Center for Atmospheric Research (NCAR), was organized by NCAR senior scientist Michael Glantz with support from the United Nations University (UNU), based in Tokyo. At the summit, more than 40 top researchers from universities and government agencies identified what is known and still unknown about La Niña and what societies need in order to issue forecasts and prepare for La Niña's impacts. An executive summary, along with papers submitted by the attending scientists, can be found via the Web site above. The site includes a wealth of La Niña links covering forecasts, impacts, and general information.

Below are some frequently asked questions about La Niña and key points from the summit, plus a list of La Niña experts and a rundown of other Web sites of interest.

NCAR's primary sponsor is the National Science Foundation. NCAR is managed by the University Corporation for Atmospheric Research, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences.

Key points from the La Niña Summit

Strong La Niñas do not always follow strong El Niños. Some strong El Niños have been followed by neutral conditions or by weak La Niñas.

Better ocean monitoring is needed for better predictions of El Niño and La Niña. The current network of buoys across the Pacific should be expanded to higher latitudes, and the Indian and Atlantic oceans need to be better monitored because of their influence on the climate signals that emanate from El Niño and La Niña. Early detection of El Niño and La Niña might be enhanced through such techniques as monitoring changes in ocean color (caused, for instance, by the growth of phytoplankton blooms) from space.

Public understanding of the probability shifts associated with El Niño and La Niña needs to be improved. Margins of error should accompany El Niño/La Niña outlooks. The public needs to understand that a location's long-term climate tendency associated with El Niño or La Niña does not guarantee a certain outcome--it merely shifts the odds in one direction or another.

Frequently asked questions

What causes La Niña? Typically, a La Niña is preceded by a buildup of cooler-than-normal subsurface waters in the tropical Pacific. Eastward-moving atmospheric and oceanic waves help bring the cold water to the surface through a complex series of events still being studied. In time, the easterly trade winds strengthen, cold upwelling off Peru and Ecuador intensifies, and sea-surface temperatures (SSTs) drop below normal. During the 1988-89 La Niña, SSTs fell to as much as 4 degrees C (7 degrees F) below normal. Both La Niña and El Niño tend to peak during the Northern Hemisphere winter.

What's the difference between La Niña and El Niño? Both terms refer to large-scale changes in sea-surface temperature across the central and eastern tropical Pacific. Usually, sea-surface readings off South America's west coast range from the 60s to 70s F, while they exceed 80 degrees F in the "warm pool" located in the central and western Pacific. This warm pool expands to cover the tropics during El Niño but shrinks to the west during La Niña. The El Niño/Southern Oscillation (ENSO) is the coupled ocean-atmosphere process that includes both El Niño and La Niña.

Is there such a thing as "normal," aside from El Niño and La Niña? Over the long-term record, sea-surface temperatures in the central and eastern tropical Pacific diverge from normal in a roughly bell- curve fashion, with El Niño and La Niña at the tails of the curve. Some researchers argue there are only two states, El Niño and non-El Niño, while others believe either El Niño or La Niña is always present to a greater or lesser degree. According to one expert, NCAR's Kevin Trenberth, El Niños were present 31% of the time and La Niñas 23% of the time from 1950 to 1997, leaving about 46% of the period in a neutral state. The frequency of El Niños has increased in recent decades, a shift being studied for its possible relationship to global climate change.

Why hasn't the public heard much about La Niña before now? For many decades, scientists have known about the oscillation in atmospheric pressure across the tropical Pacific at the heart of both El Niño and La Niña. However, La Niña's effects on fisheries along the immediate coast of South America, where El Niño was named, are benign rather than destructive, so La Niña received relatively little attention there. Research on La Niña increased after its wider impacts (often called teleconnections) were recognized in the 1980s.

When have La Niñas occurred? The answer varies depending on the definition used. According to the National Centers for Environmental Prediction, this century's previous La Niñas began in 1903, 1906, 1909, 1916, 1924, 1928, 1938, 1950, 1954, 1964, 1970, 1973, 1975, 1988, and 1995. These events typically continued into the following spring. Since 1975, La Niñas have been roughly half as frequent as El Niños.

What are some of the U.S. weather and climate impacts related to La Niña?

Hurricanes: Hurricanes are more likely to form across the Atlantic Ocean and Gulf of Mexico during La Niña than El Niño. The Atlantic's two busiest back-to-back seasons on record--1995 and 1996--occurred near the beginning and end of the last La Niña. The 1998 season also has been unusually active.

Tornadoes: Despite the intense, frequent tornadoes during this past spring across the South and East, research at Florida State University shows that outbreaks of violent tornadoes east of the Mississippi River are actually more likely during springs that follow La Niña than during those that follow El Niño.

Precipitation: The Southeast, Great Plains, and Southwest tend to be drier than normal, while the Mississippi and Ohio Valleys are often wetter than usual. The Pacific Northwest tends to be wetter during La Niña than El Niño, a pattern that sometimes extends as far south as northern California.

Temperature: On average, colder than normal conditions become more likely across the northern U.S. and milder than normal conditions across the South and East. However, the greater atmospheric variability connected to La Niña suggests a greater potential for ups and downs in temperature. In a seasonal outlook issued last month, the Climate Prediction Center of the National Oceanic and Atmospheric Administration wrote, "We anticipate that periods of strikingly cold weather [alternating with] much milder weather may occur this winter."

Experts

Specialty: The use of satellite-derived winds, ocean color, sea-surface temperature and topography, and other remotely sensed data to track El Niño and La Niña and improve forecast techniques. An oceanographer, Busalacchi is experienced in tropical ocean modeling and coupled modeling of the tropical ocean and atmosphere.

Specialty: Interaction between climate anomalies and human activities. A social scientist, Glantz has studied societal impacts of ENSO for 23 years.

Specialty: The use of numerical climate models to study El Niño and La Niña and to gauge the likelihood of weather and climate impacts in North America and other regions.

Specialty: Teleconnections (the influence of variations in sea-surface temperature across the tropical Pacific on regional and local climate regimes around the world) and the physical mechanisms behind them.

Specialty: Development of ocean observing systems for climate studies; interpretation of resulting data to understand and predict climate variability.,

Specialty: Tropical climate and climate change. Meehl has studied ENSO phenomena using observations and global climate models and has analyzed links between ENSO and the Asian and Australian monsoons. (La Niña tends to enhance both of these.)

Specialty: Impacts (hurricanes, tornadoes, forest fires, agriculture) of ENSO on North America. O'Brien and colleagues have paid particular attention to La Niña's effects on the generation and intensity of U.S. tornadoes and hurricanes.

Specialty: Global climate analysis. Trenberth studies ENSO's interaction with global change and its impact on weather and climate anomalies. He was among the first to examine connections between the 1988-89 La Niña and the severe Midwest drought of 1988.

World Wide Web links

NCAR/Environmental and Societal Impacts Group

http://www.dir.ucar.edu/esig/lanina
This Web page for the La Niña Summit includes an executive summary and the text of a number of papers presented at the meeting.

http://www.dir.ucar.edu/esig/la_nina_home
Linked from this page are a number of La Niña forecasts and summaries of regional impacts.

NOAA/PMEL/TAO: What Is La Niña?

http://pmel.noaa.gov/toga-tao/la-nina-story.html
This overview is provided by the National Oceanic and Atmospheric Administration's Pacific Marine Environment Laboratory, which monitors both El Niño and La Niña with a set of more than 70 buoys across the tropical Pacific.

International Research Institute for Climate Prediction

http://iri.ldeo.columbia.edu/
This collaborative center sponsors a variety of ENSO research, highlighted by news updates and graphics on its main Web page.

NOAA Climate Prediction Center

http://www.nnic.noaa.gov/products/analysis_monitoring/ensostuff/lanina/index.html
This page includes detailed three-month maps of temperature and precipitation deviations observed during strong La Niñas.

http://nic.fb4.noaa.gov/products/predictions/
On this page are multiseason U.S. outlooks and special ENSO discussions.

NOAA/CIRES Climate Diagnostics Center (CDC)

http://www.cdc.noaa.gov/ENSO
Research findings, graphics, and animations on La Niña, El Niño, and the evolution of each can be found on this page. The CDC is operated by NOAA and the Cooperative Institute for Research in the Environmental Sciences (CIRES).

NASA/The Current State of the Tropical Pacific

http://nsipp.gsfc.nasa.gov/enso/
This page features recent satellite images, animations, research updates, and an ENSO primer available on line in English, Spanish, French, Portuguese, and Russian.

NCAR/Climate and Global Dynamics Division

http://www.ucar.edu/communications/lasers/elnino
This layperson's introduction to the workings of ENSO, "Children of the Tropics: El Niño and La Niña," was cowritten by NCAR senior scientist Kevin Trenberth.

http://neit.cgd.ucar.edu/cas/papers/clivar98_Santiago
In this scientific paper on the development and forecasts of the 1997-98 Niño, Kevin Trenberth touches on many of the issues related to predicting La Niña and El Niño.