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Hot on the heels of El Nino, La Nina arrives

November 13, 1998

With surprising speed, the past year's El Nino has evolved into its oceanic counterpart, La Nina. 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 Nina could help trigger dramatic temperature swings across the central and eastern United States and increase the likelihood of drought across the Southeast and Southwest.

A new Web site, http://www.dir.ucar.edu/esig/lanina, brings together the latest scientific consensus on La Nina and its impacts. The site emerged from the world's first summit devoted to El Nino's less-studied counterpart. "Review of the Causes and Consequences of Cold Events: A La Nina Summit," hosted last July by the National Center for Atmospheric Research, and 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 Nina and what societies need in order to issue forecasts and prepare for La Nina'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 Nina links covering forecasts, impacts and general information.

Below are some frequently asked questions about La Nina and key points from the summit.

Key points from the La Nina Summit

• On the whole, La Nina's climatic impacts are weaker and less consistent than El Nino's.
• While some locations around the globe have a symmetric response (for instance, Indonesia's tendency toward drought during El Nino and heavy rains during La Nina), other locations show no such symmetry. The predictability of U.S. climate impacts is somewhat lower for strong La Ninas than for strong El Ninos.
• Strong La Ninas do not always follow strong El Ninos. Some strong El Ninos have been followed by neutral conditions or by weak La Ninas.
• Better ocean monitoring is needed for better predictions of El Nino and La Nina. 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 Nino and La Nina. Early detection of El Nino and La Nina 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 Nino and La Nina needs to be improved. Margins of error should accompany El Nino/La Nina outlooks. The public needs to understand that a location's long-term climate tendency associated with El Nino or La Nina does not guarantee a certain outcome -- it merely shifts the odds in one direction or another.

Frequently asked questions

What causes La Nina?
Typically, a La Nina 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 Nina, SSTs fell to as much as 4 degrees C (7 degrees F) below normal. Both La Nina and El Nino tend to peak during the Northern Hemisphere winter.

What's the difference between La Nina and El Nino?
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" in the central and western Pacific. This warm pool expands to cover the tropics during El Nino but shrinks to the west during La Nina. The El Nino/Southern Oscillation (ENSO) is the coupled ocean-atmosphere process that includes both El Nino and La Nina.

Is there such a thing as "normal," aside from El Nino and La Nina?
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 Nino and La Nina at the tails of the curve. Some researchers argue there are only two states, El Nino and non-El Nino, while others believe either El Nino or La Nina is always present to a greater or lesser degree. According to Trenberth, El Ninos were present 31 percent of the time and La Ninas 23 percent of the time from 1950 to 1997, leaving about 46 percent of the period in a neutral state. The frequency of El Ninos 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 Nina before now?
For many decades, scientists have known about the oscillation in atmospheric pressure across the tropical Pacific at the heart of both El Nino and La Nina. However, La Nina's effects on fisheries along the immediate coast of South America, where El Nino was named, are benign rather than destructive, so La Nina received relatively little attention there. Research on La Nina increased after its wider impacts (often called teleconnections) were recognized in the 1980s.

When have La Ninas occurred?
The answer varies depending on the definition used. According to the National Centers for Environmental Prediction, this century's previous La Ninas 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 Ninas have been roughly half as frequent as El Ninos.

What are some of the U.S. weather and climate impacts related to La Nina?
Hurricanes   Hurricanes are more likely to form across the Atlantic Ocean and Gulf of Mexico during La Nina than El Nino. The Atlantic's two busiest back-to-back seasons on record -- 1995 and 1996 -- occurred near the beginning and end of the last La Nina. 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 Nina than during those that follow El Nino.

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 Nina than El Nino, 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 Nina 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."

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