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Fall 1998

Whither La Niña? A Boulder summit provides some clues

by Bob Henson
UCAR Communications

Michael Glantz. (Photo by Carlye Calvin.)

NCAR's Michael Glantz really knows how to time a conference. He organized an El Niño symposium for July 1997 long before anyone knew the second "El Niño of the century" would be well under way by that time. This summer, Glantz put together an international summit on La Niña. Once again, the earth system provided an apparently related real-world event, this time pointing to a La Niña in the works.

The 15-17 July meeting brought several dozen La Niña experts from 14 countries to NCAR's Mesa Laboratory in Boulder. It was sponsored by the United Nations University in Tokyo, to launch UNU's series of "usable science" projects focused on El Niño impacts and responses in developing countries; by the United Nations Environment Programme in Nairobi, Kenya; and by NSF, in collaboration with NCAR's Environmental and Societal Impacts Group (ESIG), where Glantz is a senior scientist and long-time specialist on the El Niño/Southern Oscillation (ENSO) phenomenon and its effects on society.

From one event to another

Leading up to the summit was oceanographic drama of a high order. Sea-surface temperatures across the central equatorial Pacific plummeted 8 degrees C (14 degrees F) in May alone. Just below the surface, a huge mass of colder-than-normal water spread across most of the tropical Pacific. "We've never seen anything like this before," said Michael McPhaden (NOAA/Pacific Marine Environmental Laboratory). " 'Why [in] May?' is an interesting question." He characterized slow-paced ocean dynamics as preconditioning the upper layers of the Pacific for a La Niña, with fast atmospheric dynamics (such as a shift in trade winds) then "throwing the switch" and bringing cold subsurface water to the surface. Other possible triggers mentioned by summitgoers included Kelvin waves and Madden-Julian oscillations in the Pacific and even in the Indian Ocean.

Despite its intensity, the Pacific cooldown could not be christened a La Niña at the July summit. For one thing, warm-water remnants of the 1997-98 El Niño still clung to the South American coast, producing an unusual mixed signature at the surface. Also, it was simply too early to make the call.

Although summitgoers agreed on the need for firm criteria to define these events--positive or negative ocean-temperature anomalies in the proper locations for a sufficient length of time--they weren't ready to agree on the criteria themselves. Many institutions and researchers have created their own indexes, which has led to some differences among lists of years for El Niño and La Niña events. "There's no universally agreed-upon definition," observed McPhaden, referring to a December 1997 article in the Bulletin of the American Meteorological Society by NCAR's Kevin Trenberth (Climate and Global Dynamics Division) on that very topic.

Toward that end, one of the summit's two working groups recommended that academia and laboratories unite to create a single multivariate index and form a clearinghouse of widely accepted information on ENSO events and effects. Such a site would not rule out other scientific positions, group members said, but it would provide credibility and accountability that could reduce public confusion.

A distorted mirror

The public's embryonic view of La Niña appears to be that it is a mirror image of El Niño. Generally speaking, that's not far off, but abundant asymmetries came to light at the summit. El Niño's effect on North American climate is largely driven by the presence of intense convection in the central and eastern Pacific tropics. The stronger the El Niño, according to this idea, the more convection it causes and the greater the U.S. impact. On the other hand, La Niña tends to calm convection in the eastern Pacific tropics--and once an event is strong enough to stop the rain, it can't "not rain harder," as McPhaden put it.

The most intense El Niños and La Niñas show the most asymmetry. Martin Hoerling (University of Colorado) examined North American anomalies for the four strongest recent El Niños and La Niñas, two of each. The pattern, he noted, "looks anything but symmetric." When averaged over each event, the centers of high and low pressure at midlevels were much more intense during the El Niños than the La Niñas, implying that La Niña's signal was diluted by variability.

More variability suggests a stronger parade of storms, with greater potential for ups and downs in temperature from week to week, noted George Kiladis (NOAA). However, since La Niña is a poor sibling to El Niño in terms of research, little rigorous work has been done to quantify these suspected effects. Outlooks that focus on seasonal averages--such as the tendency toward cold in the North and warmth in the Southeast during La Niña winters--could mask a secondary tendency toward stronger cold and warm spells sweeping across the nation in series.

Precipitation during La Niña has its own subtle but important distinctions. For example, in northern and central California El Niño produces more seasonal rainfall than normal (as it does in the rest of the state), but La Niña may actually be the greater hydrologic threat. Hoerling noted that La Niñas apparently produce more-intense, week-long stretches of rain by tapping tropical Pacific moisture (the so-called Pineapple Express) more effectively than El Niño. "We've seen this in the GCMs [general circulation models], and it's very compelling," he said. Maurice Roos (California Department of Water Resources) showed that three of the five biggest floods since 1950 on the American River, which runs through Sacramento, occurred during La Niñas, but none of the five occurred during El Niños.

International attendees brought reports of ENSO impacts and concerns from their own countries. A common theme was the benefit reaped during the 1997-98 El Niño from policies and forecast tools adopted in the 15 years since the last mega-El Niño. Drought struck Ethiopia in the summer of 1997, but an early-warning system helped prevent the kind of famine that occurred after the 1982-83 El Niño and associated drought. As for the heavy rains that La Niña could bring, "The scariest thing in Ethiopia is not floods [but drought]. . . . People wish for floods," said Tsegay Wolde-Georgis (Embassy of Ethiopia). He added that government warnings reduced the imapct of La Niña floods in the summer of 1996. In Nairobi, Kenya, eight months of El Niño rains helped drive out a plague of rats--but, said the United Nations Environment Programme's Peter Usher, "The city infrastructure is broken down." Australians are preparing for heavy rains and possible outbreaks of mosquito-borne diseases, such as Ross River virus and encephalitis, although it's been 20 years since a La Niña brought the latter. For now, according to Tahl Kestin (Monash University), "We're still a bit reeling from our very wet spring." The moisture--a surprise for an El Niño year--saved the nation's wheat crop.

In the background of the summit hung the context of a record-setting global heat spike that emerged with the most recent El Niño and continued into the summer (see related article on page XX). Such warming has played tricks with the long-term patterns that scientists use to ascertain ENSO impacts. NOAA's Climate Prediction Center tweaked the standard El Niño projection of coolness in the southern United States upward for its 1997-98 forecasts. According to CPC's Tony Barnston, "It's just hard to get cold in the Southeast with El Niño any more." Added Trenberth, "The problem with analogs or composites is that they assume the climate is stationary." Trenberth led a session on the connections between ENSO and global change, including the mini-global warmings and coolings that accompany El Niño and La Niña. (See "El Niño and global warming: What's the connection?" in the Winter 1997 issue of the UCAR Quarterly.)

Participants at the summit appeared to relish the chance, perhaps their first, to discuss La Niña in such depth. The phenomenon may be riding the coattails of her Pacific counterpart to her own day in the sun, but that's fine with Glantz. "Had there been no interest in the [1997-98] El Niño, there would likely have been no interest in La Niña at this time. But interest in La Niña is better late than never."

A summary report will be posted at the summit's Web site. Seel also ESIG's La Niña page.

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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Tue Apr 4 15:03:21 MDT 2000