|For one memorable winter, El Niño made the perfect scapegoat. The climate phenomenon obscure to many in the fall of 1997 was a household word by the spring of 1998, implicated in floods, droughts, tornadoes, and a major ice storm. Before and after the deluge of media hype, NCAR experts pursued their long-term analysis of El Niño, one of several ocean-atmosphere cycles that shape our global climate and might themselves be shaped by climate change.|
Peruvian navy captain Camilo Carillo makes one of the first public statements linking the name "El Niño" to the periodic warming noted just off the nation's coastline by Peruvian sailors.
On a huge sheet of poster board, NCAR senior scientist Michael Glantz has traced the outlines of experimental and operational climate prediction as we know it. There are fifteen rows, one for each of the world's major dynamical or statistical computer models that anticipate El Niño. The eight columns represent dates from March 1996 through September 1997. Within each cell of the array is a verbal outlook for El Niño--a model's best estimate of when the trade winds will weaken, the gentle east-to-west slope of the Pacific tropics will slacken, and waters warmer than 26 degrees Celsius (79 degrees Fahrenheit) will collect atop the normally cooler seas off the coasts of Ecuador and Peru.
One model correctly predicted an El Niño for 1997-98 as far in advance as December 1996. Most others didn't call for an event until it began to take shape in March-April 1997. And one model, a pioneer that correctly predicted the onset of two previous El Niño events, steadfastly said "no El Niño" this time, even as one of the century's great ones built to its peak intensity.
As an El Niño forecast tool, history is just as imperfect as computer models. El Niños arrive roughly once every three to six years, but the interval between visits can stretch from less than a year to as much as a decade. El Niños have become more frequent and lengthier over the past 20 years. Meanwhile, La Niña--a periodic cooling of the eastern tropical Pacific that historically alternates with El Niño--has occurred less often in this period.
As a social scientist, Glantz knows how hard it can be for people and computers alike to get a handle on El Niño. He began researching its human side not long after coming to NCAR in 1974, well before El Niño was familiar to most atmospheric scientists or the public. For years Glantz has organized meetings and chaired working groups to promote awareness of El Niño and its impacts, largely through activities funded by the United Nations Environment Program. His timely 1996 book, Currents of Change, the first popular paperback overview of El Niño, became a surprise international hit.
Glantz enjoys asking questions, and El Niño provides him with lots of opportunities to do so. The term is Spanish for "the boy child"--or, when capitalized, "the Christ child"--because Peruvian sailors and fishers, who named the phenomenon, most often observed it around Christmas. Scientists refer to the coupled ocean-atmosphere event as the El Niño/Southern Oscillation, or ENSO.
As the eastern equatorial Pacific warmed through the spring of 1997, scientists around the globe put forth dramatic predictions of likely El Niño impacts for the winter of 1997-98. Their outlooks were among the best-publicized climate forecasts in history and, in large part, they were correct. Drought and fire plagued Indonesia. Floods washed away parts of South America, California, the U.S. Gulf Coast, and sub-Saharan Africa. In addition, a freak ice storm paralyzed Quebec and northern New England, and killer tornadoes plowed across the Deep South.
Still, Glantz wonders how to gauge the success of the forecasts of El Niño impacts. It's hard to determine exactly how much El Niño had to do with some of the events just listed, such as the tornadoes and ice storm. And some classic El Niño effects weren't seen at all. Contrary to expectations, the Indian monsoon arrived on schedule, and Australians got a decent winter-wheat crop, thanks to timely rains in the midst of a prolonged drought.
As El Niño builds and warm water spreads across a vast surface, the water helps to warm the atmosphere above it. The globally averaged air temperature can climb more than one-half degree F during a strong El Niño. Could the recent tendency of El Niño to linger have anything to do with global warming? Kevin Trenberth thinks it might. The NCAR senior scientist was the first to rigorously connect the 1988 U.S. drought and heat wave with atmospheric circulation changes triggered by that year's La Niña. He is also one of the first scientists to assert that global climate change may be prodding the Pacific into more frequent El Niños.
The idea put forth by Trenberth and collaborator De Zheng Sun (Cooperative Institute for Research in the Environmental Sciences) is that El Niño acts as a release valve to disperse heat that builds up in the so-called warm pool of the western tropical Pacific. By the same token, La Niña allows the warm pool to recharge. The ocean-atmosphere system oscillates between El Niño and La Niña based on the strength of each event (the amount of heat exchanged) and the time span between them. If human activities are in fact enhancing the atmosphere's natural greenhouse effect, then some of the added heat should be passing into the oceans. In fact, water temperatures have risen in recent decades across the entire tropical Pacific. However, because the warmest tropical waters are already dispersing as much heat as they can through evaporation, "something has to happen to get the [extra] heat out of the tropics," says Trenberth, "and the something that happens is El Niño."
El Niño causes droughts in some locations and floods in others. Both are apt to be exacerbated by global warming, Trenberth points out. Additional heating not only raises temperatures, but also evaporates surface moisture, if present. Dry areas thus can become drier while the added moisture in the atmosphere is overlaid onto patterns of rainfall. Since the early 1970s--as El Niño has paid eight visits and La Niña just three--the atmosphere over the United States has moistened by about 10%.
One of the challenges in diagnosing and predicting ENSO is its roots in the depths of the tropical Pacific. Ocean temperatures near the equator are monitored down to 500 meters (1,600 feet) by an array of buoys installed by the Tropical Ocean and Global Atmosphere Program in the late 1980s. The TOGA buoys correctly signaled the formation of the 1997-98 El Niño. But the buoy data are once-per-day slices through a vast ocean that is otherwise poorly observed.
|James Hurrell (left) and Kevin Trenberth are working to connect the El Nino/Southern Oscillation (ENSO) and the lesser-known North Atlantic Oscillation (NAO) with global temperature trends.|
Another NCAR researcher has focused on the Atlantic, where a separate cyclical climate pattern has its own effect on Northern Hemisphere climate, especially temperature. NCAR's James Hurrell has been studying the North Atlantic Oscillation, a seesaw in atmospheric pressure between Iceland and the Azores. The NAO switches modes even less predictably than ENSO. It alters European winters by either pulling in moist Atlantic systems (its positive mode) or shunting them away and allowing frigid Siberian air to infiltrate (its negative mode). The positive mode has dominated since the 1970s, giving northern Europe a string of warm, wet winters but helping to keep the Mediterranean region dry.
When Hurrell studied the combined effects of ENSO and the NAO, he found that they explained nearly half of the wintertime temperature variation across the Northern Hemisphere. The moisture and clouds driven inland by El Niño and the NAO's positive mode help impede the radiation from earth to space that can produce intense winter cold. Thus, the continents stay warmer than normal, while the cooling of marine air is tempered by the presence of water below. Overall, the hemisphere warms--especially near the surface.
Statistics provide another tool for making sense out of the quirks in the El Niño record. Trenberth and NCAR colleague Timothy Hoar used a statistical technique to extrapolate the past century's data and see how a million years of El Niños and La Niñas might look. The simulation suggests that an event as long-lived as the 1990-95 El Niño would be expected only about once per 2,000 years unless some long-term change was pushing the climate system in that direction. Follow-up work--and the 1997-98 El Niño--reinforced the conclusion that ENSO has been behaving strangely since the 1970s.
It's easier to make statistical statements about ENSO events as a group than to produce one in a computer model. The ocean-atmosphere twitches that set off each El Niño or La Niña are too subtle and varied to be captured by most of today's models. Glantz notes that the most confident forecasts of the 1997-98 El Niño weren't issued until ocean buoy data settled the disagreement among computer models in June 1997. Still, that provided policy makers and the public with a useful few months' warning of the potential effects. An analysis by Trenberth found that several models were on track by early 1997, most models did a good job of predicting El Niño's December 1997 peak, and at least one global circulation model successfully pointed to regions where classic and not-so-classic El Niño effects would occur.
In 1998, Glantz launched a collaboration with the United Nations University, based in Tokyo, to help Pacific Rim nations collaborate on mitigating ENSO's effects. The program kicked off in July 1998 at NCAR with the world's first summit centered on La Niña. The meeting gathered several dozen global experts to discuss what is known about El Niño's less-studied counterpart. As with his El Niño book, Glantz's timing was impeccable: the meeting took place just as a new La Niña appeared to be brewing in the Pacific.
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