The greatest El Nio of this century ended abruptly six months ago - or did it?
Satellite data suggest that recovery from the record-breaking warmth that spread across the equatorial Pacific Ocean and disrupted worldwide weather has stalled, according to the Jet Propulsion Laboratory in Pasadena, Calif.
The JPL report added a cautionary note to El Nio researchers' discussions during last month's meeting of the American Geophysical Union. As Antonio Busalacchi from the NASA Goddard Space Flight Center in Greenbelt, Md., noted, the 1997-1998 event was the first El Nio to be "observed globally from start to finish." Data from deep ocean buoys, land stations, and several satellites have given scientists unprecedented insight into El Nio phenomena. Yet El Nio's unexpected reluctance to depart reminds them that they still have a lot to learn about our planet's most important short-term climate oscillation.
Nevertheless, scientists, for the first time, have been able to link El Nio directly with droughts, floods, and other extreme weather around the Northern Hemisphere. They often gave useful warning of those events three to six months in advance. Ants Leetmaa, who heads the National Oceanic and Atmospheric Administration's Climate Prediction Center in Camp Springs, Md., says, "It's reassuring to us" that the understanding that scientists did have produced useful forecasts.
El Nio is a complex interaction of air and sea along the equatorial Pacific. Normally, easterly wind patterns shift and even reverse. Warm sea-surface water normally pooled in the western Pacific spreads eastward. More moisture rises into the air as thunderstorm activity increases. This feeds energy into the atmosphere that winds carry long distances. Dr. Busalacchi and other scientists at the meeting explained that this is a short-term climate shift. It is not weather. It causes hemispherewide shifts in jet streams and other atmospheric circulation features that "load the dice" in favor of extreme local weather, Busalacchi said.
With hindsight, scientists now can link the El Nio induced circulation changes directly to some regional weather events. These include drought in Australia and floods in parts of South America. According to the NOAA Climate Diagnostics Center, analyses now show "a strong link" between the Pacific warming and the catastrophic ice storm that hit southeastern Canada and the northeastern United States last January.
California's heavy rains last February also link to El Nio. But the Pacific warming had much less effect on the state's rains earlier that winter.
The NOAA report notes that the new understanding of El Nio "contributed not only to the very high skill of seasonal forecasts ... but also to increased skill in extended-range weather forecasts (5 days to 2 weeks)." Dr. Leetmaa reflected his colleagues' pride in this skill when he observed that "we were light years ahead of the last major El Nio" 15 years ago.
But there are caveats. Not all the forecasts were successful. And false alarms can be more dangerous than having no forecast at all, as Jennifer Philips from Columbia University in New York pointed out. She noted that forecasts of El Nio related drought in Zimbabwe so intimidated farmers that they planted minimum crops. In fact, there was adequate rain. But the poor planting caused an unnecessary food shortage. There needs to be better advance planning on how best to use long-range climate forecasts, she said.
Tim Barnett from Scripps Institution of Oceanography in La Jolla, Calif., explained that lesser climate oscillations that occur in various regions are also important. They include such effects as the North Atlantic Oscillation that brings periodic drought to parts of Africa. "The next frontier of research" is to learn how these oscillations affect distant weather and how they interact with each other and with El Nio, Dr. Barnett said.
Meanwhile, climate watchers will not be blindsided by El Nio again. The observing system now in place will continuously monitor the equatorial Pacific. The Tropical Atmosphere Ocean Array of 70 instrumented buoys strung across the Pacific is particularly important. Before TAO, "we were working blind," Busalacchi says.