The El Nino Climate Connection
Meteorologists analyze effects of Pacific Ocean-atmosphere interaction on world weather
BOSTON — FLOODS in Texas. Typhoons in Japan. Warmer than normal surface temperatures across the central and eastern equatorial regions of the Pacific Ocean. It's the El Nino connection - or, to use a geophysical term, teleconnection.
The buildup of that equatorial warm-water pool over the past half year marks the return of the recurring phenomenon meteorologists call El Nino. It's a subtle trick the ocean-atmosphere system performs every two to 10 years that has worldwide weather impact. It has caught meteorologists around the world off guard in the past. This time, they were ready for it.
El Nino investigator James O'Brien at Florida State University in Tallahassee says El Nino research has accelerated since the last event faded out around 1987. This has yielded two important results: "A: we now can forecast it; B: we can specify its weather impacts better," he says.
This information can help affected regions prepare for El Nino-related weather changes. For example, with such forewarning, farmers in Peru, where El Nino brings heavy rains, could shift to rain-tolerant crops such as rice and avoid the catastrophic losses they used to suffer.
Perhaps the most significant result for meteorologists is that they now realize how seriously they have to take what once seemed a climatic curiosity. Climatologist Timothy Barnett of Scripps Institution of Oceanography in La Jolla, Calif., agrees that "what we've learned in the past five years is really incredible." He explains that the El Nino phenomenon now "looks like a normal mode of the coupled ocean-atmosphere system." He says, "It's the biggest short-term climate event on the planet, and it affec ts billions of people."
Dr. Barnett points out, for example, that the atmospheric circulation that locked in over North America this winter "is kind of a classical El Nino pattern" with a high-pressure ridge in the West and a low-pressure trough in the East. That's a long way from the equatorial Pacific. But the tie-in seems clear.
Climate investigator Kevin Trenberth at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., agrees: "The fact that storms have penetrated that far south [into Texas] and linked up with moisture sources to cause intense flooding is consistent with an El Nino circulation pattern."
Meanwhile, Japan experienced several times the normal number of typhoons last year. The Japan Meteorological Office has blamed this on El Nino too.
Dr. O'Brien says he finds that plausible. As an El Nino condition develops, sea-surface temperatures cool in the Western Pacific and rise in central and eastern tropical regions. The result is a greatly expanded area of above-average sea surface temperatures. O'Brien explains that hurricanes and typhoons feed on energy they draw from warm water. They die out when they pass over colder water or over land. The El Nino warm water pool allows typhoons spawned in the tropics to travel thousands of kilometers.
So more of them probably would survive all the way to Japan, O'Brien says.
Meteorologists consider El Nino to be part of what they call the southern oscillation, hence they speak of an ENSO (El Nino/ Southern Oscillation) event. In this oscillation, sea-level air pressure seesaws between the Australian-Indonesian region and the eastern subtropical Pacific. This affects the trade winds. S an ENSO develops, pressure becomes unusually high in the western Pacific and low in the east. The trade winds weaken over the central and eastern regions. The warm-water pool normally found in the western equatorial Pacific migrates eastward. The result is an expanded warm pool east of the international date line with an associated strongly convective atmospheric circulation.
As Trenberth notes, "This pumps a lot of heat into the atmosphere." It sets up circulation patterns that not only affect weather thousands of kilometers away but also act to counter the ENSO itself. Thus, he says, "The ENSO has a life cycle and will eventually peter out." He classes the present event as "moderate" and expects it may well begin to weaken in the spring.
The successful forecast of the 1991-92 El Nino is a triumph of computer-model-based prediction over what O'Brien calls old-fashioned "seat-of-the-pants" informed guesswork. There are three model-based forecasting teams located, respectively, at Columbia University's Lamont-Doherty Geological Observatory near New York City, at Florida State University, and at Scripps Institution of Oceanography. Inspired guesswork predicted an El Nino starting early last year. The model-based forecasts correctly said "no"
but then predicted the current event.
O'Brien says the model-based techniques now provide reliable El Nino predictions. He also notes that meteorologists have a better knowledge of where El Nino will affect weather. It has little influence in Europe or in China north of Tibet, for example. But it has major influence in the Americas and the Southern Hemisphere Pacific region.
El Nino might seem to be a climatic "bad guy" bringing destructive weather patterns. Yet research at Florida State University and at the University of Arizona and US Geological Survey Office in Tucson shows that abnormal rains associated with El Nino reduce wild fires. El Nino also puts a lid on Atlantic tropical hurricanes. "When it suppresses forest fires and hurricanes, El Nino is actually a 'good guy, O'Brien observes.