El Nino: it's back! (Climate).

When sea-surface temperatures across the equatorial Pacific Ocean measure 0.5[degrees]C above average for any particular 3-month period--as they did this year for April, May, and June--it heralds the onset of the worldwide weather maker known as El Nino.

Early this year, an increase in ocean temperatures in the central Pacific led researchers at the National Oceanic and Atmospheric Administration (NOAA) to suggest that an El Nino was on the way. Around the same time, NASA scientists analyzed patterns of wind and rainfall southeast of India and predicted an El Nino would appear between July and October (SN: 3/2/02, p. 142). From temperature data collected by satellites, buoys, and ships, NOAA made it official on July 11: The agency announced that the latest El Nino had indeed arrived.

The last El Nino, the strongest on record, began in 1997. It lasted about 13 months and, for that period, boosted average global land temperatures by more than 0.8[degrees]C. Although NOAA scientists aren't yet sure, they suspect that this year's El Nino will be weaker than 1997's says Jim Laver, director of the agency's Climate Prediction Center in Camp Springs, Md. That's because the temperature anomalies that the center's scientists have observed aren't as large and haven't grown as quickly as during the last event.

In any case, effects of the new El Nino probably won't shown up in the United States until fall. Typical El Nino winters bring warmer-than-normal temperatures to a region stretching from southern Alaska to the upper Midwest and enhance rainfall across the country's southern tier. Other effects include increased rainfall in Peru and drought in Indonesia and other areas of the western Pacific.--S.P.