La Nina readies to steal El Nino's thunder

 Water temperatures in parts of the equatorial Pacific Ocean have dropped precipitously in the last 2 months, chilling El Nino's fever and setting the stage for the arrival of another climatic hooligan: La Nina.

 Meteorologists were divided earlier this year on whether an episode of La Nina cooling would follow El Nino's demise, but the recent Pacific shift has brought consensus. "All of the forecasts are consistently indicating that we will have a La Nina shortly and it will continue over the next winter," says Vernon E. Kousky of the National Oceanic and Atmospheric Administration (NOAA) in Camp Springs, Md.

 La Nina and its sibling, El Nino, are opposite extremes of a Pacific pendulum that causes the equatorial waters to swing from warm to cold and back again. During El Nino, warmth normally centered over Indonesia spreads eastward across the entire Pacific, dragging with it towering thunderheads that pump heat into the atmosphere. The storms redirect jet streams and turn typical weather patterns upside down around much of the globe.

 During La Nina, the equatorial Pacific waters turn cold except around Indonesia, drawing thunderstorms back to that part of the ocean basin. The Pacific jet stream weakens, and wintertime weather in the United States turns much more mercurial. "The jet stream tends to undulate more. During our winter, the jet stream may be way north in Alaska and then come diving down south in certain periods. This will be much more variable in the sense that we can go from one month to the next and have a dramatically different pattern," says Kousky.

 Measurements of water temperature, taken by buoys and satellites, show that a full-fledged La Nina has not yet developed. Between early May and early June, the equatorial temperatures fell markedly from 3@C above normal to 3@C below normal around 130@W, but the cooling stayed confined and has not spread westward. Moreover, warm remnants of El Nino linger to the north and south of the equator, helping to suppress rainfall needed to quench the fires raging in Mexico.

 Meteorologists will be watching in coming months to see whether water temperatures drop further to the west, near the International Date Line. That's a key spot affecting how the weather evolves downstream in North America, says Arthur J. Miller of the Scripps Institution of Oceanography in La Jolla, Calif.

 The recent El Nino tamed the winter weather in the United States, bringing mild temperatures to much of the country. Researchers say it is hard to forecast what kind of weather will arrive this winter as the Pacific enters its cold phase. "La Nina conditions are much more difficult to predict. It's not as simple as the reverse of El Nino," says Miller.

 La Nina tends to bring warmer-than-normal winter weather to the southeastern United States while the Northwest gets colder than normal, according to NOAA. The Pacific Northwest generally gets wet weather in late fall and early winter during La Nina, whereas the Southwest often dries out. "There is a strong potential for drought in the Southwest," says Kousky.