Blowing Hot And Cold

When scientists first spotted it last fall, it was just a wedge of chilly water, parked at a depth of 70 fathoms in the western Pacific and extending from Papua New Guinea to the international dateline. As they tracked it over the next few months, following its development through a vast network of buoys tethered to the sea floor, it slowly expanded up and east, toward South America. Now, like a spume-blowing whale, it has broken through to the surface, forcing temperatures across a 5,000-mile strip of ocean to drop more than 15[degrees]F in just four weeks.

This was the sign meteorologists had been waiting for. Not only does the sharp fall in ocean temperatures signal the breakup of the giant pool of warm water in the tropical Pacific that triggered one of the century's greatest El Ninos, but it may also signal the birth of El Nino's unruly twin sister, the climatological reversal that scientists call La Nina ("the girl").

For Americans suffering through one of the worst heat waves of the century, a break in the weather can't come too soon. El Nino is at least partly responsible for the scorching drought and record temperatures that have been blamed for 50 deaths across the U.S., 30 of them in Texas, which has hit triple digits every day for two weeks. With El Nino's help, a high-pressure zone has been anchored over the South for several months, robbing places like Texas and Florida of the thunderstorms and cooling rains that usually bring relief at this time of year.

Trouble is, La Nina is likely to bring her own set of weather problems. Last week scientists meeting in Boulder, Colo., at a La Nina summit sponsored by the National Center for Atmospheric Research (NCAR) sketched out a lengthy list: more Atlantic Ocean hurricanes. Colder winters across Canada. Wetter winters in the Pacific Northwest. Warmer, dryer winters in the Southern U.S. More wildfires in Florida. Lower wheat yields in Argentina. Torrential rains in Southeast Asia.

Broadly speaking, La Nina is the flip side of El Nino. But as the scientists at last week's workshop agreed, it is not just a mirror image. For one thing, La Ninas in general are never quite as cold as El Ninos are warm. Also, while El Ninos grow in strength with each degree of change in ocean temperature, La Ninas do not. The reason can be traced to the physics that links the atmosphere to the ocean. What allows El Nino to affect weather worldwide is the intrusion of unusually warm water into the eastern Pacific. As this happens, storms (which feed off warm water) inevitably move eastward. But once the eastern Pacific cools, storm formation in this region shuts down. At that point, any further cooling triggered by La Nina can have only a small effect.