Blowing Hot And Cold

By J. Madeleine Nash/Boulder

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.

The distinction between La Nina and so-called normal conditions is not at all obvious. El Nino, for example, is truly an aberration. It makes dry places wet, wet places dry, warm places cold and cold places warm. By contrast, La Nina appears to exaggerate conditions that are more or less normal. Thus, under La Nina's sway, the Indonesian archipelago, which is usually wet, should expect to receive substantially more rain than it got last year. But in the absence of La Nina, Indonesia would still receive a lot of rain. Similarly, Canada and Alaska, which tend to be cold in winter anyway, might well be colder under La Nina conditions. During the 1995-96 La Nina, for example, Winnipeg suffered daytime temperatures of -4[degrees]F or less for more than a week, and overnight lows fell to -22[degrees]F or below for 19 days running.

In the U.S., the big question is what's going to happen in the coming hurricane season. The westerly wind patterns that El Nino fosters tend to shear off the tops of developing Atlantic Ocean hurricanes. Last year, for example, when El Nino was firmly in control, the Atlantic hurricane season was over almost before it began. La Nina, by contrast, partners with wind patterns that favor the formation of Atlantic Ocean hurricanes. The problem is that forecasters at the moment are looking at a mixed picture. While sea-surface temperatures in parts of the tropical Pacific have dropped precipitately, there are still substantial patches of warmth hanging around South America's Pacific coast. Once this warmth dissipates, tropical storms will be able to march across the Atlantic unopposed.

The prognosis? Right now, a fading El Nino and a burgeoning La Nina appear to be locked in a struggle for dominance. As Kevin Trenberth, an NCAR climatologist, put it last week, "It's a war out there." But even if La Nina wins the battle, as many scientists now expect, she'll have a hard time overshadowing her more famous brother. In June, owing in part to El Nino and in part to some longer-term warming trends, global mean temperatures reached an all-time high. The first six months of 1998 have already entered the record books as the warmest in the past 100 years.