Blowing Hot And Cold

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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.