Scientists Hope Shift in La Nina Pattern Will End Drought

FOR drought-stricken parts of middle North America, the recent arctic air invasion is a herald of hope. It fits the United States Climate Analysis Center's long-range outlook for a change in the global air-flow patterns that have steered moisture-bearing storms away from that region.

Analyst Tony Barnston says that, while the February long-range forecast calls for lighter than normal precipitation along much of the Eastern seaboard, it projects heavier precipitation through the Midwest all the way down to southeastern Texas. He explains that this means a 60 to 65 percent probability of greater than the median precipitation in this Midwestern area. ``We are hoping it will break the drought,'' he adds.

Looking farther ahead to the three-month period February through April, Mr. Barnston's unit again projects relatively heavy precipitation for much of the Midwest, while the Southeastern and extreme Southwestern states would receive relatively light precipitation.

One of the drought-inducing global weather influences likely to change this year is a phenomenon meteorologists have only recently taken seriously. They call it La Nina (the girl) because it is in some ways the counterpart of the famed El Nino (the Christmas boy).

El Nino brings unusually warm surface water temperatures over large areas of the tropical Pacific Ocean. It influences weather worldwide. La Nina brings cold sea surface temperatures to the same region. These cold temperatures combine with warmer than normal water to the north - specifically in a zone from about 10 degrees to 20 degrees north latitude.

What meteorologists call the Inter Tropical Convergence Zone (ITCZ) also moves northward. In the ITCZ, rising moisture-laden air generates widespread convective storms. The heat released by the water vapor condensing in these storms is one of the atmosphere's major energy supplies.

Last December, Kevin Trenberth and Grant Branstator of the National Center for Atmospheric Research, and Phillip Arkin of the Climate Analysis Center, published research in the journal Science showing that La Nina also has a wide reach. They linked its influence to the general upper-air flow pattern involved in the North American drought. They noted that there was no one cause for the 1988 drought, which developed at different times in different places. However, they link the rapid development of the Midwestern drought from April to June to the La Nina phenomenon.

Now the outlook is for La Nina to fade away. Barnston explains that ocean surface temperature patterns change slowly. ``I don't know whether it will subside in spring or summer,'' he says. But he adds, ``I don't expect next winter to be like this one.''

DR. TRENBERTH agrees. He explains: ``The favorite time for a La Nina change is March/April. I expect the ocean pattern to turn back to near normal. That's encouraging from the viewpoint of summer drought, too.''

Given that time frame, meteorologists wonder why the weather pattern over North America, with its promise of more moisture, is shifting earlier than expected. Because it's still cold along the equatorial Pacific Ocean, Trenberth says he would expect storm tracks to be displaced somewhat westward and northward from the Midwest. He adds: ``Something else is going on that I can't explain. When we look back at it after the fact, we may be able to say more.''

Barnston makes the same point. He says something is overriding the La Nina effect. The present shift, which he expects to bring more moisture to the Midwest, is not the beginning of the seasonal change from winter to spring either. He explains that, whatever the change is, it has to do with changes in the weekly fluctuations in the upper-air flow that steers storms. There is a lot of persistence in the climate system, he says. Once a change sets in, it tends to last a while.

Besides hoping that the change will break the Midwest drought, Barnston's unit expects warmer than normal temperatures in the eastern US and colder than normal in many western areas.