Hot-and-cold Pacific fed Midwest drought.

Hot-and-cold Pacific fed Midwest drought

A group of meteorologists attributes this summer's severe drought in the north-central United States to a region of warm Pacific water that persisted unusually far north of the equator last spring. "The greenhouse effect almost certainly did not play a primary role in the drought," contends Kevin E. Trenberth of the National Center for Atmospheric Research (NCAR) in Boulder, Colo.

During April, May and June, water 1[deg.]F to 2[deg.]F warmer than normal occupied an area southeast of Hawaii, between 10[deg.]N and 20[deg.]N latitude and 120[deg.]W and 150[deg.]W longitude, Trenberth told an audience at the University of Maryland in College Park last week. Forecasters consider a warm Pacific region in spring normal, "but the northern location of the warm water was unusual," Trenberth told SCIENCE NEWS.

He says abnormally cold sea temperatures and dry air at the equator caused the warm water to settle so far noth. "Water at the equator went through one of the coldest periods ever recorded," with temperatures 3[deg.]F to 4[deg.]F below normal, Trenberth reports. He says the cold equatorial water resulted from feedback between the atmosphere and the ocean, which "constantly force changes in each other."

Along with NCAR colleague Grant W. Branstator and Phillip A. Arkin of the National Oceanic and Atmospheric Administration's Climate Analysis Center in Camp Springs, Md., Trenberth analyzed ocean and atmospheric data and satellite photographs of the Pacific -- a major source of U.S. weather -- for the months before and during the period of extremely low rainfall in the Midwest. The satellite images show that the warm-water region, with its thick cloud cover and heavy rainfall, acted like a rock in a river, diverting westerly winds northward.

This obstacle effect, says Trenberth, pushed the moisture-bearing jet stream into Canada and allowed a strong region of high pressure to build over the central United States. Once established, the high-pressure ridge hindered moisture from reaching the parched area. Trenberth says the greenhouse effect "could have been a small enhancement, making the summer heat waves even hotter."

Although areas in the West and Southeast have experienced prolonged periods of low precipitation during the past several years, those droughts have resulted from different climatic idiosyncrasies, Trenberth says.