Temperature Of Pacific Ocean Influences Midwest Rains, Scholar Says

"The flood of 1993 and the drought of 1988 raised serious questions about the causes of summertime climatic fluctuations over the central United States," said Mingfang Ting, professor of atmospheric sciences at the U. of I. "The closer we come to answering those questions, the better our predictions will become. That should lessen the tremendous socio-economic impacts caused by severe floods and droughts."

To identify a clearer cause-and-effect relationship between ocean and atmosphere, Ting and graduate student Hui Wang analyzed climatological data gathered from 1950 to 1990. Included in the study were precipitation measurements collected throughout the Midwest and sea-surface temperatures from both the tropical and northern regions of the Pacific Ocean. In general, the warmer the ocean, the wetter the weather, the researchers found.

But it's not a simple matter of moisture evaporating from the Pacific Ocean being dropped over the Great Plains, Ting said. "Because the mountains along the West Coast effectively block most of the moisture arriving from the Pacific Ocean, the moisture source for the central United States is actually the Gulf of Mexico. So, the temperature of the Pacific Ocean is also influencing the moisture transport from the Gulf of Mexico."

Ting believes the sea-surface temperatures in the Pacific Ocean affect both the position and the intensity of the jet stream over the central United States, which in turn modifies the circulation pattern from the gulf. "Warmer sea-surface temperatures shift the jet stream farther south, leading to more storm activity, which pumps more moisture up from the gulf," Ting said. "In contrast, cooler sea-surface temperatures shift the jet stream farther north, resulting in reduced storm activity and drier conditions."

While meteorologists have long recognized a connection between abnormally warm or cold conditions in the tropical Pacific and precipitation rates over the United States, Ting said the relationship does not account for all the fluctuations that appeared in the 40-year study. A much stronger correlation exists between the sea-surface temperatures in the north Pacific and rainfall over the Great Plains.

"Currently, only tropical Pacific sea-surface temperatures are being plugged into the forecast model to make seasonal precipitation predictions," Ting said. "By also including the north Pacific msea-surface temperatures, I think we can obtain more accurate predictions."

The researchers' findings appeared in the August issue of the Journal of Climate.