Warm U.S. months a century apart studied

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COLUMBIA, Mo., June 14 (UPI) -- The two warmest winter months in U.S. Midwest history, separated by more than a century, may be linked to important climate phenomena, researchers say.

This past March was the second-warmest winter month ever recorded in the Midwest, with temperatures 15 degrees above average; the only other winter month that was warmer was December 1889, with temperatures 18 degrees above average, University of Missouri-Columbia researchers reported Thursday.

Now, they said, they've discovered why the weather patterns during these two winter months, separated by 123 years, were so similar, and the answer could help scientists develop more accurate weather prediction models.

The preceding months for both years were also dry and warm, as well as the previous summers, leading researchers to conclude both 2012 and 1889 were La Nina years, researcher Tony Lupo said.

"During a period of La Nina the sea surface temperatures across the equatorial Eastern Central Pacific Ocean are lower than normal by 3 to 6 degrees," Lupo said. "This typically directs the jet stream from the Pacific on a northeastern path over Canada. Rain storms follow the jet stream, leaving the central and south-central states dry, while blocking air from moving south into the Midwest, resulting in higher temperatures."

Also, 2012 and 1889 both featured strong Arctic Oscillations, a pattern of air pressure that wraps itself around the North Pole and traps and keeps cold air in the arctic, the researchers said.