Summertime heat waves in the United States tend to be preceded two weeks before by a distinctive atmospheric circulation pattern, climate models suggest.
The finding, detailed in this week's issue of the journal Nature Geoscience, could lead to improved probability forecasts of U.S. heat waves beyond the 1-week weather forecast range.
"We think once this pattern forms, it favors the formation of heat waves over the U.S.," explained Haiyan Teng, a climate scientist at National Center for Atmospheric Research (NCAR) in Boulder, Colorado.
Teng and her NCAR colleagues noticed the connection between the pattern and U.S. heat waves while analyzing a 12,000-year simulation of the weather across the northern hemisphere.
After analyzing nearly 6,000 U.S. heat waves in the computer model, the team concluded that summertime heat waves in the United States were 2 to 4 times more likely if a distinctive "wavenumber-5" atmospheric pattern developed some 15 to 20 days ahead of time.
The wavenumber-5 pattern is a sequence of alternating high- and low-pressure systems (five of each, hence the name) that form a ring circling the northern midlatitudes, several miles above the Earth's surface.
"This pattern has been documented in a number of studies by other people. Our contribution is to link it to probability of heat waves with the help of a climate model," Teng said.
Why Is It Important?
The model found that during the 20 days leading up to a heat wave, the five ridges and troughs that make up a wavenumber-5 pattern are unusually strong and tend to propagate very slowly westward around the globe.
Eventually, a high-pressure ridge formed in the model that moved from the North Atlantic into the United States, shutting down rainfall and setting the stage for a heat wave.
To see whether the model results applied to the real world, the team looked at actual U.S. heat wave records extending back to 1948. They found that some historical heat waves events were indeed characterized by circulation patterns similar to the predicted wavenumber-5 pattern.
"This pattern doesn't only exist in climate models. It exists in nature," Teng said.
What Does This Mean?
The team's results could ultimately improve long-term heat wave probability forecasts, the scientists say.
"Right now, we are relying mostly on operational weather forecast, which can only look about 5 days to a week out," she added. "Beyond that, we just don't have much confidence in the forecast."
Teng says she and her colleagues plan to continue searching for other circulation patterns and environmental factors—such as sea surface temperature or soil moisture—that may foreshadow extreme weather events.
"Here, we're just showing one particular circulation pattern. There may be other sources that we're not aware of," she said.
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