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Global warming drives more nighttime heatwaves

Pacific Northwest researchers document startling increase in number of hot nights during the past few decades

Global warming map

A NASA map shows an area of above-average temperatures hugging the Pacific Northwest coast during the spring of 2013.

By Summit Voice

FRISCO — A trend of more frequent nighttime heatwaves in the Pacific Northwest may not portend killer temperatures and withering crops, but it is another sign that  global warming will have a profound effect on regional climate and weather.

Scientists with the Oregon Climate Service at Oregon State University documented 15 examples of “nighttime heat waves” from 1901 through 2009. Ten of them have occurred since 1990 and five were during a four-year period from 2006-09.

The research shows that the region west of the Cascades saw only three nighttime heat waves between 1901 and 1980, but that number quadrupled to 12 nighttime heat waves in the three decades after 1980, according to the paper published in the July issue of the Journal of Applied Meteorology and Climatology.

“In general, minimum daily temperatures have been warming faster than maximum temperatures, so we’re not surprised to see a trend in the minimum events,” said corresponding author Karin Bumbaco, a research scientist at the Joint Institute for the Study of the Atmosphere and Ocean. “Still, we were surprised to see this significant increase in the frequency of nighttime heat waves.”

Since the study was accepted for publication, another nighttime heat wave took place at the end of this June, the authors point out.

“Most people are familiar with daytime heat waves, when the temperatures get into the 100s and stay there for a few days,” said Oregon Climate Service deputy director Kathie Dello, one of the report’s co-authors. “A nighttime heat wave relates to how high the minimum temperature remains overnight.

“Daytime events are usually influenced by downslope warming over the Cascade Mountains, while nighttime heat waves seem to be triggered by humidity,” said Dello, who is in OSU’s College of Earth, Ocean, and Atmospheric Sciences. “Elevated low-level moisture at night tends to trap the heat in.”

In their study, Dello and co-authors Karin Bumbaco and Nicholas Bond from the University of Washington defined heat waves as three consecutive days of temperatures at the warmest 1 percentile over the past century. Using that standard criterion, they documented 13 examples of daytime heat waves during the time period from 1901 to 2009. Only two of those occurred in the last 20 years.

In contrast, nighttime heat waves have been clustered over the past two decades, with what appears to be accelerating frequency. A warming climate suggests the problem may worsen, studies suggest.

“If you look at nighttime temperatures in Oregon and compared them to say the Midwest, people there would laugh at the concept of a Pacific Northwest heat wave,” Dello said. “However, people in the Midwest are acclimated to the heat while in the Northwest, they are not. People in other regions of the country may also be more likely to have air conditioning in their homes.

On occasion, daytime and nighttime heat waves coincide, Dello said, as happened in 2009 when temperatures in the Pacific Northwest set all-time records in Washington (including 103 degrees at SeaTac), and temperatures in Oregon surpassed 105 degrees in Portland, Eugene, Corvallis and Medford. It was the second most-intense daytime heat wave in the last century, but lasted only three days by the 1 percentile definition.

However, that same stretch of hot weather in 2009 results in a nighttime heat wave that extended eight days, by far the longest stretch since records were kept beginning in 1901.

The latest nighttime heat wave began in late June of this year, and continued into early July, Dello said.

“Like many nighttime heat waves, a large high-pressure ridge settled in over the Northwest, while at the same time, some monsoonal moisture was coming up from the Southwest,” she pointed out. “The high swept around and grabbed enough moisture to elevate the humidity and trap the warm air at night.”

Dello frequently provides weather facts and historical data via Twitter at: www.twitter.com/orclimatesvc.

The Oregon Climate Change Research Institute is supported by the state of Oregon, U.S. Department of the Interior, National Oceanic and Atmospheric Association, and other agencies.

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