Pacific Ocean Temperature Patterns Can Predict U.S. Heat Waves 50 Days in Advance

April 1, 2016 | Joanne Kennell

Heat wave

This could lead to far more notice than current forecasts can give.

Scientists have discovered that a distinct pattern of sea surface temperatures that form in the middle of the North Pacific Ocean can be used to predict the chance of summer heat waves in the eastern half of the United States.

The pattern is described as a contrast of warmer-than-average water coming up against cooler-than-average seas. When it appears, the odds that extreme heat will occur during a particular week can more than triple, depending on how well-formed the pattern is.

Pacific Extreme Pattern: 50 days in advance of June 29, 2012

Photo credit: Karen McKinnon/NCAR. Image has been cropped

“Summertime heat waves are among the deadliest weather events, and can have big effects on farming, energy use and other critical aspects of society,” lead author Karen McKinnon of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, said in a press release. “If we can give city planners and farmers a heads-up that extreme heat is on the way, we might be able to avoid some of the worst consequences.”

For the study, the scientists divided the country into regions that often experience extreme heat at the same time, and decided to focus on the largest of the resulting areas — a section the stretches across most of the Midwest and up the East Coast. This is a region with a lot of agriculture as well as heavily populated cities.

As the researchers were looking for a relationship between global sea surface temperature anomalies — waters warmer or cooler than average — and extreme heat in the eastern half of the U.S., a pattern appeared in the middle of the Pacific at roughly 20 degrees north latitude.

The scientists found the pattern in ocean water temperatures, which they named the Pacific Extreme Pattern, not only occurred when the eastern U.S. was already hot, but before the heat arrived.

To test how well this activity could predict future heat, the researchers used data collected from 1,613 weather stations across the eastern U.S. and daily sea-surface temperatures between 1982 and 2015.

The researchers defined extreme heat in the eastern U.S. as a summer day when the temperature readings from the warmest five percent of weather stations in the region were at least 6.5 degrees Celsius (11.7 degrees Fahrenheit) hotter than average. They examined extreme heat during the region’s 60 hottest days of the year: June 24 through August 22.

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The scientists then “hindcasted,” a method of testing a mathematical model by using data from the past year in the data set to see if they could retrospectively predict extreme heat events.

The results? At 50 days out, they were able to predict an increase in the odds — from one-in-six to one-in-four — that a heat wave would hit the eastern U.S. during a given week. However, for a particularly well-formed pattern, at 30 days out, they were able to predict that a heat wave would strike on a particular day at odds better than one-in-two.

“We found that we could go back as far as seven weeks and still predict an increase in the odds of future heat waves,” McKinnon said in the release. “What's exciting about this is the potential for long-range predictions of individual heat waves that give society far more notice than current forecasts.”

Scientists are not sure why this pattern in sea surface temperatures in the Pacific predicts heat on the East Coast. However, some have hypothesized that the temperatures themselves start a weather pattern causing heat, or that they are both different results of the same phenomenon, but one does not cause the other.

"The results suggest that the state of the mid-latitude ocean may be a previously overlooked source of predictability for summer weather," McKinnon explained in the press release.

McKinnon is currently working with colleagues at NCAR to figure out what’s happening. The results of the paper were published on March 28 in the journal Nature Geoscience.

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