Heat waves are the deadliest weather threat in the United States, killing more people each year than hurricanes, tornadoes, floods and earthquakes combined.

They're also expected to become more frequent and more severe with climate change.

One way to minimize the negative effects of heat waves is to predict them earlier. This gives people and cities more time to prepare: to purchase air conditioners and do things like set up access to cooler environments for the elderly and homebound, people who are disproportionately affected by heat waves.

Improving such forecasts, however, has been hard to do. Predicting extreme events any further than 10 days in advance has long been an elusive goal for meteorologists.

Now, research led by Haiyan Teng of the National Center for Atmospheric Research has identified an atmospheric wave pattern that precedes North American heat waves by two weeks. The study, published yesterday in the journal Nature Geoscience, has the potential to improve forecasts of such extreme events.

"People haven't connected this particular wave pattern to the heat waves before," Teng said, but "many [heat waves] tend to be led by this wavenumber-5 pattern."

The wavenumber-5 pattern is a ring of five high- and low-pressure systems in the atmosphere that circles the Earth's surface above the midlatitudes, or the temperate zone, running in a direction opposite to the flow of the jet stream.

Teng's research discovered that the presence of such a pattern tends to precede heat waves by about 15 days. This makes it a possible predictor for future heat waves.

Computer model finds historical patterns
In order to learn that this atmospheric pattern exists in advance of heat waves, Teng and her co-authors had to look far back in the history of heat waves -- from before weather records were kept.

That's because heat waves do not happen often enough for researchers to find patterns in recent data.

"It's very difficult to study predictability of heat waves using observations, because there are not very many events," Teng said.

So the researchers used an atmospheric model to simulate weather conditions back 12,000 years. They defined heat waves as summertime periods with five days in a row of persistently high air temperatures in a certain area, and found 5,900 U.S. heat waves over that 12,000-year simulation.

What the researchers saw, when looking for patterns that predicted heat waves, was that heat waves tended to be preceded by wavenumber-5 patterns by 15 to 20 days.

The stronger the amplitude of the wavenumber-5 pattern, the more likely it was that a heat wave occurred 15 days later.

"The real strength of that study is that they were able to leverage those runs they had, these many thousands of years of model runs," said Siegfried Schubert, a senior research meteorologist at NASA who has also researched how these large-scale atmospheric wave patterns affect heat and drought.

The researchers looked only at heat waves over the United States, although the research could also be extended to Europe and Asia. "We think this pattern can provide a useful pattern for heat waves in other countries," Teng said.

Knowing what to look for
Previous studies investigating heat wave prediction have looked for patterns in the tropics, but this research was interesting because the predictive factor is an atmospheric phenomenon that occurs in the middle latitudes, Schubert said.

In fact, the paper stresses that researchers looked for tropical linkages and were unable to find a connection between tropical precipitation and heat waves, although they could not rule out the possibility that they may be connected in some way.

There may also be other circulation patterns that are predictors of heat waves, and this is an area Teng and her colleagues will continue to research, she said.

Randall Dole, a senior scientist at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory who was not affiliated with the research, said the paper was interesting because it demonstrated how natural variability in the atmosphere can lead to heat waves.

"The deeper significance is that it reminds us that extreme events need not have extreme causes; that is, they can arise from intrinsic atmospheric variability," Dole said.

As to whether forecasters could use this research, Dole said more testing would be needed, but it could be a phenomenon forecasters might watch for.

"The scientific value is whenever we break through what is supposed to be a predictability barrier, that tells us what we need to have in models. And it begins to provide the forecasters with something that they begin to monitor two to three weeks out," Dole said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500