“The pattern popped out at us really clearly,” said Karen A. McKinnon, a postdoctoral researcher at the National Center for Atmospheric Research, in Boulder, Colo., and the lead author of the study.
Not only did it exist on those hot days – defined as about 12 degrees hotter than normal summer temperatures – “But importantly, up to seven weeks before,” Dr. McKinnon said.
Heat waves can be predicted with confidence in short-term forecasts, but these become increasingly unreliable beyond several days. Most longer-term, seasonal forecasts, which look at weather probabilities a month or more in the future, do not focus on heat waves or other extreme events. “If we do see the pattern, we can say how similar is this to the one that leads to a heat wave.”
To test their approach, the researchers looked at the summer of 2012, when three heat waves occurred in the East in June and July.Seasonal forecasts had predicted that normal temperatures would be most likely that summer in the Northeast and Midwest.
Peter Huybers, a climate scientist at Harvard University and an author of the study, said that it was not yet clear why there was a connection between temperatures in the North Pacific and heat waves in the East.One possibility, he said, is that the temperatures on the sea surface lock in undulations in the jet stream that lead to a large mass of high-pressure air settling over the region, allowing it to bake.