The formation of a pattern of sea surface temperatures in the middle of the North Pacific can predict an increased chance of summertime heat waves in the eastern half of the US up to 50 days in advance, according to a study led by the National Center for Atmospheric Research.
The pattern is a contrast of warmer-than-average water butting up against cooler-than-average seas. When it appears, the odds that extreme heat will strike during a particular week, or even on a particular da, can more than triple, depending on how well-formed the pattern is.
"Summertime heat waves are among the deadliest weather events, and they can have big impacts on farming, energy use, and other critical aspects of society," said Karen McKinnon, a postdoctoral researcher at NCAR and the lead author of the study. "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."
The research was largely funded by the National Science Foundation, NCAR’s sponsor. In addition to McKinnon, the research team includes Andrew Rhines, of the University of Washington; Martin Tingley, of Pennsylvania State University; and Peter Huybers, of Harvard University.
For the study, the scientists divided the country into regions that tend to experience extreme heat at the same time. The scientists then focused on the largest of the resulting blocks: a swath that stretches across much of the Midwest and up the East Coast, encompassing both important agricultural areas and heavily populated cities.
The research team looked to see if there was a relationship between global sea surface temperature anomalies, waters that are warmer or cooler than average, and extreme heat in the eastern half of the U.S.
Right away, a pattern popped out in the middle of the Pacific, above about 20 degrees North latitude. The scientists found that the particular configuration of ocean water temperatures, which they named the Pacific Extreme Pattern, was not only found when it was already hot in the eastern U.S., but that it tended to form in advance of that heat.
"Whatever mechanisms ultimately leads to the heat wave also leaves a fingerprint of sea surface temperature anomalies behind," McKinnon said.
Scientists do not yet know why the fingerprint on sea surface temperatures in the Pacific predicts heat in the eastern U.S. It could be that the sea surface temperatures themselves kick off weather patterns that cause the heat. Or it could be that they are both different results of the same phenomenon, but one does not cause the other.
To learn more about how the two are connected, McKinnon is working with colleagues at NCAR to use sophisticated computer models to try and tease apart what is really happening.
The study’s findings also point toward the possibility that the Pacific Extreme Pattern, or a different oceanic fingerprint, could be used to forecast other weather events far in advance, including cooler-than-average days and extreme rainfall events.
“The results suggest that the state of the mid-latitude ocean may be a previously overlooked source of predictability for summer weather,” McKinnon said.