According to a new study led by a scientist at the National Center for Atmospheric Reseach (NCAR), a distinct pattern of sea surface temperatures in the middle of the North Pacific Ocean can predict an increased chance of summertime heat waves in the eastern half of the United States up to 50 days in advance. Although scientists do not know why that pattern predicts heat, the study suggests that the state of the mid-latitude ocean may be a previously overlooked source of predictability for summer weather.
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 day - can more than triple, depending on how well-formed the pattern is.
For the study, the scientists divided the country into regions that tend to experience extreme heat at the same time. Then they 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 US.
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 US, 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," said Karen McKinnon, a postdoctoral researcher at NCAR and the lead author of the study.
Top: Sea surface temperature anomalies in the mid-latitude Pacific 50 days in advance of June 29, 2012. The pattern inside the green box resembled the Pacific Extreme Pattern, indicating that there would be an increase in the odds of a heat wave in the eastern half of the United States at the end of June. Image courtesy of Karen McKinnon, NCAR
To test how well that fingerprint could predict future heat, the scientists used data collected from 1 613 weather stations across the eastern US between 1982 and 2015, as well as daily sea surface temperatures for the same time period.
They defined extreme heat in the eastern US as a summertime day when the temperature readings from the warmest 5 percent of weather stations in the region were at least 6.5 °C (11.7 °F) hotter than average.
The researchers "hindcasted" each year in the dataset to see if they could retrospectively predict extreme heat events—or lack of those events—during that year's summer, using only data collected during the other years as a guide. At 50 days out, the scientists were able to predict an increase in the odds—from about 1 in 6 to about 1 in 4—that a heat wave would strike somewhere in the eastern U.S. during a given week. At 30 days out or closer, the scientists were able to predict an increase in the odds—to better than 1 in 2 for a particularly well-formed pattern—that a heat wave would strike on a particular day.
This new technique could improve existing seasonal forecasts, which do not focus on predicting daily extremes. For example, the seasonal forecast issued for the summer of 2012 predicted normal heat for the Northeast and Midwest. But, the summer ended up being especially hot, thanks to three major heat waves that struck in late June, mid-July, and late July.
When the team used the Pacific Extreme Pattern to hindcast 2012, they were able to determine as early as mid-May that there were increased odds of extremely hot days occurring in late June. The hottest day of the summer of 2012, as measured by the technique used for this study, was June 29, when the warmest 5 percent of weather stations recorded temperatures that were 10.4 °C (18.7 °F) above average.
Scientists do not yet know why the fingerprint on sea surface temperatures in the Pacific predicts heat in the eastern US. 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.
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.
- "Long-lead predictions of eastern United States hot days from Pacific sea surface temperatures" - Karen McKinnon, Andrew Rhines, Martin Tingley, and Peter Huybers - Nature Geoscience - DOI: 10.1038/ngeo2687
Featured image credit: Perry McKenna (CC - Flickr)