Americans could have as many as seven weeks’ advance notice of devastating heatwaves. A telltale pattern of sea surface temperatures in the Pacific Ocean could presage weeks of intolerable temperatures in the cities and plains of the US.
Research published in Nature Geoscience reveals that a “fingerprint” of sea surface temperatures about 20 degrees north of the Equator – and dubbed the Pacific Extreme Pattern – meant that the chances of sustained and extreme heat 50 days later in the US Midwest and East Coast stepped up from one in six to one in four. And 30 days ahead, the same pattern brought the odds to better than one in two.
“Summertime heatwaves are among the deadliest weather events, and they can have big impacts on farming, energy use, and other critical aspects of society,” says Karen McKinnon, a post-doctoral fellow at the National Centre for Atmospheric Research in Boulder, Colorado.
“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.”
Climate scientists have consistently predicted that climate change will be driven by global warming that will manifest itself in ever more intense, and more frequent, extremes of heat.
In 2012, the US was hit by an unprecedented and sustained heatwave that broke thousands of local temperature records, and claimed an estimated 100 lives.
Using satellite data and meteorological records, Dr McKinnon – then at Harvard University – and her co-researchers looked for ocean temperature anomalies that could be linked with extreme heat on the eastern seaboard of the US.
They defined their extreme as at least 6.5°C hotter than the average during the 60 hottest days of the year, June 24 to August 22.
They then used their data to “hindcast” extremes of heat. They found that the Pacific Extreme Pattern, a meeting of warmer-than-average and colder-than-average ocean waters, not only coincided with US heatwaves, but tended to form in advance of the heatwaves over land.
And, sure enough, characteristic signals from the distant stretch of ocean preceded the arrival of thunderous temperatures over a great tract of the US in 2012.
It isn’t clear how the two phenomena are connected: that has yet to be explored. But one factor could be precipitation. When the Pacific Extreme Pattern forms, atmospheric circulation tends to divert rainfall from the eastern US. The less water there is to evaporate, the less it can cool the air.
The next step is to see whether other ocean patterns can be used to forecast other weather events well in advance. “The results suggest that the state of the mid-latitude ocean may be a previously overlooked source of predictability for summer weather,” Dr McKinnon says.