American heat waves can be predicted up to 50 days in advance, simply by taking the temperature of the North Pacific Ocean.
A study published in Nature Geoscience by US earth scientists shows a link between a particular pattern of warm and cool Pacific Ocean water and the odds that extreme heat will strike on a particular day across the eastern states.
"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," says Karen McKinnon from the National Centre for Atmospheric Research in Boulder, Colorado, and lead author of the study.
Case in point: three heat waves during the summer of 2012 claimed more than 80 lives across the US. But in the month before it struck, the Climate Prediction Centre's seasonal forecast was for normal temperatures for the midwest and northeast, and a 33-40% chance of above-normal temperatures for the southeast.
"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," McKinnon says.
The link between sea surface temperature and weather is well-established. For instance, distribution of warm water in the tropical Pacific Ocean – the El Niño-Southern Oscillation – can affect rainfall in Australia and on the west coast of North and South America.
So McKinnon and colleagues sought a link between global sea surface temperature oddities and extreme heat in the eastern half of the US.
They found a pattern almost immediately – when warmer-than-average water butted up against cooler-than-average water in the North Pacific Ocean, hot days were more likely to spring up in the weeks following.
To test how well the "Pacific Extreme Pattern" could predict extreme heat, they examined daily sea surface temperatures and weather station data from 1982 to 2015. Their definition of "extreme heat" was when temperature readings from the warmest 5% of weather stations were at least 6.5 ºC above average.
They then tried to "hindcast" if a heat wave hit – or not – each year during that 33-year period.
And they were remarkably accurate. At 50 days in advance, they could give one-in-four odds that a heat wave was going to hit the eastern states during a given week. And when the gap narrowed to 30 days, their odds were better than one in two that a particular day would be extremely hot.
Tellingly, when they hindcasted the summer of 2012, they could see Pacific Extreme Pattern warning signs as early as 15 May – some 40 days before the first heat wave.
And they were able to tell, 20 days out, when the heat wave was due to subside.
So how does are the North Pacific and eastern US heat linked?
Scientists aren't sure. The Pacific Extreme Pattern may initiate atmospheric patterns that trap heat over that portion of the world. But it may not be a causation effect: hot days and the Pacific Extreme Pattern may be different results of another phenomenon.
McKinnon is using sophisticated computer modelling to unpick the relationship. For now, though, those living in the eastern US may get fair warning the next time a heat wave strikes.