Ocean temperatures predict U.S. heat waves 50 days out
March 28, 2016 - 1:01 PM
Credit: NSF Central Arizona-Phoenix LTER Site
WASHINGTON, D.C. - The formation of a distinct pattern of sea surface temperatures in
the middle of the North Pacific Ocean can predict an increased chance of
summer heat waves in the eastern half of the U.S. up to 50 days in
The pattern is a contrast of warmer-than-average water coming 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
The findings were published today in the journal Nature Geoscience. The lead author is scientist Karen McKinnon of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.
"Summertime heat waves are among the deadliest weather events, and
can have big effects on farming, energy use and other critical aspects
of society," said McKinnon. "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."
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.
"This intriguing result has enormous practical implications," said
Candace Major, program director in the National Science Foundation (NSF)
Division of Ocean Sciences, which funded the research along with NSF's
Division of Atmospheric and Geospace Sciences. "The potential for
predicting the risk of dangerous heat waves more than a month in advance
is very exciting. With more time to prepare, communities have a greater
chance of avoiding the serious economic and health consequences of
A fingerprint on the ocean
For the study, the scientists divided the country into regions that
tend to experience extreme heat at the same time. They then focused on
the largest of the resulting blocks: a swath that stretches across much
of the Midwest and up the East Coast, encompassing important
agricultural areas and heavily populated cities.
The researchers looked for a relationship between global sea surface
temperature anomalies -- waters warmer or cooler than average -- and
extreme heat in the eastern half of the U.S.
A pattern popped out in the middle of the Pacific, above a point
roughly 20 degrees north latitude. The scientists could find the
particular configuration of ocean water temperatures, which they named
the Pacific Extreme Pattern, not only when the eastern U.S. was already
hot, but also in advance of that heat.
"Whatever mechanisms ultimately lead to the heat wave also leave a
fingerprint of sea surface temperature anomalies behind," McKinnon said.
Improving seasonal forecasts
To test how well that activity could predict future heat, the
scientists used data collected from 1,613 weather stations across the
eastern U.S. between 1982 and 2015, as well as daily sea surface
temperatures from the same time period.
The researchers defined extreme heat in the eastern U.S. as a summer
day when the temperature readings from the warmest 5 percent of weather
stations in the region were at least 6.5 degrees Celsius (11.7 degrees
Fahrenheit) hotter than average. They only examined extreme heat during
that region's 60 hottest days of the year: June 24 through Aug. 22.
The scientists "hindcasted" each year in the data set to see if they
could retrospectively predict extreme heat events -- or the lack of
those events -- during that year's summer.
At 50 days out, they were able to predict an increase in the odds --
from about one-in-six to about one-in-four -- that a heat wave would
strike somewhere in the eastern U.S. during a given week.
For a particularly well-formed pattern, at 30 days out or closer the
scientists were able to predict that a heat wave would strike on a
particular day at odds of better than one-in-two.
This new technique could improve existing seasonal forecasts, which
do not focus on predicting daily extremes. Seasonal forecasts typically
predict whether an entire summer is expected to be warmer than normal,
normal, or cooler than normal.
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 research team used the Pacific Extreme Pattern to hindcast
2012, they were able to determine as early as mid-May 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 degrees Celsius (18.7
degrees Fahrenheit) above average.
"We found that we could go back as far as seven weeks and still
predict an increase in the odds of future heat waves," McKinnon said.
"What's exciting about this is the potential for long-range predictions
of individual heat waves that give society far more notice than current
Scientists don't yet know why the fingerprint of sea surface temperatures in the Pacific predicts heat on the East Coast.
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
To learn more about how the two are connected, McKinnon is working
with colleagues at NCAR to use sophisticated computer models to try to
tease apart what's happening.
The study's findings also point to 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,"
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