Atmospheric circulation pattern foreshadows prolonged dry and hot weather
By Summit Voice
FRISCO — Just a few weeks after a major report from NOAA found near-certain links between global warming and intensifying heatwaves, researchers say they’ve been able to pinpoint a high-altitude atmospheric wave pattern above the northern hemisphere that can help predict heatwaves more than two weeks in advance.
“It may be useful to monitor the atmosphere, looking for this pattern, if we find that it precedes heat waves in a predictable way,” said NCAR scientist Haiyan Teng, lead author of the study. “This gives us a potential source to predict heat waves beyond the typical range of weather forecasts.”
The research team discerned the pattern by analyzing a 12,000-year simulation of the atmosphere over the Northern Hemisphere. During those times when a distinctive “wavenumber-5″ pattern emerged, a major summertime heat wave became more likely to subsequently build over the United States.
The wavenumber-5 pattern refers to a sequence of alternating high- and low-pressure systems (five of each) that form a ring circling the northern mid latitudes, several miles above the surface. This pattern can lend itself to slow-moving weather features, raising the odds for stagnant conditions often associated with prolonged spells of hot weather.
Essentially, the constantly undulating progression of ridges and troughs along the course of the jet stream gets stuck at times, enabling heat to build up across certain regions.
Heat waves are among the most deadly weather phenomena on Earth. Even in Scandinavia, public health researchers say they’ve been able to trace increased mortality to periods of above-normal temperatures. In North America, a 2006 heatwave was blamed for 600 deaths in California, and record-breaking European heat in the summer of 2003 may have killed more than 50,000 people.
The researchers tried to focus on the most extreme heat, when daily high temperatures reached the top 2.5 percent of readings for that date across at least 10 percent of the lower 48 states. Such extremes are rare — the scientists could identify only 17 events that met the criteria — not enough to tease out a reliable signal amid the noise of other atmospheric behavior.
The group then turned to an idealized simulation of the atmosphere spanning 12,000 years. The simulation had been created a couple of years before with a version of the NCAR-based Community Earth System Model, which is funded by NSF and the Department of Energy.
By analyzing more than 5,900 U.S. heat waves simulated in the computer model, they determined that the heat waves tended to be preceded by a wavenumber-5 pattern. This pattern is not caused by particular oceanic conditions or heating of Earth’s surface, but instead arises from naturally varying conditions of the atmosphere. It was associated with an atmospheric phenomenon known as a Rossby wave train that encircles the Northern Hemisphere along the jet stream.
During the 20 days leading up to a heat wave in the model results, the five ridges and five troughs that make up a wavenumber-5 pattern tended to propagate very slowly westward around the globe, moving against the flow of the jet stream. Eventually, a high-pressure ridge moved from the North Atlantic into the United States, shutting down rainfall and setting the stage for a heat wave to emerge.
When wavenumber-5 patterns in the model were more amplified, U.S. heat waves became more likely to form 15 days later. In some cases, the probability of a heat wave was more than quadruple what would be expected by chance.
In follow-up work, the research team turned again to actual U.S. heat waves since 1948. They recognized that some historical heat wave events are indeed characterized by a large-scale circulation pattern that indicated a wavenumber-5 event.
The research finding suggests that scientists are making progress on a key meteorological goal: forecasting the likelihood of extreme events more than 10 days in advance. At present, there is very limited skill in such long-term forecasts.
Previous research on extending weather forecasts has focused on conditions in the tropics. For example, scientists have found that El Niño and La Niña, the periodic warming and cooling of surface waters in the central and eastern tropical Pacific Ocean, are correlated with a higher probability of wet or dry conditions in different regions around the globe.
The wavenumber-5 pattern does not rely on conditions in the tropics. However, the study does not exclude the possibility that tropical rainfall could act to stimulate or strengthen the pattern.
Now that the new study has connected a planetary wave pattern to a particular type of extreme weather event, Teng and her colleagues will continue searching for other circulation patterns that may presage extreme weather events.
“There may be sources of predictability that we are not yet aware of,” she said. “This brings us hope that the likelihood of extreme weather events that are damaging to society can be predicted further in advance.”
The study is being published next week in Nature Geoscience. It was funded by the U.S. Department of Energy, NASA, and the National Science Foundation (NSF), which is NCAR’s sponsor. NASA scientists helped guide the project and are involved in broader research in this area.