In the messy, chaotic atmosphere of our planet meanders the jet stream, a wiggly belt of air circling the mid-latitudes. As the belt moves south, it pulls cool air from the Arctic toward the tropics. Then it switches direction, pulling warm air from the tropics toward the poles.

Sometimes, in response to natural climate patterns, the jet stream becomes abnormally wavy. Such amplified waves have coincided with heat and cold waves, droughts and flooding across the world, according to a study published yesterday in Nature Climate Change.

Take the past winter in North America, for example. The eastern and central United States were plunged into a deep freeze, Alaska was unusually warm and California was dry. Above the continent, the jet stream was indulging in some unusual behavior. Its northward swing was so big that it sucked warmer air right into Alaska, which was positively balmy in the wintertime. Then, the wave turned toward the south with a big swing, bringing Arctic air into the central United States.

Weather watchers have always assumed that the jet stream might account for some weather extremes, but this is the first paper to demonstrate that conclusively.

"I think [the paper] has done a fabulous job of basically documenting a relationship that most people believed existed," said Jennifer Francis, an atmospheric scientist at Rutgers University, who was not affiliated with the new study. "[It] has shown that, indeed, many of the extreme events that have occurred in the past, going back to the late 1970s, are associated with very large waves in the jet stream."

The study is particularly relevant in the context of a controversial hypothesis championed by Francis that the jet stream will get more wavy in the future with climate change (ClimateWire, April 3).

Understanding the climate link requires knowledge of the jet stream's evolution, which most people do not have, because, "why would you?" asked James Screen, a climate researcher at the United Kingdom's University of Exeter and the lead author of the new study, with a laugh.

A possible connection to Arctic warming?
The jet stream, also known as "Rossby waves," is result of the Earth spinning. As the planet turns, blobs of air begin moving from west to east. As the blobs encounter accidents of topography—such as a mountain range—they deflect, sometimes toward the Equator.

Another little-known fact is that the Earth—and the air above—has more spin at the poles than at the Equator. A blob from the north that is deflected south will find itself in a region with less spin. It then attempts to return to the latitude it belongs and curves back. But, it overshoots its mark and has to turn back southward. A wavy jet stream forms.

When the waves get very large, they move more slowly. That means the weather they create also move more slowly, which leads to very extreme weather that hangs around oppressively for weeks.

Francis has suggested that the frequency of the wavy patterns in the jet stream is increasing as the Arctic warms due to climate change. But there is not enough data to prove this hypothesis.

"No one has come out and said this is wrong and presented results showing that it's wrong," Francis said. "But there is uncertainty at this point whether we can see this happening in the real world or not."

If the theory is true, then the new study would seem to suggest that heat and cold waves would occur more frequently in the future.

Droughts, extreme rainfall and temperature extremes
Screen, the primary author, stressed that his new study does not deal with the controversial link between the Arctic, climate change and the jet stream. Rather, it simply uses historical weather data to connect the jet stream and extreme weather, which is the noncontroversial part of the equation.

Screen and his colleagues identified 40 extreme weather events, including heat waves, cold temperatures, droughts and heavy rainfall, that occurred since 1979 throughout the world. That date was chosen because satellites began providing quality meteorological data around then.

The scientists then used computer models to reconstruct the jet stream as it must have existed during those weather events. They found that, in general, extreme weather appeared to coincide with amplified jet streams.

The type of extreme weather appeared to differ with geography. This is because the jet stream is just one link in the chain of climatic events influencing local weather.

The study found that the jet stream increased the likelihood of hot months in western North America and central Asia, and extreme cold months in eastern North America. It also increased the likelihood of droughts in central North America, Europe and central Asia, and extreme rainfall in western Asia.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500