Extra waves may be great for surfers, but they can lead to tumultuous weather when they start showing up in jet streams.
That's because jet streams - blowing ribbons of wind that encircle the earth - play a critical role in the location and severity of weather events, such as the recent floods that devastated Kentucky. Even a slight change in the "waviness" of the polar or the subtropical jet stream can lead to dramatic weather changes in mid-latitude regions, from northern California to Moscow.
Recent decades have seen just such an increase in jet stream waves, leading scientists to pose the question: Are these windy wanderings being caused by a warming planet?
A noted Yale scientist and a team of international colleagues say yes - and in a new study, published in the journal Proceedings of the National Academy of Sciences, they offer a theory to prove it.
"As the planet warms, we predict that the land-ocean contrast of atmospheric heating enhances the meandering in the jet stream and that implies more of these extreme weather events, such as what we have experienced this summer in Kentucky," said John Wettlaufer, the A.M. Bateman Professor of Geophysics, Mathematics, and Physics at Yale, the co-corresponding author for the study.
A certain amount of waviness has always been a feature of jet streams. Swedish-American meteorologist Carl-Gustaf Rossby predicted the atmospheric wave concept in the late 1930s; since then, the meanders have been commonly known as "Rossby waves."
In the past 30 years, scientists have observed an intensification of the waves, coinciding with increased global warming. More waviness in the jet stream means that rain and wind remain in a region longer than if the jet stream simply traveled due east with no detours.
Various researchers have made the correlation between climate change and greater waviness in jet streams, however, the mechanism to explain the connection has been debated.
In the new study, Wettlaufer and his colleagues developed a mathematical theory that explains jet stream waviness and then created a simulation of atmospheric circulation under warming conditions to test their theory.
"Because polar regions of the planet are warming faster than the mid-latitudes, the typical north-south temperature difference is lower," Wettlaufer said. As this temperature difference decreases, it causes a slight drop in zonal winds in the jet stream - which, in turn, leads to more meandering of the jet stream.
"This means that the cyclones and anticyclones associated with the meanders are more stationary - there are weather systems 'parked' over some place on the planet," Wettlaufer said. "Thus, if a low-pressure system is sitting over eastern Kentucky for a long time, moisture is just focused there until the meanders start to move and the weather system moves eastward."
The first author of the study is Woosok Moon of Stockholm University. Co-authors are Baek-Min Kim and Gun-Hwan Yang of Pukyong National University.
