What does the jet stream, a band of fast-moving air in the upper atmosphere, have to do with the blazing heat waves this summer in the Northern Hemisphere? Not only have these heat waves broken temperature records, they have also fueled wildfires and caused travel disruptions. The jet stream is the leading driver of Earth’s weather. Think of it like a conveyor, delivering storm after storm. Sometimes, waves and wobbles form in the jet stream, allowing heat to get trapped in the “bends” and to persist over regions for weeks at a time. This year, an interesting shape in the jet stream, dubbed by scientists as “wavenumber 5”, has formed: this is a global pattern of five big waves circling the globe, leading to simultaneous heat waves across continents. In this case, China, the US and Europe are being affected.
The heat waves are not just a product of this shape in the jet stream, but also due to an increase in overall global temperatures, as well as dry soils, which, in turn, have resulted from prolonged hot weather. Over the long-term, the jet stream appears to be changing its behavior, particularly by slowing down in Northern Hemisphere summer, allowing for these persistent heat waves. Why? It’s due to the rapid warming of the Arctic. The reason there’s a jet stream at all is due to a temperature difference between the cold of the north and the warmth of the south. With rising temperatures, the temperature difference lessens between these regions and causes the wind to decrease. This leads to a slowdown of the jet stream and creates more unstable wobbles, bends and waves. Think about a spinning top. When it starts to slow down, it wobbles and destabilizes. Future jet stream pattern behavior is hard to predict given that long periods of observation are necessary. But climate models are showing that as global average temperatures increase, the heat waves will get hotter.
photo: U. Peña, shot on an airplane on the way to Juneau, Alaska