Tree rings show extreme weather due to jet stream changes since 1960s

Tree rings show extreme weather due to jet stream changes since 1960s

Sarah Chong

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Researchers from the University of Arizona have reconstructed historical changes in the North Atlantic jet stream going back almost 300 years, using information extracted from tree rings. The trees were from the British Isles and the northeastern Mediterranean. According to Valerie Trouet, a researcher at the University of Arizona, swings between northern and southern positions of the jet have become more frequent in the second half of the 20^th century. (1)

Trees can store data of the environment through growth. Densities of latewood, the part of an annual tree ring that forms in the latter part of the growing season, reflect the temperature of August. By taking samples from trees from a range of locations, the researchers could form a map of the jet’s location.

Extreme North Atlantic Jet positions result in extreme weather such as heat waves, droughts, wildfires, and floods. When positioned extremely far north, the jet stream causes summers in the British Isles and western Europe to have heat waves while southeastern Europe faces heavy rainfall with resulting floods. When in the southern extreme position, the jet stream causes western Europe to have heavy rains while southeastern Europe suffers droughts and wildfires from abnormally high temperatures. (2) Trouet points out that “The heat waves and drought that are related to such jet stream extremes happen on top of already increasing temperatures and global warming – it’s a double whammy.”(1)

The North Atlantic Jet drives temperature and precipitation extremes in Europe by controlling Atlantic storm tracks. It achieves this by influencing the occurrence and duration of almost stationary atmospheric pressure fields, known as atmospheric blocking. A decrease in atmospheric blocking causes the increase in precipitation whereas an increase in atmospheric blocking results in the higher chance of heatwaves. (2)

The reconstruction of historical weather fluctuations serves as a multi-century benchmark against which scientists can compare more recent weather anomalies. Research of the North Atlantic Jet also helps studies of extreme summer weather events in North America, specifically the American Midwest. “What we’re experiencing now in North America is part of the same jet stream system,” says Trouet. More specifically, the North Pacific Jet is what dictates the cold winters of the North American Northeast and scorching summers of the Southwest.

Trouet hopes that the discovery of much older trees in the Balkans and the British Isles will mean more data that can help further reconstruct the path of the North Atlantic jet stream – almost as much as a thousand years into the past. The team of researchers hope to use this data to help settle the debate on whether the increased variability of the jet stream is connected to man-made global warming and the faster warming of the Arctic compared to the tropics. (1)

1. University of Arizona. “Jet stream changes since 1960s linked to more extreme weather.” ScienceDaily. ScienceDaily, 12 January 2018. <www.sciencedaily.com/releases/2018/01/180112091209.htm>.

2. Trouet, F. Babst, M. Meko. Recent enhanced high-summer North Atlantic Jet variability emerges from three-century context. Nature Communications, 2018; 9 (1) DOI: 10.1038/s41467-017-02699-3