The summer of 2018 brought a series of extreme weather events that occurred almost simultaneously around the Northern Hemisphere - from record-breaking heatwaves and droughts in North America and Western Europe, to torrential rainfalls and floods in Southeastern Europe and Japan.

A new study by scientists from NCAS, the University of Oxford, Potsdam Institute for Climate Impact Research (PIK) and Vrije Universiteit Amsterdam shows that these extreme weather events had something in common. The scientists identified a stalled wave pattern of the jet stream, which made weather conditions more persistent and extreme in the affected regions - regions which constitute major crop production sites and places where the majority of people live in the Northern Hemisphere.

The same pattern also occurred during European heatwaves in 2015, 2006 and 2003, which rank among the most extreme heatwaves ever recorded. In recent years, a clear increase in these patterns has been observed. The identified jet stream wave pattern also provides an opportunity for improving the early-prediction of future extreme weather events for vulnerable regions in the Northern Hemisphere.

The jet stream, a narrow band of fast flowing air about 10 km high up in the atmosphere, steers large scale weather systems from west to east around the globe. The wind system can develop large meanders, so-called Rossby waves, and occasionally these waves stay in place for weeks. Under these conditions warm sunny days can turn into a heat wave and drought, and rainy days into floods.

Lead author Kai Kornhuber, an NCAS researcher based at the University of Oxford and PIK explains: “Our study shows that the specific locations and timing of the 2018 summer extremes weren’t random but directly connected to the emergence of a re-occurring pattern in the jet stream that stretches around the entire Northern Hemisphere

The researchers find a strong relation between the pattern and persistent heat extremes in Western Europe, North America and the Caspian Sea region. This pattern was present during other years with extreme weather events such as the heat waves in Europe during summer 2015, 2006 and 2003. Moreover, they report that its frequency and duration have increased over the last two decades.

Scott Osprey, an NCAS scientist based at the University of Oxford, explains “These persistent heatwaves occurring through stalled wave patterns come on top of the general increase in temperature observed due to global warming. This creates the real possibility of particularly extreme heatwaves, most notably in regions like North America and Europe.

The observed wave pattern is anticipated to re-occur more frequently in future because of climate change and human-caused global warming. The cooler than normal North Atlantic, likely a result of a slowdown of the Atlantic Meridional Overturning Circulation (a large system of ocean currents driven by differences in temperature and salt content) could also affect the emergence of the newly identified wave pattern.

 

Study: Kai Kornhuber, Scott Osprey, Dim Coumou, Stefan Petri, Vladimir Petoukhov, Stefan Rahmstorf, Lesley Gray (2019): Extreme weather events in early summer 2018 connected by a recurrent hemispheric wave-7 pattern. Environmental Research Letters, Volume 14, Number 5. [DOI: 10.1088/1748-9326/ab13bf]