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Summer weather extremes linked to stalled Rossby waves in the jet stream

14 May 2019
Wildfire
Heatwave: the Ferguson fire burned in Yosemite, California in July and August 2018. That year was the worst ever for wildfires in the US state, something that may have been related to stalled Rossby waves. (Courtesy: Blake Scott/US Forest Service)

Early summer heatwaves in Western Europe and North America set new temperature records in 2018, while other regions of the northern hemisphere were hit with torrential rain and severe flooding. Now researchers in the UK, Germany and the Netherlands say that these events were linked by a pattern of stalled waves in the jet stream. They add that this wave pattern appears to have increased in frequency and persistence in recent years and may occur more frequently in the future due to climate change.

The northern jet stream is a river of fast-moving air that circles the northern hemisphere in the mid-latitudes. Travelling from east to west at an altitude of around 10 km, these winds drive large-scale weather systems around the globe.

Jet-stream winds generally travel at the same latitude, but they can shift into a wave-like pattern, known as Rossby waves, where they meander from north to south and back again. When this happens, warm air fills the peaks of the wave, while cold polar air drops into the troughs. Rossby waves normally continue to move from east to west – shifting high- and low-pressure weather systems with them. However, they can also stall – which can lead to heatwaves, droughts and floods as the regions of hot and cold air hover over the same regions for days, or even weeks.

Severe temperatures and rainfall

In June and July 2018 extreme heatwaves hit North America, Western Europe and the Caucasus, while south-east Europe and Japan experience heavy rain and flash flooding. Norway set a new maximum temperature record and received just half of its average July rainfall, the United Kingdom experienced the second hottest July since records began, and various locations in the western United States broke temperature records. Meanwhile in Japan severe floods and landslides caused by heavy rain destroyed more than 10,000 houses.

In the past it has been suggested that such simultaneous events could be caused by a pattern of stationary Rossby waves around the northern hemisphere. Now, by studying climate data from the US National Oceanic and Atmospheric Administration, Kai Kornhuber, a climate scientist at the University of Oxford, and colleagues have showed that the heatwaves and floods of 2018 were linked by a stalled pattern of seven peaks and troughs in the jet stream. Writing in Environmental Research Letters, they also point-out that this “wave-7” pattern was also present during extreme weather events in the summers of 2003, 2006 and 2015.

“We can say with some confidence that these events have something in common,” Kornhuber told Physics World. “We identified a recurring pattern, the regions where high- and low-pressure systems rest are always the same. So, you have heat in Western Europe, over the Caspian Sea region and the western US, and rainfall on the east coast of the US and over the Balkans.”

Stalls becoming longer

Further analysis also showed that the number of these wave-7 events has increased significantly over the last twenty years, and that they appear to hang around for longer. In the two decades before 1999, there were no summers where this stalled system lasted for two or more weeks, but since then it has happened seven times.

Kornhuber believes that a high contrast between land and ocean temperatures, and the winds that develop along that gradient, favours the stalling of Rossby waves. This he explains, suggests a link with climate change, because this temperature gradient is predicted to get stronger as temperatures warm. Kornhuber adds, “We need more research to support this hypothesis. We see an increasing trend in these patterns, but the issue with that is that we have a relatively short record of reliable atmospheric data.” He warns, however, that even if these wave-7 patterns are not influenced by rising temperatures, they are still relevant to climate change. “If the frequency of these events is unchanged the extremes that are created by this circulation pattern will become more extreme just because of the mean temperature rise,” he explains.

Ted Shepherd, a climate scientist at the UK’s University of Reading, who was not involved with the research, agrees. He says, “It is an interesting scientific question if the waves themselves might get stuck even more, but even if they don’t it is still an important phenomenon to be aware of.”

Shepherd says that this recent work adds to the evidence for this stalled wave-7 pattern. He believes that the idea that these fixed waves will get more intense with climate change is “very plausible”. He adds, however, that “it would be nice to have a bit more of a rigorous theoretical basis for it, but that is something that is an area of investigation”.

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