Scientists study how rippling jet stream plus “extra heat” drives extreme weather

Map animation showing the undulating jet stream pattern in the Northern Hemisphere that has fueled extreme weather in the US and Europe

Scientists are struggling to understand the effects of climate change on the rippling jet stream that has brought record temperatures to Europe and torrential rain and snow in the western United States, as extreme weather events around the world mark the beginning of 2023.

Scientists say the fast-moving band of air is behind many of the extreme weather events that have occurred in the northern hemisphere in recent weeks.

In North America, the late December freeze was followed by a “bomb cyclone” or bombbogenesis—an explosive storm caused by the collision of a cold air mass with a warm air mass—”atmospheric flows,” or air currents that carry moisture from the ocean that affect the western US states.

The weather anomalies continued into the weekend The US National Weather Service warned a “relentless parade of hurricanes” from the Pacific that would bring more rain and mountain snow to the West Coast, with Northern California the main focus.

Waves created by a massive Pacific storm smashed ashore in central California’s Santa Cruz, while in San Francisco to the north, residents barricaded windows and storefronts as they braced for more wild weather. © DANE NABAL via REUTERS

In Europe, at least eight countries have set new record January temperatures so far, including Poland, Latvia and Denmark.

“It’s no coincidence that North America had a cold spell and Europe had a heatwave,” said Paul Williams, professor of atmospheric sciences at the University of Reading. “It’s still looking pretty wavy today, with a big wave over the US and a slight wobble over Europe.”

“The jet stream causes all of our weather,” said Jennifer Francis, an atmospheric scientist at the Woodwell Climate Research Center. “These two things cannot be separated.”

The fast-moving bands of air blowing from west to east separate the cold air from the polar regions from the warmer air from the tropics and typically occur in the mid-latitudes. However, the jet stream can develop deep waves that push colder air south and warmer air north.

These waves can stay in place for weeks and bring with them more extreme and unusual weather patterns like those seen in North America and Europe.

In the southern hemisphere, it coincides with the La Niña phenomenon, which is also wreaking havoc. The three-year unusually recurring event involving the convection of air across the Pacific Ocean has caused flooding in Australia and droughts in the Horn of Africa and South America.

The Australian Bureau of Meteorology said this week that while climate change has seen some signs of weakening of La Niña conditions due to changes in the Western Pacific Oscillation, it “continues to affect Australian and global climates.”

The UK Met Office reported this week that the country’s mean annual temperature was 10.03C in 2022, the highest record since 1884. This put the year 0.89°C above the 1991-2020 average and 0.15°C above the previous record of 9.88°C set in 2014.

Global temperatures are estimated to have risen by at least 1.1°C since pre-industrial times.

January temperature records broken in Europe and US. Map showing the 2 meter mean air temperature anomaly for January 1, compared to the 1991-2020 average temperature for January. Big storms swept through much of the US this week, causing flooding, heavy snowfall, tornadoes and record temperatures in the South.

In mid-2022, the jet stream formed into five major waves, resulting in simultaneous heat waves around the world. This pattern is known as wavenumber 5.

While scientists are clear that climate change will make extreme weather events more frequent and intense, the effects of the warming planet on the jet stream are less clear and an active area of ​​research.

One theory suggests that the increased warming in the Arctic is reducing the temperature differential between the hot and cold air that the jet stream separates, slowing the flow and making it more rippled. But that’s far from consensus.

“How exactly waves in the jet stream will change due to climate change – there is still a lot of uncertainty,” said Dim Coumou, a climate scientist at the Vrije Universiteit Amsterdam.

According to the scientists, even without climate change, many of the extreme weather events of recent weeks could be attributed to the undulating jet stream.

Big waves in the jet stream have “always happened,” although right now we have “a very distinct wave pattern,” Coumou said. It’s “certainly too early to say it would be outside of the normal behavior of jet stream variability,” he said.

Last week, scientists at Nasa said Recent weather extremes have been associated with changes in the behavior of the Arctic polar vortex, the band of strong winds over the jet stream.

The vortex had “expanded” from its usual more circular formation, as it often did contributed to the jet stream becoming more rippled, they said.

Adam Scaife, head of monthly to decadal forecasting at the UK Met Office, said warm temperatures in Europe and the UK were likely the result of jet stream behavior combined with the effects of climate change.

“It’s not that the waves are unprecedented, it’s that we’re adding extra warmth now,” he said. “The same wave pattern 100 years ago would have given us much lower temperatures.”

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Adam Bradshaw

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