Scientists are struggling to understand the effects of climate change on the wavy jet stream that has fueled record heat in Europe and heavy rain and snow in the western US, as global extreme weather marks the start of 2023.
A fast-moving band of air is behind much of the extreme weather experienced in recent weeks in the northern hemisphere, according to scientists.
In North America, the end of December freezes from “bomb cyclones”, or bombogenesis – explosive storms caused by cold air masses meeting warm air masses – has been followed by “atmospheric rivers”, or air currents that bring moisture from the sea. , which has hit the western states of the US.
Weather anomalies continue into the weekend as the US National Weather Service warns of a “relentless parade of hurricanes” from the Pacific that will bring torrential rain and mountain snow to the West Coast, with a major focus on northern California.
Waves brought by a massive Pacific storm crashed on land in Santa Cruz in the center of California, while in San Francisco for residents of the north barricaded windows and shopfronts while braced for more wild weather. © DANE NABAL via REUTERS
In Europe, at least eight countries have set record high temperatures for January so far including Poland, Latvia and Denmark.
“It is no coincidence that North America has had a cold spell and Europe has had a hot spell,” said Paul Williams, professor of atmospheric science at the University of Reading. “It still looks a bit wavy, with big waves in the US and a bit of a shake in Europe.”
“The jet stream affects all of our weather,” said Jennifer Francis, an atmospheric scientist at the Woodwell Climate Research Center. “You can’t separate those two things.”
A band of air moving rapidly from west to east separates the cold air of the polar regions from the warmer air of the tropics, and is usually located in the middle latitudes. However, the jet stream can develop deep waves, which push colder air to the south and warmer air to the north.
These waves can remain in place for weeks, bringing more extreme and unusual weather to North America and Europe.
In the southern hemisphere, coinciding with the La Niña phenomenon also causes havoc. Unusually repeated for three years, events involving air convection across the Pacific Ocean have been behind floods in Australia and droughts in the Horn of Africa and South America.
The Australian Bureau of Meteorology said this week that while it is seeing some signs of a weakening La Niña situation, as a result of changes in the western Pacific Ocean oscillation, climate change “continues to affect the Australian and global climate.”
The UK Met Office this week reported that the country’s 2022 annual average temperature will be 10.03C, the highest on record since 1884. This makes the year 0.89C above the 1991-2020 average and 0.15C above the previous record. 9.88C. set in 2014.
Global temperatures are estimated to have risen by at least 1.1C since pre-industrial times.
In the middle of 2022, the jet stream is configured into five large waves and causes simultaneous heat waves around the world. This pattern is known as wave number 5.
While scientists are clear that climate change will make extreme weather events more common and intense, the effect of a warming planet on the jet stream is less clear, and an area of active research.
One theory is that the enhanced warming in the Arctic reduces the temperature difference between the hot and cold air separated by the jet stream, which slows the current and makes it thinner. But that is far from a consensus.
“How exactly the waves in the jet stream are changing due to climate change – there is still a lot of uncertainty,” said Dim Coumou, a climate scientist at the Vrije Universiteit Amsterdam.
Even without climate change, much of the extreme weather of the past few weeks could have been caused by the turbulent jet stream, according to scientists.
Large waves in the jet stream have “always happened,” although now “we have a very clear wave pattern,” Coumou said. It is “definitely too early to say that it will be outside the normal behavior of the jet stream variability,” he said.
Last week, scientists at NASA said that today’s extreme weather is linked to changes in the behavior of the polar vortex in the Arctic, a band of strong winds above the jet stream.
The vortex has “stretched” out of its usual more circular formation, which often contributes to the jet stream becoming wavier, they said.
Adam Scaife, head of monthly to decadal forecasting at the UK Met Office, said the warm temperatures in Europe and the UK could be a result of the behavior of the jet stream combined with the effects of climate change.
“It’s not an unprecedented wave, but now we’re adding extra warmth,” he said. “The same wavy pattern 100 years ago would have given us lower temperatures.”
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