Jet stream makes summer weather hard to forecast

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Scientists say recent work to unravel the mysteries of the Atlantic jet stream could pay off with better long-term forecasts of summer weather.

The strength of the jet stream—a ribbon of very strong winds caused by the temperature difference between tropical and polar air masses—plays a big role in summer weather in the UK and northwest Europe.

A northward shift in the Atlantic jet stream tends to direct low-pressure systems northwards and away from the UK, leading to warm and dry weather during summer. But, if the summer jet slips southwards it can lead to the jet shifting the low-pressure systems directly over the UK, causing miserable weather.

The big question is “why does the jet stream shift?”

A new study, led by PhD student Richard Hall and Professor Edward Hanna from the University of Sheffield, suggests that up to 35 percent of this variability may be predictable—a significant advance which may help in the development of seasonal forecasting models.

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“There is nothing people in the UK like to discuss more than the weather. This is because it can fluctuate so drastically—we can be basking in high temperatures and sunshine one week only to be struck by heavy downpours and strong winds the next,” says Hall.

“Our study will help forecasters to predict further into the future, giving a clearer picture of the weather to come.”

The findings suggest the latitude of the Atlantic jet stream in summer is influenced by several factors, including sea surface temperatures, solar variability, and the extent of Arctic sea-ice. This suggests a potential long-term memory and predictability in the climate system.

“Working with the Met Office, we were able to look at the different factors which may influence the jet stream, which paves the way for improvements in long-term forecasting,” says Hanna, a professor of climate change.

Professor Adam Scaife, head of long range forecasting at the Met Office, says that while scientists have made inroads into long-range forecasts for winter, “we are still limited to shorter-range weather forecasts in summer. Studies like this help to identify ways to break into the long-range summer forecast problem.”

The study appears in the journal Climate Dynamics.

Source: University of Sheffield