Jet stream ‘may be changing’

Pallab Ghosh: “We may have to get used to winters where spells of weather go on for weeks – or even months”

New research suggests that the main system that helps determine the weather over Northern Europe and North America may be changing.

The study shows that the so-called jet stream has increasingly taken a longer, meandering path.

This has resulted in weather remaining the same for more prolonged periods.

The work was presented at the annual meeting of the American Association for the Advancement of Science (AAAS) in Chicago.

The observation could be as a result of the recent warming of the Arctic. Temperatures there have been rising two to three times faster than the rest of the globe.

According to Prof Jennifer Francis of Rutgers University in New Jersey: “This does seem to suggest that weather patterns are changing and people are noticing that the weather in their area is not what it used to be.”

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We can expect more of the same and we can expect it to happen more frequently”

Prof Jennifer FrancisRutgers University

The meandering jet stream has accounted for the recent stormy weather over the UK and the bitter winter weather in the US Mid-West remaining longer than it otherwise would have.

“We can expect more of the same and we can expect it to happen more frequently,” says Prof Francis

The jet stream, as its name suggests, is a high-speed air current in the atmosphere that brings with it the weather.

It is fuelled partly by the temperature differential between the Arctic and the mid-latitudes.

If the differential is large then the jet stream speeds up, and like a river flowing down a steep hill, it ploughs through any obstacles – such as areas of high pressure that might be in its way.

If the temperature differential reduces because of a warming Arctic then the jet stream weakens and, again, like a river on a flat bed, it will meander every time it comes across an obstacle.

This results in weather patterns tending to becoming stuck over areas for weeks on end. It also drives cold weather further south and warm weather further north. Examples of the latter are Alaska and parts of Scandinavia, which have had exceptionally warm conditions this winter.

In the UK, storm after storm has rolled across the country
In the UK, storm after storm has rolled across the country

With the UK, the US and Australia experiencing prolonged, extreme weather, the question has been raised as to whether recent patterns are due to simple natural variations or the result of manmade climate change? According to Prof Francis, it is too soon to tell.

“The Arctic has been warming rapidly only for the past 15 years,” she says.

“Our data to look at this effect is very short and so it is hard to get a very clear signal.

“But as we have more data I do think we will start to see the influence of climate change.”

Prof Francis was taking part in a session on Arctic change involving Mark Serreze, the director of the US National Snow and Ice Data Center in Colorado.

He said the idea that changes in the polar north could influence the weather in middle latitudes – so-called “Santa’s revenge” – was a new and lively area of research and somewhat controversial, with arguments for and against.

“Fundamentally, the strong warming that might drive this is tied in with the loss of sea-ice cover that we’re seeing, because the sea-ice cover acts as this lid that separates the ocean from a colder atmosphere,” Dr Serreze explained.

“If we remove that lid, we pump all this heat up into the atmosphere. That is a good part of the signal of warming that we’re now seeing, and that could be driving some of these changes.”

Chicago 6 January
Chicago is now warming after being gripped by frigid polar air in January

The Maths Behind the Heat Wave.

A heat wave in the UK is usually measured by temperatures reaching in excess of 30 degrees Celsius for more than a week. The current heat wave – the longest spell of hot weather in seven years – is welcomed by most after one of the most prolonged winters on record; but if temperatures remain high, health risks and environmental hazards, such as crop failure, wildfires, and water shortages, become a real concern. So how good are we at predicting heat waves? Are they related to global warming? And do we know when it will rain again?


Maths helps us to answer these questions. Forecasting a heat wave relies on an accurate prediction of the jet stream, and the attendant troughs of low pressure and ridges of high pressure. The present spell of hot weather in the UK is related to the dramatic shift of the jet stream in early July to a northerly position relative to the British Isles.

Jet streams were among the first meteorological phenomena to be understood and quantified by mathematics. The breakthrough was made by a Swedish meteorologist who moved to the United States in 1925 and subsequently changed the face of meteorology, in both the US and worldwide. His name was Carl-Gustaf Rossby, and his name is immortalized in the Rossby wave – a key mechanism in controlling extreme weather such as heat waves.

Rossby waves are similar to water waves, but instead of water moving up and down, the air stream undulates northwards and southwards. Rossby waves are revealed by distinctive patterns of the troughs and ridges. These mostly westward propagating waves are quirkily associated with eastward-moving weather. However, from time-to-time the waves come to a halt – remaining stationary over a particular region.

Forecasters call this “blocking”, which is when a weather event such as a heat wave persists in an area for a long period of time. One undulation of a Rossby wave generally has an east-west extent of about five thousand kilometres (roughly across the northern Atlantic Ocean). By studying how the actual spacing of the troughs and ridges compares to an undulation calculated for the average winds, we estimate their subsequent motion.

Understanding how patterns in the jet streams change with the climate is critical. One of the major factors that influence the dynamics of a jet stream is the thermal wind equation. This equation states that the vertical rate of change of the horizontal wind is proportional to the horizontal rate of change of temperature. This means that the decrease in temperature towards the North Pole causes winds to develop a larger eastward component with altitude. The strong eastward moving jet streams are in part a consequence of the simple fact that the equator is warmer than the north and south poles. So, if the equator to pole temperature differences were to be modified as a consequence of climate change, this would most likely have a major impact on the behaviour of the jet streams.

Because jet streams are large-scale features of weather, forecasters can be reasonably confident in their ability to predict how the jets will change from day to day. The computer models used in forecasting divide the entire atmosphere into millions of ‘weather pixels’. Each pixel covers a horizontal area roughly 15km by 15km, and there are about 70 vertical layers of these pixels throughout the depth of the atmosphere. One of the crucial factors in determining whether or not a weather system can be forecast on the computer accurately is the ratio of the size of the system to the size of a typical pixel: weather phenomenon that are many more times larger than a weather pixel, and will therefore be represented by perhaps many thousands of pixels, are likely to be forecast well. This is the case for the jet streams.

On the other hand we know that prolonged periods of hot weather invariably give rise to severe thunderstorms. Thunderstorms are usually very local phenomenon, and they are not well resolved by the current computer models. For this reason, the periods of hot weather can be forecast with considerable accuracy, while forecasters may only be able to indicate the likelihood of isolated, but often damaging, thunderstorms.