Understanding climate
for the benefit of society

Jetstrømmen: Figuren viser vinden ved 300 hPa, representativt for jetstrømmens høyde. Det er den sterkeste vinden i verden og blåser en mil over hodene våre. Høyt der oppe styrer den lavtrykkenes bane - rett mot oss, eller lengre mot nord eller sør. Derfor må vi finne ut hvordan klimaet styrer denne vinden. Illustrasjon: Haltenbanken

Chasing the strongest winds

The passengers on British Airways’ flight BA114 saved almost an hour on the trip from New York to London on 8 January this year. The pilot was riding the jet stream, a wind that is responsible for both storms in Norway and eager discussion among climate scientists.


The passengers on British Airways’ flight BA114 saved almost an hour on the trip from New York to London on 8 January this year. The pilot was riding the jet stream, a wind that is responsible for both storms in Norway and eager discussion among climate scientists.

Rosendal, Norway, 25 March 2015: Ninety scientists are gathered to talk about the weather. Over snow-covered mountain peaks there is a touch of blue sky among gray clouds. But it’s not the weather in Rosendal this March day they’re discussing, it’s heat waves and icy winters in Europe, Russia and North America during the last decades. Could both long-lasting heat and cold here be a result of higher temperatures in the Arctic?

One of the promoters of this theory is Jennifer Francis, professor at Rutgers University in New Jersey. In several studies she and colleagues have presented data indicating that the winds high up in the atmosphere may have changed during the last decades. In Rosendal she leads the discussion together with med Dr. James Screen, researcher at the University of Exeter. He is not convinced.  

James Screen and Jennifer Francis in Rosendal, Norway. Photo: Ellen Viste 

Almost as fast as sound

Eleven kilometers above the North Atlantic, 8 January 2015: Pushed by the strongest winds on the globe, a passenger plane from British Airways approaches the speed of sound, relative to the ground.

Flying from New York to London, the pilot had maneuvered the plane into the jet stream, a ribbon of strong westerly wind encircling the globe seven to twelve kilometers above the ground. In the core of the stream the wind speed is normally about 300 kilometers per hour – 2.5 times as much as in the weakest hurricanes. That this influences air traffic is so well known that it is taken into account when setting up plane schedules. The airline Norwegian’s route from New York to Oslo is 35 minutes shorter than that from Oslo to New York.

This winter day the jet stream was extra strong above the northeastern Atlantic Ocean, and according to The Telegraph, the plane whizzed toward London at a speed of 1200 kilometers per hour. The jet stream contributed to one third of this.


Bergen, Norway, 10 January 2015: Shingles and large parts of roofs fly through the air and parts of the city center have to be shut off. The storm Nina has hit western Norway, and the county of Hordaland gets the strongest winds since 1994. It’s the same, strong jet stream that has driven Nina across the Atlantic and toward Norway.

The jet stream is located at the polar front – the border between cold, polar air and warmer air to the south. This is also the region where lows pressure centers form. And the location of this border is crucial. Whether the jet stream lies far to the south or north determines where the lows end up. Often the jet stream drives lows into the Norwegian Sea, but it can also lie farther to the south and send them into southern or central Europe.

These days in January the jet stream was particularly strong, and Nina swooped into the Norwegian Sea. But climate scientists have also started to wonder whether a weak jet stream could cause trouble – because they extend periods with weather of the same kind.

The jet stream does not always blow straight eastward. It forms waves in the north-south direction, with crests and troughs. That the icy winter of 2013/2014 in North America should have anything in common with the heat wave in Russia during the summer of 2010, is not obvious. But both the stable northerly winds during that winter and the weeks without rain during the summer, occurred at a time when there were large bulges in the jet stream. In both cases persistent highs blocked the lows and clouds for a long time.

Such repeated concurrences have triggered the curiosity of scientists. Could it be more than a coincidence?

Stuck in the cold

Jennifer Francis was one of the first to ask whether rapid Arctic warming may already have influenced the jet stream. In several studies she has coupled increasing temperatures in the Arctic to extreme cold and heat in North America and Europe.

Together with colleagues, Francis has presented data showing that the wave crests in the jet stream are reaching further north, which through atmospheric theory means that they should move eastward more slowly than before.

– This means that for example cold spells during winter may last longer, says Francis.

All regions of the world have become warmer over the last century. But temperatures have not increased uniformly. The largest increases are seen in the Arctic, mainly because of diminishing sea ice. Both the jet stream and the lows over the North Atlantic form because the Arctic is normally colder than the regions farther south. As the temperature has increased more in the north, the difference to the middle latitudes is now smaller. All other things aside, a weaker temperature difference could mean weaker westerlies in the jet stream.

Francis’ calculations indicate that the jet stream has already become weaker, with more frequent occurrences of large bends, so that it takes longer time for highs, lows and fronts to pass. The system gets stuck.

Under global warming, the heat records fall readily, while extremely cold days become less common. Jennifer Francis still fears that the duration of long-lasting cold spells, as we have seen in the US and Europe the last years, may increase. And the consequences of low temperatures do not necessarily depend on whether cold records are broken.

– Cold spells could have an even larger effect if people are used to warmer conditions, even if they’re less cold than they used to be.

James Screen agrees, but thinks that it’s dangerous to draw conclusions at this stage. The recent cold winters may have several causes, and it’s difficult to separate them.

– The warming in the Arctic must necessarily have an influence on the weather, says Screen. – The question is how large it is compared to other factors.

 In an opinion article, Screen and a colleague conclude that less sea ice and increased warming in the Arctic may influence the jet stream and the weather at latitudes of North America and Eurasia, but that we so far do not know how this occurs, nor how large the effect may be. So far, the effect has been small compared with natural variations in the jet stream, and it is also uncertain how large it will be with the projected future warming. One of Screen’s reasons for being skeptical is that the co-occurrence of Arctic warming, a weaker jet stream and persistent weather at mid-latitudes, may have other causes.

– It’s easy to show that two things occur at the same time, Screen says. – But that does not mean that one of them causes the other.

But what if there is a plausible, physical mechanism? Francis thinks that this should allow us to suspect a relationship. Then it’s all about confirming or disproving the theory.

–  It’s not cast in stone, she admits. – But more evidence is emerging.

– In my opinion, the evidence is inconclusive, says Screen. – We cannot rule out other explanations.

Screen’s main objection is that the Arctic is such a small part of the world, and that the conditions in many other regions also influence the weather in the USA and Europe. The weather in North America is among other things influenced by sea surface temperatures in the Pacific. Could the warming in the Arctic perhaps strengthen weather patterns caused by variations in Pacific temperatures? Francis nods. But, both of them point out, we do not have many years with observations, and there are no previous examples of the combination of Arctic sea ice and Pacific sea surface temperatures that we see now. We don’t have anything to compare with.

– There has never been so little sea ice before, says Screen.


A need for explanations

Scientists from Europe, USA, Russia and Japan gathered in Rosendal. Francis’ hypothesis has received a lot of attention, both in the media and in the scientific community.

– The attention has been good for science in general, says Thomas Spengler. He is the organizer of the conference, and professor at the Bjerknes Centre for Climate Research and the Geophysical Institute at the University of Bergen.

Spengler is very satisfied with the discussions in Rosendal. He underlines the importance of figuring out how the Arctic interacts with the rest of the climate system, in particular as we are seeing and projecting so enormous changes in the Arctic for the coming years. He is less impressed by the general jet stream debate, which he thinks has been misleading and skewed at times.

– The claims that Arctic warming influences the jet stream must be supported with physics, says Spengler. – We need to move beyond statistics and hand-waving for arguments and come closer back to our climate science roots with dynamics and physics.

Spengler calls for more research, not just on whether there is a connection between Arctic warming and long-lasting extreme weather, but on how this could occur. The mechanisms are yet unclear.

Detectives of science

Jennifer Francis and James Screen both believe the coming five years will take us closer to an answer, not necessarily through major breakthroughs, but through a regular flow of new discoveries.

– Science works in increments, says Screen. – Over longer time-scales, evidence will stack up in favor or against a connection.

– Yes, it’s more like a murder case trial, Francis adds.

And if no conclusion is reached about the effect of Arctic warming on the jet stream and mid-latitude weather, the concentrated effort may lead to breakthroughs in related fields.

– This is how science progresses, says Screen.