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© NASA/Goddard Space Flight Center Scientific Visualization Studio

Shifting Winds: How a wavier polar jet stream causes extreme weather events

October 28, 2024
Explaining the effects of a changing polar jet stream with Dr. Jennifer Francis, senior atmospheric scientist at the Woodwell Climate Research Center and award-winning science communicator.

True to its name, the polar jet stream can shave some 30-45 minutes off your transatlantic flight going from North America to Europe. The high-altitude wind current, circulating the Northern Hemisphere at the height commercial airplanes travel, has acted as a reliable barrier between icy Arctic and warmer southern air. But climate change is disrupting the wind system, and the once relatively stable waves of the polar jet stream are now more often plunging deeper and climbing higher, leading to severe consequences for billions of people.

The polar jet stream is a river of wind, flowing from the West to the East and encircling the Northern Hemisphere. Its meandering waves create a boundary between the cold Arctic air and the warmer air to the South. Yet, with the Arctic warming three times faster than the average for rest of the world, the temperature difference between North and South decreases and so does the speed of the jet stream’s westerly winds. As these winds slow down, its waves typically grow bigger and extend farther to the North and the South. Larger jet-stream waves move eastward more slowly, affecting weather patterns across North America to Central Europe and Asia.

“The big swings of the jet stream tend to be very persistent and to stay in the same place for a long time. As each wave brings either warm or cold air to a region, this slow movement can cause extreme weather patterns, such as heat waves, floods, cold spells, storms and droughts – and we are seeing these wavey patterns happen more frequently”, explains Dr. Jennifer Francis, senior atmospheric scientist at the Woodwell Climate Research Center.

On the right (or wrong) side of the wave

The archipelago of Svalbard experienced the coldest winter in 20 years in 2024, followed by a persistent warm-up and quick disappearance of sea ice in spring, leading to yet another record-breaking warm summer. This has a direct correlation with the stream’s wild north-south swings, says Dr. Francis. While parts of the Arctic, including Svalbard and Northern Norway, recorded the warmest month (Longyearbyen, Svalbard: an average of 11 degree Celsius in August at 78°N) and most summer days above 20 degrees Celsius (Tromsø, Norway: 36 days compared to an average of 12), Central Europe saw devastating floods in later summer, impacting 2 million people on the continent.

“These extremes come in pairs. While one region might experience abnormally warm temperatures, you just need to look to the east, downstream, and you will likely face the opposite weather conditions. Whether you are experiencing a dry spell or flooding, largely depends on where on the trajectory of the polar jet stream waves are placed relative to you: If you are located on the part of a wave where the winds come down from the northwest, you're typically looking at dry conditions and clear blue skies”, shares Dr. Francis. On the other side of that wave, southwestly winds bring warmth and moisture, which tend to form storms. “So, you can look at the location of the jet stream and get a pretty good idea of how your weather will behave in the near future.”

Scientists and Arctic residents often fail to appreciate the strong effects that the extra moisture can create. For starters, water vapor is a greenhouse gas, so in a dry Arctic winter atmosphere, a little more moisture can trap substantially more heat. Moreover, when that moisture condenses into clouds, it releases latent heat, further warming the air. Dr. Jennifer Francis

Warm oceans, bigger jet stream waves

In addition to the rapidly warming Arctic, another reason for the jet stream becoming wavier is the drastic warming of the midlatitude oceans, Dr. Francis theorizes. “The oceans are absorbing much of the heat trapped by greenhouse gases – up to 90 percent. Across the North Pacific and the Atlantic, we have observed record-breaking water temperatures over the past years”.

In the higher latitudes, the warming is even more pronounced. In only 40 years, about 75 percent of sea-ice volume has been lost. As the sea ice retreats and thins, it provides less of a barrier for the ocean’s heat to enter the air, which further contributes to the warming.

Dr. Francis also emphasizes the role of water vapor and wrote in a 2018 Scientific American article: “Scientists and Arctic residents often fail to appreciate the strong effects that the extra moisture can create. For starters, water vapor is a greenhouse gas, so in a dry Arctic winter atmosphere, a little more moisture can trap substantially more heat. Moreover, when that moisture condenses into clouds, it releases latent heat, further warming the air. Finally, more clouds trap more warmth below them, one more factor contributing to the Arctic meltdown.” [1]

Add to this the jet stream’s more extensive north-south oscillation, which has brought pulses of record warmth and moisture to the high latitudes, and you are beginning to piece together the complex picture of Arctic amplification.

[1] Francis, J., 2018. Meltdown. The Arctic Climate is Shattering Record after Record, altering Weather Worldwide. Scientific American, April 2018.

The interplay of wind systems

In addition to the polar jet stream, there is also a subtropical jet stream in the Northern Hemisphere. While the polar jet stream affects weather patterns in the mid-latitudes, through the United States to Europe, its southern equivalent has more influence on areas between the equator and 30 degrees North. “Sometimes these two jet streams interact, layer on top of each other; and when they do, we tend to get the really big storms. That is because the subtropical jet stream brings tropical heat and moisture in contact with very cold Arctic air,” describes Dr. Francis.

It's a complex system and, as Dr. Francis explains, the two wind systems form for different reasons and respond differently to climate change. “It’s all very interesting. We still have a lot of questions about how these wind systems will change and in turn influence our weather.”

In wintertime, a third circling stream joins the scene: The stratospheric polar vortex. This pool of cold air sits over the North Pole only during winter months, way above the jet stream (30 kilometers versus 10). It starts to form in late autumn and usually spins around the top of the world – not bothering anybody, as Dr. Francis puts it.

Normally, the polar vortex has little influence on our weather, but every once in a while, the it stretches into a bean shape, or it splits into two or three pockets. When that happens, those smaller pools of cold air can descend southward and reinforce a dip in the jet stream, making a cold spell much worse. Dr. Jennifer Francis

A wobbling polar vortex causing deadly freezing events

“Normally, the polar vortex has little influence on our weather, but every once in a while, the it stretches into a bean shape, or it splits into two or three pockets. When that happens, those smaller pools of cold air can descend southward and reinforce a dip in the jet stream, making a cold spell much worse.”

This is exactly what happened in Texas in February 2021, when the U.S. state recorded the longest freezing streak in its history. Lasting for almost 9 full days, the Great Texas Freeze saw temperatures drop below -26 degrees Celsius and cause a power outage that left millions without electricity.

While polar vortex splits happen naturally, researchers like Dr. Francis are observing these events more frequently now. “We think one of the reasons it's happening more often is the loss of sea ice, especially in the Barents-Kara-Sea area east of Svalbard. This is one of the fastest warming places in the Arctic and it’s located right underneath the westerly winds of the polar vortex. It creates like a bubble of hot air over the region, which can cause the spinning vortex to wobble, stretch and ultimately to break apart”.

While reversing these changes and bringing the jet stream back to its original state may be unrealistic, we can strive to slow down these changes as much as possible. The actions we take today will make the future less dire for our children and grandchildren. Dr. Jennifer Francis

The future jet stream – and the chances of getting it back on track

When asked how she thinks the polar jet stream will change in the future, Dr. Francis predicts that wavier jet-stream patterns, causing more persistent weather regimes, will occur more often. “That’s bad news, because it means that we will see more droughts, more heat waves, more wildland fires, more flooding.” All of these effects are connected to unusual and persistent weather patterns. “The underlying message here is that we are going to see weather regimes becoming more persistent, leading to extremes that result from long-lived weather conditions. Unfortunately, we are going to experience even more extreme events and record-breaking conditions that will affect billions of people across the Northern Hemisphere,” Dr. Francis warns.

So, is there a possibility of getting the jet stream back on track? “The disease that needs to be treated is the build-up of heat-trapping greenhouse gases in the atmosphere, which are mainly coming from burning fossil fuels, but also from cutting down forests and removing wetlands”, Dr. Francis emphasizes. “We’ve got all the tools, we just need the will of people, governments and communities to embrace the goal. In the meantime, we have to prepare for the changes that we know are coming.”

As Dr. Francis states, we need to identify and address the risks and vulnerabilities communities face due to increasing and intensifying extreme events. “While reversing these changes and bringing the jet stream back to its original state may be unrealistic, we can strive to slow down these changes as much as possible. The actions we take today will make the future less dire for our children and grandchildren.”

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