Icy waves – Magazine ?

The Beast from the East and the Polar Vortex were two extreme cold waves that occurred in 2018 and 2019 in Europe and North America, respectively. Despite having different names, these events were caused by the same stratospheric phenomenon.

Last winter, 250 million people suffered from cold temperatures that for some days were worse than those at the North Pole. In Chicago and Boston the temperature plummeted to minus 35°C below zero. The rivers froze. Buildings and bridges were covered in ice. In Minnesota the temperature reached a record of -49 ° C, which had not happened in more than a century. The cold claimed the lives of more than 40 people.

The impact of the phenomenon was such that the media considered it something extraordinary – which was surely due to climate change – and began to call the episode “Polar Vortex”, as if this term were synonymous with cold wave. In fact, they had already done it during another cold wave in 2014, also related by meteorologists to a polar vortex.

But for atmospheric scientists the polar vortex is not an extreme weather event. It’s not even weird at all. The polar vortex is a current of air that forms every year around the poles (or near the polar regions) and weakens with some frequency—approximately two out of every five winters. We are beginning to understand that cold waves like the one in 2019 are a consequence of the weakening of the polar vortex.

polar air trap

The first record of the polar vortex—or at least the term—dates back to 1853 and is found in a weekly magazine edited by the English writer Charles Dickens. In an article titled “Wind Maps,” a merchant named John Capper, based in Ceylon (now Sri Lanka), explained a new science that made it possible to identify the planet’s large air currents. One of these currents, Capper wrote, “spins around the pole in a continuous circular gale that reaches the great polar vortex.” Capper was referring to the south pole, but today we know that vortices form around both poles. The polar vortex, as we understand it today, is a mass of icy air that forms in the stratosphere, the layer of the atmosphere that is between 12 and 40 kilometers high. This vortex appears every winter due to the low temperatures of the polar stratosphere. It begins to form in late autumn, when sunlight wanes, and strengthens as winter progresses.

During the polar night there is no light at the poles for several weeks and temperatures plummet. Due to the great contrast between polar temperatures and those of mid-latitudes (in the northern hemisphere, the latitudes of Europe and the United States, for example), a wind barrier is formed. This barrier flows rapidly around the poles, forming a barrier for polar air and preventing it from mixing with warmer mid-latitude air. Once the circulation closes and the polar air is trapped, temperatures in the polar stratosphere drop even further, increasing the speed of the circular flow in a chain of events that reinforce each other. This forms a wind vortex that rotates from west to east and encloses and keeps intact the polar air mass.

The polar vortex disappears in early spring, when the Sun warms the pole again and the temperature difference with the mid-latitude air decreases, weakening the wind barrier until it breaks and all the air mixes.

Sudden stratospheric warming

In the 1950s, the fashionable toy of atmospheric scientists were weather balloons, which today are launched around the world to measure the state of the atmosphere. In 1952 Richard Scherhag, from the Free University of Berlin, detected by means of these devices an anomalous heating of the stratosphere over Berlin. A few years later, Scherhag had documented several cases of sudden warming of up to 40 ºC, which had then moved westward. In an article published in 1960, the researcher mentions that other scientists found a simultaneous phenomenon: a weakening of the polar current.

Over time we have been able to confirm that in certain years, although not all, the polar vortex current suddenly weakens at the north pole. It doesn’t happen at the south pole because the polar vortex there is much stronger. When the vortex weakens at the north pole, the stratospheric polar air mass suffers two effects: the first, as I mentioned, is that it mixes with the surrounding air that is warmer, and the second is that the cold air coming from above compresses it downwards. The combined effect is that the polar vortex heats up very quickly. This phenomenon is known as sudden stratospheric warming. When it occurs, the vortex may split into two “daughter” streams, or may be displaced from its typical position.

fast jet

The jet stream is another very fast flow of air that surrounds the entire planet, but is located a few kilometers below the stratosphere, at an average height of between eight and 10 kilometers. It is normally observed as an undulating flow surrounding the poles between latitudes 40 and 60°N. This flow of air rotates from west to east like the polar vortex, and is so fast that some commercial airplanes take advantage of it to reach their destination faster.

In winter the jet stream gains strength with the help of the stratospheric polar vortex, which rotates in the same direction, only at different heights in the atmosphere. When the polar vortex warms and weakens, it no longer moves as fast and may even begin to flow in reverse, from east to west, which in turn weakens the jet stream. The effect propagates to the Earth’s surface and affects weather. When sudden stratospheric warming occurs, the air just above the poles receives pressure from above, forcing it to move laterally; that is, towards lower latitudes.

In 2001, Mark Baldwin and Timothy Dunkerton documented the typical effects of sudden stratospheric warming using 42 years of data provided by the US National Oceanic and Atmospheric Administration (NOAA). Baldwin and Dunkerton found that these phenomena can drastically modify the course of winter storms and that they are related to the coldest periods of the boreal winter. In fact, they are so related that they can even be used to make forecasts: a sudden warming is a precursor to a cold wave.

In 2013, Amy Butler of NOAA and her collaborators documented that these warmings occur on average two out of every five winters, although there is a lot of variation: sometimes many years go by without them occurring.

The Beast from the East

One of the most drastic and recent examples of the impact of sudden stratospheric warming occurred in mid-February 2018 (the previous one happened in 2013). This phenomenon caused the polar vortex to split in two. Just 10 days later, the stratospheric anomaly spread to the lower layers, affecting the jet stream. This fragmented over the United States and Europe and the polar air spilled southward.

With temperatures below -10°C, polar air invaded the mid-latitudes over Siberia and was quickly predicted to move westward across Europe. From that moment on, the newspapers warned that one of the most intense cold waves in the recent history of the continent was coming. They called it the Beast from the East. Snowfall, frigid air and ice covered the surface of Europa for two weeks, all due to a sudden stratospheric warming that had occurred days before, 25 kilometers high. The impact was such that the following snowfalls, typical of a European winter and unrelated to the polar vortex, were called the Beast from the East 2.

In early 2019, just 10 months after the Beast from the East snow melted, weather forecasts called for another sudden stratospheric warming. The polar vortex split in two around December 28, 2018. One part headed toward Siberia and the other toward North America. The effects of this movement of the polar vortex propagated to the surface within two weeks. Surface polar air migrated southward across Canada and Russia.

By mid-January 2019 the effects were noticeable. Temperatures in Canada and the United States began to plummet, reaching minus 25°C by January 20; and the worst was still to come. In the following days, temperatures would drop between five and 15 degrees more, depending on the region. While North America and Russia froze, the North Pole was warmed by air blowing in from the Atlantic. Thus, for a few historic days, the North Pole was hotter than most of North America. They were the lowest temperatures that an entire generation, that of people under 40 years old, had seen. For days on end, most of Canada and the northeastern United States were covered in snow and ice, like a scene from The day after tomorrow.

hot but cold

We call climate change the change in the averages of variables such as the temperature of a region over a period of 30 years or more. Global warming does not mean that there will no longer be wild cold waves like the Beast from the East and the Polar Vortex, or even that the frequency of snowfall will decrease, but only that the average temperature is increasing. Global warming may be local cooling in some regions. You cannot argue that there is no climate change by pointing out that one winter was especially cold or one snowfall was particularly heavy, such as…