Why what happens at the North Pole can cause destructive storms in Spain

One of the biggest unknowns about the so-called gota fría or DANA — an autumn weather event typical of eastern Spain — is whether what happened in Valencia last week can be said to be caused by conditions over 3,100 miles away, near the North Pole. It is a real possibility. For years, changes have been observed in the polar jet stream, the regime of west-to-east winds that blows at high latitudes in the northern hemisphere and is responsible for transatlantic flights taking about 40 minutes less to travel from New York to Madrid than vice versa.

This current is essential for regulating the climate of the northern hemisphere. It acts as a border between the cold polar air and the warm air currents that rise from the equator. This border is not straight, but has undulations that let in warm air from the south or cold air from the north. In recent years, these undulations seem to have become much more pronounced, so that the polar air pockets descend like river bends to much lower latitudes, and more frequently. When they reach southern Europe, the cold air pockets at high altitude encounter the heat and humidity rising from the warmer-than-normal waters of the Mediterranean and generate much more intense and longer rainstorms, as they are recharged again and again.

This anomaly in the polar current would also cause the opposite effect. As it moves back north, masses of warm air rise more and more frequently, causing heat waves and droughts. Jennifer Francis, a meteorologist at the Woodwell Climate Research Center in the United States, is one of the main proponents of this theory, which fits with what has been witnessed in Spain. “A current with large undulations like the one that caused the horrible floods in Spain will become more and more frequent due to the accelerated warming of the Arctic,” the researcher explains to this newspaper.

The changes in the current are producing anomalies that last much longer than usual. The jet stream moves from west to east across the hemisphere, undulating north and south, and those waves create the areas of high and low pressure that we see on weather maps, says Francis. “When the waves are small, the climate changes quickly, but if they grow and move more slowly, persistent weather patterns are created. This can lead to droughts, prolonged heat waves, or continuous rains that can cause flooding, as the same climate stays in one region for longer.”

Recent studies show that these fluctuations are becoming more abrupt, in the form of so-called climate whiplashes. “A warmer planet tends to suffer more droughts and heat waves. What our studies suggest is that these periods are abruptly interrupted by torrential rains, which are also connected to the climate crisis,” Francis explains. Her studies link changes in the jet stream with recent snow and ice storms at low latitudes, such as in Texas, or with the transition from drought and fires to torrential rains in Italy or Greece, which suffered a terrible storm in 2023.

One of the keys would be the uncontrolled warming of the Arctic Ocean, where temperatures are rising about four times faster than in the rest of the planet. The key to the jet stream functioning normally is the temperature difference between the polar and equatorial regions, which is shortening, which would cause a polar jet stream with increasingly more meanders that release large masses of icy air that meet the warmer-than-normal waters of the Atlantic and the Mediterranean and generate large storms. “The Arctic region north of Europe is unusually warm, more than 15 degrees above its average. This dome of warm air reinforces the northward movements of the jet stream, such as the one affecting Western Europe and generating the DANA that has caused intense rain in eastern Spain and continues to bring even more rain to the Iberian Peninsula,” explains Francis.

The disruption of the polar current is worrisome, as there is currently great uncertainty about its magnitude and future scope on a warmer planet. Scientists are dealing here with one of the most complex systems imaginable, the regime of wind currents and their interaction with oceans and topography, something that is difficult to model accurately even for the largest known computers. The United Nations’ international panel of experts on climate change, the IPCC, is studying the issue, but for now it says the level of uncertainty is high.

The big question is whether what is happening is due to human activities in the burning of solid fuels, or part of the natural variability of the jet stream, which changes substantially every year, Paulo Ceppi, a climatologist at Imperial College London, tells this newspaper. “In a system like this it is very difficult to detect the mark of climate change, although the models tell us that in general the stream will retreat to the north, which would bring more rain to northern Europe and more drought to the south,” he explains. Esteban Rodríguez, from the Spanish State Meteorological Agency, emphasizes the current doubts due to the complexity of the process. “It is still a matter of discussion. Evidence of these effects has been seen in summer, but in winter it is not yet so clear. What we are sure of is that from now on, there will be fewer days of rain, but it will be more torrential rain,” he explains.

Robert Vautard, co-chair of the IPCC’s atmospheric physics working group, points out that, regardless of changes in the jet stream, “there is no doubt that when the cold drops arrive, they dump more and more rain across Europe.” A recently published preliminary study by the Global Climate Attribution Group suggests that the rainfall from this kind of weather event in Spain was 12% more intense due to climate change, and will be twice as intense as usual. Vautard, who was not involved in the study, says that this is a reasonable estimate: “Our studies in France suggest 20% more precipitation due to warming. This may not seem like much, but it is enough to produce a tremendous disaster, especially since the water falls in very specific areas.”

The French meteorologist and oceanographer points to two other factors that have worsened the impact of the storm. The first is that, unlike many other cold drops, this one has fallen in highly populated and urbanized areas, so the water has had nowhere to permeate the ground and has caused “terrifying” floods. The researcher also points to the increasing construction of homes near river beds and flood zones. “It is something that we suffer a lot from in France; houses have eaten up part of the river beds, which are constricted, and rivers need space.” The tragedy in Valencia should “serve as a lesson to us,” he emphasizes. “We have to better predict the scale of current floods and not base our policies on past storms, but on future ones.”

Geographer Sergio Vicente-Serrano co-authored the chapter on extreme weather events in the latest IPCC report. He explains that for current science, understanding atmospheric dynamics, which include phenomena such as fluctuations in the polar jet stream, is a challenge; and there is still great uncertainty about their magnitude. On the other hand, the thermodynamic impacts of global warming are becoming increasingly clear. An increasingly warm atmosphere due to rising temperatures is already generating a situation of more torrential rains, since the air's capacity to absorb moisture is greater.

An event like the one in Valencia has a return time of about 50 years, which means that on average similar events will be repeated every half-century, which is not that much, according to preliminary calculations by the researcher, who works at the Pyrenean Institute of Ecology (IPE-CSIC). Vicente highlights that the great novelty of this flood is not that it has occurred, but that it has fallen in an area of high population density crossed by many roads and communication routes. “In 1983, similar rainfall was recorded in Oliva (also in the Valencia region), but in this case it was far from metropolitan areas, unlike now,” he explains.

The main reason why the DANA has caused such destruction and mortality is “a dantesque lack of city planning,” says Vicente. “It cannot be that the largest shopping center in Valencia is built on a ravine. We are no longer talking about buildings from before the days when there was an orderly urban zoning system,” he says. “What I find shameful and regrettable is that a region like Valencia does not yet have a flood forecasting system based on a hydrological model that is nested with the weather forecast model, and that in turn nested with a hydraulic model to determine the height of the areas that will be affected by rising waters. Calculating the height of the sheet of water in these cases is something that can already be done with current technology, and it should be a priority over other issues such as the America’s Cup, soccer or bullfighting.”

Fluctuations in the polar jet stream have shaped the lives of most Europeans for at least the past 700 years, according to a study published a few weeks ago. Researchers estimated the variability of the stream by analyzing tree rings, and showed that anomalies in this atmospheric circulation are linked to droughts, famines, and even plague epidemics. “Given the possibility that the jet stream has been altered and intensified extreme events,” the authors said, studying its variability is key to understanding “the climate risks of the future.”

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