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One of the biggest unknowns about the so-called have a fria or DANA – an autumn weather event typical of eastern Spain – is or what happened in Valencia It can be said that last week was caused by conditions more than 5,100 kilometers away, near the North Pole. It’s a real possibility. For years, changes have been observed in the polar jet stream, the regime of west-east winds that blow at high latitudes in the Northern Hemisphere and make transatlantic flights take about 40 minutes less to travel from New York to Madrid than vice versa.
This current is essential for regulating the climate in the Northern Hemisphere. It acts as a boundary between the cold Arctic air and the warm air currents rising 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 waves seem to have become much more pronounced, so that the polar air pockets descend like river bends to much lower latitudes, and more often. When they reach southern Europe, the high-altitude cold air pockets are confronted by the heat and humidity rising from the air warmer than normal waters of the Mediterranean Sea and generate much more intense and longer rain showers, because they are charged again and again.
This anomaly in the polar current would also cause the opposite effect. As the air moves north again, masses of warm air rise more and more often, causing heat waves and droughts. Jennifer Francis, a meteorologist at the Woodwell Climate Research Center in the United States, is one of the leading proponents of this theory, which echoes what has been observed in Spain. “A current with large waves, like the one that caused the terrible floods in Spain, will become increasingly common due to the accelerated warming of the Arctic,” the researcher explained to this newspaper.
The changes in the current cause deviations that last much longer than normal. 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 we see on weather maps, Francis says. “When the waves are small, the climate changes quickly, but when they grow and move more slowly, persistent weather patterns emerge. This can lead to drought, prolonged heat waves or persistent rain that can cause flooding, as the same climate remains 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 usually suffers more from droughts and heat waves. What our studies suggest is that these periods are abruptly interrupted by torrential rains, which are also linked to the climate crisis,” Francis explains. Her studies link changes in the jet stream to recent snow and ice storms at low latitudes, such as in Texas, or to the transition from drought and fires to heavy rainfall in Italy and Greece, where a terrible storm occurred in 2023.
One of the keys would be the uncontrolled warming of the Arctic Oceanwhere temperatures are rising about four times faster than the rest of the planet. The key to the normal functioning of the jet stream is the temperature difference between the polar and equatorial regions, which is becoming shorter and shorter, which would lead to a polar jet stream with more and more meanders that release large amounts of icy air that meet the warmer than normal water. of the Atlantic Ocean and the Mediterranean Sea and cause large storms. “The Arctic region north of Europe is unusually warm, more than 15 degrees above average. This dome of warm air amplifies the northward movements of the jet stream, such as the one that hits Western Europe and generates the DANA that has caused heavy rainfall in eastern Spain and continues to bring even more rain to the Iberian Peninsula,” Francis explains.
Video: NASA
The disruption of the Arctic Current is of concern because there is currently great uncertainty about its size and future reach on a warmer planet. Scientists here are dealing 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 IPCCis studying the issue, but for now says the degree of uncertainty is high.
The big question is whether what is happening is due to human activities in the combustion of solid fuels, or to 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 signs of climate change, although the models tell us that the flow will generally retreat northwards, which would bring more rain to northern Europe and more drought to the south to take.” he explains. Esteban Rodríguez, from the Spanish Meteorological Agency, highlights the current doubts due to the complexity of the process. “It’s still a matter of discussion. Evidence of these effects has been seen in summer, but in winter this is not yet so clear. What we are sure of is that from now on there will be fewer days of rain, but there will be more downpours,” 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 as the cold droplets arrive, they will dump more and more rain over Europe.” A recently published preliminary study by the Global Climate Attribution Group suggests that rainfall from these types of weather events in Spain was 12% more intense due to climate change, and will be twice as intense as normal. Vautard, who was not involved in the research, says 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 cause a huge disaster, especially as the water falls in very specific areas.”
The French meteorologist and oceanographer points to two other factors that exacerbated the storm’s impact. The first is that, unlike many other cold drops, this one fell in densely populated and urbanized areas, preventing the water from penetrating the ground anywhere and causing “frightening” floods. The researcher also points to the increasing construction of houses near riverbeds and flood areas. “It is something that bothers us a lot in France; houses have eaten up some of the riverbeds, which have become narrowed, and rivers need space.” The tragedy in Valencia should “serve as a lesson for us,” he emphasizes. “We need to better predict the extent of current flooding and base our policies not on past storms, but on future storms.”
Geographer Sergio Vicente-Serrano co-authored the chapter on extreme weather events in the latest IPCC report. He explains that it is a challenge for current science to understand atmospheric dynamics, including phenomena such as fluctuations in the polar jet stream; and there is still great uncertainty about its magnitude. On the other hand, the thermodynamic consequences of global warming are becoming increasingly apparent. An increasingly warm atmosphere due to rising temperatures is already creating a situation of more torrential rain, because the air’s ability to absorb moisture is greater.
An event like the one in Valencia has a return time of about 50 years, which means that similar events will repeat on average every half century, which is not that much, according to preliminary calculations of the researcher, who works at the Pyrenean Institute of Ecology (IPE-CSIC ). Vicente emphasizes that the great novelty of this flood is not that it occurred, but that it fell 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 the metropolitan areas, unlike today,” he explains.
The main reason DANA has caused such destruction and deaths is “a huge lack of urban planning,” Vicente says. “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 time when there was an orderly city 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 nested with the weather forecast model, which in turn is nested with a hydraulic model to calculate the height of the water mass determine. areas that will be affected by rising waters. Calculating the height of the water surface in these cases is something that is already possible with current technology, and should be a priority over other issues such as America’s Cupfootball or bullfighting.”
Fluctuations in the polar jet stream have shaped the lives of most Europeans over the past seven hundred years, according to a study published a few weeks ago. Researchers estimated the variability of the flow 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 changed and intensified extreme events,” the authors said, studying its variability is essential to understanding “the climate risks of the future.”
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