Our planet’s fridge is leaking

When the air masses mix increasingly

Using complex data analyses, the Vienna-based research group determined the residence time of air masses in the lower air layers north of latitude 70 degrees north , which is approximately as far north as the Arctic city of Tromsø. The result: between 1980 and 2023, these air masses resided in the Arctic for an average of around 12 days in summer and around seven days in winter.

Over the past four decades, the Arctic has become increasingly similar to mid-latitudes in this respect – particularly in spring and fall. Between March and May, cold air masses remained near the surface for 1.4 days less than in the 1980s. Between September and October, they remained for 0.9 days less. “Our data shows that the transport of air masses from mid-latitudes to the Arctic is becoming more efficient throughout the year,” says Andreas Stohl. This is not only happening near the ground. Even at an altitude of eight kilometers above sea level, the air in April is now moving away from the Arctic ten percent faster than it did around 40 years ago.

Why the fridge is leaking

But why is that? The most important reason is what is known as Arctic amplification. This means that the temperature in the Arctic is rising faster than the global average. Feedback processes play an important role in this regard. Arctic sea ice and white land areas covered by glaciers and snow reflect radiation back into the atmosphere.

“When the ice and snow melt, the dark sea surface is revealed, which absorbs a large part of the radiation. This in turn leads to greater warming,” explains Andreas Stohl. Rising surface temperatures and the decline of snow and ice on the sea and mainland are also causing the lower atmospheric layers of the Arctic to become warmer. The temperature difference between these layers and the air from the mid-latitudes makes the Arctic dome more permeable.

“Our findings show that air masses are now moving much more efficiently between the Arctic and the mid-latitudes,” reports Stohl. The Arctic is becoming less and less atmospherically isolated. It would be fair to say that the refrigerator has a leak.

Dramatic, but not surprising

Andreas Stohl and his colleagues were not surprised by these results. “But so far, no one had documented them,” says Stohl. One of the reasons was that there was a lack of appropriate data to substantiate these developments.

Many meteorological studies analyze existing data, such as data sets on temperature, air pressure, humidity, wind direction, and wind speed, which have been recorded for decades in many places by measuring instruments such as barometers, weather balloons, satellites, weather buoys, aircraft, and weather observation ships. These data enable researchers to track how the temperature at a given location has changed over a long period of time.

“The thing is, we cannot use these data sets to plot air movements,” explains Stohl. Researchers can use them only to a limited extent to track how polluted air particles are transported through the atmosphere or where water that falls as heavy precipitation comes from. Or, indeed, how fast air from the Arctic moves on.

A data set called LARA

Under the lead of astrophysicist Lucie Bakels, who currently conducts research at Stockholm University, Stohl and his team have now created a globally unique data set named LARA, short for Lagrangian Reanalysis.

For their project, the team used a grid point representation of a data set from the European Center for Medium-Range Weather Forecasts (ECMWF). With this representation they operated a model that can represent air movements. LARA allows the movement of six million virtual air particles to be tracked hourly – covering a period from 1940 to 2023. “It took several months to calculate the data set,” says Andreas Stohl. The set comprises around 320 terabytes of data and has enabled the researchers for the first time to prove that the Arctic is less isolated atmospherically now than it was a few decades ago.

This development has far-reaching, complex consequences. “For example, we are now seeing a growing number of what are called atmospheric rivers in the Arctic. This is a direct consequence of the fact that the Arctic is becoming less isolated,” notes Stohl. These moisture-saturated bands of air are several hundred kilometers long and transport water vapor from the tropics. By now they are also reaching the far north. “If precipitation falls more often as rain, the atmospheric rivers can cause melting events over Greenland, for example,” explains Stohl.

A tool for future extremes

The LARA data set is now freely accessible to researchers across the globe. Andreas Stohl assumes that it could become the basis for many further studies. It helps, inter alia, to trace the origin of air pollutants or heavy precipitation events, as well as the movement and change of jet streams, global wind systems, and heat flows. Andreas Stohl and his colleagues are currently using LARA to find out where the moisture came from that caused flooding in large parts of Lower Austria in September 2024. In a warming world, there will be no shortage of further applications. “We have many ideas to pursue,” says the meteorologist.