The mathematics behind navigation apps has helped reveal the paths of atmospheric rivers
An atmospheric river is heading toward the western United States in December 2023. These weather events can cause devastating flooding while replenishing water supplies. A new study has mapped these rivers on a global scale.
A new global map shows how gigantic swaths of water vapor move along networks of interconnected highways across the sky.
When these atmospheric rivers make landfall, they can cause floods, blizzards, landslides, and even heat waves. But their timing and intensity can be difficult to predict. The first detailed atmospheric river network covering the world could help change that, researchers report June 12 in Earth system dynamics.
Although the map is not ready to serve as a forecasting tool, it has revealed previously unknown locations where rivers intensify by attracting moisture as they travel a few kilometers above the Earth’s surface. Such a gas station, on Australia’s east coast, would not have been detected by normal mapping methods, says atmospheric scientist Kimberley Reid, who was not involved in the study. Some of Australia’s worst floods have been associated with atmospheric rivers, says Reid, of the University of Melbourne.
Seen from the ground, atmospheric rivers might look like endless gray skies. But they extend far beyond the horizon, averaging about 2,000 kilometers long and 500 kilometers wide, and carry about the same amount of water that flows from the mouth of the Amazon River. Although they sometimes flooding places like the western United States and Portugal, if they don’t go that way, can mean drought. Previous attempts to map the world’s atmospheric rivers have been spotty, stopping at the point where they weaken by shedding rain and snow.
Tobias Braun, a physicist at the Institute for Earth System Sciences and Remote Sensing at the University of Leipzig in Germany, wanted to follow them around the world.
Mapping the rivers of the sky
Each colored drop in this animation shows a different atmospheric river based on nearly a century of recordings, and a corresponding colored line follows its path. The researchers covered the globe with hexagonal grids, represented by white lines, to help them track the paths of the rivers. Major highways appear as bright blue and white dotted lines; Atmospheric rivers capture steam along these corridors, bringing rain and snow as they crisscross the globe.
With colleagues, Braun first compiled a catalog of traces of atmospheric rivers dating back to 1940. To make the map, he adapted ideas from the branch of mathematics known as graph theorywhich mobile phones use to find the shortest path between two points. The research team divided the globe into thousands of hexagonal grids and counted each time an atmospheric river passed between two hexagons. By following the path of all the rivers in their catalog, the scientists constructed a sort of road network, with major intersections, highways and interconnected regions.
Parts of this network – where atmospheric rivers crisscross the Atlantic, Pacific and Indian Oceans – are already well known, telling the researchers that their method works. It also revealed hot spots that had previously received little or no attention. These include tangles of interconnected atmospheric riverways in the Middle East and the Mediterranean Sea, as well as gas stations in Central Asia and the southern tip of South Africa.
A flood in Europe is “the downstream end of a path we can follow across the Atlantic,” Braun says. “If you know about highways and how they change with the seasons and other important climate oscillations, like an El Niñoyou have solid evidence about the direction an atmospheric river might take.
Atmospheric rivers can deviate from the trajectories described in the study and those trajectories are likely to change as the planet warms, Reid says. But this approach, she says, shows their typical journey and growth and decline, anticipating a future where “we can better predict and prepare for these extreme events.”
