Atmospheric flows run at altitudes of 1,000 to 3,000 meters and act like water vapor highways, transporting moisture poleward from the tropics. Although they only cover about 10 percent of the Earth’s surface, they contain 90 percent of the water vapor that travels south. In most cases, they are rather harmless or even useful, explain Reid and King: Without the water of the atmospheric rivers, drought would prevail in agriculturally important areas of Australia such as the Murray-Darling Basin.
But when atmospheric flows hit mountain ranges or collide with cold fronts, they suddenly and quickly rain down the stored water. Heavy thunderstorms can also occur.
© NASA / Earth Observatory / Jesse Allen (detail)
Atmospheric flow off California (archive image) | The satellite image shows an atmospheric flow that supplies moisture to the California coast. In January 2021, due to such a weather phenomenon, the US west coast experienced record rainfall and blizzards, which resulted in a landslide.
Warm air holds even more water
Australia is now experiencing its second consecutive La Niña summer. The weather phenomenon is the counterpart of El Niño, causing the waters off Australia’s Pacific coast to warm, resulting in a rise in moisture which then moves inland and rains down. As a result, aquifers were already well stocked when the atmospheric flow spilled its wet cargo across the land.
Climate change has made such a collision – and thus the catastrophic floods – a lot more likely, the experts at the University of Melbourne calculate using the example of a similar flood around Sydney last March. At that time, an atmospheric river had passed over the city on the east coast, transporting around 800 liters of water per second.