These Invisible Rivers Carry 90% of Earth’s Polar-Bound Water: Scientists Finally Mapped Them

Nearly 90% of all the water vapour that moves from the tropics towards Earth’s poles travels through a single type of weather system: the atmospheric river. Despite carrying more water than any river on land, exactly where these airborne channels gather their moisture has remained something of a mystery — until now. A new study in Climate and Atmospheric Science has produced the first global map of the ocean regions where atmospheric rivers pick up the water vapour they later release as rain.

To build the map, researchers drew on more than 40 years of global atmospheric data, combining satellite observations, weather reanalysis datasets and computer modelling to trace atmospheric rivers across their full lifespans. The study found that these systems do not absorb moisture evenly along their route. Instead, they pass through a small number of ocean regions where warm surface waters and favourable winds cause large-scale evaporation.

Some of the most important of these regions sit in the eastern Indian Ocean and western Pacific, close to Australia. Moisture gathered here can be carried onward to Australia itself, New Zealand, South America or Antarctica, depending on the atmospheric conditions steering the system. The researchers also found that while some atmospheric rivers draw from multiple ocean regions, others depend almost entirely on a single moisture source.

Atmospheric rivers themselves are long, narrow bands of concentrated water vapour, sometimes stretching thousands of kilometres while remaining only a few hundred kilometres wide. When one of these systems makes landfall and is pushed upward by mountains or a weather front, the vapour condenses into rain or snow. If the system is strong or moves slowly, the result can be days of heavy rainfall, raising the risk of flooding and landslides.

Past floods linked to atmospheric rivers have struck Australia, California, western Europe, South America and New Zealand, in several cases causing billions of dollars in damage. Researchers say that by tracking the ocean ‘source regions’ identified in the new study, forecasters could gain extra lead time before a flood-producing system even reaches the coast.

The study’s authors also point to a longer-term concern: as ocean temperatures rise with climate change, evaporation from these moisture hotspots is expected to increase, meaning atmospheric rivers of the future could carry even more water than the ones responsible for past disasters.

Image credit: Wikimedia Commons