Roberta De Carolis A recent study led by the University of California at Santa Barbara (USA) showed that atmospheric rivers have moved about 6-10 degrees towards the two poles in the last four decades. This is causing waves of drought but also floods, putting the water resources of entire communities at risk (and changing climate patterns around the world). But what are these atmospheric rivers?
©NOAA
One of the studies led by the University of California, Santa Barbara, showed that atmospheric rivers are shifting poleward and, in most instances, have implications on weather patterns globally. These cause floods but at the same time droughts that put in jeopardy the security of water resources for entire communities.
What are atmospheric rivers, and where are they found?
According to NOAA, atmospheric rivers are relatively narrow, long areas in the atmosphere that act like “rivers in the sky” in transporting most of the water vapors outside of the tropics.
These vary greatly in size and strength, but the typical atmospheric river carries the equivalent in water vapor as does the average flow of the Mississippi River, whereas very strong ones can carry as much as 15 times that volume.
Most atmospheric rivers at least deposit part of their moisture as rainfall or snowfall on land when they impinge on a coast. The ones with the most vapor combined with the coldest air masses can lead to extreme rainfall, flooding, and landslides along with causing widespread damage. A famous one is the “Pineapple Express“, an intense atmospheric river that steers moisture from tropical regions near Hawaii northeastward to the U.S. West Coast.
For instance, California relies on atmospheric rivers for up to 50% of its yearly precipitation, and a series of winter atmospheric rivers often gives enough rain and snow to wipe out droughts.
But they aren’t a solely U.S. phenomenon. Atmospheric rivers form over various areas of the world, including the southeastern and western U.S., Southeast Asia, New Zealand, northern Spain, Portugal, the United Kingdom, and central-southern Chile.
Why the shift in atmospheric rivers is changing global and local climates
©NOAA
The study reports, “atmospheric rivers have moved about 6-10 degrees towards the poles over the last four decades, a trend going in a very alarming direction.”.
This shift is making the drought worse in some places and the flooding worse in others, which threatens the supplies of water on which many communities rely,” writes Zhe Li, lead author of the study, in The Conversation. “When the atmospheric rivers reach as far north as the Arctic, they even melt sea ice, which has implications for global climate.”
That said, the large-scale behavior of atmospheric rivers is far more common within both hemispheres at extratropical latitudes between 30 and 50°, which span much of the continental U.S., southern Australia, and Chile.
A shift in atmospheric rivers can provide a strong impact on regional climates, as well. This could lead to very long droughts and severe water shortages in subtropical areas where atmospheric rivers are becoming less common.
This may cause the equatorward-moving atmospheric rivers to produce rainfalls, flooding, and landslides that are far more extreme than previously seen in the Pacific Northwest, Europe, and even polar regions at higher latitudes.
More atmospheric rivers could hasten the melting of sea ice in the Arctic, adding further to global warming and affecting species dependent on Arctic ices for survival.
What Is causing this shift?
According to scientists, the primary cause of this shift is a change in sea surface temperatures in the eastern tropical Pacific. Above-average cool waters in the eastern tropical Pacific have, since 2000, favored a global shift in atmospheric circulation. This cooling, due to La Niña, pushes atmospheric rivers poleward.
Sea surface temperatures along the equator in the eastern Pacific fluctuate during the year. The warmer-than-usual phases involve El Niño, and the cooler-than-usual phases involve La Niña.
On the other hand, there is a 2023 study done in Australia that purports human-made global warming amplifies atmospheric and oceanic phenomena such as El Niño and La Niña.
In addition, global warming will supposedly be increasing both the frequency and intensity of atmospheric rivers since the atmosphere is able to hold more moisture when it is at a warmer temperature.
As the planet continues to warm up, atmospheric rivers and the precious rains they bring will keep shifting course. The ability to adapt to those changes is the key to successful communities in a changing climate.
How that might change as the planet continues to warm up is less clear. The study brings out the urgent need to narrow the range of climate models.
As the researcher summarizes, “These changes raise key questions about how climate models project future changes in atmospheric rivers. Current projections may be underestimating natural swings-things like fluctuations in the tropical Pacific-that can make a big difference in atmospheric rivers. Understanding this linkage could help meteorologists make better predictions about future rainfall and water supply.”
Science Advances published the study.
