What are the ‘atmospheric rivers’ hitting the US West Coast?

Huge ‘atmospheric rivers’ have resulted in torrential downpours, high winds and floods.

Atmospheric rivers are storms akin to rivers in the sky that dump massive amounts of rain and can cause flooding, trigger mudslides, and result in loss of life and enormous property damage.

The latest in a series of atmospheric river storms has soaked California, causing flooding and mandatory evacuations for about 27,000 residents in 10 counties.

Here is what to know about the weather phenomenon:

What is an ‘atmospheric river’?

An “atmospheric river” is a relatively long, narrow region that transports columns of water vapour away from the tropics in the atmosphere.

According to the National Oceanic and Atmospheric Administration (NOAA), these columns can carry roughly 7.5 to 15 times the average flow of water at the mouth of the Mississippi River.

When the atmospheric rivers reach land they often release these columns of water vapour in the form of rain or snow.

On average, the West Coast receives between 30-50 percent of its annual precipitation through “atmospheric rivers”, according to the NOAA.

Are ‘atmospheric rivers’ dangerous?

Atmospheric rivers are usually harmless. Weaker storms provide beneficial amounts of rain for the water supply.

Atmospheric rivers are ranked on a scale of 1-5 based on how much moisture they transport from the tropics to the mid-latitudes.

However, category 4 or 5 atmospheric rivers that produce large quantities of rain can result in severe flooding, induce mudslides and cause catastrophic damage to life and property, according to the NOAA.

In 2019, an atmospheric river nicknamed the “Pineapple Express” hit California. The water vapor from near Hawaii brought rain and triggered mudslides that forced motorists to swim for their lives and sent homes sliding downhill.

In 2021, an atmospheric river dumped a month’s worth of rain on British Columbia in two days, prompting fatal floods and landslides, devastating communities and severing access to Canada’s largest port.

How common are atmospheric rivers?

These “rivers in the sky” are relatively common, with about 11 present on Earth at any time, according to NASA. Most atmospheric rivers are weak and do not cause damage. In fact, they can provide much needed rain or snow.

One such storm last year in drought-stricken California triggered mudslides, toppled utility poles and blocked roadways, but also helped replenish depleted reservoirs and reduced the risk of wildfires by saturating the state’s parched vegetation.

What impact will climate change have on them?

Atmospheric rivers of the kind that drenched California and flooded British Columbia in recent years will become larger — and possibly more destructive — because of climate change, scientists say.

Columns in the atmosphere hundreds of kilometres long carry water vapor over oceans from the tropics to more temperate regions in amounts more than double the flow of the Amazon River, according to the American Meteorological Society.

There are projected to be 10 percent fewer atmospheric rivers in the future, but they are expected to be 25 percent wider and longer and carry more water.

Source: Al Jazeera and news agencies