Breakdown: Why a newly discovered weather phenomenon has scientists scratching their heads

MEMPHIS, Tenn. (WMC) - You’ve likely heard of an Atmospheric River, a meteorological phenomenon known for bringing large amounts of moisture to the U.S. West Coast each year.

Recently, atmospheric scientist Brian Mapes discovered a new type of meteorological phenomenon dubbed “atmospheric lakes,” the river’s calmer cousin.

Mapes and his team made the discovery while studying weather patterns over the Indian Ocean, an area of sea that surrounds Indonesia and the Philippines.

Unlike rivers, these lakes are slow-moving compact pools of moisture that originate over the Indo-Pacific and bring water to dry lowlands along East Africa’s coastline.

Atmospheric lakes start as rivers, scientists say, but at a certain point they pinch off, forming an isolated, concentrated mass of water vapor. These lakes then drift very slowly across the sky, in areas where the wind speed is around zero. They can bring a lot of rain to the surface below – if the “lake” moniker was taken literally, the team says they hold enough water to form a puddle a few centimeters deep and about 620 miles wide.

They occurred in all seasons, and usually within 10 degrees of the equator.

They can occur further from the equator as well, and these storms sometimes turn into tropical cyclones.

Streams of water vapor flow from the western side of the South Asian monsoon and then detach, floating peacefully away in an isolated patch. The atmospheric lakes eventually connect with coastal regions near the equator, providing significant precipitation for the arid regions of eastern coastal Africa, home to millions of people.

“It’s a place that’s dry on average, so when these [atmospheric lakes] happen, they’re surely very consequential,”Mapes said.

This region of the world hasn’t had extensive studies of daily precipitation patterns, according to the researchers. Mapes and his team specifically examined rain and water vapor patterns occurring on a day-to-day scale.

In an initial survey to catalog such storms, Mapes used five years of satellite data to spot 17 atmospheric lakes lasting longer than six days in all seasons.

The atmospheric lakes last for days at a time and occur several times a year.

He is working to understand why atmospheric lakes pinch off from the river-like pattern from which they form, and how and why they move westward. This might be due to some feature of the larger wind pattern, or perhaps that the atmospheric lakes are self-propelled by winds generated during rain production.

“The winds that carry these things to ashore are so tantalizingly, delicately near zero [wind speed], that everything could affect them,”Mapes said.“That’s when you need to know, do they self-propel, or are they driven by some very much larger-scale wind patterns that may change with climate change.”

The team plans to study these atmospheric lakes further to determine if they can occur in other parts of the world and how they might be affected by climate change.

Mapes presented his findings Dec. 16, 2021 at the American Geophysical Union meeting in New Orleans. Click here to read the full article.

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