Scientists find new weather phenomenon drifting slowly over western Indian Ocean

Meteorologists reported a new weather phenomenon called “atmospheric lakes” drifting slowly over the western Indian Ocean and bringing water to dry lowlands along East Africa’s coastline.

Similar to the better-known streams of humid, rainy air called atmospheric rivers that deliver large amounts of rainfall, the researchers from the University of Miami say the newly discovered long-lived pools of water vapour start as filaments in the Indo-Pacific.

The weather phenomenon, the scientists say, begins as water vapour streams that flow from the western side of the South Asian monsoon and pinch off to become their own measurable, isolated objects.

“These vapour bodies sometimes drift west over the East African coast, bringing rain to that semi-arid area,” the scientists noted in their study, presented at the American Geophysical Union Fall Meeting 2021 (AGU21) earlier this month.

Atmospheric lakes, they say, are defined by the presence of water vapour concentrated enough to produce rain, rather than being formed and defined by a vortex, like most storms on Earth.

And unlike atmospheric rivers, they say the new phenomenon detaches from its source as it moves at a “sedate pace” toward the coast, lasting for days at a time, and occurring several times a year.

“By contrast to rain-bearing ‘atmospheric rivers’ of vapour, which are contiguous from source to coastline at an instant, we call these disconnected and drifting water bodies ‘atmospheric lakes,’” they wrote in the research.

If all the water vapour from the atmospheric lakes were liquified, the researchers say it would form a puddle only a few centimetres deep and around 1,000 kilometres wide – an amount that can create significant rainfall for the dry lowlands of eastern African countries where millions of people live.

“It’s a place that’s dry on average, so when these atmospheric lakes happen, they’re surely very consequential,” atmospheric scientist Brian Mapes from the University of Miami said in a statement.

In the study, the scientists assessed five years of satellite data to spot 17 atmospheric lakes lasting longer than six days and within 10 degrees of the equator, in all seasons.

They say lakes farther off the equator also occur, which sometimes become tropical cyclones.

“This amount of water can create significant precipitation for the dry lowlands of eastern African countries where millions of people live,” Dr Mapes said.

In future studies, the scientists hope to understand why atmospheric lakes pinch off from the river-like pattern from which they form and why they move westward towards the East African coast.

“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,” Dr Mapes said.