Atmospheric lakes are long-lived pools of water vapor that occur over the western Indian Ocean and bring water to dry lowlands along East Africa’s coastline.
Long-lived lakes of water vapor in the atmosphere are observed to occur over the western Indian Ocean. Their dynamics resemble those of a well-studied theoretical and simulation paradigm for tropical disturbances called self-aggregation. Image credit: Mapes & Tsai.
Like the better-known streams of humid, rainy air called atmospheric rivers that are famous for delivering large amounts of precipitation, atmospheric lakes start as filaments of water vapor in the Indo-Pacific.
These phenomena are defined by the presence of water vapor concentrated enough to produce rain, rather than being formed and defined by a vortex, like most storms on Earth.
Unlike the fast-flowing atmospheric rivers, the smaller atmospheric lakes detach from their source as they move at a sedate pace toward the coast.
Atmospheric lakes begin as water vapor streams that flow from the western side of the South Asian monsoon and pinch off to become their own measurable, isolated objects.
They then float along ocean and coastal regions at the equatorial line in areas where the average wind speed is around zero.
In an initial survey to catalog such storms, University of Miami atmospheric scientists Brian Mapes and Wei-Ming Tsai used 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.
Lakes farther off the equator also occur, and sometimes those become tropical cyclones.
The atmospheric lakes last for days at a time and occur several times a year.
If all the water vapor from these lakes were liquified, it would form a puddle only a few centimeters (a couple inches) deep and around 1,000 km (620 miles) wide.
“This amount of water can create significant precipitation for the dry lowlands of eastern African countries where millions of people live,” Dr. Mapes said.
“It’s a place that’s dry on average, so when these atmospheric lakes happen, they’re surely very consequential.”
“I look forward to learning more local knowledge about them, in this area with a venerable and fascinating nautical history where observant sailors coined the word monsoon for wind patterns, and surely noticed these occasional rainstorms, too.”
“Weather patterns in this region of the world have received little attention from meteorologists, limited mostly to studies of rain and water vapor on a monthly rather than day-to-day scale,” he added.
“I’m 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 researchers presented their results this month at the American Geophysical Union Fall Meeting 2021 (AGU21).
_____
Brian E. Mapes & Wei-Ming Tsai. Long-lived vapor lakes over the Indian Ocean: closest outdoor phenomenon to the self-aggregation paradigm? AGU21, abstract # A42B-04
