New atmospheric phenomenon discovered – “Atmospheric lakes” transport concentrated moisture to East Africa –

Researchers have discovered a new meteorological phenomenon – “atmospheric lakes”. These consist of hundreds of kilometers of diffuse zones of high water vapor density that form near the equator over the Indian Ocean and then slowly move west. You can then bring heavy rains to East Africa. Why these “lakes” arise and what determines their trajectories is still a mystery.

The atmosphere of our planet is a dynamic place: invisible to us, it is traversed by countless smaller and larger currents that have a decisive impact on our weather. The fast and high-speed jet stream transports the high and low pressure areas in our latitudes. Atmospheric rivers, thousands of kilometers long, can also arise over the Pacific – huge, rapid flows of water-vapor-saturated air that can bring enormous amounts of rain over the west coast of North America.

The atmospheric lakes cut off from the damp currents south of India and then slowly migrate west – why is still unclear. © Brian Mapes/ NOAA ERA-Interim reanalysis

Water vapor lakes over the Indo-Pacific

Another previously unrecognized weather phenomenon over the Pacific and Indian Oceans has now been discovered by Brian Mapes from the University of Miami and his colleagues. When analyzing satellite images that had been taken over the course of five years over the Indo-Pacific, they encountered an abnormality: they repeatedly observed compact zones of unusually steam-rich air over the equatorial region.

These zones contain so much water vapor that if they rain down they could cover a thousand kilometers of area with several centimeters of water, as Mapes and his team report. Typically, these wetlands are preserved for several weeks. Because of this abundance of water and their rounded shape, they have christened the new phenomenon “atmospheric lakes”.

Overlooked rainbringers

Closer analysis revealed that these lakes are formed from humid air south of the Indian subcontinent when they are choked off by one of the faster flowing atmospheric rivers. Once such a “lake” has formed, it slowly drifts over the equatorial Pacific towards the west. As the team observed, some of these zones can also develop further away from the equator and then become headwaters for tropical storms.

Most of the time, however, the atmospheric lakes are driven to the east coast of Africa. “They are therefore rainbringers for the semi-arid areas of the coastal East Africa,” says Mapes. However, this meteorological phenomenon has never been discovered before. “The precipitation climatology of this region has been thoroughly investigated, but only on a monthly basis,” explains the researcher. “In addition, one mostly concentrates on wind-driven and long-lasting weather phenomena.”

Education and transportation puzzles

But even after their discovery, the atmospheric lakes raise a number of questions: Although they are so saturated with water vapor that they already rain down on the way, they do not get smaller and remain for days or weeks. “How do you manage to stay so wet even though it’s raining? And how do they stay so spatially compact? ”Asks Mapes. The answer is still pending. It is also unknown why these structures cut themselves off from the atmospheric rivers at all.

It is also puzzling so far why these rain-bringing areas are drifting steadily to the west, even though there is almost no wind in this zone. “The air masses that drive these lakes towards the coast have a wind speed close to zero,” says Mapes. “We therefore have to find out whether they somehow propel themselves or whether they are moved by large-scale wind patterns.” (AGU Fall Meeting 2021, A42B-04)

Quelle: American Geophysical Union (AGU)

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