Water that evaporates on the steamy Equator flows toward the Earth's poles in massive airborne "rivers" of vapor that can be equal in volume to the Amazon, scientists at the Massachusetts Institute of Technology and NASA reported after analyzing satellite data.
Reginald E. Newell of MIT's Earth, atmospheric and planetary sciences department said these lower-atmosphere "water vapor jet streams" sometimes are bordered by towering cloud banks. But, he said, it is not clear if the phenomena are related or coincidental.
Now that the atmospheric rivers have been discovered, he added, he and his colleagues are trying to determine how and why they occur, and what role, if any, they might play in cyclones and other global weather phenomena.
"One always thinks of the atmosphere as very turbulent and atmospheric phenomenon as being very broad horizontally. But these rivers are very narrow," Newell said Thursday from his office in Cambridge, Mass. "Now we're doing our Sherlock Holmes bit. We're looking at it piece by piece, trying to figure it out."
Scientists have long known that warm, moist air from the Equator circulates toward the polar regions; some have even recorded curious streaks in high-altitude clouds at certain latitudes. Newell and his colleagues found discrete vapor flows over much longer distances at much lower altitudes, less than 10,000 feet.
In the Dec. 24 issue of Geophysical Research Letters, Newell and his co-authors--Nicholas Newell of Arlington, Mass.; Yong Zhu of MIT and Courteney Scott of the National Aeronautics and Space Administration's Goddard Space Flight Center in Greenbelt, Md.--wrote that the tropospheric rivers are present at all times and develop and move in a coherent fashion.
There are usually five such rivers flowing at any one time in the Southern Hemisphere, and four or five in the Northern Hemisphere, they reported. Each river flows for at least 10 days before being succeeded by a new, nearby stream, they reported.
One river, which originated over Brazil and flowed over the south Atlantic, carried almost as much moisture as the Amazon River, they reported.
Another river of moisture, which originated in South Africa, flowed into a violent rainstorm over the Indian Ocean southwest of Australia. Reginald Newell said he does not know if the vapor river fed the storm, but said he is trying to find out.
"We think (the vapor flows) terminate in the polar regions, in precipitation, but we're not sure that some don't spread out over a wider area."
Newell and his colleagues discovered the tropospheric rivers using data collected in 1984 and 1991 by NASA's Measurement of Atmospheric Pollution by Satellite program and analyzed by the European Center for Medium-Range Weather Forecasts.