Ocean currents originating near the poles drives tropical rainfall
By Summit Voice
FRISCO — Rainfall amounts in the tropics may be influenced by ocean currents originating thousands of miles away, in polar regions, according to an international team of climate scientists trying to track down how global warming might affect precipitation in different regions.
Most tropical rains fall in the northern hemisphere — Palmyra Atoll, at 6 degrees north, gets 175 inches of rain a year, while comparable locations at similar latitudes south of the equator only get 45 inches annually.
Scientists have long thought that this was due to a quirk in the Earth’s geometry — with the spin of the Earth pushing tropical rain bands north across diagonally tilted ocean basins. But the study, led by University of Washington researchers suggests the pattern is driven by ocean currents originating from the poles, thousands of miles away.
The findings, published Oct. 20 in Nature Geoscience, explain a fundamental feature of the planet’s climate, and show that icy waters affect seasonal rains that are crucial for growing crops in such places as Africa’s Sahel region and southern India.
The results of the study suggest that the global conveyor belt of cold water sinking to the bottom of the ocean in the Arctic and flowing south toward Antarctica; then gradually warming as it flows back toward the north, is responsible for the difference between rainfall in the northern and southern hemisphere tropics.
Altogether, the current carries about 400 trillion watts of power across the equator.
The recent IPCC global climate assessment indicates that the current could slow down by 2100, with significant implications for rainfall patterns. The slowdown of the currents is predicted because increasing rain and freshwater in the North Atlantic would make the water less dense and less prone to sinking.
“This is really just another part of a big, growing body of evidence that’s come out in the last 10 or 15 years showing how important high latitudes are for other parts of the world,” said corresponding author Dargan Frierson, a UW associate professor of atmospheric sciences.
In general, hotter places are wetter because hot air rises and moisture precipitates out.
“It rains more in the Northern Hemisphere because it’s warmer,” Frierson said. “The question is: What makes the Northern Hemisphere warmer? And we’ve found that it’s the ocean circulation.”
Frierson and his co-authors first used detailed measurements from NASA’s Clouds and Earth’s Radiant Energy System, or CERES, satellites to show that sunlight actually provides more heat to the Southern Hemisphere. By atmospheric radiation alone, the Southern Hemisphere should be the soggier one.
After using other observations to calculate the ocean heat transport, the authors next used computer models to show the key role of the huge conveyor-belt current that sinks near Greenland, travels along the ocean bottom to Antarctica, and then rises and flows north along the surface. Eliminating this current flips the tropical rain bands to the south.
Frierson’s earlier work shows how the changing temperature balance between hemispheres influences tropical rainfall. A recent study by Frierson and collaborators looked at how pollution from the industrial revolution blocked sunlight to the Northern Hemisphere in the 1970s and ’80s and shifted tropical rains to the south.
“A lot of the changes in the recent past have been due to air pollution,” Frierson said. “The future will depend on air pollution and global warming, as well as ocean circulation changes. That makes tropical rainfall particularly hard to predict.”
The research was funded by the National Science Foundation, the Japan Agency for Marine-Earth Science and Technology, the National Aeronautic and Space Administration, the National Oceanic and Atmospheric Administration and the U.S. Department of Defense.