For many years, slanting ocean basins have been the accepted reason for the asymmetry in tropical rainfall.
"But at the same time, a lot of people didn't really believe that explanation because it's kind of a complicated argument. For such a major feature there's usually a simpler explanation," Frierson said.
The ocean current they found to be responsible was made famous in the 2004 movie "The Day After Tomorrow," in which the premise was that the overturning circulation shut down and New York froze over. While a sudden shutdown like in the movie won't happen, a gradual slowing which the recent United Nations report said was "very likely" by 2100 could shift tropical rains south, the study suggests, as it probably has in the past.
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," Frierson said.
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."
|Contact: Hannah Hickey|
University of Washington