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Global warming could shift tropical rainfall

Ongoing burning of fossil fuels could flip which portion of the tropics receive more rainfall: the southern hemisphere or the northern. Currently, the northern hemisphere tropics is the wetter of the two, but why this is has long baffled scientists. Now, new research in Nature Geoscience has discovered that rainfall in the tropics is in part driven by massive ocean currents that travel back-and-forth between the Arctic and Antarctic, a process known as ocean overturning circulation.



“Ocean overturning circulation brings a large amount of heat northward across the equator, which makes the Northern Hemisphere warmer,” lead author Dargan Frierson with the University of Washington told mongabay.com. “Some of the heat spreads into the Northern Hemisphere tropics, and since warmer ocean waters are where the heaviest rains occur, precipitation peaks in the Northern Hemisphere.”



Frierson and his team employed data from NASA’s Clouds and Earth’s Radiant Energy System (CERES) and computer modeling to determine that this large ocean current likely produces a wetter tropics in the Northern Hemisphere. However, that could change in the next century: many scientists believe that global climate change will slowdown the ocean circulation, potentially changing where rain falls in the tropics.



“Just a few hundred kilometers separates the Sahara Desert from the African rainforests, so even a small shift of the tropical rain belt can have a devastating effect,” notes Frierson.




NASA animation showing precipitation concentrated in the tropics in the form of  average daily rainfall rates during the month of January from 1998-2007.

NASA animation showing precipitation concentrated in the tropics in the form of average daily rainfall rates during the month of January from 1998-2007.

The theory that global warming is set to slowdown—or even stop—this massive marine current was the driving force behind the Hollywood disaster movie, The Day After Tomorrow. Although scientists dismissed how the filmmakers employed climate science (in the film the world enters an implausible new Ice Age), a slowdown in the current is certainly a theoretical possibility in the future.



Still, Frierson cautions that rainfall patterns in the tropics remain complex and therefore difficult to predict.



“Many aspects of climate change affect tropical rain. We aren’t sure which effect will be most important in the future, so tropical rainfall forecasts remain uncertain for now.”



John Fasullo, a researcher at the National Center for Atmospheric Research who was unaffiliated with the research, says that the discovery that ocean circulation likely drives rainfall in the tropics is an “important step” in predicting shifting rainfall on a hotter world. However, he also agrees that much more research will be needed to untangle the complexities driving tropical rainfall.



“It is important to recognize that this work does not explain rainfall’s regional features, such as for example the contrast between the western Pacific Ocean and the eastern Pacific, nor does it say anything about whether more rainfall will fall over land or over ocean,” Fasullo told mongabay.com.



He also says that to date scientists have not observed any slowdown in ocean circulation, however it is “extremely important” for researchers to continue studying this possibility.







Rainforest in Borneo. If tropical rainfall patterns shift, ecosystems like rainforests could be hugely impacted. Photo by: Rhett A. Butler.






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