Dozens of previous computer simulation systems have indicated, on average, a weak wet trend for the Sahel, if climate change proceeds unabated. In the new study, the scientists took a closer look at the simulations that show the greatest rainfall increases–40 to 300 percent. They found that in these, as the surrounding oceans warm, Sahel rainfall would increase suddenly and substantially. During the same time, monsoon winds that blow from the Atlantic ocean to the African interior would get stronger and extend northwards. The authors say that this pattern is reminiscent of periods in earth’s history during which paleoclimate records show that African and Asian monsoon systems alternated between wet and dry, sometimes quite abruptly.
The scientists previously identified a self-amplifying mechanism behind the sudden rainfall changes. When the ocean surface temperature increases, more water evaporates. The moist air drifts onto land, where vapor is released as rainfall. When the vapor turns into rain, heat gets released. This increases the temperature difference between the generally cooler ocean and the warmer landmasses, sucking more moist winds into the continent’s interior. This again will produce more rain, and so on. “Temperatures have to rise beyond a certain point to start this process,” said Schewe. “We find that the threshold for this ‘Sahel monsoon’ is remarkably similar across different models.”
Levermann said that while more rain could be good news, it could pose challenges, particularly in the transition period between the dry conditions of today and conceivably much wetter conditions. Levermann said the Sahel might experience years of hard-to-handle variability between drought and flood. “Obviously, agriculture and infrastructure will have to meet this challenge,” he said.
(Adapted from a press release by the Potsdam Institute for Climate Impact Research.)