Rainy tropics could face unprecedented droughts as Atlantic Ocean circulation slows down
New research warns that global rainfall patterns could shift dramatically as a result of climate change.
Some of the rainiest places on Earth could see their annual precipitation nearly halved if climate change continues to alter the way ocean water moves around the globe. In a new study published in Nature, an international team of scientists revealed that even a modest slowdown of the Atlantic Ocean circulation could dry out rainforests, threaten vulnerable ecosystems and upend livelihoods across the tropics. Ute Merkel and Matthias Prange from MARUM – Center for Marine Environmental Sciences and the Faculty of Geosciences at the University of Bremen are participating in the study with their climate model simulations.
“It’s a stunning risk we now understand much better,” said lead author Pedro DiNezio, associate professor in University of Colorado Boulder’s Department of Atmospheric and Oceanic Sciences, adding that parts of the Amazon rainforest could see up to a 40 percent reduction in annual precipitation.
The ocean conveyor belt
The Atlantic Meridional Overturning Circulation (AMOC) is a massive system of ocean currents that moves water through the Atlantic Ocean, transporting warm, salty water from the tropics to the North Atlantic. The AMOC plays an important role in regulating the climate by redistributing heat from the southern to the northern hemisphere. It also makes sure the tropical rain belt, a narrow band of heavy precipitation near the equator, stays north of it.
As the climate warms, melting polar ice and increasing rainfall will dilute the ocean’s surface waters, making them less dense and potentially slow down the circulation. The impact of a weakened AMOC on the tropics remains uncertain, because scientists have only been monitoring the system directly for two decades.
“A few years ago, this monitoring system recorded signs of a decline in the AMOC, but it later rebounded. So we weren’t sure if it was just a fluke. The problem is, we haven’t been measuring the ocean long enough to detect meaningful long-term change,” DiNezio said.
While scientists are uncertain whether the AMOC has already begun to decline, climate models predict that the system will eventually weaken because of climate change.
The longterm fluctuations of the AMOC are among the key research topics of Matthias Prange and Ute Merkel, both from the Geosystem Modelling Department at MARUM, who co-authored the study. Their simulations with the CESM1.2 Earth-system model were included into the multi-model comparison of DiNezio and team of authors. The MARUM contribution to the study builds on work conducted as part of the PalMod project which is funded by the German Federal Ministry of Research, Technology and Space (Bundesministerium für Forschung, Technologie und Raumfahrt – BMFTR).
“The study innovatively combined a large set of Earth-System model simulations for past and future climate states with proxy data. This provided new insights into large-scale relationships between high and low latitudes and potential future climatic changes,” Ute Merkel comments.
Predicting the future
DiNezio and his team set out to explore how a future slowing of the Atlantic Meridional Overturning Circulation could impact global precipitation patterns. “Changes in rainfall are very difficult to predict, because so many factors are involved in making rain, like moisture, temperature, wind and clouds. Many models struggle to predict how the pattern will change in a warming world,” DiNezio said.
The team turned to climate records from about 17,000 years ago, when the AMOC last slowed down significantly due to natural causes. Evidence of precipitation preserved in cave formations, as well as lake and ocean sediments revealed how rainfall patterns responded to the slowdown during that period.
Drawing on that data, the authors identified the computer models that best captured those ancient rainfall shifts and used them to predict how the patterns could change in the future.
Their best models predict that as the AMOC weakens and cools the northern Atlantic, this temperature drop would spread toward the tropical Atlantic and into the Caribbean. This change, on top of rising global temperatures, will lead to significant reductions in precipitation over Central America, the Amazon, and West Africa.
Matthias Prange: “This study shows that even moderate changes in the AMOC could significantly disrupt tropical rainfall patterns, profoundly impacting ecologically and economically vulnerable regions across the Global South.”
