Contemporary climate models show wide variation in 21st century projections of Atlantic Meridional Overturning Circulation, or AMOC, weakening. This study aimed to reconcile these discrepancies.Pixabay
The Atlantic Meridional Overturning Circulation, or AMOC, is a system of ocean currents that plays a crucial role in regulating Earth's climate by transporting heat from the Southern to Northern Hemisphere. Confined to the Atlantic basin, the AMOC modulates regional weather - from mild summers in Europe to monsoon seasons in Africa and India.
Climate models have long predicted that global warming will cause the AMOC to weaken, with some projecting what amounts to a near-collapse relative to the AMOC strength in observations today. But a new study from a team of researchers that included the University of Washington shows that the AMOC is likely to weaken to a much lesser extent than current projections suggest. The study was published May 29 in Nature Geoscience.
A severe weakening would have far-reaching consequences, including changes in regional sea level rise, and major shifts in regional climate, such as colder conditions in northern Europe and drier weather in parts of the Amazon and West Africa.
"Our results imply that, rather than a substantial decline, the AMOC is more likely to experience a limited decline over the 21st century - still some weakening, but less drastic than previous projections suggest," says David Bonan, lead author of the study and a UW postdoctoral research fellow in the Cooperative Institute for Climate, Ocean and Ecosystem Studies.
The researchers developed a simplified physical model based on fundamental principles of ocean circulation - specifically, how sea water density differences and the depth of the overturning circulation are related - that also incorporates real-world measurements of the ocean current's strength. The real-world data was collected over 20 years with monitoring arrays and other observations of the Atlantic basin.
Results show that the AMOC will weaken by around 18-43% by the end of the 21st century. While this represents some weakening, it's not the near-collapse that more extreme climate model projections suggest.
Paleoclimate records, like ocean sediments that record past climate conditions, indicate that the AMOC experienced weakening in the past. One example is during the last ice age 20,000 years ago, which led to major swings in the climate that affected North America and Europe.
Contemporary climate models show wide variation in 21st century projections of AMOC weakening. This study aimed to reconcile these discrepancies by better understanding the physical mechanisms governing the AMOC behavior in climate models. Through this work, researchers shed light on a previously unexplained feature of climate models: the link between the present-day and future strength of the AMOC.
Climate models that simulate a stronger present-day AMOC tend to project greater weakening under climate change. Researchers found that this relationship stems from the depth of the AMOC. A stronger AMOC typically extends to greater depths and allows changes in surface water temperature and salinity properties - caused by global warming and freshwater input - to penetrate deeper into the ocean and drive greater weakening.
A climate model with a stronger and deeper AMOC is less resilient to surface changes and experiences proportionally more weakening than one with a shallower current. Climate models with a shallower present-day AMOC still show weakening under climate change, but to a lesser extent than those with a deeper present-day AMOC.
The researchers used the ocean observations to show that the real-world AMOC is relatively shallow when compared to most climate models. The results indicate that the AMOC will experience only limited weakening, even in the highest emissions scenarios. The study also suggests that much of the previous uncertainty and more extreme weakening projections stemmed from biases in how climate models simulate the ocean's current state, particularly its density stratification.
"There is immense value in doing basic research," Bonan said. "It can give us a better indication of what the future might look like, as our study shows."
Bonan emphasized the need to examine higher-resolution climate models that also include more sophisticated processes. Higher-resolution models might offer deeper insights into AMOC behavior and improve projections of its future changes. The study provides a framework to interrogate and evaluate more sophisticated models.
Kyle Armour, professor of atmospheric & climate science and oceanography at the UW, was a co-author. Other co-authors are Tapio Schneider and Andrew Thompson of the California Institute of Technology, Laure Zanna of New York University and Shantong Sun of Laoshan Laboratory in Qingdao, China. The study was funded by the National Science Foundation, the David and Lucile Packard Foundation and Schmidt Sciences LLC.
This story is adapted from a release by the California Institute of Technology.
