University of California-Riverside (UCR) scientists studying a massive, mysterious cold spot south of Greenland in the Atlantic Ocean have determined the most likely cause is the slowing of the Atlantic Meridional Overturning Circulation (AMOC), which delivers warmer, saltier waters from the tropical latitudes to northern latitudes.
“People have been asking why this cold spot exists,” said UCR climate scientist Wei Liu, who led the study with doctoral student Kai-Yuan Li. “We found the most likely answer is a weakening AMOC.”
If correct, the researchers say understanding how this slowing current, which results in a reduced salinity, or freshening, of polar ocean water, affects marine life and other ocean dynamics could be crucial when preparing societies for potential impacts like extreme weather events caused by the change.
Because direct observational data have not existed for the last 20 years, Liu and Li turned to historical salinity and temperature records. If the AMOC current was slowing down, the differences should appear in the data.
“We don’t have direct observations going back a century, but the temperature and salinity data let us see the past clearly,” Li said.
First, the team combed through 100 years’ worth of data to reconstruct changes in the circulation system. Next, those temperature and salinity-based reconstructions were compared with 100 different climate models simulating stronger and weaker AMOCs. According to a statement announcing the team’s findings, only the models simulating a weakened AMOC matched the real-world data. Conversely, models simulating a stronger circulation “didn’t come close.”
“It’s a very robust correlation,” Li said. “If you look at the observations and compare them with all the simulations, only the weakened-AMOC scenario reproduces the cooling in this one region.”
The research team says the precise cause of the slowing that may also be causing the mysterious cold spot is still a hotly debated scientific topic. Some climate modelers have favored ocean dynamics and atmospheric factors as primary causes, while others think factors like aerosol pollution could be driving the change. The team believes their findings can help settle that debate, since aerosol-based prediction models showing declining emissions should reduce the effect were inaccurate.
“Our results show that only the models with a weakening AMOC get it right,” Liu said. “That means many of the recent models are too sensitive to aerosol changes, and less accurate for this region.”
While the study is inconclusive, the team says that if a slowing AMOC is behind the mysterious cold spot in the Atlantic Ocean, the consequences could be widespread and dramatic. For example, this region is susceptible to ocean circulation changes, including the people who live and work in and around the ocean. It also alters rainfall patterns and the jet stream, affecting European weather. Changes in water salinity and temperature will also likely alter habitats and ocean life that depend on them.
Moving forward, the team says that the continuing change in climate due to global warming could amplify the effects of a slowing AMOC. This could result in an increasingly large mysterious cold spot in the Atlantic, driving even more extreme changes in the global ecosystem.
“This work shows the AMOC has been weakening for more than a century, and that trend is likely to continue if greenhouse gases keep rising,” Li explained. “The technique we used is a powerful way to understand how the system has changed, and where it is likely headed if greenhouse gases keep rising.”
The research team believes that by “unlocking its origins,” climate scientists can “better prepare societies for what lies ahead.”
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.