The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken in the 21st century, although there has been limited (and conflicting) evidence of this trend in the observational record. The freshening and warming of waters in the deep water formation regions of the North Atlantic (NA) Ocean are expected to drive future weakening of the AMOC. In this study, we utilize the growing set of in-situ observations from depth-profiling ARGO floats along with historical, SSP585 and single-forcing simulations from 12 DAMIP (Detection and Attribution Model Intercomparison Project) General Circulation Models to detect and attribute climate change signals in temperature and salinity trends within the NA Ocean from 1950-1970 to 2002-2020.

Our analysis indicates that observed changes over the historical period (based on ARGO floats) are significantly different from the expected natural variability and can be attributed to anthropogenic greenhouse gas emissions. Distinct fingerprints of heat and salt accumulation around 40°N and freshening and cooling around 55°N are detectable in both modeled and observational datasets within the water column of the NA. These trend patterns are consistent with the theorized weakening of the AMOC upper cell.