Changing properties of NADW cause AMOC weakening at 26.5°N

The RAPID-MOCHA array monitors the Atlantic Meridional Overturning Circulation (AMOC) at 26.5°N by combining contributions from wind-driven Ekman transport, the Florida Current, and the mid-ocean geostrophic flow derived from tall moorings. While the Florida Current transport has been remarkably steady over the past decades and the Ekman transport has been strengthening since 2004, the AMOC has been weakening since 2004 when the RAPID-MOCHA observations started. This study investigates the role of buoyancy anomalies along the deep western boundary (DWB) on the observed AMOC decline. The DWB presents density features typical of both upper and lower North Atlantic Deep Water (uNADW and lNADW, respectively), which are water masses formed in the subpolar North Atlantic and tend to flow southward along the North Atlantic western boundary until reaching 26.5°N. The uNADW can be divided into upper Labrador Sea Water (uLSW) and classical LSW (cLSW). Since 2004, the DWB has been getting lighter, largely due to warming, but with varying effects of salinity on the uNADW and lNADW layers. To isolate the influence of these water mass changes on the AMOC, we recalculated the AMOC by substituting the western boundary density profiles in a given layer (e.g., uNADW, lNADW, uLSW, cLSW) with monthly climatological values and compared the resulting estimates to those derived from the full RAPID-MOCHA data set. Between 2004 and 2023, the observed AMOC weakened at a rate of −0.8±0.7 Sv/decade, with DWB density anomalies accounting for 77% of this trend (−0.6±0.1 Sv/decade). Further breakdown reveals that the lNADW contributes 47% (−0.39±0.07 Sv/decade) and the uNADW contributes 33% (−0.27±0.07 Sv/decade) to the overall AMOC decline. When the uNADW is subdivided, the cLSW influences the AMOC weakening by −0.38±0.09 Sv/decade, similar to the influence of the lNADW, while uLSW acts to strengthen the AMOC by 0.11±0.03 Sv/decade. Experiments isolating temperature and salinity anomalies indicate that temperature anomalies drive approximately two-thirds of the DWB-induced AMOC weakening, with salinity playing a secondary but important role in the lNADW. These findings suggest that southward advection of buoyancy anomalies formed in the Labrador and Nordic Seas account for about 75% of the AMOC weakening observed at 26.5°N between 2004 and 2023, particularly highlighting the influence of cLSW and lNADW water mass changes. The trend of the residual signal (joint influence of Florida Current, upper ocean, eastern boundary and Ekman transports) is not statistically significant, whereas the DWB and individual water mass influence trends are significant at the 99% confidence level.