Attribution of Subtropical Versus Subpolar Atlantic Overturning Variability

The Atlantic meridional overturning circulation (AMOC) is important for regional and global climate due to its key role in the uptake and redistribution of heat, carbon and other tracers. Establishing the causes of historical variability in the AMOC can help us understand how the circulation responds to natural and anthropogenic changes at the ocean surface. However, attributing observed AMOC variability and inferring causal relationships is challenging because the AMOC is influenced by multiple factors which co-vary and whose impacts can persist for several years. Here we unambiguously attribute observed and modelled variability in the AMOC at the latitudes of two recent observational arrays to surface winds, temperatures and salinities. We use algorithmic differentiation of an ocean general circulation model that has been optimized using historical observations (the ECCO-v3 state estimate) to compute the time-dependent response of the AMOC at 26N and across the OSNAP East mooring array to a change in sea surface temperature, sea surface salinity and surface wind stress. We convolve these Green’s functions with best-estimates of 1992-2016 surface properties to reconstruct variability in the overturning circulation. We find that, on inter-annual timescales, AMOC variability at 26N is overwhelmingly dominated by a linear response to local wind stress. Surface density anomalies in the Arctic and subpolar gyre contribute to a longer term trend. Inter-annual AMOC variability across OSNAP East is, however, generated by both local surface density anomalies and remote wind stress, with non-linear processes also important, making it harder to reconstruct and predict. Our results provide powerful motivation for the continued observation of the overturning circulation in the subpolar North Atlantic.