Variability of the eastern subpolar North Atlantic meridional overturning circulation

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the Earth’s climate. According to the last IPCC reports, it is expected to weaken as the mean atmosphere temperature increases, with considerable impact around the world. There are few long time series of observations of the AMOC, and the study of the mechanisms driving its variability depends mainly on numerical simulations. Understanding the seasonal to decadal variability is required to properly detect a trend in the observed AMOC timeseries. Here, we use four ocean circulation estimates produced by different data-driven approaches of increasing complexity to analyse the seasonal to decadal variability of the subpolar AMOC across the Greenland–Portugal OVIDE (Observatoire de la Variabilité Interannuelle à DÉcennale, GO-SHIP line A25) line since 1993. We decompose the MOC strength variability into a velocity-driven component due to circulation changes and a volume-driven component due to changes in the depth of the overturning maximum isopycnal. We show that the variance of the time series is dominated by seasonal variability, which is due to both seasonal variability in the volume of the AMOC limbs and to seasonal variability in the transport of the Eastern Boundary Current. The decadal variability of the subpolar AMOC is mainly caused by changes in velocity, which after the mid-2000s are partly offset by changes in the volume of the AMOC limbs. This compensation means that the decadal variability of the AMOC is weaker and therefore more difficult to detect than the decadal variability of its velocity-driven and volume-driven components.