Recent slowing of Atlantic overturning circulation as a recovery from earlier strengthening

The Atlantic meridional overturning circulation (AMOC) has weakened substantially over the past decade(1). Some weakening may already have occurred over the past century(2), and global climate models project further weakening in response to anthropogenic climate change(3). Such a weakening could have significant impacts on the surface climate(4). However, ocean model simulations based on historical conditions have often found an increase in overturning up to the mid-1990s, followed by a decrease(5). It is therefore not clear whether the observed weakening over the past decade is part of decadal variability or a persistent weakening(6). Here we examine a state-of-the-art global-ocean reanalysis product, GloSea(5), which covers the years 1989 to 2015 and closely matches observations of the AMOC at 26.5 degrees N, capturing the interannual variability and decadal trend with unprecedented accuracy. The reanalysis data place the ten years of observations-April 2004 to February 2014-into a longer-term context and suggest that the observed decrease in the overturning circulation is consistent with a recovery following a previous increase. We find that density anomalies that propagate southwards from the Labrador Sea are the most likely cause of these variations. We conclude that decadal variability probably played a key role in the decline of the AMOC observed over the past decade.