A decline in Atlantic meridional overturning circulation (AMOC) strength has been observed between 2004 and 2012 by the RAPID-MOCHA-WBTS (RAPID – Meridional Overturning Circulation and Heatflux Array – Western Boundary Time Series, hereafter RAPID array) with this weakened state of the AMOC persisting until 2017. Climate model and paleo-oceanographic research suggests that the AMOC may have been declining for decades or even centuries before this; however direct observations are sparse prior to 2004, giving only “snapshots” of the overturning circulation. Previous studies have used linear models based on upper-layer temperature anomalies to extend AMOC estimates back in time; however these ignore changes in the deep circulation that are beginning to emerge in the observations of AMOC decline. Here we develop a higher-fidelity empirical model of AMOC variability based on RAPID data and associated physically with changes in thickness of the persistent upper, intermediate, and deep water masses at 26∘ N and associated transports. We applied historical hydrographic data to the empirical model to create an AMOC time series extending from 1981 to 2016. Increasing the resolution of the observed AMOC to approximately annual shows multi-annual variability in agreement with RAPID observations and shows that the downturn between 2008 and 2012 was the weakest AMOC since the mid-1980s. However, the time series shows no overall AMOC decline as indicated by other proxies and high-resolution climate models. Our results reinforce that adequately capturing changes to the deep circulation is key to detecting any anthropogenic climate-change-related AMOC decline. […]
In conclusion, this study shows that the dynamics of the AMOC can be represented by an empirical linear regression model using boundary density anomalies as proxies for water mass layer transports. More than one layer, represented by boundary density anomalies, is required to capture lower-frequency changes to UMO transport. Deep density anomalies combined with Ekman transport are successful in reconstructing LNADW transport, the deepest limb of the AMOC in the subtropical North Atlantic. Previous proxies for AMOC or UMO at 26∘ N that rely on single-layer dynamics (e.g. Frajka-Williams, 2015; Longworth et al., 2011) cannot capture this low-frequency variability. This is also the case for similar reconstructions at other latitudes, for example Willis (2010). Single-layer dynamics are also fundamental to estimates of the AMOC that use fixed levels of no motion such as the MOVE (Meridional Overturning Variability Experiment) array (Send et al., 2011) or inverted echo sounders (see McCarthy et al. (2020) for details). We have shown the importance of the inclusion of deep density measurements in AMOC reconstructions and believe these to be key to identifying the fingerprint of anthropogenic AMOC change (e.g. Baehr et al., 2008).
Our model, applied to historical hydrographic data, has increased the resolution of the observed AMOC between 1981 and 2004 from approximately decadal to approximately annual, and in doing so we have shown decadal and 4-yearly variability of the AMOC and its associated layer transports. The result is the creation of an AMOC time series extending over 3 decades, including for the first time deep density anomalies in an AMOC reconstruction.
Our model has not revealed an AMOC decline indicative of anthropogenic climate change (Stocker et al., 2013) nor the long-term decline reported in sea-surface-temperature-based reconstructions of the AMOC (Caesar et al., 2018). It has accurately reproduced the variability observed in the RAPID data, showing that the downturn between 2008 and 2012 (McCarthy et al., 2012) marked not only the weakest AMOC of the RAPID era but the weakest AMOC since the mid-1980s. Since this minimum, the strength of the AMOC has recovered in line with observations from the RAPID array (Moat et al., 2020). In fact, according to our model, southward flowing LNADW has regained a vigour not seen since the 1980s. Recent cold and fresh anomalies in the surface of the North Atlantic subpolar gyre seemed to indicate a return to a cool Atlantic phase associated with a weak AMOC (Frajka-Williams et al., 2017). However, a weakened AMOC was not the primary cause of these anomalies (Josey et al., 2018; Holliday et al., 2020). Whether a restrengthened AMOC will ultimately have a strong impact on Atlantic climate such as was believed to have occurred in the 1990s (Robson et al., 2012) remains to be seen.