The origin of the observed multi-decadal swings in the North Atlantic sea-surface temperatures (SSTs), conventionally referred to as Atlantic multidecadal variability (AMV), is at the core of a highly controversial debate (Clement et al., 2015;Mann et al., 2021;Zhang et al., 2019). In the context of the historical climate record (past 150 years), different processes have been suggested as drivers of the observed AMV-related fluctuations, including internal variations of the Atlantic meridional overturning circulation (AMOC; Gulev et al., 2013), natural forcings (Otterå et al., 2010;Swingedouw et al., 2015) and changes in anthropogenic aerosols and green-house gases (Bellucci et al., 2017;Booth et al., 2012). Under fixed external forcing conditions, the AMOC-AMV connection emerges as a key element of the low-frequency variability in the Atlantic region (Ba et al., 2014;Delworth & Mann, 2000;Knight et al., 2005) with changes in the AMOC strength leading, through a delayed response in the poleward heat transport, to a modulation of the SSTs on a basin-wide scale. This causal chain has relevant implications for the climate predictability of the Atlantic region on decadal and longer time scales. Decadal hindcasts performed with global climate models initialized with realistic estimates of the climate state display significant skill in retrospective predictions of SSTs over the North Atlantic subpolar gyre, suggesting a role for the initialization of the ocean through synchronization of predicted and observed AMOC and heat transport fluctuations (Robson et al., 2014;Yeager & Robson, 2017). The stability of the AMOC-AMV connection is therefore instrumental for the predictability of the North Atlantic climate anomalies: a temporary weakening or disruption of this mechanism might negatively affect the predictive ability of decadal forecast systems in the Atlantic sector.
<p>The connection between the Atlantic meridional overturning circulation (AMOC) and the Atlantic multidecadal variability (AMV) is inspected in a suite of pre-industrial integrations from the 6th phase of the Coupled Model Inter-comparison Project (CMIP6), using a change-point detection method to identify different AMOC-AMV co-variability regimes. A key finding of this study is that models robustly simulate multi-decadal windows where the AMV and the AMOC are essentially uncorrelated. These regimes coexist with longer periods with relatively high correlation. Drops and recoveries of correlation are found to be often abrupt and confined in a temporal window of the order of 10 years. Phenomenological evidence suggests that the no-correlation regimes may be explained by drops in the variance of the AMOC: a less variable meridional heat transport leads to a suppressed co-variability of the AMV, leaving a larger role for non-AMOC drivers, consistent with a non-stationary AMOC-stationary noise interpretative framework.</p>
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