Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis

Abstract

The notion that the Atlantic Meridional Overturning Circulation (AMOC) can have more than one stable equilibrium emerged in the 1980s as a powerful hypothesis to explain rapid climate variability during the Pleistocene. Ever since, the idea that a temporary perturbation of the AMOC—or a permanent change in its forcing—could trigger an irreversible collapse has remained a reason for concern. Here we review literature on the equilibrium stability of the AMOC and present a synthesis that puts our understanding of past and future AMOC behavior in a unifying framework. This framework is based on concepts from Dynamical Systems Theory, which has proven to be an important tool in interpreting a wide range of model behavior. We conclude that it cannot be ruled out that the AMOC in our current climate is in, or close to, a regime of multiple equilibria. But there is considerable uncertainty in the location of stability thresholds with respect to our current climate state, so we have no credible indications of where our present‐day AMOC is located with respect to thresholds. We conclude by identifying gaps in our knowledge and proposing possible ways forward to address these gaps.

The Atlantic Meridional Overturning Circulation in High‐Resolution Models

“…Weijer et al, 2012), has been a key research question for climatologists for decades; a collapse of the AMOC would have significant climate implications, with large societal repercussions. Despite the potentially high impact, there is still significant uncertainty in the likelihood of such an event (see Weijer et al, for a review).…”

“…However, whether the current generation of low‐resolution climate models adequately simulates the sensitivity of the AMOC to a warming climate is still a matter of debate (e.g., Caesar et al, ; Rahmstorf et al, ) with some authors suggesting that the sensitivity in nature could be higher than previously thought (Heuzé, ; Reintges et al, ). For more exhaustive overviews about the AMOC and mechanisms driving its strength and variability we refer the reader to comprehensive reviews such as Lumpkin and Speer (2007), Kuhlbrodt et al (), Buckley and Marshall (), Zhang et al (), Johnson et al (2019) and Weijer et al ().…”

Atlantic Meridional Overturning Circulation and δ13C Variability During the Last Interglacial

“…Such constraints are needed to assess, for example, whether it is mechanistically plausible for changes in water mass distribution to have driven abrupt centennial‐scale variability in bottom water δ 13 C and, furthermore, whether such tracer field adjustments could be linked to AMOC variability. Determining the characteristic pattern or fingerprint in interior ocean δ 13 C related to AMOC changes would provide a more realistic and physically consistent framework for interpreting past circulation variability and its implication for the carbon cycling but also in understanding its recent (Caesar et al, ) and potential future changes (Weijer et al, ). Here, we span this knowledge gap by using iLOVECLIM, an Earth system model of intermediate complexity (EMIC), to perform a transient simulation of the LIG (125–115 ka) under past natural variations in natural greenhouse gases and transient orbital forcings.…”

Global Oceans

“…flux into the South Atlantic may control the stability of the entire AMOC system (e.g., Rahmstorf 1996;Dijkstra 2007;Drijfhout et al 2011;Garzoli et al 2013;Weijer et al 2019). To monitor the impact of inter-ocean exchanges on the AMOC, the SAMBA moorings at 34.5°S began being deployed in 2009 (e.g., Meinen et al 2013;Ansorge et al 2014).…”