| 20 May 2026
Multidecadal North Atlantic Circulation Shifts under Historical Anthropogenic Forcing in CESM2-LE
Abstract. The North Atlantic subpolar gyre is a critical region for global climate, yet the mechanisms driving its multidecadal circulation variability under anthropogenic forcing remain poorly understood. This study investigates the physical processes underlying large multidecadal shifts in density overturning strength at 55°N using a 100-member ensemble of the Community Earth System Model version 2 (CESM2). Using change point and composite analyses, we identify three distinct categories of circulation shifts: strengthening events, and two types of weakening events separated by a 1985 regime shift. Strengthening shifts are driven by an internal positive feedback where enhanced surface heat loss triggers deep convection and strengthened horizontal gyre circulation, subsequently increasing northward heat and salt transport that sustains the anomaly. Weakening shifts before 1985 are primarily driven by internal density-gradient adjustments between the subpolar and subtropical gyres. In contrast, post-1985 weakening events are characterized by basin-wide thermodynamic changes, where greenhouse gas-induced warming and reduced surface buoyancy loss suppress convection across the Irminger Sea and eastern subpolar North Atlantic. Our results reveal that these shifts are non-linear and asymmetric, reflecting a transition from a salinity-dominated internal variability regime to a forced, temperature-driven regime. These findings suggest that the North Atlantic circulation is undergoing a fundamental change in its governing dynamics, highlighting the increasing influence of anthropogenic forcing on the stability of the Atlantic Meridional Overturning Circulation.
Dear editor and authors,
The well-written manuscript under consideration presents important results about multidecadal periods of strengthening and weakening of the North Atlantic circulation. The authors furthermore make a qualitative distinction between the pre- and post-1985 weakening events with compelling evidence. The emphasis in this paper is on subpolar gyre, inter-gyre, and basin-wide thermodynamic drivers of circulation anomalies.
However, I think the authors should also discuss the synchronous, coherent coevolution of the maximum overturning between the subpolar and subtropical latitudes seen very clearly in their results. The authors should furthermore discuss the role of the salinity anomalies along the western boundary of the Labrador Sea in driving these coherent overturning anomalies. These salinity anomalies stand out in some of the figures showing the authors’ results. In addition to their present analysis of inter-gyre thermodynamic gradients, the authors should also examine in greater detail the horizontal density gradients between the western boundary and the interior of the subpolar gyre. I therefore recommend minor but strongly recommended revisions.
Major comments:
Figures 4 and 9: The authors should discuss and interpret the synchronous, coherent coevolution of the maximum overturning between 55N and 26N seen in these figures.
Figures 3, 6, and 8: The authors should discuss the role of salinity anomalies along the western boundary of the Labrador Sea seen in these figures, and the role of these anomalies in driving meridionally coherent overturning variability in the context of Kostov et al. (2023).
Figure 10: Once again, in this figure, the western boundary of the Labrador Sea is a region of particularly strong density anomalies that also extend along the intergyre boundary. Please discuss in the context of Kostov et al. (2023) and the possible fast mechanism that give rise to the coherent response you see between 55N and 26N.
Lines 225-229, 257-260, and 275-276: You talk about “density within the SPG”, “in the STG”, the “inter-gyre density gradient,” and “the meridional gradient.” However, you do not mention the dynamical importance of density anomalies along the western boundary. What about the horizontal density gradients between the western boundary and the interior?
Lines 293-294: Finally, there is a brief mention of the western SPG margin in this single sentence. Could you also cite Kostov et al. (2023) here in the context of fast north-to-south connectivity along the western boundary?
Minor comments:
Lines 41-42: “in the subtropical gyre (STG), where it loses heat and salt.” – How is salt lost in the subtropical gyre? I think freshwater is lost there, evaporatively.
Lines 51-52: “it is ultimately density that governs the movement of water masses” – The statement should be qualified in the context of the study. Otherwise, winds play a first order role.
Line 85: “a pressure of 2000 dbar” – Is this reference level appropriate for subpolar North Atlantic overturning
Lines 92-93: “by taking the maximum of the annual mean overturning strength at 55N in the annual mean” – Is the maximum overturning strength close to a dept of 2 km or in a density layer that includes that depth.
Line 110: “sea surface height” – Cite Yeager et al.(2021) and Kostov et al. (2023) who discuss the importance of sea surface height anomalies in the North Atlantic and their role in setting the variability in overturning.
Figures 4 and 9: Define the positive direction.
Lines 300-301: Could you please cite Buckley and Marshall (2016) in the context of the Mann Eddy and inter-gyre exchange. Could you please cite Buckley et al. (2023) in the context of eddy exchange between the boundaries and the gyre interior?
Additional references:
Buckley, M. W. and J. Marshall (2016), Observations, inferences, and mechanisms of the Atlantic Meridional Overturning Circulation: A review, Rev. Geophys., 54, doi:10.1002/2015RG000493.
Buckley MW, Lozier MS, Desbruyères D, Evans DG. 2023 Buoyancy forcing and the subpolar Atlantic meridional overturning circulation.Phil. Trans. R. Soc. A 381: 20220181. https://doi.org/10.1098/rsta.2022.0181
Kostov, Y., Messias, MJ., Mercier, H. et al. Fast mechanisms linking the Labrador Sea with subtropical Atlantic overturning. Clim Dyn 60, 2687–2712 (2023). https://doi.org/10.1007/s00382-022-06459-y