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Abstract North Atlantic meridional density gradients have been identified as a main driver of the Atlantic meridional overturning circulation (AMOC). Due to the cabbeling effect, these density gradients are increasingly dominated by temperature gradients in a warming ocean, and a direct link exists between North Atlantic mean temperature and AMOC strength. This paper quantifies the impact of this mechanism in the Stommel and Gnanadesikan models. Owing to different feedback mechanisms being included, a 1°C warming of North Atlantic mean ocean temperature strengthens the AMOC by 3% in the Gnanadesikan model and 8% in the Stommel model. In the Gnanadesikan model that increase is equivalent to a 4% strengthening of Southern Hemisphere winds and can compensate for a 14% increase in the hydrological cycle. Furthermore, mean temperature strongly controls a freshwater forcing threshold for the strong AMOC state, suggesting that the cabbeling effect needs to be considered to explain past and future AMOC variability. 

Abstract. It is widely accepted that orbital variations areresponsible for the generation of glacial cycles during the latePleistocene. However, the relative contributions of the orbital forcingcompared to CO2 variations and other feedback mechanisms causing thewaxing and waning of ice sheets have not been fully understood. Testingtheories of ice ages beyond statistical inferences, requires numericalmodeling experiments that capture key features of glacial transitions. Here,we focus on the glacial buildup from Marine Isotope Stage (MIS) 7 to 6covering the period from 240 to 170 ka (ka: thousand years before present). Thistransition from interglacial to glacial conditions includes one of thefastest Pleistocene glaciation–deglaciation events, which occurred during MIS 7e–7d–7c (236–218 ka). Using a newly developed three-dimensional coupledatmosphere–ocean–vegetation–ice sheet model (LOVECLIP), we simulate thetransient evolution of Northern Hemisphere and Southern Hemisphere ice sheets duringthe MIS 7–6 period in response to orbital and greenhouse gas forcing. For arange of model parameters, the simulations capture the evolution of globalice volume well within the range of reconstructions. Over the MIS 7–6period, it is demonstrated that glacial inceptions are more sensitive toorbital variations, whereas terminations from deep glacial conditions needboth orbital and greenhouse gas forcings to work in unison. For someparameter values, the coupled model also exhibits a critical North Americanice sheet configuration, beyond which a stationary-wave–ice-sheettopography feedback can trigger an unabated and unrealistic ice sheetgrowth. The strong parameter sensitivity found in this study originates fromthe fact that delicate mass imbalances, as well as errors, are integratedduring a transient simulation for thousands of years. This poses a generalchallenge for transient coupled climate–ice sheet modeling, with suchcoupled paleo-simulations providing opportunities to constrain suchparameters. 

Abstract Revolutionary observational arrays, together with a new generation of ocean and climate models, have provided new and intriguing insights into the Atlantic Meridional Overturning Circulation (AMOC) over the last two decades. Theoretical models have also changed our view of the AMOC, providing a dynamical framework for understanding the new observations and the results of complex models. In this paper we review recent advances in conceptual understanding of the processes maintaining the AMOC. We discuss recent theoretical models that address issues such as the interplay between surface buoyancy and wind forcing, the extent to which the AMOC is adiabatic, the importance of mesoscale eddies, the interaction between the middepth North Atlantic Deep Water cell and the abyssal Antarctic Bottom Water cell, the role of basin geometry and bathymetry, and the importance of a three‐dimensional multiple‐basin perspective. We review new paradigms for deep water formation in the high‐latitude North Atlantic and the impact of diapycnal mixing on vertical motion in the ocean interior. And we discuss advances in our understanding of the AMOC's stability and its scaling with large‐scale meridional density gradients. Along with reviewing theories for the mean AMOC, we consider models of AMOC variability and discuss what we have learned from theory about the detection and meridional propagation of AMOC anomalies. Simple theoretical models remain a vital and powerful tool for articulating our understanding of the AMOC and identifying the processes that are most critical to represent accurately in the next generation of numerical ocean and climate models.