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AMOC (changes)

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Idea

The Atlantic Meridional Overturning Circulation or AMOC is a current that carries warm water north, where it cools, falls to the bottom and returns south. As you can see here, it is part of a larger worldwide current system, often called the thermohaline circulation:

A failure of the AMOC is believed to have caused the Younger Dryas episode. The possibility of another shut-down, caused by global warming, has been widely studied. See for example:

Details of the ‘standard model’ of the AMOC have been questioned here:

Abstract: To understand how our global climate will change in response to natural and anthropogenic forcing, it is essential to determine how quickly and by what pathways climate change signals are transported throughout the global ocean, a vast reservoir for heat and carbon dioxide. Labrador Sea Water (LSW), formed by open ocean convection in the subpolar North Atlantic, is a particularly sensitive indicator of climate change on interannual to decadal timescales. Hydrographic observations made anywhere along the western boundary of the North Atlantic reveal a core of LSW at intermediate depths advected southward within the Deep Western Boundary Current (DWBC). These observations have led to the widely held view that the DWBC is the dominant pathway for the export of LSW from its formation site in the northern North Atlantic towards the Equator. Here we show that most of the recently ventilated LSW entering the subtropics follows interior, not DWBC, pathways. The interior pathways are revealed by trajectories of subsurface RAFOS floats released during the period 2003–2005 that recorded once-daily temperature, pressure and acoustically determined position for two years, and by model-simulated ‘e-floats’ released in the subpolar DWBC. The evidence points to a few specific locations around the Grand Banks where LSW is most often injected into the interior. These results have implications for deep ocean ventilation and suggest that the interior subtropical gyre should not be ignored when considering the Atlantic meridional overturning circulation.

A shutdown of the AMOC will affect different parts of Europe in different ways. There are multiple equilibria in the AMOC model (“on” and “off” states). For a description of how they arise in a simple case, and for maps of the regional temperature anomaly simulated by some climate models, see:

  • T. F. Stocker “Climatic change. North-South Connections”Science

    T. F. Stocker “Climatic change. North-South Connections” Science 297 (2002)

    297 (2002)
  • W. Weijer, W. Cheng, S.S. Drijfhout, A.V. Fedorov, A. Hu, L.C. Jackson, W. Liu, E.L. McDonagh, J.V. Mecking, J. Zhang, “Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis” Journal of Geophysical Research: Oceans (2019)

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.

category: climate