25 Mar 2024
 | 25 Mar 2024
Status: this preprint is open for discussion and under review for Climate of the Past (CP).

Surface buoyancy control of millennial-scale variations of the Atlantic meridional ocean circulation

Matteo Willeit, Andrey Ganopolski, Neil R. Edwards, and Stefan Rahmstorf

Abstract. Dansgaard-Oeschger (DO) events are a pervasive feature of glacial climates. It is widely accepted that the associated changes in climate, which are most pronounced in the North Atlantic region, are caused by abrupt changes in the strength and/or latitude reach of the Atlantic meridional overturning circulation (AMOC), possibly originating from spontaneous transitions in the ocean-sea-ice-atmosphere system. Here we use an Earth System Model that produces DO-like events to show that the climate conditions under which millennial-scale AMOC variations occur are controlled by the surface ocean buoyancy flux. In particular, we find that the present day-like convection pattern with deep water formation in the Labrador and Nordic Seas becomes unstable when the buoyancy flux integrated over the northern North Atlantic turns from negative to positive. It is in the proximity of this point that the model produces transitions between different convection patterns associated with strong and weak AMOC states. The buoyancy flux depends on the surface freshwater and heat fluxes and on sea surface temperature through the temperature dependence of the thermal expansion coefficient of seawater. We find that larger ice sheets tend to stabilize convection by decreasing the net freshwater flux while CO2-induced cooling decreases buoyancy loss and destabilizes convection. These results help to explain the conditions under which DO events appear, and are a step towards an improved understanding of the mechanisms of abrupt climate changes. 

Matteo Willeit, Andrey Ganopolski, Neil R. Edwards, and Stefan Rahmstorf

Status: open (until 20 May 2024)

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Matteo Willeit, Andrey Ganopolski, Neil R. Edwards, and Stefan Rahmstorf
Matteo Willeit, Andrey Ganopolski, Neil R. Edwards, and Stefan Rahmstorf


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Short summary
Using an Earth system model that can simulate Dansgaard-Oeschger-like events, we show that the conditions under which millenial-scale climate variability occurs is related to the integrated surface buoyancy flux over the northern North-Atlantic. This newly defined buoyancy measure explains why millenial-scale climate variability arising from abrupt changes in the Atlantic Meridional Overturning Circulation occurred for mid-glacial conditions but not for interglacial or full glacial conditions.