17 Oct 2023
 | 17 Oct 2023
Status: this preprint is currently under review for the journal CP.

Multiple thermal AMOC thresholds in the intermediate complexity model Bern3D

Markus Adloff, Frerk Pöppelmeier, Aurich Jeltsch-Thömmes, Thomas F. Stocker, and Fortunat Joos

Abstract. Variations of the Atlantic Meridional Overturning Circulation (AMOC) are associated with Northern Hemispheric and global climate shifts. Thermal thresholds of the AMOC have been found in a hierarchy of numerical circulation models, and there is an increasing body of evidence for the existence of highly sensitive AMOC modes where small perturbations can cause disproportionately large circulation and hence climatic changes. We discovered such thresholds in simulations with the intermediate complexity Earth system model Bern3D, which is highly computationally efficient allowing for studying this non-linear behaviour systematically over entire glacial cycles. By simulating the AMOC under different magnitudes of orbitally-paced changes in radiative forcing over the last 800,000 years, we show that up to three thermal thresholds are crossed during glacial cycles in Bern3D, and that thermal forcing could have destabilised the AMOC repeatedly. We present the circulation and sea ice patterns that characterise the stable circulation states between which the model oscillates during a glacial cycle, and assess how often and when thermal forcing could have preconditioned the AMOC for abrupt shifts over the last 800 kyr.

Markus Adloff et al.

Status: open (until 12 Dec 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on cp-2023-82', Marlene Klockmann, 16 Nov 2023 reply
  • RC2: 'Comment on cp-2023-82', Anonymous Referee #2, 24 Nov 2023 reply
  • RC3: 'Comment on cp-2023-82', Sam Sherriff-Tadano, 30 Nov 2023 reply

Markus Adloff et al.

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Data required to reproduce the figures Markus Adloff, Aurich Jeltsch-Thömmes, Frerk Pöppelmeier, Thomas F. Stocker, Fortunat Joos

Markus Adloff et al.


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Short summary
The Atlantic Meridional Overturning Circulation (AMOC) is an ocean current that transports heat into the North Atlantic and is important for the regional and global climate. Over the ice age cycles, AMOC changed strength and shape. By simulating the temperature changes of the last eight glacial cycles, we tested the role of heat forcing for AMOC changes. In our model, AMOC shifts between four circulation states, caused by changes in the distribution of heat and salinity in the ocean.