10 May 2022
10 May 2022
Status: this preprint is currently under review for the journal CP.

Unraveling the mechanisms and implications of a stronger mid-Pliocene AMOC in PlioMIP2

Julia E. Weiffenbach1, Michiel L. J. Baatsen1, Henk A. Dijkstra1,2, Anna S. von der Heydt1,2, Ayako Abe-Ouchi3, Esther C. Brady4, Wing-Le Chan3, Deepak Chandan5, Mark A. Chandler6, Camille Contoux7, Ran Feng8, Chuncheng Guo9, Zixuan Han10,11, Alan M. Haywood12, Qiang Li11, Xiangyu Li13, Gerrit Lohmann14,15, Daniel J. Lunt16, Kerim H. Nisancioglu17,18, Bette L. Otto-Bliesner4, W. Richard Peltier5, Gilles Ramstein7, Linda E. Sohl6, Christian Stepanek14, Ning Tan7,19, Julia C. Tindall12, Charles J. R. Williams16,20, Qiong Zhang11, and Zhongshi Zhang9,13 Julia E. Weiffenbach et al.
  • 1Institute for Marine and Atmospheric research Utrecht (IMAU), Department of Physics, Utrecht University, 3584 CC Utrecht, The Netherlands
  • 2Centre for Complex Systems Science, Utrecht University, 3584 CE Utrecht, the Netherlands
  • 3Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, 277-8564, Japan
  • 4National Center for Atmospheric Research, (NCAR), Boulder, CO 80305, USA
  • 5Department of Physics, University of Toronto, Toronto, M5S 1A7, Canada
  • 6CCSR/GISS, Columbia University, New York, NY 10025, USA
  • 7Laboratoire des Sciences du Climat et de l’Environnement, LSCE/IPSL, CEA-CNRS-UVSQ Université Paris-Saclay, 91191 Gif-sur-Yvette, France
  • 8Department of Geosciences, College of Liberal Arts and Sciences, University of Connecticut, Storrs, CT 06033, USA
  • 9NORCE Norwegian Research Centre, Bjerknes Centre for Climate Research, 5007 Bergen, Norway
  • 10College of Oceanography, Hohai University, Nanjing, China
  • 11Department of Physical Geography and Bolin Centre for Climate Research, Stockholm University, Stockholm, 10691, Sweden
  • 12School of Earth and Environment, University of Leeds, Woodhouse Lane, Leeds, West Yorkshire, LS2 9JT, UK
  • 13Department of Atmospheric Science, School of Environmental studies, China University of Geoscience, Wuhan 430074, China
  • 14Alfred-Wegener-Institut – Helmholtz-Zentrum für Polar and Meeresforschung (AWI), 27570 Bremerhaven, Germany
  • 15Department of Environmental Physics and MARUM, University of Bremen, 28359 Bremen, Germany
  • 16School of Geographical Sciences, University of Bristol, Bristol, BS8 1SS, UK
  • 17Bjerknes Centre for Climate Research, Department of Earth Science, University of Bergen, 5007 Bergen, Norway
  • 18Centre for Earth Evolution and Dynamics, University of Oslo, 0315 Oslo, Norway
  • 19Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
  • 20NCAS-Climate, Department of Meteorology, University of Reading, RG6 6ET Reading, UK

Abstract. The mid-Pliocene warm period (3.264–3.025 Ma) is the most recent geological period in which the atmospheric CO2 concentration was approximately equal to the concentration we measure today (ca. 400 ppm). Sea surface temperature (SST) proxies indicate above-average warming over the North Atlantic in the mid-Pliocene with respect to the pre-industrial period, which may be linked to an intensified Atlantic Meridional Overturning Circulation (AMOC). Earlier results from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) show that the ensemble simulates a stronger AMOC in the mid-Pliocene than in the pre-industrial. However, no consistent relationship between the stronger mid-Pliocene AMOC and either the Atlantic northward ocean heat transport (OHT) or average North Atlantic SSTs has been found. In this study, we look further into the drivers and consequences of a stronger AMOC in mid-Pliocene compared to pre-industrial simulations in PlioMIP2. We find that all model simulations with a closed Bering Strait and Canadian Archipelago show reduced freshwater transport from the Arctic Ocean into the North Atlantic. The resulting increase in salinity in the subpolar North Atlantic and Labrador Sea drives the stronger AMOC in the mid-Pliocene. To investigate the dynamics behind the ensemble's variable response of the total Atlantic OHT to the stronger AMOC, we separate the Atlantic OHT into two components associated with either the overturning circulation or the wind-driven gyre circulation. While the ensemble mean of the overturning component is increased significantly in magnitude in the mid-Pliocene, it is partly compensated by a reduction of the gyre component in the northern subtropical gyre region. This indicates that the lack of relationship between the total OHT and AMOC is due to changes in OHT by the subtropical gyre. The overturning and gyre components should therefore be considered separately to gain a more complete understanding of the OHT response to a stronger mid-Pliocene AMOC. In addition, we show that the AMOC exerts a stronger influence on North Atlantic SSTs in the mid-Pliocene than in the pre-industrial, providing a possible explanation for the improved agreement of the PlioMIP2 ensemble mean SSTs with reconstructions in the North Atlantic.

Julia E. Weiffenbach et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on cp-2022-35', Anonymous Referee #1, 17 Jun 2022
  • RC2: 'Comment on cp-2022-35', Anonymous Referee #2, 26 Jun 2022

Julia E. Weiffenbach et al.

Julia E. Weiffenbach et al.


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Short summary
We study the behavior of the Atlantic Meridional Overturning Circulation (AMOC) in the mid-Pliocene. The mid-Pliocene was about 3 million years ago and had a similar CO2 concentration to today. We show that the AMOC is stronger during this period due to changes in geography and that this has a significant influence on ocean temperatures and heat transported northwards by the Atlantic Ocean. Understanding the behavior of the mid-Pliocene AMOC can help us to learn more about our future climate.