21 Nov 2020

21 Nov 2020

Review status: a revised version of this preprint is currently under review for the journal CP.

Impact of Southern Ocean surface conditions on deep ocean circulation at the LGM: a model analysis

Fanny Lhardy1, Nathaëlle Bouttes1, Didier M. Roche1,2, Xavier Crosta3, Claire Waelbroeck4, and Didier Paillard1 Fanny Lhardy et al.
  • 1Laboratoire des Sciences du Climat et de l’Environnement, LSCE/IPSL, CEA-CNRS-UVSQ-Université Paris-Saclay, F-91198 Gif-sur-Yvette, France
  • 2Vrije Universiteit Amsterdam, Faculty of Science, Cluster Earth and Climate, de Boelelaan 1085, 1081HV Amsterdam, The Netherlands
  • 3Univ. Bordeaux, CNRS, EPOC, EPHE, UMR 5805, F-33600 Pessac, France
  • 4Laboratoire d’Océanographie et du Climat: Expérimentation et Approches Numériques (LOCEAN), IPSL, Université Pierre et Marie Curie, Paris, France

Abstract. Changes in water mass distribution are considered to be a significant contributor to the atmospheric CO2 concentration drop to around 186 ppm recorded during the Last Glacial Maximum (LGM). Yet simulating a glacial Atlantic Meridional Overturning Circulation (AMOC) in agreement with paleotracer data remains a challenge, with most models from previous Paleoclimate Modelling Intercomparison Project (PMIP) phases showing a tendency to simulate a strong and deep North Atlantic Deep Water (NADW) instead of the shoaling inferred from proxy data. Conversely, the simulated Antarctic Bottom Water (AABW) is often reduced compared to its pre-industrial volume, and the Atlantic Ocean stratification is underestimated with respect to data. Inadequate representation of surface conditions, driving deep convection around Antarctica, may explain inaccurate simulated bottom water properties in the Southern Ocean. We investigate here the impact of a range of surface conditions in the Southern Ocean, using nine simulations obtained using different modelling choices and/or boundary conditions in the iLOVECLIM model. Based on data-model comparison of key parameters (sea-surface temperatures and sea ice), we find that only simulations with a cold Southern Ocean and a quite extensive sea-ice cover show an improved agreement with proxy data, despite systematic model biases in the seasonal and regional patterns. We then show that the only simulation which does not display a much deeper NADW is obtained by parameterizing the sinking of brines along Antarctica, a modelling choice reducing the open ocean convection in the Southern Ocean. These results highlight the importance of the representation of convection processes, which have a large impact on the water masses properties, while the choice of boundary conditions appears secondary for the model resolution and variables considered in this study.

Fanny Lhardy et al.

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Fanny Lhardy et al.

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Model outputs for figures in Climate of the Past "Impact of Southern Ocean surface conditions on deep ocean circulation at the LGM: a model analysis" Fanny Lhardy

Fanny Lhardy et al.


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Short summary
Climate models struggle to simulate a LGM ocean circulation in agreement with paleotracer data. Using a set of simulations, we test the impact of boundary conditions and other modelling choices. Model-data comparisons of sea-surface temperatures and sea-ice cover support an overall cold Southern Ocean, with implications on the AMOC strength. Changes in implemented boundary conditions are not sufficient to simulate a shallower AMOC, other mechanisms to better represent convection are required.