Highly stratified mid-Pliocene Southern Ocean in PlioMIP2

Julia E. Weiffenbach*, Henk A. Dijkstra, Anna S. von der Heydt, Ayako Abe-Ouchi, Wing-Le Chan, Deepak Chandan, Ran Feng, Alan M. Haywood, Stephen J. Hunter, Xiangyu Li, Bette L. Otto-Bliesner, William Richard Peltier, Christian Stepanek, Ning Tan, Julia C. Tindall, Zhongshi Zhang

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

During the mid-Pliocene warm period (mPWP; 3.264–3.025 Ma), atmospheric CO2 concentrations were approximately 400 ppm, and the Antarctic Ice Sheet was substantially reduced compared to today. Antarctica is surrounded by the Southern Ocean, which plays a crucial role in the global oceanic circulation and climate regulation. Using results from the Pliocene Model Intercomparison Project (PlioMIP2), we investigate Southern Ocean conditions during the mPWP with respect to the pre-industrial period. We find that the mean sea surface temperature (SST) warming in the Southern Ocean is 2.8 °C, while global mean SST warming is 2.4 °C. The enhanced warming is strongly tied to a dramatic decrease in sea ice cover over the mPWP Southern Ocean. We also see a freshening of the ocean (sub)surface, driven by an increase in precipitation over the Southern Ocean and Antarctica. The warmer and fresher surface leads to a highly stratified Southern Ocean that can be related to weakening of the deep abyssal overturning circulation. Sensitivity simulations show that the decrease in sea ice cover and enhanced warming is largely a consequence of the reduction in the Antarctic Ice Sheet. In addition, the mPWP geographic boundary conditions are responsible for approximately half of the increase in mPWP SST warming, sea ice loss, precipitation, and stratification increase over the Southern Ocean. From these results, we conclude that a strongly reduced Antarctic Ice Sheet during the mPWP has a substantial influence on the state of the Southern Ocean and exacerbates the changes that are induced by a higher CO2 concentration alone. This is relevant for the long-term future of the Southern Ocean, as we expect melting of the western Antarctic Ice Sheet in the future, an effect that is not currently taken into account in future projections by Coupled Model Intercomparison Project (CMIP) ensembles.
Original languageEnglish
Pages (from-to)1067-1086
Number of pages20
JournalClimate of the Past
Volume20
Issue number4
DOIs
Publication statusPublished - 2 May 2024

Bibliographical note

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Funding

Christian Stepanek acknowledges funding from the Helmholtz Climate Initiative REKLIM and the research program PACES-II of the Helmholtz Association. Alan M. Haywood, Stephen J. Hunter, and Julia C. Tindall acknowledge the FP7 Ideas program from the European Research Council (grant no. PLIO-ESS, 278636), the Past Earth Network (EPSRC, grant no. EP/M008.363/1), and the University of Leeds Advanced Research Computing service. Julia C. Tindall was also supported through the Centre for Environmental Modelling And Computation (CEMAC), University of Leeds. Deepak Chandan and W. Richard Peltier have been supported by Canadian NSERC Discovery Grant(grant no. A9627), and they wish to acknowledge the support of the SciNet HPC Consortium for providing computing facilities. SciNet is funded by the Canada Foundation for Innovation under the auspices of Compute Canada, the Government of Ontario, the Ontario Research Fund \u2013 Research Excellence and the University of Toronto. This research has been supported by the Netherlands Earth System Science Centre (NESSC), which is financially supported by the Ministry of Education, Culture, and Science (OCW, grant no. 024.002.001). The work by Julia E. Weiffenbach, Henk A. Dijkstra, and Anna S. von der Heydt was carried out under the program of the Netherlands Earth System Science Centre (NESSC), financially supported by the Ministry of Education, Culture and Science (OCW, grant no. 024.002.001). CCSM4-Utr simulations were performed at the SURFsara Dutch national computing facilities and were sponsored by NWO-ENW (Netherlands Organisation for Scientific Research, Exact Sciences) under the project nos. 17189 and 2020.022. Ran Feng acknowledges support from the U.S. National Science Foundation (grant nos. NSF-2103055 and NSF-2238875). The CCSM4 and CESM1 and CESM2 simulations are performed with high-performance computing support from Cheyenne ( 10.5065/D6RX99HX ) provided by NCAR's Computational and Information Systems Laboratory, sponsored by the National Science Foundation. Ayako Abe-Ouchi and Wing-Le Chan acknowledge funding from JSPS KAKENHI (grant no. 17H06104) and MEXT KAKENHI (grant no. 17H06323) and are grateful to JAMSTEC for use of the Earth Simulator. Xiangyu Li acknowledges financial support from the National Natural Science Foundation of China (grant nos. 42005042 and 42275047) and the China Scholarship Council (201804910023). Zhongshi Zhang acknowledges financial support from the National Natural Science Foundation of China (grant nos. 41888101 and 42125502).

FundersFunder number
Helmholtz Association
Seventh Framework Programme
Ontario Research Foundation
Netherlands Earth System Science Centre
University of Toronto
Canada Foundation for Innovation
University of Leeds
Government of Ontario
Past Earth Network
Helmholtz Climate Initiative REKLIM
NWO-ENW17189, 2020.022
European Research Council278636
European Research Council
National Science FoundationNSF-2103055, NSF-2238875
National Science Foundation
Natural Sciences and Engineering Research Council of CanadaA9627
Natural Sciences and Engineering Research Council of Canada
Japan Society for the Promotion of Science17H06104
Japan Society for the Promotion of Science
National Natural Science Foundation of China42275047, 42005042
National Natural Science Foundation of China
China Scholarship Council201804910023, 41888101, 42125502
China Scholarship Council
Engineering and Physical Sciences Research CouncilEP/M008.363/1
Engineering and Physical Sciences Research Council
Ministry of Education, Culture, Sports, Science and Technology17H06323
Ministry of Education, Culture, Sports, Science and Technology
Ministerie van onderwijs, cultuur en wetenschap024.002.001
Ministerie van onderwijs, cultuur en wetenschap

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