08 Nov 2023
 | 08 Nov 2023
Status: this preprint is currently under review for the journal CP.

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, W. Richard Peltier, Christian Stepanek, Ning Tan, Julia C. Tindall, and Zhongshi Zhang

Abstract. During the mid-Pliocene (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 mid-Pliocene 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 mid-Pliocene 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 of the Antarctic ice sheet. In addition, the mid-Pliocene geographic boundary conditions are responsible for approximately half of the increase in mid-Pliocene 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 mid-Pliocene has a substantial influence on the state of the mid-Pliocene 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 CMIP ensembles.

Julia E. Weiffenbach et al.

Status: open (until 03 Jan 2024)

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Julia E. Weiffenbach et al.

Julia E. Weiffenbach et al.


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Short summary
Elevated atmospheric CO2 concentrations and a smaller Antarctic ice sheet during the mid-Pliocene (~3 million years ago) cause the Southern Ocean surface to become fresher and warmer, which affects the global ocean circulation. The CO2 concentration and the smaller Antarctic ice sheet both have a similar and approximately equal impact on the Southern Ocean. The conditions of the Southern Ocean in the mid-Pliocene could therefore be analogous to those in a future climate with smaller ice sheets.