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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.
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Evaluation of Arctic warming in mid-Pliocene climate simulations
Abstract. Palaeoclimate simulations improve our understanding ofthe climate, inform us about the performance of climate models in adifferent climate scenario, and help to identify robust features of theclimate system. Here, we analyse Arctic warming in an ensemble of 16simulations of the mid-Pliocene Warm Period (mPWP), derived from thePliocene Model Intercomparison Project Phase 2 (PlioMIP2). The PlioMIP2 ensemble simulates Arctic (60–90∘ N) annual meansurface air temperature (SAT) increases of 3.7 to 11.6 ∘Ccompared to the pre-industrial period, with a multi-model mean (MMM) increase of7.2 ∘C. The Arctic warming amplification ratio relative to globalSAT anomalies in the ensemble ranges from 1.8 to 3.1 (MMM is 2.3). Sea iceextent anomalies range from −3.0 to -10.4×106 km2, with a MMManomaly of -5.6×106 km2, which constitutes a decrease of 53 %compared to the pre-industrial period. The majority (11 out of 16) of models simulatesummer sea-ice-free conditions (≤1×106 km2) in their mPWPsimulation. The ensemble tends to underestimate SAT in the Arctic whencompared to available reconstructions, although the degree of underestimationvaries strongly between the simulations. The simulations with the highestArctic SAT anomalies tend to match the proxy dataset in its current formbetter. The ensemble shows some agreement with reconstructions of sea ice,particularly with regard to seasonal sea ice. Large uncertainties limit theconfidence that can be placed in the findings and the compatibility of thedifferent proxy datasets. We show that while reducing uncertainties in thereconstructions could decrease the SAT data–model discord substantially,further improvements are likely to be found in enhanced boundary conditionsor model physics. Lastly, we compare the Arctic warming in the mPWP toprojections of future Arctic warming and find that the PlioMIP2 ensemblesimulates greater Arctic amplification than CMIP5 future climate simulationsand an increase instead of a decrease in Atlantic Meridional OverturningCirculation (AMOC) strength compared topre-industrial period. The results highlight the importance of slow feedbacks inequilibrium climate simulations, and that caution must be taken when usingsimulations of the mPWP as an analogue for future climate change.
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- Award ID(s):
- 1903650
- PAR ID:
- 10280521
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Date Published:
- Journal Name:
- Climate of the Past
- Volume:
- 16
- Issue:
- 6
- ISSN:
- 1814-9332
- Page Range / eLocation ID:
- 2325 to 2341
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/cp-16-2325-2020
