The Climate of the Eocene-Oligocene-Transition during the Antarctic Ice Sheet Buildup: Modern Climate Model Simulations

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The Eocene-Oligocene-Transition (EOT) marks the significant change from a ”Green- house” to an ”Icehouse” climate during the Cenozoic (Bohaty et al., 2012). This work presents modelled climate conditions during the EOT and contributes to the discussion about Antarctic glaciation. The strength of this work lies in the use of a modern fully- coupled Earth System Model (ESM), the high resolution applied to the polar regions and the strong comparison to proxy data. The model results combined with new West Antarctic proxy data from Klages et al., under review strongly support the hypothesis of an asymmetric Antarctic Ice Sheet (AIS) and West Antarctic deciduous forest vege- tation. Observed CO2 thresholds for Antarctic glaciation are > 840 ppm for the onset of glaciation and < 560 ppm for a continental-scale ice sheet reaching into West Antarc- tica. For the Southern ocean heat transport an intermediate state between a subpolar gyre and an Antarctic Circumpolar Current (ACC) regime is modelled. West Antarc- tic temperatures are several degrees lower compared to proxy data but agree with other modelled temperatures. Summer air temperatures are between 95 − 5◦C. Summer sea surface temperature (SST) ranges within 91 − 5◦C and allow only few summer sea ice. The modelled EOT climate is several degrees warmer than the pre-industrial (PI) cli- mate on all latitudes. The global annual mean air temperature lies almost 5◦C above the reference PI simulation. With 18.0◦C it is relatively low compared to other modelled temperatures. Global vegetation known from proxy data can partly be resolved by the model. Overall, this model supports an ice free West Antarctic during the EOT found in Klages et al., under review and provides a detailed framework for Antarctic ice in global EOT climate dynamics. The largely good agreement with EOT proxy data encourages deep time modelling with AWI-ESM-2.1.

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Knahl, H. S. (2022): The Climate of the Eocene-Oligocene-Transition during the Antarctic Ice Sheet Buildup: Modern Climate Model Simulations / G. Lohmann , J. Klages and P. Gierz (editors) Master thesis,

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