CPD Climate of the Past Discussions CPD 1814-9359 Copernicus GmbH Göttingen, Germany 10.5194/cpd-5-1297-2009 Warm Paleocene/Eocene climate as simulated in ECHAM5/MPI-OM Heinemann M. 1 2 Jungclaus J. H. 1 Marotzke J. 1 Max Planck Institute for Meteorology (MPI-M), Hamburg, Germany International Max Planck Research School on Earth System Modelling (IMPRS-ESM), Hamburg, Germany 05 05 2009 5 3 1297 1336 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.clim-past-discuss.net/5/1297/2009/cpd-5-1297-2009.html The full text article is available as a PDF file from http://www.clim-past-discuss.net/5/1297/2009/cpd-5-1297-2009.pdf

We investigate the late Paleocene/early Eocene (PE) climate using the coupled atmosphere-ocean-sea ice model ECHAM5/MPI-OM. The surface in our PE control simulation is on average 297 K warm and ice-free, despite a moderate CO<sub>2</sub> concentration of 560 ppm. Compared to a pre-industrial reference simulation (PR), low latitudes are 5 to 8 K warmer, while high latitudes are up to 40 K warmer. This high-latitude amplification is in line with proxy data, yet a comparison to sea surface temperature proxy data suggests that the Arctic surface temperatures are still too low. <br><br> To identify the mechanisms that cause the PE-PR temperature difference, we fit a zero-dimensional energy balance model to the ECHAM5/MPI-OM results. Doubled <i>p</i>CO<sub>2</sub> in PE compared to PR, increased atmospheric water vapour, and a slightly increased longwave cloud radiative forcing together cause about 2/3 of the PE-PR temperature difference; planetary albedo changes cause about 1/3. Our results support the hypothesis that local radiative effects as well as topographic changes, rather than increased meridional heat transports, were responsible for the "equable" PE climate.

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