Warm Paleocene/Eocene climate as simulated in ECHAM5/MPI-OM

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 atmospheric CO2 concentration of 560 ppm. Compared to a pre-industrial refere...

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Bibliographic Details
Published in:Climate of the Past
Main Authors: Heinemann, M., Jungclaus, J. H., Marotzke, J.
Format: Text
Language:English
Published: 2018
Subjects:
geo
envir
Antarctic
Arctic
albedo
Antarc*
Sea ice
Online Access:https://doi.org/10.5194/cp-5-785-2009
https://cp.copernicus.org/articles/5/785/2009/
Description
Summary: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 atmospheric CO2 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 in our PE simulation. To identify the mechanisms that cause the PE-PR surface temperature differences, we fit two simple energy balance models to the ECHAM5/MPI-OM results. We find that about 2/3 of the PE-PR global mean surface temperature difference are caused by a smaller clear sky emissivity due to higher atmospheric CO2 and water vapour concentrations in PE compared to PR; 1/3 is due to a smaller planetary albedo. The reduction of the pole-to-equator temperature gradient in PE compared to PR is due to (1) the large high-latitude effect of the higher CO2 and water vapour concentrations in PE compared to PR, (2) the lower Antarctic orography, (3) the smaller surface albedo at high latitudes, and (4) longwave cloud radiative effects. Our results support the hypothesis that local radiative effects rather than increased meridional heat transports were responsible for the "equable" PE climate.

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