Journal cover Journal topic
Climate of the Past An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/cp-2017-161
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
09 Jan 2018
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Climate of the Past (CP).
Climate sensitivity and meridional overturning circulation in the late Eocene using GFDL CM2.1
David K. Hutchinson1, Agatha M. de Boer1, Helen K. Coxall1, Rodrigo Caballero2, Johan Nilsson2, and Michiel Baatsen3 1Department of Geological Sciences, Stockholm University, 10691 Stockholm, Sweden
2Department of Meteorology, Stockholm University, 10691 Stockholm, Sweden
3IMAU, Utrecht University, Princetonplein 5, 3584CC Utrecht, the Netherlands
Abstract. The Eocene–Oligocene Transition (EOT), approximately 34 Ma ago, is an interval of great interest in Earth's climate history, due to the inception of the Antarctic ice sheet and major global cooling at the time. Climate simulations of the transition are needed to help us interpret proxy data, test mechanistic hypotheses for the transition, and determine the climate sensitivity at the time. However, model studies of the EOT thus far typically employ control states designed for a different time period, or ocean resolution on the order of 3 degrees. Here we developed a new higher resolution paleoclimate model configuration based on the GFDL CM2.1 climate model adapted to a late Eocene (38 Ma) paleogeography reconstruction. We employ an ocean resolution of 1 × 1.5 degrees, and an atmosphere resolution of 3 × 3.75 degrees. This represents a significant step forward in resolving the ocean geography, gateways and circulation in a coupled climate model of this period. We simulate the model under 3 different levels of CO2; 400, 800 and 1600 ppm. The model exhibits relatively high sensitivity to CO2 compared with other recent model studies, and thus can capture the expected Eocene high latitude warmth within observed estimates of atmospheric CO2. However, the model does not capture the low meridional temperature gradient seen in proxies. Equatorial sea surface temperatures are too high in the model (30–37 degrees C) compared with observations (max 32 degrees C), though observations are lacking in the warmest regions of the western Pacific. The model exhibits robust bipolar sinking in the North Pacific and Southern Ocean, which persists under all levels of CO2. North Atlantic salinities are too fresh to permit sinking (25–30 psu), due to surface transport from the very fresh Arctic (~ 20 psu), whose salinities approximately agree with Eocene proxy estimates. North Atlantic salinity increases by 1–2 psu when CO2 is halved, and similarly freshens when CO2 is doubled, due to changes in the hydrological cycle.
Citation: Hutchinson, D. K., de Boer, A. M., Coxall, H. K., Caballero, R., Nilsson, J., and Baatsen, M.: Climate sensitivity and meridional overturning circulation in the late Eocene using GFDL CM2.1, Clim. Past Discuss., https://doi.org/10.5194/cp-2017-161, in review, 2018.
David K. Hutchinson et al.
David K. Hutchinson et al.
David K. Hutchinson et al.

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Short summary
The Eocene–Oligocene transition was a major cooling event 34 million years ago. Climate model studies of this transition have used low resolution, or topography that roughly approximates the time period. We present a new climate model simulation of the late Eocene, with higher ocean resolution, and topography that is accurately designed for this time period. These features improve the ocean circulation and gateways that are thought to be important for the climate transition.
The Eocene–Oligocene transition was a major cooling event 34 million years ago. Climate model...
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