A model-data comparison for a multi-model ensemble of early Eocene atmosphere-ocean simulations: EoMIP

Abstract

The early Eocene (~55 to 50 Ma) is a time period which has been explored in a large number of modelling and data studies. Here, using an ensemble of previously published model results, making up "EoMIP" – the Eocene Modelling Intercomparison Project, and syntheses of early Eocene terrestrial and SST temperature data, we present a self-consistent inter-model and model-data comparison. This shows that the previous modelling studies exhibit a very wide inter-model variability, but that at high CO<sub>2</sub>, there is good agreement between models and data for this period, particularly if possible seasonal biases in some of the proxies are considered. An energy balance analysis explores the reasons for the differences between the model results, and suggests that differences in surface albedo feedbacks, water vapour and lapse rate feedbacks, and prescribed aerosol loading are the dominant cause for the different results seen in the models, rather than inconsistencies in other prescribed boundary conditions or differences in cloud feedbacks. The CO<sub>2</sub> level which would give optimal early Eocene model-data agreement, based on those models which have carried out simulations with more than one CO<sub>2</sub> level, is in the range 2000 ppmv to 6500 ppmv. Given the spread of model results, tighter bounds on proxy estimates of atmospheric CO<sub>2</sub> during this time period will allow a quantitative assessment of the skill of the models at simulating warm climates, which could be used as a metric for weighting future climate predictions

Ocean biogeochemistry in the warm climate of the late Paleocene

“…The ocean has a mean temperature of 14.7 • C (preindustrial: 5.6 • C) and a global annual mean SST of 24.8 • C, which is in agreement with results of other climate models (Lunt et al, 2012). The northern high latitudes reach maximum SST of 12.8 • Cin Northern Hemisphere summer (JJA), but the sea surface of the central Arctic Ocean does not get warmer than 4 • C. The southern high latitudes show maximum SST of 17.8 • C in austral summer (DJF).…”

“…Based on Heinemann et al (2009), we use a 560 ppm CO 2 late Paleocene atmospheric forcing to achieve a plausible background climate for the PETM. The applied atmospheric CO 2 concentrations and late Paleocene boundary conditions cause a new equilibrium climate state, which fits the proxy-record-based SST quite well (Lunt et al, 2012). However, the pre-PETM ocean biogeochemistry is not only affected by modifications in temperature and atmospheric conditions at the oceanatmosphere boundary (Archer et al, 2004), but also by alterations in the general ocean physical state.…”

“…The pole-to-equator temperature gradient was smaller, displayed in sea surface temperatures (SST) of more than 30 • in the tropics (Pearson et al, 2001) and up to 20 • at high latitudes (Sluijs et al, 2006;Lunt et al, 2012). Deep ocean water masses were up to 10 • warmer compared to modern values (Kennett and Stott, 1991;Zachos et al, 2008;Tripati and Elderfield, 2005).…”

“…The late Paleocene climate state in our simulation is characterized by a global annual mean temperature of 23.6 • C (at 2 m in height), using the atmospheric forcing described above (for a comparison, see Lunt et al, 2012). Maximum annual average temperatures are reached along the Equator over Africa, Asia and South America, with temperatures close to 40 • C (Fig.…”

“…Studies of the PETM background climate show a wide range of intermodel variability, using prescribed atmospheric CO 2 concentrations ranging from 2× to 16× pre-industrial CO 2 (Lunt et al, 2012). In previous studies, the late Paleocene ocean biogeochemistry has been addressed exclusively with earth system models of intermediate complexity (EMIC) or box models (e.g., Panchuk et al, 2008;Zeebe et al, 2009;Ridgwell and Schmidt, 2010;Winguth et al, 2012).…”