Experimental design for DeepMIP-Eocene Phase 2 - impact of new paleogeography and vegetation

Warm, high-CO₂ climates of Earth's past provide an opportunity to both evaluate climate models under extreme forcing and to explore mechanisms that lead to such warmth.  One such time period is the early Eocene, when global mean surface temperatures were 10-16 ^oC higher than preindustrial, and  CO₂ concentrations were about ~1500 ppmv.

In this presentation we present the experimental design for Phase 2 of the Eocene component of the Deep-time Model Intercomparison project (DeepMIP-Eocene-p2).  The aim is to provide a framework within  which modelling groups can carry out a common set of simulations, thereby facilitating exploration of inter-model dependencies.  Focus is on the early Eocene Climatic Optimum (EECO, ~53.3-49.1 million years ago).  Relative to Phase 1 of DeepMIP, we provide a new paleogeography (topography, bathymetry) derived from four independent reconstructions, a new vegetation derived from vegetation model simulations that have been evaluated with paleobotanical data, and a new CO₂ specification derived from the boron isotope proxy.  The core set of simulations consists of a preindustrial control, an abrupt increase to 4x preindustrial CO₂ concentrations under modern conditions, a standard control EECO simulation at 5x preindustrial CO₂ concentrations, and an EECO simulation with preindustrial CO₂ concentrations.  In addition to these core simulations, we suggest a suite of optional sensitivity studies, which allow various sensitivities to be explored, such as to topography/bathymetry, greenhouse gases, land-surface parameters, astronomical and solar forcings, and internal model parameters.  Overall, we hope that the updated boundary conditions and guidance on initialisation in Phase 2 will allow more robust model-data comparisons, more accurate insights into mechanisms influencing early Eocene climate, and increased relevance for informing future climate change projection. 

In addition to the exprimental design, we present intitial simulations with the HadCM3 model with the new boundary conditions, and compare with the results from Phase 1, illustrating the sensitivity to the new paleogeography and vegetation.