Session: Warm Climates (Mid-Holocene, Last interglacial, Deep-time, Pliocene)
Author: Patrick J. Bartlein / email@example.com / University of Oregon
Co-author: Sandy P. Harrison, University of Reading;
Kenji Izumi, IPSL;
Climate-model simulations uniformly show drier and warmer summers in central Eurasia during the mid-Holocene, a regional signal which is not consistent with palaeoenvironmental observations. The simulated climate results from a reduction in the zonal temperature index, which weakens westerly flow and reduces moisture flux and precipitation in the mid-continent. As a result, evaporation and latent heating are reduced and sensible heating increased, resulting in substantial surface-driven atmospheric warming. Thus, the discrepancy with the palaeoenvironmental evidence arises initially from a problem in the simulated circulation and is exacerbated by land-surface feedback. Analyses show that this region is also drier and warmer than indicated by observations in the pre-industrial control simulations, and this bias arises in the same way: zonal flow and hence moisture flux into the mid-continent is too weak and land-surface feedback results in dry conditions and surface-driven warming. These analyses pinpoint the processes underlying discrepancies between simulated and observed central Eurasian climates and suggest the need to improve aspects of the model that affect the strength of westerly circulation.