The simulated response of the climate system to changes in soil moisture parameterization under paleoclimatic boundary conditions at 6000 years before present

We present the results of a sensitivity study involving modifications to th e simple land surface scheme implemented in the second-generation atmospher ic general circulation model (GCMII) of the Canadian Climate Centre for Mod elling and Analysis (CCCma), under paleoclimatic boundary conditions charac teristic of 6000 calendar years before present (6 ka BP). The land surface parameterization is modified in two primary respects. Firstly, we modify th e space dependant bucket depth scheme in the original model to one in which this depth is taken to be constant. Secondly, we modify the evapotranspira tion parameterization from the space dependant form employed in the control model to a more conventional space independent scheme. In all, 4 experimen ts have been performed to enable us to resolve both the modern control and the 6 ka BP response to the land surface modifications. A subset of these s imulations is also compared with results obtained using other models in the context of the Paleoclimate Model Intercomparison Project (PMIP) to invest igate the mid-latitude behaviour of these models to reveal the extent to wh ich model response to a change in radiative forcing may be significantly in fluenced by changes in the land surface parameterization. These comparisons reveal that the original Canadian GCMII model is an extreme outlier among the members of the set of all models in that its Northern Hemisphere mid-la titude surface continental response to the 6 ka BP insolation anomaly is si gnificantly cold biased in the summer season. We investigate the extent to which this anomalous behaviour may be explained as a consequence of modific ations to the land surface parameterizations employed in GCMII. Our results reveal a strong sensitivity in the modern control model to changes in buck et depth, but not to the modification of the evapotranspiration scheme. Mor e interesting, however, is the fact that the model climate sensitivity at 6 ka BP is influenced both by changes in bucket depth and by changes in the evapotranspiration scheme. A detailed investigation of the surface energy b alance, cloud cover, surface albedo, and snow cover reveals the role of eac h of the components that contribute to the 6 ka BP surface temperature resp onse. Comparison of the predictions of the CCCma model with proxy climate i ndicators of lake levels and surface temperature over Canada, furthermore, demonstrate the extreme sensitivity of climate predictions for this geograp hical region to changes in the manner in which land surface processes are r epresented.