How the simulated change in monsoon at 6 ka BP is related to the simulation of the modern climate: results from the Paleoclimate Modeling Intercomparison Project

Using the results of the Paleoclimate Modeling Intercomparison Project (PMIP) for the mid-Holocene (6000 years ago, 6 ka BP), we investigate the impact of the control (present-day) simulation of the models on the climate change simulated for 6 ka BP. The focus is mainly on the Northern Hemisphere summer monsoon, which is enhanced at 6 ka BP in response to increased incoming solar radiation at the top-of-the-atmosphere. We first discuss the relationship between the spatial patterns of the simulated changes at 6 ka BP and those of the control simulation for surface temperature, precipitation and sea level pressure.We show that models with the smallest land-sea contrast for present day tend to have the smallest increase in land-sea contrast at 6 ka BP. However, the differences between the responses of the models to the 6 ka insolation forcing cannot simply be inferred from the control simulations. We then show that the magnitude of the change in monsoon precipitation depends on several factors: the continental warming, the model efficiency at producing precipitation (which is the signature of model parametrisations of convection and rainfall), and the mean temperature of the control climate. Our results show that the correlation found across model results between precipitation in northern India and temperature over land (Joussaume et al. 1999) is not simply a linear relationship between those two fields, but results from small groups of models having similar behaviour, but not necessarily for the same reasons. Our analysis also shows that, to study the response of monsoon to a climatic perturbation, the seasonal evolution of rainfall over monsoon areas and the mean temperature need to be properly represented in the control simulation because they affect the monsoon response.