Energetics of the 6000-yr BP Atmospheric Circulation in Boreal Summer, from Large-Scale to Monsoon Areas: A Study with Two Versions of the LMD AGCM

Abstract

The mechanisms of northern summer monsoon changes at 6 kyr BP and their dependency to model parameterizations is investigated using two versions of the atmospheric general circulation model developed by the Laboratoire de Météorologie Dynamique, CNRS, Paris (LMD), within the Paleoclimate Modeling Intercomparison Project (PMIP). These two versions differ in their horizontal resolution and in their treatment of some surface processes: one model has a low horizontal resolution and rather simple surface treatments; the other model has a better resolution and uses more complex surface parameterizations. The authors focus on the energy budgets, which are useful tools to understand the impact of the model parameterizations. As a response to mid-Holocene insolation change, the June?September monsoon is enhanced over northern Africa, northern India, and the western Pacific, both in terms of precipitation and low-level convergence changes;convection decreases over Central America. These changes, which are simulated with different amplitudes and locations by the two LMD versions, are associated with large-scale changes in the Hadley?Walker circulation?export of energy released by the precipitation in the monsoon regions to the northern extratropics and to the Southern Hemisphere. Due to its simple treatment of the evaporation, the first model simulates large changes in the hydrological cycle, but negative feedbacks compensate for the gain of energy associated with the latent heat release: because of a strong evaporation increase, the sensible heat flux decreases, and the high-albedo clouds formed in low levels have a negative impact on atmospheric heat gain. With a more complex surface treatment, the other model simulates less evaporation changes and thus a weaker hydrological response, but similar energy source changes, because of increased sensible heat flux and the formation of more middle-level clouds that have a positive effect on the radiative budget. The differences between both versions are particularly large for the African monsoon, which is mainly fed by local recycling. It is shown that the mechanism of past monsoon intensification is sensitive to model surface parameterization, mainly because of the strong coupling between the surface evaporation and the cloud formation and optical properties. This methodology will help to compare the different models that participate to PMIP.