Simulation of the East Asian Summer Monsoon during the Last Millennium with the MPI Earth System Model

The decadal-centennial variations of East Asian summer monsoon (EASM) and the associated rainfall change during the past millennium are simulated using the earth system model developed at the Max Planck Institute for Meteorology. The model was driven by up-to-date reconstructions of external forcing including the recent low-amplitude estimates of solar variations. Analysis of the simulations indicates that the EASM is generally strong during the Medieval Warm Period (MWP; A.D. 1000-1100) and weak during the Little Ice Age (LIA; A.D. 1600-1700). The monsoon rainband exhibits a meridional tripolar pattern during both epochs. Excessive (deficient) precipitation is found over northern China (35 degrees-42 degrees N, 100 degrees-120 degrees E) but deficient (excessive) precipitation is seen along the Yangtze River valley (27 degrees-34 degrees N, 100 degrees-120 degrees E) during the MWP (LIA). Both similarities and disparities of the rainfall pattern between the model results herein and the proxy data have been compared, and reconstructions from Chinese historical documents and some geological evidence support the results. The changes of the EASM circulation including the subtropical westerly jet stream in the upper troposphere and the western Pacific subtropical high (WPSH) in the middle and lower troposphere are consistent with the meridional shift of the monsoon rain belt during both epochs. The meridional monsoon circulation changes are accompanied with anomalous southerly (northerly) winds between 208 and 50 degrees N during the MWP (LIA). The land-sea thermal contrast change caused by the effective radiative forcing leads to the MWP and LIA monsoon changes. The "warmer land-colder ocean" anomaly pattern during the MWP favors a stronger monsoon, while the "colder land-warmer ocean" anomaly pattern during the LIA favors a weaker monsoon.