East Asian summer monsoon precipitation variations in China over the last 9500 years: A comparison of pollen-based reconstructions and model simulations

To better understand the long-term changes of the East Asian summer monsoon precipitation (P-jja), quantitative reconstructions and model simulations are needed. Here, we develop continental-scale pollen-based transfer functions for P-jja with weighted averaging-partial least squares (WA-PLS) regression and a Bayesian multinomial regression method. We apply these transfer functions to a set of fossil pollen data from monsoonal China for quantitatively reconstructing the P-jja changes over the last 9500 years. We compare the reconstructions with P-jja simulations from a coupled atmosphere-ocean-sea ice general circulation model (the Kiel Climate Model, KCM). The results of cross-validation tests for the transfer functions show that both the WAPLS model (r2 = 0.83, root mean square error of prediction (RMSEP) = 112.11 mm) and the Bayesian model (r2 = 0.86, RMSEP = 107.67 mm) exhibit good predictive performance. We stack all P-jja reconstructions from northern China to a summary curve. The stacked record reveals that P-jja increased since 9500 cal. yr BP, attained its highest level during the Holocene summer monsoon maximum (HSMM) at similar to 7000-4000 cal. yr BP and declined to present. The KCM output and the reconstructions differ in the early-Holocene (similar to 9500-7000 cal. yr BP) where the model suggests higher P-jja than the reconstructions. Moreover, during the HSMM, the amplitude of the P-jja changes (similar to 20-60 mm above present) in simulations is lower than the reconstructed changes (similar to 70-110 mm above present). The rising (declining) P-jja patterns in reconstructions before (after) the HSMM are more pronounced and fluctuating than in simulations. Other palaeohydrological data such as lake-level reconstructions indicate substantial monsoon precipitation changes throughout the Holocene. Our results therefore show that the KCM underestimates the overall amplitude of the Holocene monsoon precipitation changes.