Precipitation variability in the north fringe of East Asian Summer Monsoon during the past millennium and its possible driving factors

摘要

Summer precipitation in the north fringe of East Asian Summer Monsoon (NFEASM) accounts for the majority of annual regional precipitation and plays an important role in regional climatology and agriculture development. Here we analyze variations in summer precipitation in the NFEASM over the past millennium using several simulations with Earth System Models and compare with two dendroclimatological hydroclimate reconstructions that partly cover the past millennium. Both reconstructed records show good agreement on past hydroclimate variations but do not show overly anomalous hydroclimate periods, except for clear drying trends in the second half of the twentieth century. The reconstructed decadal hydroclimate variations are not correlated with any of the simulations, and the simulations are not correlated among themselves either, which strongly suggests that the decadal variability is not linked to the external climate forcing. In addition, the superposed epoch analysis also does not identify a response of simulated precipitation to volcanic eruptions. Therefore, precipitation variability in this region over the past millennium seems to have been driven by internal climate processes. In the simulations, regional summer precipitation is positively and significantly correlated with sea-level-pressure (SLP) over the North Pacific and is negatively and significantly correlated with SLP in southwestern China at both interannual and decadal time scales. This agrees with the teleconnection patterns identified from meteorological reanalysis datasets, indicating that this dipole correlation pattern is robust. Therefore, the SLP difference between these two areas is used as an index to identify the atmosphere circulation pattern favorable to summer precipitation in the NFEASM. This SLP index shows spatial variable correlations with sea-surface-temperature (SST) in the North Pacific: there is a significant positive correlation in the northern North Pacific, while a significant negative correlation in the southern North Pacific. Although the underlying mechanisms for these correlations likely differ between two subareas, it appears that the atmosphere drives SST anomalies in the both subareas.