Mechanisms for Spatially Inhomogeneous Changes in East Asian Summer Monsoon Precipitation during the Mid-Holocene

Abstract

AbstractThe East Asian summer monsoon (EASM) intensified during the early to mid-Holocene relative to the present primarily due to orbital forcing. However, on the regional scale, changes in the monsoonal precipitation exhibit considerable spatial disparity, and the underlying mechanisms remain unresolved. In this study, the dynamic processes responsible for the difference of the EASM precipitation between the mid-Holocene and preindustrial period are systematically examined using the CMIP5 multimodel simulations. The moisture budget diagnostic identifies vertical motion as the key factor determining the cross-like precipitation pattern in East Asia. Relative to the preindustrial period, the mid-Holocene anomalous ascending motion corresponds well with the excessive precipitation over northern and southern China, and vice versa for west-central China, the Korean peninsula, Japan, and its marginal seas. In the framework of the moist static energy budget, the increased insolation and the attendant intensification of land–sea thermal contrast give rise to anomalous ascending motions, while descending motions are fundamentally forced by the decreased latitudinal insolation gradient. In particular, thermodynamic changes, namely, the reduced pole–equator temperature and humidity gradients, account for the downward motions over the northwestern Pacific. Dynamic changes, namely, the weakened westerlies, play a leading role in suppressing updrafts in west-central China. This study highlights that the orbital-scale monsoonal precipitation changes are not solely determined by local radiative forcing as repeatedly emphasized before. The latitudinal uneven distribution of insolation is crucial to explain the spatial inhomogeneity in the EASM precipitation changes during the Holocene.