Transient Simulation and Dynamics of the 4.2 ka BP Event in the East Asian Monsoon Region

The 4.2 ka BP event (approximately 4200~3900 years before present) was a widespread episode of global drought and cooling during the middle-late Holocene, which has attracted significant attention due to the temporal coincidence with the evolution and sociocultural transformations of early human civilizations in the Indus Valley, the Yellow River basin, and Mesopotamia, among other regions. However, the global consistency and the climate change mechanisms of the 4.2 ka BP event remain debated, particularly regarding the spatial hydroclimatic patterns and principal driving factors within the East Asian monsoon region. Here we conducted a set of transient simulations for the middle-late Holocene (6~3 ka BP) using the Community Earth System Model version 1.0.4 (CESM1.0.4), incorporating orbital parameters (precession, eccentricity, and obliquity), greenhouse gases (CO₂, CH₄, and N₂O), and solar irradiance as external forcings. The Weather Research and Forecasting (WRF) model was employed for dynamical downscaling to produce a high-resolution (~40 km) dataset over the East Asian monsoon domain. These data are synthesised to reveal the spatial modes of temperature and precipitation in the East Asian monsoon region and their connection to global ocean-atmosphere system variations during the 4.2 ka BP event.

Preliminary results indicate that: (1) multicentennial variations in the Atlantic Meridional Overturning Circulation (AMOC) likely dominated winter-half year (October to March) and summer (July to September) precipitation anomalies in North China during 4.2~3.9 ka BP; and (2) multicentennial variations in AMOC and the Pacific Decadal Oscillation (PDO) may be linked to shifts between “north-dry/south-wet” and “north-wet/south-dry” rainfall patterns over East Asia. The underlying mechanisms, however, require further investigation. This study aims to advance the understanding of abrupt climate change events and to provide a scientific basis for examining the relationship between climatic variability and the development of human civilizations.