Do climate simulations support the existence of East Asian monsoon climate in the Late Eocene?

Early synthesis of geologic evidence demonstrated that a zonal climate pattern once dominated China in the Paleogene. The zonal climate pattern is very different from the non-zonal modern monsoon climate pattern. It has been hypothesized that the transition from a zonal to non-zonal pattern is related to the initiation of the East Asian monsoon. The earliest timing of East Asian monsoon initiation is suggested to be the Late Eocene, although this is still the subject of hot debate. Here, we use the low-resolution Norwegian Earth System Model (NorESM-L) and the high-resolution Community Atmosphere Model version 4 (CAM4) to simulate the climate in China for the Early and Late Eocene and further evaluate the climate effect of topography and sea-surface temperature (SST) on East Asian regional climate. Our simulations supported a zonal/zonal-like arid desert/steppe climate band appearance in China in the Early Eocene, but the zonal band very likely disappeared in the Late Eocene due to increased precipitation over East China. The increased precipitation is caused by intensified summer southerlies, associated with the westward extension of the western Pacific subtropical high (WPSH) in the Late Eocene. However, the disappearance of a zonal climate band does not sufficiently indicate formation of an East Asian monsoon climate in the Late Eocene: simulated wind and precipitation seasonality is still much weaker in the Late Eocene and should be distinguished from the modern monsoonal seasonality.