Monsoon and Tropical Cyclones shape mid-latitude precipitation in East Asia during the early Eocene hothouse climate

Abstract The Asian mid-latitudes experienced a subtropical arid belt during most of the Eocene hothouse climate. This interpretation is supported by climate model simulations, but is in contrast with new proxy reconstructions, which indicate greening during the early Eocene. Here, we investigate the discrepancy between models and proxies by analysing the atmospheric circulation in the modelled climate. We assess the contribution of monsoonal circulation, extratropical and tropical cyclones to precipitation, and investigate the sensitivity of precipitation patterns to sea surface temperature (SST) changes. We find that the precipitation in our selected target region is mainly associated with monsoonal circulation and that fewer extratropical cyclones reach the target region from west than in the pre-industrial climate. Additionally, we find tropical cyclones in the model simulation, which reach the mid-latitude target region and contribute significantly to the summer precipitation. The co-existence of an extremely warm tropical ocean and the large-scale monsoonal circulation promotes tropical cyclones to reach the continental mid-latitudes, presenting an additional pathway for moisture transport into the mid-latitudes. Our sensitivity experiments show that the precipitation in our mid-latitude continental target area is sensitive to the land-sea temperature contrast, and that lowering SST values enhance the monsoonal circulation which then transports more moisture inland. Our study thus highlights that the early Eocene climate simulation underestimates the monsoonal circulation and precipitation across the continental interior of East Asia, which is likely connected to a discrepancy in the land sea thermal contrast.