Early Eocene Asian climate dominated by desert and steppe with limited monsoons

Paleoenvironmental reconstructions illustrated that Asia was dominated by a zonal climate pattern in the Eocene epoch, with an almost latitudinal arid/semiarid band extending from the eastern coast of China to Central Asia. However, the mechanism behind the formation of the broad zonal arid/semiarid region and the possible occurrence of a monsoonal system remains unclear for that period. Here, using a reconstructed land-sea distribution and topography of 50 Ma, we employed a global climate model to simulate the Early Eocene climate in China. A set of experiments with consideration to different topography, orbital parameters, vegetation, sea surface temperature and atmospheric CO2 concentration demonstrated that a large extent of arid/semi-arid region with annual precipitation less than 800 mm was distributed latitudinally between 20°N and 40°N paleolatitude in China. This arid region was controlled by desert (BWh) and steppe (BSh) climates (according to the Köppen climate classification systems). On the north of the zonal desert/steppe region, a Mediterranean-like climate occurred in Central Asia. These simulations agree well with the geological reconstructions of a zonal desert/steppe region and indicate that this pattern was primarily caused by subtropical high pressures associated with strong descending air. However, it is likely that the aridity in the zonal desert/steppe was reduced occasionally by intensification of monsoon circulations over East Asia. The change in orbital parameters, causing enhanced insolation in summer, was the most likely factor that could enlarge the land-sea thermal contrast and further intensify the monsoon circulations in the Eocene epoch, though its effect was reversible and impermanent. In addition, simulations show that East Asian monsoon did not play a leading role in Chinese Paleogene climate although it may have affected Eastern China occasionally.

► Asia was dominated by a zonal desert (BWh) and steppe (BSh) region in the Eocene.
► The zonal arid region was cause by subtropical high pressure in the Eocene.
► Mediterranean-like climate was an important climate type in Central Asia in the Eocene.
► It was likely that monsoon appeared occasionally over East Asia in the Eocene.