Large dry-humid fluctuations in Asia during the Late Cretaceous due to orbital forcing: A modeling study

Cretaceous stratigraphy in the mid- to low-latitude Asia indicates that the climate was overall dry with large variations in aridity in timescales of 1-100 ka, different from the present-day monsoonal humid climate. To identify the possible mechanism for such variation, we modeled the Late Cretaceous climate using a fully coupled atmosphere-ocean general circulation model (AOGCM), Community Earth System Model version 1.2 (CESM1.2), under different CO2 concentrations and orbital configurations. The simulation results show that the mid- to low-latitude Asia was generally dry during that time, consistent with reconstructions. The arid area decreases and the semi-arid area increases with the increase of CO2 concentration, but the change in total area is small even when CO2 is increased by 8 folds. In contrast, the area of dryland can increase by similar to 67% or 500% depending on how dryness is defined, when the orbital configuration changes from one in which the northern hemispheric summer receives the most solar insolation to one in which it receives the least solar insolation. Therefore, the mid- to low-latitude Asia was likely going through dramatic dry-humid cycles at orbital timescale during the Late Cretaceous, similar to the Saharan region during the Late Miocene-Pleistocene.