Response of Decadal Droughts on the Mongolian Plateau to External Forcings and Internal Variability over the Last Millennium

Since the mid-20th century, the Mongolian Plateau (MP) has experienced decadal droughts coupled with extreme heatwaves, severely affecting regional ecology and social development. However, the mechanisms behind these decadal-scale compound heatwave–drought events remain debated. Here, using reconstructions and simulations from the Community Earth System Model Last Millennium Ensemble, we demonstrate that, over the last millennium, decadal droughts on the MP occurred under both warm and cold conditions, differing from recent compound heatwave–drought events. We found that by examining temperature changes during these drought periods, the distinct influences of external forcings and internal variability can be simply and effectively distinguished. Specifically, cold–dry events were primarily driven by volcanic eruptions that weakened the East Asian summer monsoon and midlatitude westerlies, reducing moisture transport to the MP. In contrast, warm–dry events were predominantly induced by internal variability, notably the negative phase of the Atlantic Multidecadal Oscillation and the expansion of the Barents Sea ice extent. The recent extreme compound event was probably influenced by the combined effects of anthropogenic forcings and internal variability. These findings deepen our understanding of how external forcings and internal variability affect decadal drought events on the MP and highlight that recent compound events are unprecedented in the context of the last millennium.