Global terrestrial monsoon area variations since Last Glacial Maximum based on TraCE21ka and PMIP4-CMIP6 simulations

Abstract

Global monsoon over land (GM_L) have significant variations in their dominated area (GMA_L). The quantitative assessment of changes in GMA_L is essential for understanding spatial environment impacts and the underlying driving mechanism of GM_L evolution. Simulation results of a suite of transient climate evolution simulation over the last 21,000 years (TraCE21ka) and five-model ensemble outputs of the Paleoclimate Modelling Intercomparison Project in their latest phase (fourth, PMIP4-CMIP6) are analyzed to study the changes in global, hemispheric and six sub-monsoon areas over the past 21 ka over land. Both datasets can reproduce large-scale characteristics of GMA_L variability, although there are differences in the magnitudes of their changes. Compared to 0 ka, TraCE21ka simulations reveal that GMA_L reduced by about 4.89 × 106 km2 (decreased by 10.89%) during the Last Glacial Maximum (LGM, 21–18 ka), and reached a maximum of 46.61 × 106 km2 (increased by 3.83%) during the middle Holocene (MH, 9–6 ka). In comparison, the results of PMIP4-CMIP6 yield a smaller change of GMA_L during the LGM (decreased by 3.72%) and a larger magnitude of GMA_L during the MH (increased by 4.73%). The analysis of GMA_L over the past 21 ka reveals the varying influences of external forcing, with insolation playing a dominant role. Rising atmospheric greenhouse gas concentrations (GHG) and ice sheet melting have induced a stepwise expansion in GMA_L during the last deglaciation, superimposed with several abrupt millennial-scale weakened monsoon events under meltwater fluxes (MWF) forcing. After that, insolation caused a gradually declining trend in GMA_L during the Holocene. The position of the Intertropical Convergence Zone (ITCZ) under orbital forcing, leading to an anti-phase reaction between Northern Hemisphere monsoon areas (NHMA_L) and Southern Hemisphere monsoon areas (SHMA_L), particularly after 9 ka. Among the six sub-monsoons, they show diverse variations and regional characteristics under different forcings. Although regional monsoons like Indonesia-Australia monsoon (IAM) exhibit nonlinear behavior due to internal variabilities, the overall response of global monsoons to external forcing is predominantly linear. In addition, changes in the hydrological processes can indicate the formation and expansion of monsoon domains. Moisture budget shows that thermodynamic effect has always dominated monsoon hydrological cycles over the past 21 ka. The differences in regional monsoon hydrological processes arise from the dynamic effect. Moreover, δ18O records have been used to test these numerical simulation results. Which, together with diverse thermodynamic and dynamic component configurations of terrestrial paleo-monsoon variations, would help to better understand the underlying mechanism of global monsoon and its influences in the future.