Understanding global monsoon precipitation changes during the 8.2 ka event and the current warm period

Abstract

Global monsoon (GM) precipitation has profound impacts on water resources, food security, and the livelihood of about two-thirds of the world's population. Understanding the contrasting changes of GM precipitation (GMP) during the 8.2 ka cold event and the present-day warm event helps better comprehend the common origin of the GMP change and its future projection. We analyzed a suite of transient climate evolutions (TraCE-21 ka simulation). We show that the simulated monsoon rainfall changes during the 8.2 ka abrupt cooling event are qualitatively consistent with the paleoclimate archive collected worldwide. The simulated Northern Hemisphere monsoon (NHM) precipitation significantly decreased by 12.4% per one degree of global mean temperature change (12.4%/°C) while the Southern Hemisphere monsoon (SHM) precipitation increased by 4.2%/°C. The cooling-induced suppressed upward motion plays a dominant role in reducing NHM precipitation, and the reduced moisture adds to the circulation effect, whereas the enhanced SHM precipitation is mainly due to the moisture increase. In the 8.2 ka event, the circulation response reinforces the moisture-induced drought over the NHM region, resulting in an excessive precipitation sensitivity to temperature change (12.4%/°C). In contrast, during the present warm period, the greenhouse warming-induced moisture and circulation effects cancel each other, resulting in a moderate sensitivity (1.8%/°C). Although meltwater and greenhouse gas forcings induce contrasting global temperature change patterns, the GMP changes are governed by common root causes: forced NH-SH thermal contrast, land-ocean thermal contrast, and the tropical SST gradients. The moisture change plays a crucial role in altering precipitation amount but not spatial distribution. We suggest that the external forcing-induced warming (cooling) pattern drives the circulation changes (dynamic effects), determining the spatial structure of the monsoon rainfall change in the past, present, and future.