Contrasting responses of summer precipitation to orbital forcing in Japan and China over the past 450 kyr

Understanding orbital-scale changes in East Asian summer monsoon (EASM) precipitation is a fundamental issue in paleoclimate research as it helps assess the response of the East Asian monsoon to different climatic forcings, such as insolation, ice volume, and greenhouse gases. However, due to inconsistencies between different proxies, the fundamental driving force for EASM variability remains controversial. The present study simulated the global climate under given insolation changes over the past 450 kyr using a climate model, version 2.3 of the Meteorological Research Institute's Coupled General Circulation Model (MRI-CGCM2.3). Changes in summer insolation over East Asia resulted in distinct climatic responses in China and Japan: an increase in summer insolation led to increased summer precipitation in China and decreased summer precipitation in Japan. Composite analyses of simulated climate under strong boreal-summer insolation suggest that warming of the Indian Ocean occurs under intense insolation, resulting in the intensification of the North Pacific subtropical high (sub-high). The northern shift in the monsoon front, associated with the intensified sub-high, leads to an increase in rainfall in the coastal area of China. In contrast, the intensity of the EASM around Japan is affected by the strength of the North Pacific High. Under strong insolation, the increase in thermal contrast between the North American continent and the North Pacific Ocean intensifies the North Pacific High, decreasing summer precipitation around Japan. Thus, strong regional differences in the effects of solar-insolation variability on summer precipitation in East Asia exist due to interactions with different ocean basins.