The effects of orbital forcing on the East Asian Summer Monsoon for the past 450 kyr
Abstract. Understanding orbital-scale changes in East Asian summer monsoon (EASM) precipitation is a fundamental issue in paleoclimate research for assessing the response of the East Asian monsoon to different climate forcings, such as insolation, ice volume, and greenhouse gases. However, owing to the inconsistencies between different proxies, the fundamental driving force for EASM variability remains controversial. In the present study, the global climate under the given insolation changes over the past 450 kyr was calculated using a climate model, Meteorological Research Institute Coupled General Circulation Model version 2.3 (MRI–CGCM2.3). The calculated change in summer precipitation is dominated by a 20-kyr precession cycle over China, highly consistent with cave δ18O records in southeast China. The proxy data from northern China (Chinese Loess Plateau) and Japan (Lake Biwa) cannot be fully explained by the calculation results, implying the importance of other forcing such as ice-sheet volume. A strong positive correlation was observed between insolation and precipitation over the coastal area of China and a negative correlation between insolation and precipitation around Japan. The results imply that the EASM is affected by the insolation intensity; however, the effect can vary between regions. The positive correlation between boreal summer insolation and precipitation over China results from the atmosphere-ocean interaction over the Indian Ocean and the western Pacific. Under intense insolation, the northern shift of the monsoon front associated with the intensification of the North Pacific subtropical high causes an increase in rainfall in the coastal area of China. The intensification of the subtropical high is caused by the integration of local wind–evaporation–sea surface temperature (WES) feedback with the Kelvin wave response to the warm Indian Ocean (IPOC mode). In contrast, the EASM intensity around Japan was affected by the strength of the North Pacific High. Under strong insolation, the North Pacific High intensified, causing a decrease in summer precipitation around Japan.
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