Asian monsoon change in the Mid-Holocene: Influence of ocean coupling for GCM simulations

In order to clarify the role of ocean for changes on Asian monsoon in response to the insolation forcing in 6000 years before present (mid-Holocene, 6ka), simulations using (A) an CCSR/NIES/FRCGC ocean-atmosphere coupled GCM, (B) an AGCM extracted from the coupled GCM coupled with a mixed ocean model, and (C) the AGCM with prescribed SST are compared. Paleoclimate evidences of lake status records show conditions wetter than present in central Asia, from India to northern China and Mongolia and more arid condition along the coast of China (Yu and Harrison 1996). However precipitation in Asia in all three simulations becomes more vigorous with 6ka insolation forcing, ocean feedbacks in (A) and (B) simulations suppress the enhancement of precipitation. These suppressions are not consistent to cooler SST along the coast of India in (A) and (B) simulations suggesting more vigorous monsoon. This suggests that more than simple understanding of monsoon mechanisms plays a role in the change in Asian monsoon. Effect of developed anticyclone by cooler SST in Indian Ocean on relationship between African and Asian monsoons may have to be considered. The wetter conditions from India to northern China and the more arid condition along the coast of China are represented by the three model configurations. Precipitation reduction in the coastal China is expressed better by (A) and (B) than by (C). This may be resulting from weaker development of the Pacific summer anticyclone with ocean feedbacks, attributed to the warmer SST in the Pacific. It is found that the introduction of ocean dynamics strengthens the tendency of changes of Asian monsoon in 6ka but it is less important than the introduction of heat exchange on the ocean surface.