Asynchronous Holocene optimum of the East Asian monsoon

The spatial and temporal distribution of summer monsoon precipitation (or e ffective moisture) during the Holocene has been reconstructed on the basis of geological data, including lake levels, pollen profiles, and loess/paleo sol records. In addition, the summer (July) precipitation increment, effect ive precipitation, and monsoon strength index have been obtained from numer ical modeling experiments. Both geological data and numerical modeling indi cate that the Holocene optimum, as defined by peak East Asian summer monsoo n precipitation, was asynchronous in central and eastern China, reaching a maximum at different times in different regions, e.g., ca. 10,000-8000 yr a go in northeastern China, 10,000-7000 yr ago in north-central and northern east-central China, ca. 7000-5000 yr ago in the middle and lower reaches of the Yangtze River, and ca. 3000 yr ago in southern China. In southwestern China the maximum appeared ca. 11,000 yr ago, but probably was related to t he maximum landward extension of the Indian summer monsoon. The regional sh ift in the maximum precipitation belt from northwest to southeast over the past 10,000 yr is interpreted as a response to changing seasonality related to orbital forcing of the climate. The southeastward shift of the East Asi an summer monsoon maximum is consistent with the progressive weakening of t he summer monsoon as the summer solar radiation anomaly decreased progressi vely through the Holocene and the East Asian monsoon index declined, while the early maximum in southwestern China matches the maximum of the Indian m onsoon index. (C) 2000 Elsevier Science Ltd. All rights reserved.