Northwestward migration of the East Asian monsoonal limit during the Medieval Warm Period

The East Asia summer monsoon variability and changes in monsoonal rainfall are critical for hydrology and ecology conditions in water-stressed regions of northern China. Due to complex interactions of monsoonal and westerlies circulation, hydroclimatic conditions at the boundary between two circulation systems, commonly defined as the 300 mm annual rainfall line, remain poorly understood. Here we report alkenone records (U^K'₃₇, %C_37:4, and RIK₃₇) from Lake Eastern Juyanze in northern China to assess temperature and hydrological changes in marginal monsoon region over the last millennium. Our records show wet conditions during the Medieval Warm Period, with the presence of alkenone C_37:3 isomer at the interval of ~ 700-1,050 AD and thus evident freshening of lake water, and dry conditions during the Little Ice Age. The paired records follow the warm-wet and cold-dry association in monsoonal regions over the last millennium, opposite to the warm-dry and cold-wet association in westerlies region, although Lake Eastern Juyanze is located to the northwest of current monsoonal limit. Our results, together with other hydrological records from marginal monsoon regions, collectively indicate northwestward migration of East Asia monsoonal limit during the Medieval Warm Period associated with relatively high solar irradiance and enhanced summer monsoon circulation. Findings in this study highlight the complexity of hydroclimatic changes in marginal monsoonal regions, and further investigations focusing on the Holocene epoch are highly recommended.