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Regulation of the monsoon climate by two different orbital rhythms and forcing mechanisms

¹ Department of Geography, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK
² Natural History Museum and Institute of Chiba, Chiba 260-8682, Japan
³ College of Education, Naruto University of Education, Naruto 772-8502, Japan
⁴ Department of Applied Science, Faculty of Science, Okayama University of Science, Okayama 700-0005, Japan
⁵ Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan
⁶ Faculty of Policy Informatics, Chiba University of Commerce, Chiba 272-8512, Japan
⁷ Institute for Geological Sciences, Freie Universität Berlin, Malteserstrasse 74-100, Haus D, 12249 Berlin, Germany
⁸ Botanical Garden, Research Institute of Natural Sciences, Okayama University of Science, Okayama 700-0005, Japan
⁹ Institute for Geothermal Sciences, Faculty of Science, Kyoto University, Beppu 874-0903, Japan

The East Asian monsoon is responsible for transferring huge amounts of heat and moisture between the land and the adjacent ocean. Significant changes in its capacity to do this will have direct impacts on regional climatic gradients and global atmospheric circulation. Determining the mechanisms that force long-term variation in monsoon behavior is therefore important for understanding global climate change. Competing theories vary in the degree of importance attached to glacial forcing, other orbital rhythms, and internal feedback mechanisms as primary drivers of change. There is, however, no convincing explanation as to why different proxy records from closely neighboring regions are tuned to different orbital rhythms. Here we present quantitative climatic reconstructions for the past 450 k.y. based on a long pollen record from Lake Biwa in Japan. The data suggest that continental and oceanic air mass temperatures respond predominantly to the 100 k.y. orbital rhythm, whereas the land-ocean temperature gradient and monsoon vigor oscillate mainly at the 23 k.y. insolation cycle. We suggest that the mechanisms for this behavior lie in the differential response of land and ocean to solar forcing, and conclude that the 100 k.y. signal dominates monsoon intensity only when the amplitude of solar forcing falls below a threshold level.

Key Words: monsoon • Milankovitch theory • land-ocean temperature gradient • climate change • glacial interglacial cycles • 23 k.y. El Niño—Southern Oscillation • ENSO