Climate of the Past Discussions www.clim-past-discuss.net 1814-9340 1814-9359 4 3 2008 10.5194/cpd-4-719-2008 http://www.clim-past-discuss.net/4/719/2008/ http://www.clim-past-discuss.net/4/719/2008/cpd-4-719-2008.html http://www.clim-past-discuss.net/4/719/2008/cpd-4-719-2008.pdf 719 740 2008-06-09 Influence of orbital forcing on the seasonality and regionality of the Asian Summer monsoon precipitation M. E. Hori mhori@hyarc.nagoya-u.ac.jp M. Abe T. Yasunari A. Kitoh Graduate School of Environmental Sciences, Nagoya University, Nagoya, Japan National Institute for Environmental Studies, Tsukuba, Japan Hydrosphere/Atmosphere Research Center, Nagoya University, Nagoya, Japan Meteorological Research Institute, Tsukuba, Japan The response of Asian monsoon precipitation to contrasting orbital parameters is simulated using the MRI-CGCM climate model. Results show that for the 125 kya B. P. experiment, a large continental heating due to obliquity forcing is apparent and accounts for the strengthened cross equatorial flow, stronger monsoon westerly over the Arabian Sea, and an enhanced precipitation over the Indian subcontinent. For the 115 kya B. P. experiment, while the monsoon westerly becomes weaker in the Arabian Sea, the overall strength of the monsoon westerly becomes stronger in the Bay of Bengal. This eastward extension of the monsoon westerly converges with the equatorial trade wind to give rise to an increased precipitation over the maritime continent and Indochina peninsula. Such increase in precipitation is accompanied with an earlier onset of the Asian monsoon, and an earlier warming of the tropical SST due to precessional forcing. It is concluded that while the obliquity forcing creates the baseline land-sea contrast which maintains the Asian monsoon westerly, when such forcing is comparably weaker, the Indian monsoon is diminished and the precessional forcing becomes more dominating to create a distinct earlier warming of the tropical SST which leads to an earlier onset of the maritime monsoon over the western Pacific. This study implies that even under weaker insolation forcing, the precessional signal may act to enhance certain regional precipitation and onset timing of the Asian monsoon. Berger A. and Loutre, M. F.: Insolation values for the climate of the last 10 million of years, Quaternary Sci. Rev., 104, 297–317, 1991. Clemens, S. C., Murray, D. W., and Prell, W. L.: Nonstationary phase of the plio-pleistocene Asian monsoon, Science, 274(5289), 943–948, 1996. Clemens, S., Prell, W., Murray, D., Schimmield, G., and Weedon, G.: Forcing Mechanisms of the Indian-Ocean Monsoon, Nature, 353(6346), 720–725, 1991. Clement, A. C., Seager, R., and Cane, M. A.: Orbital controls on the El Nino/Southern Oscillation and the tropical climate, Paleoceanography, 14(4), 441–456, 1999. Cubasch, U., Zorita, E., Kaspar, F., et al.: Simulation of the role of solar and orbital forcing on climate, Particle Acceleration, Space Plasma Physics, Solar Radiation and the Earth's Atmosphere and Climate, 37(8), 1629–1634, 2006. Lestari, R. K. and Iwasaki, T.: A GCM study on the roles of the seasonal marches of the SST and land-sea thermal contrast in the onset of the Asian summer monsoon, J. Meteorol. Soc. Jpn., 84(1), 69–83, 2006. Prell, W. L. and Kutzbach, J. E.: Sensitivity of the Indian Monsoon to Forcing Parameters and Implications for Its Evolution, Nature, 360, 647–652, 1992. Wang, B. and LinHo: Rainy season of the Asian-Pacific summer monsoon, J. Climate, 15(4), 386–398, 2002. Yukimoto, S., Noda, A., Kitoh, A., Sugi, M., Kitamura, Y., Hosaka, M., Shibata, K., Maeda, S., and Uchiyama, T.: The New Meteorological Research Institute Coupled GCM (MRI-CGCM2) – Model Climate and Variability, Pap. Meteorol. Geophys., 51(2), 47–88, 2001.