The Influence of Obliquity in the Early Holocene Asian Summer Monsoon
Chi-Hua Wu1, Shih-Yu Lee1, John C. H. Chiang2, and Huang-Hsiung Hsu1
1Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan
2Department of Geography and Berkeley Atmospheric Sciences Center, University of California, Berkeley, California
The early-Holocene climatic optimum is associated with perihelion precession and high obliquity, though most studies emphasize the former over the latter. Asian monsoon proxy records only decisively show the precessional impact. To explore the obliquity effect, four climate simulations are conducted by fixing orbital parameters of present-day (0K), early Holocene (11K), the lowest obliquity (31K), and 11K’s precession and eccentricity with 31K’s obliquity (11Kp31Ko). We show that high obliquity significantly augments the precessional impact by shifting the Asian monsoon farther north than present. By contrast, the present-day monsoon seasonality can still be identified in the simulations with low obliquity. We argue that the upper-tropospheric (South Asian) and lower-tropospheric (North Pacific) high-pressure systems are affected by the subtropical atmospheric heating changes responding to obliquity. As a consequence, associated with the changes in meridional gradients of geopotential height and temperature made by the heating, midlatitude transient eddies and monsoon-midlatitude interactions are modulated.
Wu, C.-H., S.-Y. Lee, J. C. H. Chiang, and H.-H. Hsu, 2016: The influence of obliquity in the early Holocene Asian summer monsoon, Geophys. Res. Lett., 43, doi:10.1002/2016GL068481. (First online: 20 April, 2016)
Seasonal evolution of total column heating of the atmosphere (Wm-2) in the northern South Asian region (a, 60°E–110°E, 20°N–35°N, NSA) and southern South Asian region (b, 60°E–110°E, 5°N–20°N, SSA); black dot lines for 0K, red solid lines for 11K, purple solid line for 11Kp31Ko, and blue dashed lines for 31K simulations. (c) Schematic diagram of the heating changes (relative to 0K) in mid-June.