Dominant control of the South Asian monsoon by orographic insulation versus plateau heating


The Tibetan plateau, like any landmass, emits energy into the atmosphere in the form of dry heat and water vapour, but its mean surface elevation is more than 5 km above sea level. This elevation is widely held to cause the plateau to serve as a heat source that drives the South Asian summer monsoon, potentially coupling uplift of the plateau to climate changes on geologic timescales1,2,3,4,5. Observations of the present climate, however, do not clearly establish the Tibetan plateau as the dominant thermal forcing in the region: peak upper-tropospheric temperatures during boreal summer are located over continental India, south of the plateau. Here we show that, although Tibetan plateau heating locally enhances rainfall along its southern edge in an atmospheric model, the large-scale South Asian summer monsoon circulation is otherwise unaffected by removal of the plateau, provided that the narrow orography of the Himalayas and adjacent mountain ranges is preserved. Additional observational and model results suggest that these mountains produce a strong monsoon by insulating warm, moist air over continental India from the cold and dry extratropics. These results call for both a reinterpretation of how South Asian climate may have responded to orographic uplift, and a re-evaluation of how this climate may respond to modified land surface and radiative forcings in coming decades.

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Figure 1: Observational estimates of June–August thermodynamic structure, precipitation and wind.
Figure 2: Thermodynamic structure from balloon soundings for June–August.
Figure 3: Thermodynamic structure, precipitation and wind from the atmospheric model.
Figure 4: Results from model runs with modified topography and surface albedo.


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We thank P. Molnar for conversations motivating this work. Computing time was provided by the NCAR, which is sponsored by the NSF. W.R.B. was supported by the Reginald A. Daly Postdoctoral Fellowship in the Department of Earth and Planetary Sciences at Harvard University, and the John and Elaine French Environmental Fellowship at the Harvard University Center for the Environment. Z.K. was supported by NSF grant ATM-0754332.

Author Contributions Both authors contributed to designing the research and interpreting results. W.R.B. performed the observational analyses and model runs, and wrote the manuscript.

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Correspondence to William R. Boos.

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Boos, W., Kuang, Z. Dominant control of the South Asian monsoon by orographic insulation versus plateau heating. Nature 463, 218–222 (2010).

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