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

The Tibetan Plateau emits energy into the atmosphere in the form of dry heat and water vapour, like any landmass, but its mean surface elevation is over 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 time scales. However, observations of the present climate 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. We show here 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 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.