Distinct Orographic Controls on the Asymmetrical Onset of the South Asian Summer Monsoon: Hindu Kush versus Himalaya-Tibet

The relative impacts of the Tibetan Plateau-Himalaya (TPH) and the adjacent narrow mountains on the South Asian summer monsoon (SASM) are still unclear. We used a general circulation model to evaluate the relative effects of the TPH, the Hindu Kush Mountains (HKM), and the Iranian Plateau on the SASM from the perspective of the asymmetrical onset of the Bay of Bengal (BOB) summer monsoon (BOBSM) and the Indian summer monsoon (ISM). The results show that the abrupt onset of the BOBSM in May is affected by the TPH and HKM, which significantly strengthen precipitation over the BOB. By contrast, precipitation over the Indian subcontinent is suppressed by the TPH. In June, the TPH and the HKM contribute to strengthening and expanding precipitation over the Indian subcontinent and the ISM is significantly intensified. The strengthening of the SASM onset induced by the HKM is comparable with, or even larger than, that induced by the TPH. The effect of the Iranian Plateau on the SASM is much weaker and localized. The distinct role of the HKM is caused by the appearance of an anomalous low-level cyclone around the HKM and an anticyclone to the south, strengthening the India-Burma trough and the Somali jet. The topographic control of the HKM depends on the pre-existence of the TPH. We show that the sensible heat from the HKM accounts for 78.6% and 31.6% of the HKM-induced increase in precipitation during the onset of the BOBSM and ISM, respectively.