Revisiting the impact of Asian large-scale orography on the summer precipitation in Northwest China and surrounding arid and semi-arid regions.

How the summer precipitation in Northwest China (NWC) changes with the Asian large-scale orographic condition is a hot research topic. In this study, the influence of the Tibetan–Iranian Plateau (TIP) is investigated based on the modeling data from the Global Monsoons Model Intercomparison Project (GMMIP) endorsed in the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Two models, FGOALS-f3-L and FIO-ESM v2.0, which can well replicate the observed climatic conditions in the Eurasian continent, are analyzed. After removing the TIP, the change in the summer precipitation in NWC and its adjacent arid and semi-arid regions exhibits an asymmetric pattern, characterized by a drying tendency over the southern portion in contradiction to a wetting one over the north. This dipole pattern indicates an inhomogeneous influence of the TIP. With the flattened TIP, significantly negative heat anomalies arise over the original orographic regions. As a result, the meridional land–ocean thermal contrast is reduced, which consequently weakens the South Asian summer monsoon, with less water vapor transported from tropical oceans into NWC through the southern boundary. On the other hand, a large-scale anomalous anticyclone surrounding the flattened orography is formed in the lower troposphere, and westerlies in the northern portion of the anomalous anticyclone are intensified, which enhance the mid-latitude moisture transport into NWC through the western and northern boundaries. Collocated with the anomalous descending motion related to the weakened elevated heating in the flattened TIP, a drying tendency occurs in southern NWC. However, compensating upward motions coupled with positive incoming water vapor lead to more precipitation to the north. This study highlights a meridional asymmetric pattern in precipitation response to the flattened TIP through both the thermodynamic and dynamic processes, which helps us understand the physical mechanism of precipitation formation in NWC more comprehensively. [ABSTRACT FROM AUTHOR]