Model sensitivity of Tibetan Plateau surface potential vorticity and the Asian summer monsoon system to Asian orographic perturbation in FGOALS-f2

Abstract

The thermodynamic forcing of the Tibetan Plateau (TP) is important for the regulation of the Asian summer monsoon (ASM). However, the monsoon responses to orographic perturbation simulations show controversial results in previously published literature. One reason for this is that the monsoon responses to orographic forcing differ among climate models. Another reason is that the quantitative changes in the TP thermodynamic forcing remain unknown when the orography is modified. Therefore, the relationship between the TP forcing and the ASM may be different among climate models. In this paper, the surface potential vorticity (SPV) is used to quantify the surface thermodynamic forcing on the TP in the FGOALS-f2 climate model for both the standard Atmospheric Model Intercomparison Project (AMIP) and Coupled Model Intercomparison Project (CMIP) experiments with a modified orography, and various aspects of the ASM responses are also examined. Finally, their relationships with TP-SPV changes are quantitively estimated. The results indicate that the intensity of TP-SPV is reduced by nearly 100% in the AMIP runs, while it is reduced by 68% in the CMIP runs when the Asian mountains are removed. Overall, the responses of the monsoon system are more sensitive when air-sea interactions are considered. When the mountains are removed, the precipitation over the southern slope of the TP decreases by 73% and increases by nearly 30% over the tropical Indian Ocean in the CMIP runs. Moreover, the precipitation responses exhibit the largest difference between the AMIP and CMIP runs over the East Asian and Western Pacific regions. The precipitation over the Western Pacific remains almost unchanged when the orography is removed in the AMIP runs but is reduced by 20% in the CMIP runs. We propose that the calculation of SPV over the TP is helpful for understanding the TP orographic forcing and monsoon dynamics in the future.