Impact of the Tibetan Plateau on the Asian monsoon. I: Formation of the Meiyu front

The Tibetan Plateau is believed to play a fundamental role in shaping the Asian climate, primarily by heating the overlying atmosphere during the warm season. Recent work (Boos and Kuang 2010, Molnar et al. 2010) however suggests that the Tibetan Plateau might influence the Asian monsoon more by acting as an obstacle to the flow aloft than as an elevated heat source, calling for a re-evalutation of how the interaction of the Tibetan Plateau with the subtropical jet might shape the summer monsoon.

Here we explore the impact of the Tibetan Plateau on the East Asian monsoon, i.e. the Meiyu-Bayu front, by performing and comparing comprehensive GCM simulations with (full-Tibet) and without (no-Tibet) the Tibetan Plateau. In Part II, we focus on the impact of the Plateau on the South Asian monsoon. We find that the Meiyu-Baiu front almost disappears in late spring and early summer if the Tibet is removed. Using a stochastic advection model for the large-scale condensation associated with large-scale extratropical eddies (O'Gorman and Schneider 2006, 2008), we show that the subtropical jet impinging on the Tibetan Plateau induces regions of enhanced meridional moisture gradient and stationary waves downstream of the Plateau, which conspire to concentrate the large-scale precipitation (accounting for a large fraction of the total precipitation in the Meiyu front) in a narrow and intense precipitation band over East Asia in May and June. As the subtropical jet retreats poleward and northward in July, this Tibetan Plateau-induced stationary wave pattern weakens, leading to the mid-summer demise of the Meiyu front.