The Effect of High-Mountain Asia Topography on Northern Hemisphere Atmospheric Flow

Large mountain regions influence local- and global-scale atmospheric flow through mechanical forcing and changes in temperature and pressure fields. The topography of High-Mountain Asia (HMA), for example, is critical for the development of important characteristics of the Indian Monsoon. In this study, global climate model sensitivity experiments are applied to quantify the magnitude and geographical extent of the effects of HMA topography on Northern Hemisphere atmospheric flows. A series of ECHAM5-wiso (climate model) experiments were set up, in which HMA topography is reduced incrementally by 25% of its current height. All other boundary conditions, such as greenhouse gas concentrations and ice cover, are kept constant. The impact of the simulated topographic changes on the Eurasian Wave Train (EWT), which is critical for Northern Hemisphere atmospheric transport, is evaluated by examining the loading patterns from empirical orthogonal functions analyses conducted on the simulated pressure and wind fields. The impact of HMA topography on the intensity and extent of meridional flow in South Asia is assessed by examining wind speeds at different pressure levels. Changes in the EWT are particularly prominent in (Central) Asia. These may be attributed to the significant changes in pressure fields west of the Tibetan Pleateau as the topography in the HMA region is varied. Furthermore, increasing HMA topography from 0% to 100% significantly increases 1) average meridional summer wind speeds (by ≤10 m/s) at the 200hPa and 1000hPa levels, and 2) the extent of northward, monsoonal flow over the Indian subcontinent. The simulations only predict notable northward flow over western and northern India in summer if HMA topography is set to ≥50% of its modern height. The flow’s northeastern extent is restricted to 25°N-65°E in the absence of HMA topography, but reaches 33°N-75°E when it is fully developed.