­­­The Role of the Tibetan Plateau in the South Asian Monsoon Atmospheric Circulation

Abstract:

The role of the Tibetan Plateau in the South Asian Monsoon circulation is the focus of this study. Typically, the Tibetan Plateau is thought to affect the circulation by acting as an elevated heat source (Molnar et al. 1993). Through radiative effects, the Tibetan Plateau would induce a meridional pressure gradient at upper levels initiating the monsoon circulation. Indeed, numerical experiments have shown that global orography affects the timing of the monsoon onset (Chakraborty et al. 2006), and observations have shown significant correlations between the moist static energy of the Tibetan Plateau’s lower atmosphere and the summer monsoon rainfall around the onset and withdraw periods (Rajagopalan and Molnar 2013). Yet, this notion has been recently questioned, and the shielding effect of the orography has been suggested to be the dominant effect in the circulation. This latter theory is supported by numerical experiments suggesting that summer precipitation does not change considerably when removing the Plateau while retaining the Himalayas (Boos and Kuang 2010). Nonetheless, both the Himalayas and the Plateau are likely to play important roles, and further experiments are needed.

In this study we construct numerical experiments to further study the role of the Tibetan Plateau in the atmospheric circulation. For the experiments we use SPEEDY, a global climate model of intermediate complexity developed at the Abdus Salam International Centre for Theoretical Physics (Molteni 2003). The experiments are conducted with different regional orographic conditions, so that we can evaluate the impact orography has in determining the characteristics of the monsoon circulation. In all experiments the atmosphere is started from a state of rest and we avoid using climatological fields for sea surface temperature, diabatic heating, and land temperature. This setup is particularly important as we wish to evaluate how the system evolves under different conditions without imposing a climatological solution to any field. The importance rests in the fact that these fields are determined by the dynamics of the coupled system and might influence how the statistics of the system, such as the mean onset dates of the monsoon or the climatological distribution of rainfall, vary with changes in orography.