Additional Possible Cause of the Erratic 2009 Monsoon Over South and East Asia: Large-Scale Change in the Upper Tropospheric Temperature

ABSTRACT It has been recognized by the scientific community for many years that erratic monsoon attribution should be prioritized in order to meet the growing demand for reliable monsoon forecasts. It is believed that the severe 2009 drought in the South and East Asian regions was caused by an erratic monsoon season, which has sparked enormous interest in its cause. In this study, we have identified a factor in the upper troposphere, i.e. the large-scale upper tropospheric temperature, which may have potentially functioned as a new cause. We have observed that a major change in the upper tropospheric temperature occurred, with one region over Central Asia and another over the eastern Tibetan Plateau; also observed is the interaction between these two local systems. The thermal wind link suggests that the 2009 circulations over the two different regions at 250 hPa – are characterized by a cyclone and an anti-cyclone over Central Asia and the eastern Tibetan Plateau, respectfully, and that this could be the dynamic cause of the 2009 monsoon prediction failure. Subtropical jets provide a means of connecting the dynamical systems in the two different regions, which have enhanced the convection over Central Asia and reduced the convection over South and East Asia during that period. Also revealed is the role of the thermodynamic processes, specifically the role of the vertical thermal contrast (VTC) over Central Asia and the eastern Tibetan Plateau, which is responsible for changing the convective patterns during the 2009 monsoon season. This is further substantiated by the inferred causal relationship between the upper VTC and longwave fluxes (LWFs) at the top of the atmosphere.