Changing Convection in Central Asia during the Seasonal Transitional Period and Region to Its West before and after 1999: Role of Upper Vertical Thermal Contrast

The objective of this study is to investigate and understand the changes in thermal contrast in the upper troposphere over Central Asia before and after 1999. It was observed that there was a discernible increase/decrease in upper tropospheric temperature (TT) in the 100 hPa/250 hPa over the Central Asian region during PRE99, whereas during POST99, it was found to be the other way around. A clear increase and decrease in the upper TT pattern in 100 hPa and 250 hPa can be attributed to anti-cyclonic and cyclonic circulations in the wind shear, respectively. The subtropical jet has been shown to act as a significant dynamical system generating cyclonic circulation over Central Asia, resulting in dynamical features favorable for enhancing convection during PRE99. This is evinced by the structure of the 200hPa zonal winds, which serve as a surrogate for a subtropical jet. It can be seen that the westerly winds over the southern part and the easterly winds over the northern part of Central Asia culminate in a significant cyclonic circulation over the region. In summary, PRE99 showed an increase in convection over Central Asia and the regions to its west. The anatomy, using geopotential height, relative humidity, total cloud area (TCA) fraction, longwave fluxes (LWF), and shortwave fluxes (SWF) through the top of the atmosphere, is consistent with the above results. To further support these findings, an updated and thorough causality analysis is performed, and it is noteworthy to mention that the variation of thermal temperature contrast is found to be causally related to LWF and SWF.