The Out-of-Phase Variation in Vertical Thermal Contrast Over the Western and Eastern Sides of the Northern Tibetan Plateau

Abstract

The upper tropospheric thermal contrast over the northern Tibetan Plateau and the Tianshan Mountains during the peak summer monsoon is found to have experienced a significant change since the late 1990s, and the change is out of phase on the western and eastern sides of the northern Tibetan Plateau. Using the National Center for Environmental Prediction (NCEP)–DOE AMIP-II reanalysis daily data sets, we found that in the late 1990s, there was an increase/decrease in the tropospheric temperature (TT) at 100 hPa/300 hPa on the western side, whereas on the eastern side the TT changed in the opposite fashion. Associated with the TT change is an increase (decrease) in convection in the west (east). This is in agreement with the patterns of humidity fluxes and ascending/descending favorable winds, and has been further substantiated in a causal inference, which identifies a dominant causality from the TT variation over the western side of the northern Tibetan Plateau to the longwave and shortwave fluxes through the top of the atmosphere. The lack of causality from the eastern TT variation suggests a reduction in convection, hence cloudiness, over that region, which may account for the flash drought over China during the recent global warming hiatus.