Evaluation of regional climate change and surface warming mechanism over the Tibetan Plateau

Abstract

Tibetan Plateau (TP), the highest land on the earth, is one of the most active regions sensitive to climate change and global warming. Motivated by understanding TP regional climate change and safeguarding water security, the thesis conducts a systematic exploration of the features of climate change, and mechanisms of surface warming, over the TP. The investigation includes analysing climate change features, revealing the warming mechanisms, and modeling and identifying the possible climate change drivers. Multiple datasets are adopted to study the variability of TP climate variables, mainly temperature and precipitation. Linear regression and the correlation coefficient are used to explore the changing trends of temperature and precipitation. The Weather Research Forecasting (WRF) model is used to explore the impacts of the meridional wind on the regional climate over the Southern TP. The Climate Feedback Response Analysis method (CFRAM) based on the Rapid Radiative Transfer Model is used to evaluate the contributions of radiative and non-radiative processes to the TP surface temperature change. The study investigates the elevational features of temperature and precipitation over TP using five datasets. The results suggest that the TP has undergone significant surface warming and slightly enhanced precipitation, especially over the western TP. There is a significant change-point around 1997 for surface temperature and precipitation demonstrate, especially in the winter. Using WRF simulations, this study discloses that the increased meridional wind would cause a substantial increase of precipitation in the middle and lower reaches of the Yarlung Zangbo River (YR) basin in the southern TP but a moderate decline in the upper reaches. Further, a case study of a rainstorm in August 2015 suggests that the weakened/enhanced meridional wind would have impacts on the rainstorm centre and rainfall peak. In addition, based on the CFRAM and RRTM simulations, the study attributes the TP summer surface temperature change to different radiative and non-radiative processes. The study discloses that the substantial increase in the water vapour input to the TP due to the weakened westerlies from 1979 to 2019 determines the elevation-oriented summer warming. The increasing snowfall at high elevations in summer enhances the surface albedo and causes a notable cooling effect, while the rising rainfall at middle and low elevations decreases the surface albedo and causes a warming effect. The non-radiative factors play an offsetting role in surface warming through rebalancing regional energy flow. Finally, using the similar methods in summer temperature analysis, the study explores the features of TP winter temperature change. The study discloses the critical role of the surface albedo in the latitude-dependent winter surface warming. The substantial decline in the surface albedo at the TP with low latitudes due to snow cover decrease has contributed to a dramatic surface warming. In contrast, the increasing albedo arising from the growing snowfall and degraded vegetation at the TP with high latitudes has caused a surface cooling effect. Similar to the summer warming, the non-radiative processes have dampened the albedo’s influence in the winter climate system to maintain a proper warming level over the TP.