Impacts of anthropogenic forcing and internal variability on the rapid warming over the Tibetan Plateau

Abstract

This study investigates the roles of anthropogenic forcing and internal variability in the recent rapid warming over the Tibetan Plateau (TP) using a 30-member ensemble of CESM1 simulations. The ensemble-mean surface air temperature (SAT) over the TP shows a warming trend, but there is a remarkable diversity among individual members, suggesting a strong effect of internal variability. The ratio of the ensemble mean to the standard deviation of SAT trends among the ensemble is larger than 3 in summer and ranges from 0.5 to 1.5 in winter over most regions of TP. Our analysis reveals that internal atmospheric variability exerts crucial influences on SAT increase over the TP in both summer and winter by modulating surface heat fluxes involving cloud-radiation and snow-albedo feedbacks. A fingerprint pattern matching method illustrates that internal variability has augmented the observed TP warming in recent decades due to anthropogenic forcing. Internal variability is estimated to account for 25% of the observed warming trend in summer and approximately 50% in winter over the TP. The dynamical adjustment method reveals that the enhanced warming in the observations over northeastern TP in summer and over central TP in winter is partly dynamically induced. Further analysis indicates that SAT changes over the TP are closely tied to the multidecadal fluctuation of the Silk Road Pattern-like and the Scandinavian Pattern-like atmospheric circulation anomalies in summer and winter, respectively.