Role of the internal atmospheric variability on the warming trends over Northeast Asia during 1970–2005

Summer surface air temperature (SAT) over Northeast Asia exhibits widespread warming trends during the past according to observational and reanalysis data. This study examines the contribution of the internal variability to the SAT trend over Northeast Asia during 1970–2005 based on 30-member large ensemble simulations of CCSM4. The 30-member simulations of CCSM4 are forced by the same external forcing, with only slight differences in the initial atmospheric conditions. Thus the discrepancies in the SAT trends among the 30 members have resulted from the internal atmospheric variability. The 30-member simulations can well reproduce the warming trend over Northeast Asia in terms of the ensemble mean, but show large spreads in magnitude and spatial pattern among different members, suggesting a notable impact on the summer SAT trend over Northeast Asia of the internal atmospheric variability. The first mode of the internally-induced SAT trend presents coherent warming trends over most parts of Northeast Asia. The second mode is characterized by a dipole pattern, with an opposite trend between western and eastern parts of Northeast Asia. The first and second modes of the internally-induced SAT trend have a close relation with the large-scale internal atmospheric wave train over Eurasia, with spatial distributions resembling the Scandinavian pattern and the eastern Atlantic/western Russia, and the British-Baikal Corridor pattern, respectively. We further employed a dynamical adjustment technique to reduce the impact of the internal atmospheric variability on the summer SAT trend over Northeast Asia. The adjusted SAT trends show significantly weakened diversities among the 30 ensemble members, with domain-wide warming trends noticed in most simulations. Following this, the observational SAT series is also dynamically adjusted. The obtained SAT trends show a weaker magnitude than the raw observational results, indicating that the internal atmospheric variability has a destructive interference with the warming trends over Northeast Asia in recent decades.