Assessing the internal variability in multi-decadal trends of summer surface air temperature over East Asia with a large ensemble of GCM simulations

This study investigates the impact of internal variability on East Asian summer (June-July-August) surface air temperature (SAT) trends on the multidecadal time scale based on a 30-member ensemble of simulations that share the same external forcing from 1970 to 2005. The ensemble-mean SAT in East Asia shows a positive trend, but the patterns and the magnitudes in the individual members are remarkably diverse, highlighting the strong effect of internal variability. The first two leading empirical orthogonal function (EOF) modes of the SAT trends among ensemble members are used to represent the leading patterns of internally generated SAT change in East Asia. The first EOF mode displays a south-north dipole structure, associated with a zonally banded circulation pattern over East Asia and the North Pacific. The second mode represents coherent trend in North China, Korea and Japan, accompanied by the Northern Hemisphere annular mode (NAM)-like circulation changes. A dynamical adjustment method is applied to reduce circulation-induced internal variability in SAT, and the adjusted SAT trends are much less variable among ensemble members and more in line with the ensemble mean than the raw trends. Observed evidences show that the summertime SAT in most of East Asia, especially in northern East Asia, has experienced rapid warming in recent decades. After dynamical adjustment, the residual trends of SAT in observations are weaker than the raw trends, especially at high and middle latitudes, suggesting the enhanced warming in northern East Asia over the recent decades was not entirely anthropogenic but partly caused by internal variability.