Contributions of internal variability and external forcing to interdecadal variations of East Asian winter temperature during the past millennium

This study investigates the effect of internal variability and external forcing on the interdecadal variations of winter East Asian averaged surface air temperature (EASAT) during the past millennium (i.e., 850 − 1850) based on the Community Earth System Model–Last Millennium Ensemble simulations. The simulated winter EASAT exhibits periods of 10 − 40 and 80 − 90 years, which are consistent with the reconstructions. Internal variability explains 76% of the interdecadal EASAT variations, in which the Arctic Oscillation (AO) plays a vital role. The positive AO phase-related Rossby waves at northern mid-latitudes feature enhanced equatorward propagation in the middle–upper troposphere and reduced upward propagation into the stratosphere. The decreased wave amplitudes at northern mid-latitudes denote the weakened Siberian high, Aleutian low, and East Asian trough, enhancing the warm horizontal temperature advection in the lower troposphere and downward longwave flux at the surface. Thus, the positive AO phase favors warmer-than-normal EASAT, and vice versa. In comparison, external forcing, especially volcanic forcing, mainly affects the EASAT variations at 40 and 80 − 90 years and plays a secondary role.