Interdecadal variations similar to the Arctic Oscillation (AO) are investigated for internal variability (INTV) and response to external forcing (REXT) in an ensemble simulation of twentieth century climate. The significant trend in REXT implies that a sizeable part of the observed AO trend in recent decades can also be attributed to anthropogenic forcing. INTV is characterized by a barotropic dipole of zonal wind anomalies and associated wave propagation, suggesting a mechanism similar to the month-to-month AO. Its thermal structure can be attributed to dynamic processes. REXT exhibits a thermal structure that can be explained by responses to the forcing due to increased greenhouse gases. A westerly wind anomaly in the stratosphere as a thermal response corresponds to anomalies in wave propagation and meridional circulation that are similar to INTV, which may induce the AO-like annular pattern.