The sensitivity of Arctic atmospheric simulations to initial and boundary conditions was investigated with the high horizontal resolution regional climate model HIRHAM driven by observational data analyses at the lateral and lower boundaries. Investigation of the constraint of the synoptic-scale flow in the specific circumpolar Arctic domain showed that even though the same domain size as in mid-latitude RCM (regional climate model) simulations was used, a weaker constraint of the synoptic scales and a smaller predictability were obtained. A simple spectral analysis showed that only scales with wavelengths longer than 1000 km contribute to the higher root-mean-square difference. Due to the weaker lateral boundary control a pronounced sensitivity of the Arctic simulations to uncertainties in initial conditions was found. The changes in the monthly mean atmospheric structures due to internal processes were of the same order as those due to inaccurate physical parameterizations. Sensitivity experiments concerning changed sea ice thickness showed a substantial impact of the lower boundary conditions on the monthly mean atmospheric structures up to the middle troposphere.