The sensitivity of Arctic atmospheric simulations to initial and boundary conditions has been investigated withthe high horizontal resolution regional climate model HIRHAM driven by observational data analyses at thelateral and lower boundaries. The investigation of the constraint of the synoptic-scale flow in the specificcircumpolar Arctic domain shows that even though the same domain size is used as in mid-latitude RCMsimulations, a weaker constraint of the synoptic scales and a smaller predictability have been obtained.A simple spectral analysis shows that only scales with wavelengths longer than 1000 km contribute to thehigher root mean square difference. Due to the weaker lateral boundary control a pronounced sensitivity ofthe Arctic simulations concerning uncertainties of initial conditions has been found. The changes of the monthlymean atmospheric structures due to internal processes are of the same order like those due to inaccuratephysical parameterizations. Sensitivity experiments concerning changed sea ice thickness show a substantialimpact of the lower boundary conditions on the monthly mean atmospheric structures up to the middletroposphere.