[1] Ensemble simulations of Arctic circulation can develop multiple dynamical regimes. We use ensemble simulations for June–December 2007 by the Advanced Research Weather Research and Forecasting model to examine regime development and to understand differences in the atmospheric circulation regimes caused by sea ice. Multiple regimes are common in our ensemble simulations, although there are differences through the period. Multiple-regime states tend to be preferred slightly more in June, July, and August than October, November, and December. September has the fewest multiple-regime periods. September is also the month of sea ice minimum, suggesting that open ocean may inhibit the occurrence of multiple regimes in ensemble simulations compared to periods when substantial sea ice is present. The regime behavior occurring here suggests that as future summer ice cover wanes in the Arctic, the predictability of the atmosphere may increase.