In this paper, by using a dynamic model with synoptic eddy and low-frequency flow (SELF) interaction, we show that the SELF feedback is of significant importance for the formation of the AO/NAO in both winter and summer. Although background storm activity is significantly weaker in summer than that in winter, the positive SELF feedback associated with the AO/NAO mode in summer can be as strong as that in winter. The seasonal variations of the AO/NAO patterns are found to resemble those of the leading singular modes simulated by a linear dynamic system with the SELF interaction in different seasons, and the seasonal shifts in spatial patterns of the AO/NAO are the result of the seasonal changes in climatological basic state and synoptic eddy activity.