Seasonality of Synoptic Eddy Feedback and Arctic Oscillation/North Atlantic Oscillation

The climatological background flow and synoptic eddy activity are much stronger in winter than that in summer. At the same time, Arctic Oscillation (AO)/North Atlantic Oscillation (NAO) is also known of substantial seasonality. In this paper, we propose that the observed seasonality of AO/NAO may be attributed to the seasonal changes in the background climate state and the two-way interaction between synoptic eddy and low-frequency flow (SELF) which depends on climatological cycles of the background flow and the synoptic eddy activity. By using a dynamic closure for the SELF feedback, we show that both in winter and summer the positive SELF feedback is of significant importance for the formation of AO/NAO. Although background storm activity is significantly weaker in summer than in winter, the positive SELF feedback associated with AO/NAO mode in summer can be as strong as that in winter. The seasonal variations of AO/NAO patterns are found to resemble those of the leading singular mode simulated by a linear dynamic system with the SELF feedback. The seasonal shifts in spatial patterns of AO/NAO are found as the result of the seasonal changes in climatological stationary waves and storm tracks patterns.