<p>Phase asymmetry in dynamic synoptic-scale eddy feedback on the winter North Atlantic Oscillation (NAO) has been observed in previous studies. However, its causes are yet unclear due to the phase asymmetry of NAO itself in observation. Therefore, in this study, sensitivity experiments based on a storm track model are conducted to clarify causes of the phase asymmetry in the NAO-related dynamic eddy feedback with constantly symmetric positive and negative NAOs specified as the background states. Despite the background NAO’s exact symmetry, the dynamic synoptic eddy feedback is still stronger in the negative NAO phase than the positive one. This phase asymmetry is primarily caused by the differences in the kinematic mechanisms of dynamic eddy feedback between the two NAO phases, resulting from contrasting background baroclinicity. In the positive phase, the stronger background baroclinicity favors eddy origination and growth, therefore making the eddy structure stronger and thus less sensitive to being deformed by the NAO flow. In contrast, the relatively weaker eddy structure in the negative phase shows a more pronounced deformation by the NAO flow, resulting in a stronger anomalous eddy vorticity structure. Thus, the forcing originated from the anomalous eddy vorticity advected by basic eddy winds, as the primary contributor to dynamic eddy feedback, is stronger in the negative NAO phase.</p>