A Dynamical Linkage between the NAO and Its Downstream Development

Phase-asymmetry in the growth of NAO and its downstream development are examined. In contrast to positive NAO events which mostly begin to grow over the western North Atlantic, negative persistent NAO events tend to be preceded by a blocking ridge in the vicinity of northern Europe. The blocking ridge contributes positively to the growth of negative NAO events through a low-frequency eddy interaction, and affects persistence of NAO events. The existence of the blocking ridge also causes a noticeable difference in the downstream anomalies between positive and negative NAO. While the blocking ridge exists over inland Eurasia, it induces anticyclonic circulation near surface over Siberia. As the NAO grows, a stationary wave train originates from the North Atlantic and propagates toward downstream Siberia. Then, the surface anticyclone is reinforced by the wave train and begins to migrate eastward. This study suggests an important role of the blocking ridge in the phase-asymmetric growth of NAO events and the downstream anomalies. The blocking ridge serves as a linkage between the NAO and the downstream development through dynamical contributions.