A linkage between the North Atlantic Oscillation and its downstream development due to the existence of a blocking ridge

Abstract

In this work the spatial growth of the North Atlantic Oscillation (NAO) and its relationship to downstream development are examined. In contrast to positive NAO events, which are primarily initiated over the western North Atlantic, negative persistent NAO events tend to be preceded by a blocking ridge in the vicinity of northern Europe. This blocking ridge contributes to the growth of negative NAO events through a low-frequency eddy interaction. It is also shown that a noticeable downstream development is dominant only for negative persistent NAO events that are preceded by the blocking ridge. While the blocking ridge exists over inland Eurasia, it induces anticyclonic circulation near the surface over Siberia. As the NAO grows, a stationary wave train that originates from the North Atlantic transfers wave energy downstream toward Siberia. The surface anticyclone is then reinforced by the wave train and begins to migrate eastward. This process explains the distinct impact of the negative NAO on the downstream Eurasian region.