Phase asymmetric downstream development of the North Atlantic Oscillation and its impact on the East Asian winter monsoon

Abstract

The phase asymmetry in a downstream pattern of the North Atlantic Oscillation (NAO) is examined during boreal winter time. We compared the downstream anomaly patterns associated with the positive and negative persistent NAO events over several days. Both phases of the persistent NAO events accompany quasi-stationary wave trains in downstream upper levels. However, a distinct low level anomaly appears only for the negative persistent NAO events. The low level anomaly migrates eastward as a surface anticyclone that is confined to the low troposphere. A few days later, the surface anticyclone reaches central Siberia and induces cold advection over East Asia. Therefore, the phase-dependent downstream development of the NAO expands the impact of the NAO to the East Asian region. We also discuss the possible interaction between the upper and lower level anomalies, a necessary condition for the amplification of the Siberian high.