Abrupt evolution of the summer Northern Hemisphere annular mode and its association with blocking

Using reanalysis data from the National Centers for Environmental Prediction-National Center for Atmosperic Research, Boulder, Colorado, for the period from 1958 to 2005, we statistically analyzed the relationships of the summer Northern Hemisphere annular mode (summer NAM) with hemispheric-scale anomalous summer weather and the occurrence of blocking highs. The anomalous positive NAM (low-pressure anomaly in the Arctic and high-pressure anomaly in midlatitudes) accounts well for the hemispheric-scale weather associated with anomalous blocking between the polar and subtropical jets, whereas blocking rarely occurs during negative NAM periods. The double jet stream structure is more evident during periods of anomalous positive NAM than during periods of negative NAM. The surface temperatures associated with the anomalous positive NAM clearly show Europe to be hot and East Asia to be cool, as was the case during the anomalous summer of 2003. The occurrence of a positive summer NAM is therefore consistent with the hemispheric-scale anomalous summer weather associated with blocking in 2003. We investigated the abrupt evolution of atmospheric patterns and the geographic distribution of blocking highs associated with the development, maintenance, and decay periods of an anomalous positive NAM. During the development period, blocking tends to occur over Europe and the Atlantic Ocean, but no significant blocking signature is evident over eastern Eurasia. During the maintenance stage, blocking tends to occur in the Far East. During the decay stage, blocking over the Pacific region is obvious. This longitudinal migration of blocking phenomena may be used to predict the evolution through time of the NAM.