The ultra-long waves on the 500-mb level in the colder season, October to April, during 20 years were separated into transient and standing waves, and the synoptic and statistical behavior of transient waves were studied. The transient waves for the first and second harmonics at high latitudes move generally westward as has been shown by the author. The retrogression of transient waves persists very often during a few 5-day periods or more and large amplification of these waves occurs more frequently in low- than high-index states. Several pronounced developments and retrogressions of blocking waves coincided well with those of transient waves. Generally speaking, the westward speed of blocking waves and positive anomaly centers are slower than those of transient waves.When the ridge of a developed transient wave for the first or second harmonics, accompanied by a blocking wave, moves westward at high latitudes and approaches to a trough in the standing (25-day mean) field in a persistent and severe low-index situation, a new ridge or an already existing ridge is intensified upstream on the 500-mb level and extends northward. Then the northern part of this ridge is covered by extensive positive anomalies with large-scale cut-off lows or negative anomaly centers to the south. Thus a typical flow pattern, called "bridge blocking", is established. The bridge blocking occurs frequently in the Far East, Europe and North America and disappears as transient and blocking waves shift further westward. As the bridge blocking makes the regional zonal index much below normal, the persistent retrogressionof developed transient waves indicates the persistence and intensification of the present severe lowindex situations.The transient waves defined in the present study are ultra-long waves with a time scale of about a month and the transient waves for the first two harmonics at high latitudes, associated with blocking waves and bridge blockings, play an important role in the general circulation.
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