LARGE-AMPLITUDE FLOW ANOMALIES IN NORTHERN-HEMISPHERE MIDLATITUDES

The composite of large amplitude flow anomalies identified from extremely large amplitudes of the planetary-scale waves is examined in terms of the temporal and spatial evolution of both the large-scale flow and the storm tracks. The characteristic spatial patterns, growth and decay rates, and persistence characteristics that the individual large amplitude anomaly cases share come out naturally in the analysis. The composite anomaly's growth and decay are very rapid, taking an average of only 4 days to develop local anomalies of 200-300 m. The spatial evolution of the flow suggests a rapidly growing standing wave over the North Pacific Ocean and North America. After a persistence of random duration (averaging 8.4 days), the composite anomaly's decay is accompanied by simultaneous retrogression of the pattern from western North America to eastern Asia and eastward progression of the pattern over Europe and western Asia. Substantial disruption of the Pacific storm track and enhancement of the Atlantic storm track accompanies the life cycle of this flow regime. A residual effect of the regime life cycle is a reduction in the low-level meridional temperature gradient, particularly over eastern Asia in the entry region of the Pacific jet stream.