Life Cycles of Persistent Anomalies. Part I: Evolution of 500 mb Height Fields

Abstract

We have conducted observational analysts to identify systematic aspects of the life cycles of persistent anomalies of the extratropical Northern Hemisphere wintertime circulation. In the present study, we focus on the typical characteristics of the 500 mb height anomaly and flow patterns accompanying the development and breakdown of large-scale flow anomalies in two key regions, the eastern North Atlantic (ATL) and the northern Soviet Union (NSU), and then compare the results for these regions with results described previously for cases located over the North Pacific (PAC) region. Throughout their life cycles, the positive anomaly and negative anomaly cases for a given region display a number of striking similarities. The primary anomaly center develop rapidly, with little indication of a significant anomaly over the key region until just prior to onset. Following establishment of the major anomaly center over the key region, anomaly centers develop and intensify in sequence downstream, leading to the establishment of the persistent anomaly pattern. Much of this downstream intensification occurs with little evidence of phase propagation. Once established, some of the anomaly patterns strongly resemble certain prominent teleconnection patterns (e.g., the Eastern Atlantic and Pacific-North American teleconnection patterns). The associated flow patterns are often characterized synoptically by the development of either blocking patterns or anomalously intense zonal flows over the key regions. For all three regions, the gross features of the development downstream from the main center qualitatively resemble the behavior seen in simple barotropic models of energy dispersion on a sphere away from a localized, transient source of vorticity, suggesting that quasi-horizontal energy dispersion by Rossby waves is likely to account for important aspects of the downstream developments. In addition, the NSU cases are typically also preceded by a well-defined upstream wavetrain. Precursors to the ATL pattern appear as characteristic anomaly patterns located to the southeast and to the southwest of the key region, the latter partly reflecting changes in the intensity and structure of the jet over the western Atlantic. The most systematic precursors to the PAC cases are related to variations in the jet intensity and structure over eastern Asia and the southwestern North Pacific and to an eastward-propagating, intensifying synoptic-scale disturbance. In both the ATL and PAC regions, the main anomaly centers are located downstream from the climatological mean jet maxima and tend to have zonally-elongated structures (i.e., typically U?2>v?2, suggesting that barotropic energy conversions from the time mean flow provide one possible source for their developments. Breakdowns of the patterns also occur rapidly. Until a few days prior to breakdown, the patterns rather closely resemble the patterns immediately following development. The centers of the NSU pattern appear to weaken systematically in sequence downstream along the wavetrain. The breakdowns of the patterns in the other regions appear less well-defined, however. there is some indication that the main centers tend to drift generally northwestward while weakening.