SIMULATED LIFE-CYCLES OF PERSISTENT ANTICYCLONIC ANOMALIES OVER THE NORTH PACIFIC - ROLE OF SYNOPTIC-SCALE EDDIES

This study examines the role of synoptic-scale eddies during the devel opment of persistent anticyclonic height anomalies over the central No rth Pacific in a general circulation model under perpetual January con ditions. The GCM replicates the basic characteristics of the evolution of the anomaly patterns found in observations. The life cycle is char acterized by the rapid establishment of the major anomaly center and c onsiderably longer maintenance and decay phases, which include the dev elopment of downstream anomaly centers. The simulation also shows a re alistic evolution of synoptic-scale activity beginning with enhanced a ctivity off the east coast of Asia prior to onset, followed by a north ward shift of the Pacific storm track, which lasts throughout the-main tenance phase. The initial enhancement of synoptic-scale eddy activity is associated with a large-scale cyclonic anomaly that develops over Siberia several days prior to the onset of the main anticyclonic anoma ly over the central North Pacific. The observations, however, show con siderable interdecadel variability in the details of the composite ons et behavior; it is unclear whether this variability is real or whether it reflects differences in the data assimilation systems.The role of the time mean flow and synoptic-scale eddies in the development of the persistent Pacific anomalies is studied within the context of a kinet ic energy budget in which the flow is decomposed into the time mean, l ow-frequency (timescales longer than 10 days), and synoptic (timescale s less than 6 days) components. The budget, which is carried out for t he simulation at 500 mb, shows that the initial growth of the persiste nt anticyclonic anomalies is associated with barotropic conversions of energy, with approximately equal contributions coming from the mean f low and the synoptic-scale eddies. After onset the barotropic conversi on from the mean flow dominates, whereas the decay phase is associated with baroclinic processes within the low-frequency flow.