PERSISTENT NORTH PACIFIC CIRCULATION ANOMALIES AND THE TROPICAL INTRASEASONAL OSCILLATION

A composite analysis of multiyear (1985-93) global reanalyses produced by the NCEP/NCAR and the NASA/DAO is used to show that the developmen t of persistent North Pacific (PNP) circulation anomalies during NH wi nter is linked to tropical intraseasonal oscillations. The development is initiated over the tropical west Pacific by anomalous convection ( characterized by an east-west dipole structure) one to two weeks prior to the extratropical onset time in both reanalyses. As tropical heati ng moves eastward toward the central Pacific, anomalous divergent outf low associated with the local Hadley circulation generates an anomalou s Rossby wave sink (source) in the subtropics, consistent with the ret raction (extension) of the Pacific jet. Prior to onset the signature o f the forced anomalies is a pair of cyclonic (anticyclonic) circulatio n anomalies centered near the node of the tropical heating dipole. Wav e trains extending from the region of anomalous convection into the ex tratropics set the stage for the subsequent rapid development of the P NP anomalies. After onset, the mature PNP anomalies extend equatorward to feed back (through modifications to the moisture transport) on the tropical precipitation anomalies. Throughout the evolution, the tropi cal precipitation anomalies and the extratropical PNP anomalies evolve coherently with tropical intraseasonal oscillations in both reanalyse s.