SIMULATIONS OF PERSISTENT NORTH PACIFIC CIRCULATION ANOMALIES AND INTERHEMISPHERIC TELECONNECTIONS

Evidence is presented, from a composite analysis of a 14-year general circulation model simulation, that persistent North Pacific (PNP) circ ulation anomalies during boreal winter are part of a larger-scale meri dional development extending into the Tropics and the Southern Hemisph ere. Lagged composites suggest that the development is initiated over the tropical Pacific by anomalous convection (characterized by an east -west dipole structure centered at the date line) one to two weeks pri or to the extratropical onset time. Relatively weak wave trains, exten ding from the region of anomalous convection into the extratropics, ap pear to set the stage for the subsequent rapid development of the PNP anomalies. After onset, the PNP anomalies extend into the Tropics and enhance moisture transports that tend to supply moisture to, and thus reinforce, the associated tropical precipitation anomalies. The mature stage is characterized by a strong coupling between hemispheres, incl uding twin low-level cyclonic (anticyclonic) circulations straddling t he equator with westerly (easterly) wind ''bursts'' on their equatorwa rd flanks. The tropical precipitation anomalies and the extratropical PNP anomalies evolve coherently with tropical intraseasonal oscillatio ns reminiscent of the Madden-Julian oscillation. Results from a simila r composite analysis of a shorter (5 year) assimilated atmospheric dat aset are generally consistent with the simulated results, despite the substantially smaller sample size. The assimilation, however, position s the tropical heating dipole farther west, in better agreement with p revious observational studies of intraseasonal tropical/extratropical teleconnections. As a consequence, the pre-onset Extratropical ''respo nse'' to the tropical anomalies in the simulation has significant phas e errors. The remarkably similar evolution in die extratropics after o nset suggests that the tropical forcing acts primarily as a catalyst f or the development of the PNP anomalies and that the most useful predi ctors of PNP events may lit not in the extratropics but in the tropica l western and central Pacific.