Simulations of Persistent North Pacific Circulation Anomalies and Interhemispheric Teleconnections

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Abstract
Evidence is presented from a composite analysis of a 14-year general circulation model simulation, that persistent North Pacific (PNP) circulation anomalies during boreal winter are part of a larger-scale meridional development extending into the Tropics and the Southern Hemisphere. 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 prior to the extratropical onset time. Relatively weak wave trains. extending from the region of anomalous convection into the extratropics, appear 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, including twin low-level cyclonic (anticyclonic) circulations straddling the equator with westerly (easterly) wind “bursts” on their equatorward flanks. The tropical precipitation anomalies and the extratropical PNP anomalies evolve coherently with tropical intraseasonal oscillations reminiscent of the Madden–Julian oscillation. Results from a similar composite analysis of a shorter (5 year) assimilated atmospheric dataset are generally consistent with the simulated results, despite the substantially smaller sample size. The assimilation, however, positions the tropical heating dipole farther west, in better agreement with previous observational studies of intraseasonal tropical extratropical teleconnections. As a consequence. the pre-onset extratropical “response” to the tropical anomalies in the simulation has significant phase errors. The remarkably similar evolution in the extratropics after onset suggests that the tropical forcing acts primarily as a catalyst for the development of the PNP anomalies and that the most useful predictors of PNP events may lie not in the extratropics but in the tropical western and central Pacific.

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