Abstract
Abstract Although considerable research attention has been devoted to examination of the Northern Hemisphere polar and subtropical jet streams, relatively little has been directed toward understanding the circumstances that conspire to produce the relatively rare vertical superposition of these usually separate features. This study investigates the structure and evolution of large-scale environments associated with jet superposition events in the northwest Pacific. An objective identification scheme, using NCEP–NCAR Reanalysis 1 data, is employed to identify all jet superpositions in the western Pacific (30°–40°N, 135°–175°E) for boreal winters (DJF) between 1979/80 and 2009/10. The analysis reveals that environments conducive to western Pacific jet superposition share several large-scale features usually associated with East Asian winter monsoon (EAWM) northerly cold surges, including the presence of an enhanced Hadley cell–like circulation within the jet entrance region. It is further demonstrated that several EAWM indices are statistically significantly correlated with jet superposition frequency in the western Pacific. The life cycle of EAWM cold surges promotes interaction between tropical convection and internal jet dynamics. Low–potential vorticity (PV), high- air, appearing to be associated with anomalous convection in the western Pacific lower latitudes, is advected poleward toward the equatorward side of the jet in upper-tropospheric isentropic layers, resulting in anomalous anticyclonic wind shear that accelerates the jet. This, along with geostrophic cold air advection in the left jet entrance region that drives the polar tropopause downward through the jet core, promotes the development of the deep, vertical PV wall characteristic of superposed jets. A conceptual model synthesizing the results of this analysis is introduced.
Published Version
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