Composite Analysis of Large-Scale Environments Conducive to West Pacific Polar/Subtropical Jet Superposition

While extensive literature exists on the Northern Hemispheric polar and subtropical jet streams, little is known about the development of the relatively rare vertical superposition of these jets. These jet superposition events are often associated with the development of high impact weather either at the time of the superposition or some time downstream from the superposition. This study investigates the structure and evolution of the large-scale environments that are associated with jet superposition events in the west Pacific.

We employ an objective identification scheme to identify all jet superpositions in the west Pacific (30-40°N, 135-175°E) for all boreal winters (DJF) between 1979/80 – 2009/10 using NCEP/NCAR Reanalysis 1 data. We find that environments that encourage west Pacific jet superpositions are associated with several large-scale features usually associated with East Asian Winter Monsoon (EAWM) cold surges. For example, anomalous lower tropospheric northerly winds and cold temperatures are present east of China, extending as far south as the South China Sea. Anomalous anticyclonic flow exists on the anticyclonic shear side of the composite jet superposition location in the upper troposphere along with an upper tropospheric trough feature east of Japan. The superposed jet is associated with an enhanced Hadley Cell like circulation in the jet entrance region. Furthermore, we show that several EAWM indices are statistically significantly correlated with the frequency of occurrence of jet superposition events in the west Pacific region of interest. Finally, we introduce a conceptual model of the evolution of the various large-scale features that conspire to produce west Pacific jet superpositions.