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