Composite Life Cycle of West Pacific Jet-Superposition Events and the Large-Scale Environmental Response over Western North America

Abstract

AbstractVertical alignment of the polar and subtropical jet streams in the west Pacific basin occurs most often during the boreal cold season. Recent work has revealed that the large-scale environment conducive to producing such superpositions involves interaction between East Asian winter monsoon cold-surge events, lower-latitude convection, and internal jet dynamics. The evolution of the large-scale environments associated with these events post-superposition as well as the significance of that evolution on aspects of the wintertime Northern Hemisphere general circulation is examined through construction of a 44-case composite. The post-superposition west Pacific jet extends eastward associated with an anomalous positive–negative geopotential height couplet straddling the jet’s exit region. This jet extension results in ridge building over Alaska and northwestern Canada. The large-scale evolutions associated with the composite post-superposition environment occur consistently among the majority of cases considered within this analysis. The positive–negative geopotential height anomaly couplet, enhanced jet entrance circulation, low-latitude convection, and internal jet dynamics present in the pre-superposition environment weaken post-superposition. As a result, the characteristic vertical PV “wall” associated with the composite vertically superposed jet weakens. Last, investigation of the value of using the two most dominant modes of west Pacific jet variability in observing the evolution of the superposed west Pacific jet post-superposition reveals that, while the extension of the jet is exhibited, significant variability exists when analyzing each of the 44 cases of interest individually.