An event‐based jet‐stream climatology and typology

Abstract A novel climatology is developed for upper‐tropospheric jet streams, which is complementary to and an alternative for the traditional depictions of the time‐mean jets. It entails identifying the occurrence of a jet event at a given location and then compiling the spatial frequency distribution of such events. The resulting climatology, derived using the ERA‐15 reanalysis data set of the ECMWF for the period 1979–1993 indicates that (1) in both hemispheres the annual cycle of jet events takes the form of comparatively smooth transition from a quasi‐annular structure in summer to a more spiral‐like structure in winter with a temporally asymmetric return to the summer pattern; (2) the hemispheres differ primarily in the amplitude of the frequencies and the longitudinal overlap of the spiral portion of the pattern. In addition, the jet events are subdivided using a two‐class typology comprising shallow and deep jets whose vertical shear (sic. baroclinicity) are/are not confined principally to the upper troposphere. This provides a conceptually simple and dynamically meaningful classification since deep jets are more likely to spawn tropospheric‐spanning cyclones. The accompanying climatology displays important longitudinal variations and significant inter‐hemispheric differences. A comparison is drawn between these new and conventional climatologies and typologies. Also, comments are proffered on the relationship between, on the one hand, the patterns of jet frequency including the differing distributions of the shallow and deep types and, on the other hand, the location of the time‐mean jets and the downstream storm tracks. Copyright © 2006 Royal Meteorological Society.