The atmosphere often exhibits a three-step pole-to-equator tropopause structure, with each break in the tropopause associated with a jet stream. The polar jet (PJ) stream resides in the break between the polar and the subtropical tropopause and is positioned above the strongly baroclinic, tropospheric-deep polar front at ~50°N. The subtropical jet (SJ) stream resides in the break between the subtropical and the tropical tropopause and is situated on the poleward edge of the Hadley cell at ~30°N. On occasion, the latitudinal separation between the PJ and the SJ vanishes, resulting in the formation of a two-step pole-to-equator tropopause structure and a jet superposition. Composite analyses of North American jet superposition events demonstrate that jet superpositions often occur within a dynamical and thermodynamic environment that is particularly conducive to high-impact weather. The composite analyses also demonstrate that, while the dynamical mechanisms that support jet superposition depend primarily on the latitude at which jet superposition occurs, subsidence beneath the confluent entrance region of the developing jet superposition is a common element among a large majority of cases. This subsidence acts to steepen the tropopause locally and completes the formation of the two-step pole-to-equator tropopause structure that accompanies a jet superposition. 

This study investigates the synoptic-scale patterns that are most conducive to the development of subsidence beneath the confluent entrance region of jet superpositions. North American jet superposition events are identified using the Climate Forecast System Reanalysis dataset during November–March 1979–2010 and are subsequently classified into three characteristic event types: “Polar Dominant,” consisting of events during which only the PJ is characterized by a substantial excursion from its climatological latitude band; “Subtropical Dominant,” consisting of events during which only the SJ is characterized by a substantial excursion from its climatological latitude band; and “Hybrid,” consisting of events characterized by a modest excursion of both the PJ and SJ from their climatological latitude bands. Following their classification, piecewise quasigeostrophic potential vorticity (QGPV) inversions are performed for each event type to diagnose the fraction of subsidence attributed to QGPV anomalies that reside along the polar and subtropical waveguides during the production of jet superpositions for each event type. The psi vector developed by Keyser et al. (1989) is applied to each event type to diagnose the fraction of subsidence attributed to across-and along-front circulations in the vicinity of developing jet superpositions.