Current and future perspectives on North American polar–subtropical jet superposition events

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 typically resides in the break between the polar and the subtropical tropopause and is positioned atop 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 can vanish, resulting in a vertical jet superposition. The leading characteristics of a jet superposition include the development of anomalously strong wind speeds that can exceed 100 m s–1, the consolidation of the pole-to-equator baroclinicity into a narrow zone of contrast in the vicinity of the jet, and the development of a steep, single-step tropopause structure. Prior work indicates that the dynamical and thermodynamic environments accompanying jet superpositions often promote the development of vigorous transverse vertical circulations that can induce high-impact weather, such as heavy precipitation and extreme near-surface winds. This presentation will review the dynamical processes that facilitate the development of jet superpositions, as well as linkages between jet superpositions with high-impact weather events and midlatitude blocking. We will then use reanalysis data and climate model projections to explore historical trends and projected changes in jet superposition frequency in conjunction with anthropogenic climate change.