A Survey of North American Polar–Subtropical Jet Superposition Events: Development, Impacts, Predictability, and Future Changes

October 30, 2025

Andrew Winters

Hosted by Russ Schumacher

Abstract

The polar and subtropical jet streams are among the most recognizable structures within the Earth’s atmosphere and are responsible for modulating near-surface weather conditions at midlatitudes. The polar and subtropical jets typically remain separate from one another, but occasionally merge on synoptic time scales to form a “polar–subtropical jet superposition”. A jet superposition features strong wind speeds that can exceed 100 m s–1, a single-step pole-to-equator tropopause structure, and a consolidation of the pole-to-equator baroclinicity into a narrow zone of contrast within the near-jet environment. Consequently, a particularly potent dynamical and thermodynamic environment is assembled during jet superpositions that supports the development of extreme weather events, such as anomalously strong surface cyclones and extreme precipitation. This presentation will introduce the dynamical and thermodynamic processes that lead to the development of jet superposition events over North America, with a specific focus on the respective roles of tropopause polar vortices and moist diabatic processes equatorward of the jet, and highlight mechanisms that favor the occurrence of extreme weather in their vicinity. Motivated by the linkage between jet superpositions and high-impact weather, we also consider the influence of diabatic processes on their prediction and how the frequency of jet superpositions may change under various climate change scenarios. Finally, we will introduce the NASA-sponsored North American Upstream Feature-Resolving and Tropopause Uncertainty Reconnaissance Experiment (NURTURE), which will be conducted during Winter 2026 and 2027 to observe the properties of jet superposition events in unprecedented detail.

Prof. Winters is an Assistant Professor in the Department of Atmospheric and Oceanic Sciences (ATOC) at the University of Colorado Boulder and has led the synoptic meteorology research group at CU Boulder since August 2019. His research interests are broadly focused on understanding the dynamics, variability, and predictability of synoptic-scale environments conducive to the development of high-impact weather events. In his free time, Prof. Winters enjoys running, hiking, cooking, and cheering on all Wisconsin sports teams.