Climatology and Dynamical Mechanisms of Stratospheric Air Intrusions over the West U.S. Hotspot

Descending air streams are a fundamental component of atmospheric circulation. One particularly impactful type of large-scale descending air stream is dry air intrusion (DI). This flow can transport air from the upper troposphere, or in some cases from the lower stratosphere (i.e., stratospheric intrusions, SI), into the planetary boundary layer and even to the surface. These flows can impose hazards such as intense precipitation, strong winds, high ozone concentrations, and wildfires. The West Coast of the United States is a global hotspot for SIs, which are associated with the largest fires in California.

Despite previous studies examining the synoptic conditions during specific types of surface hazards, the connection to SIs, while noticed before, needs further confirmation. Additionally, the broader dynamical evolution of SIs under non-hazardous conditions remains poorly understood. Addressing these gaps, our study seeks to (1) uncover the mechanisms controlling the occurrence, evolution, and variability of DIs and SIs; (2) enhance our understanding of the precursor dynamical environments involved in stratosphere-troposphere exchanges; and (3) quantify weather hazards induced by DIs and SIs.

To achieve these objectives, we are taking a Lagrangian-climatological approach to identify DI and SI air trajectories above North America’s West Coast and classify them into synoptic patterns with similar potential vorticity (PV) conditions using the Self-Organizing Map (SOM) algorithm. We find that distinct Rossby wave breaking patterns correspond to the initiation of DIs, and, differently, SIs. These results emphasize the role of upper-level forcing for surface extremes, and their link through SIs. This research deepens our understanding of key atmospheric processes, all while highlighting a hazardous phenomenon with the potential for vast amounts of damage and danger to human life, as evidenced by the recent Los Angeles wildfires.