Understand the Role of the Stratosphere in North American Cold Air Outbreaks

In the Northern Hemisphere cool-season (DJFM), surface temperature and pressure anomalies can be present up to 60-days following a Sudden Stratospheric Warming (SSW) onset date. The stratosphere can further influence surface temperature extremes during Rossby wave reflection events, where upward wave-activity flux (WAF) originating in the troposphere encounters a reflecting layer in the mid- and upper-stratosphere and subsequently is reflected downward into the troposphere, leading to locally amplified flow.

This research investigated the associations between various types of stratospheric variability (e.g., SSW and strong vortex events), the WAF in the Pacific hemisphere (specifically over Siberia and Alaska), and its relationship to the wintertime cold air outbreaks (CAO) over the United States (US). Using ERA5 reanalysis data, the results show that SSW events and wave reflection events have statistically significant impacts on the characteristics of CAOs that occur in their wake compared to the full population of US CAOs in the 1959-2021 period. Next, the analysis establishes upstream dynamical patterns specific to these CAO, to create a climatological baseline to examine the physical mechanisms responsible for the significant differences in the stratosphere-influenced CAOs. This research presents a comprehensive catalogue of WAF prior to US CAOs and highlights the variability in Alaskan ridge building (the critical synoptic feature for CAO in the United States) between the events.

The analysis is presented in the context of our current understanding of the impact of weak and strong vortex conditions on the tropospheric flow with an emphasis on impacts in North American and the United States.