Sensitivity of the February 2021 U.S. Cold-Air Outbreak to Tropopause Polar Vortex Intensity

Tropopause Polar Vortices (TPVs) are coherent long-lived vortices along the tropopause, with cyclonic TPVs identified by a local minimum in temperature and local maximum in potential vorticity along the dynamic tropopause. TPVs are most often found in high latitudes poleward of 60ºN but can be transported into the midlatitudes, where they can impact a wide variety of synoptic-scale phenomena. One common TPV pathway out of the high latitudes is via Canada, where TPVs and their associated cold pools can then contribute to major midlatitude Cold Air Outbreaks (CAOs).

February 2021 featured a historic CAO across the United States, both in terms of magnitude and duration. This CAO and associated winter storms resulted in substantial societal impacts, with extensive power and water outages across Texas and parts of Oklahoma, record cold, and multiple snow and ice events. This CAO was associated with multiple TPVs transported southward into southern Canada, with a downstream upper-tropospheric block resulting in the TPVs lingering over the same region for multiple days before merging, building a reservoir of cold air continually advected southward into the South Plains leeward of the Rockies. It is hypothesized here that the magnitude and southward extent of the CAO is sensitive to the intensity and subsequent evolution and merging of the TPVs.

A series of numerical modeling experiments using the Model for Prediction Across Scales (MPAS) is used to assess the sensitivity of the CAO evolution to the antecedent TPVs. Experiments are designed to weaken and strengthen the TPV by imposing heating and cooling rates of 5, 10, and 15 K per day near the tropopause, respectively. The stronger TPVs merged more quickly but farther east with a smaller CAO spatial extent, while the weaker TPVs took longer to merge and subsequently merged farther east. Results show a non-linear relationship exists between TPV intensity and track with the southward extent and magnitude of the cold air outbreak. We conclude that the optimal balance maximizing the impacts of the February 2021 CAO in the Southern Plains is the rate at which multiple TPVs merge and subsequently move eastward.