The Role of a Tropopause Polar Vortex in the January 2019 Arctic Outbreak

An extreme cold air outbreak took place across the Midwest, Great Lakes, and Northeast during 29 January to 1 February 2019. This event broke numerous long-standing minimum temperature records with wide-reaching societal impacts. These rare and dangerous conditions were a direct consequence of a tropopause polar vortex (TPV) originating within the Arctic and subsequently tracking southward into the United States. TPVs are coherent sub-synoptic-scale vortices that are identifiable by a closed cyclonic circulation around a minimum in potential temperature at the dynamic tropopause. TPVs can last for several weeks to even months at high latitudes and are driven by longwave radiative cooling near the tropopause associated with a sharp vertical gradient in water vapor.

This study uses observations, reanalysis, and simulations with the Model for Prediction Across Scales (MPAS) to investigate the dynamics and evolution of this TPV in January 2019 and the large-scale pattern that steered the vortex into the middle latitudes. By perturbing the strength of the TPV in MPAS simulations, we demonstrate the significant role it played in the cold air outbreak. The spatial extent of the extreme cold air in this case was more limited than often observed with large Arctic high-pressure systems that can cover wide portions of the U.S. east of the Rocky Mountains. Despite the highly anomalous and small-scale nature of the event, the skill of medium-range NWP in forecasting the event was impressive. Our study examines the influence of TPVs in extremes as well as intrinsic predictability. The climatological distribution of TPV excursions into middle latitudes and their attendant sensible weather impacts is also explored and suggests that incursions of TPVs into the middle latitudes during the winter are becoming more frequent. This talk will also discuss possible mechanisms for the increase in TPV incursions.