Temporal Phasing of Stronger Warm Air Mass Transport into the Polar Stratosphere and Cold Air Outbreaks in North America

Severe cold air outbreak events over North America (CAO_NA) are often accompanied with a weakening of the stratospheric polar vortex. One of the main drivers causing this weakening is the intensified mass transport into the polar stratosphere (ST+). However, the robustness of the linkage between CAO_NA and ST+ events has yet to be fully established. Here, the Empirical Orthogonal Function (EOF) method is applied to daily time series of mass transport into the polar stratosphere, using lead/lag days from -10 to 10 days around the peak dates of CAO_NA as the spatial domain and the number of individual CAO_NA events as the temporal domain. The results indicate that out of 227 CAO_NA events during the 41 winters (November-March) from 1979 to 2020, only 64 are not accompanied with ST+ events. The remaining 72% (163) coupled with ST+ events can be categorized into three types: two types are characterized by ST+ events occurring a few days before (ST+_lead) or after (ST+_lag) the peak dates of CAO_NA events, while the third type corresponds to ST+ events occurring a week before (ST+_week) CAO_NA events. Both “ST+_week” and “ST+_lead” events are associated with a strengthening of wavenumber-1 waves, followed by a strengthening of wavenumber-2 waves. The “ST+_lead” and “ST+_week” events are featured by a “warm Arctic & cold mid-latitudes” pattern in the Western Hemisphere with a reversed meridional dipole pattern in the Eastern Hemisphere. “ST+_lag” events are purely driven by wavenumber-2 waves, corresponding to the co-occurring coldness over mid-latitudes of both North America and Eurasia and a warmer Arctic. Further combining with the stratospheric polar vortex and circumpolar wind conditions in each type, a set of stratospheric weather regimes for CAO_NA events are identified.