Strong Stratospheric Wave Events Precede North American Cold Extremes (Invited Presentation)

North American cold events have been suggested to be associated with stratospheric waves. However, the underlying mechanisms and governing factors are not well understood. We introduce a simple method to identify events characterized by extreme stratospheric wave activity in reanalysis and model output, finding that strong stratospheric wave events are associated with intraseasonal swings from warm to cold spells over North America in both reanalysis and CMIP6 models. Notably, strong wave events increase the risk of North American cold extremes by 50% to 90% with a lag of 5-25 days.

The causality between stratospheric wave activity and cold extremes is examined by idealized nudging experiments in an AGCM with a well-resolved stratosphere. The stratosphere in the nudging run is fully relaxed to its counterpart in a free-running control simulation. By comparing the strong wave events between the two runs, we find that despite having different surface patterns prior to event onset, the nudging run replicates the surface fingerprints observed in the control run after event onset under nearly identical stratospheric evolutions. This indicates that the increased risk of cold events can be attributed to the strong stratospheric wave activity rather than being solely driven by tropospheric processes.

Further analysis with ERA5 shows that the phase of the quasi-biennial oscillation (QBO) can modulate the surface signature of strong stratospheric wave events. Strong wave events during westerly QBO (wQBO) are followed by surface cold anomalies over North America. In contrast, during easterly QBO (eQBO), we find no North American cooling following strong wave events. The different surface responses are attributed to the differences in vertical wave coupling. Strong wave events during eQBO exhibit a vertical structure of a westward tilt with increasing altitudes, while strong wave events during wQBO transition from a westward to an eastward tilt. These distinctions suggest that wQBO favors the downward coupling of strong stratospheric wave events while eQBO suppresses it. Interestingly, while CMIP6 models resemble ERA5 during wQBO, they struggle to replicate the impact of eQBO on strong wave events. In other words, CMIP6 models do not differentiate between wQBO and eQBO for the surface signals following strong wave events. Models tend to overestimate the strength of the Alaskan ridge following strong stratospheric wave events under eQBO, suggesting excessive downward coupling in models compared to ERA5. Our findings carry important implications for understanding the stratospheric contribution to surface cold extremes.