Abstract:

North American (NA) cold extremes in winter have been linked to stratospheric phenomena, including sudden stratospheric warmings (SSWs) and extreme stratospheric waves. However, the reliability of these linkages and underlying mechanisms are not well understood. The role of the stratosphere in NA cold extremes continues to spark significant discussion.

In this talk, I introduce a simple measure of stratospheric wave activity, suitable for both reanalyses and model outputs. This approach reveals that unlike SSWs—which are known to lengthen the duration of weather patterns—strong stratospheric wave activity is accompanied by transitions from warm to cold spells over North America in reanalyses and climate models. Notably, strong stratospheric wave events increase the risk of NA cold extremes by up to 90%. Furthermore, idealized simulations corroborate that strong stratospheric wave activity precedes NA cold spells through vertical wave coupling. In addition, analysis of reanalysis shows that the phase of the quasi-biennial oscillation (QBO) can modulate the surface impacts of strong stratospheric wave events; but CMIP6 models exhibit potential biases. Overall, these findings underscore the importance of strong stratospheric wave activity in stratosphere-troposphere coupling. Strong stratospheric wave events, distinct from SSWs, offer valuable insights that potentially benefit the prediction of high-impact winter cold extremes over North America.