Assessing Stratosphere-troposphere Coupling of Extreme Stratospheric Wave Activity in CMIP6 Models

Recent studies have suggested that extreme stratospheric wave activity is connected to surface temperature anomalies, with a dynamical mechanism distinct from the canonical downward influence of stratospheric polar vortex events. However, some key processes regarding the underlying dynamics and timescales are not well understood. In this study, we show in observations that the stratospheric events featured by weaker-than-normal wave activity are associated with increased cold extreme risks over North America before and near the event onset, accompanied by less frequent atmospheric river (AR) events on the west coast of the U.S. Strong stratospheric wave events, on the other hand, exhibit a tropospheric weather regime transition. North American warm anomalies and increased AR frequency over the west coast are observed before strong wave events, while an increased risk of cold extremes over North America and north-shifted ARs over the Atlantic occurs after the events. Historical simulations from CMIP6 can capture the extreme stratospheric wave events and their overall tropospheric fingerprints, with evident uncertainties across different models.

These links between the stratosphere and troposphere are attributed to the vertical structure of wave coupling. Weak wave events are accompanied by a wave structure tilting westwards with increasing altitude, while strong wave events show a shift from westward tilt to eastward tilt during the life cycle of events. This wave phase shift indicates vertical wave coupling and likely regional planetary wave reflection. Further examination shows that models with a degraded representation of stratospheric wave structure exhibit biases in the troposphere during strong wave events. More specifically, models with a stratospheric ridge weaker than the reanalysis exhibit a weaker tropospheric signal. Our findings suggest that the vertical coupling of extreme stratospheric wave activity should be evaluated in the model representation of stratosphere-troposphere coupling.