This paper compares surface signatures of the zonally symmetric and asymmetric modes of stratospheric variability, which describe the strength of the polar vortex and a planetary wave‐1 pattern, respectively. Unlike a weak polar vortex followed by negative Arctic Oscillation–like anomalies, strong stratospheric wave activity features a polar vortex displacement with a deep planetary wave‐1 structure, resulting in positive North Atlantic Oscillation–like North American cooling in about 10 days. Moreover, the linkage between the stratosphere and surface is examined in two reanalyzes and four models of different configurations, which show more robust North American cooling following the displacement of the polar vortex due to strong stratospheric wave activity than the zonally symmetric weakening of the polar vortex. This suggests strong stratospheric wave activity acts as a better predictor for cold spells in the northern U.S. and Canada compared with a weak polar vortex.
Extreme cold events over North America such as the February 2021 cold wave have been suggested to be linked to stratospheric polar vortex stretching. However, it is not resolved how robustly and on which timescales the stratosphere contributes to the surface anomalies. Here we introduce a simple measure of stratospheric wave activity for reanalyses and model outputs. In contrast to the well-known surface influences of sudden stratospheric warmings (SSWs) that increase the intraseasonal persistence of weather regimes, we show that extreme stratospheric wave events are accompanied by intraseasonal fluctuations between warm and cold spells over North America in observations and climate models. Particularly, strong stratospheric wave events are followed by an increased risk of cold extremes over North America 5–25 days later. Idealized simulations in an atmospheric model with a well-resolved stratosphere corroborate that strong stratospheric wave activity precedes North American cold spells through vertical wave coupling. These findings potentially benefit the predictability of high-impact winter cold extremes over North America.
<p>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.</p> <p>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.&#160;</p> <p>&#160;</p>
Extreme stratospheric wave activity has been suggested to be connected to surface temperature anomalies, but some key processes are not well understood. Using observations, we show that the stratospheric events featuring weaker‐than‐normal wave activity are associated with increased North American (NA) cold extreme risks 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. They are preceded by NA warm anomalies and increased AR frequency over the west coast, followed by increased risks of NA cold extremes and north‐shifted ARs over the Atlantic. Moreover, these links between the stratosphere and troposphere are attributed to the vertical structure of wave coupling. Weak wave events show a wave structure of westward tilt with increasing altitudes, while strong wave events feature a shift from westward tilt to eastward tilt during their life cycle. This wave phase shift indicates vertical wave coupling and likely regional planetary wave reflection. Further examinations of CMIP6 models show that models with a degraded representation of stratospheric wave structure exhibit biases in the troposphere during strong wave events. 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.
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