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Strong stratospheric wave events in ERA5 reanalysis As in Fig. 1, but for composites of strong stratospheric wave events. The strong wave events are defined by the 95th percentile of the first principal component of the zonally asymmetric component of 10 hPa geopotential height. See details in Methods.
Source publication
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...
Citations
... Recently, several studies used the cluster analysis and the empirical orthogonal function method to identify the SPV stretching events (Cohen et al., , 2022Ding et al., 2022Ding et al., , 2023, Liang Z et al., 2023 and found that their reflective mechanism and tropospheric response closely resemble those of reflecting SSWs (Matthias & Kretschmer, 2020;Messori et al., 2022;Shen et al., 2023;Zou et al., 2024). Further efforts have been made to unveil possible causes and impacts of the SPV stretching events. ...
... Zhang et al., 2016). Further studies also use a nudging method in modeling to highlight the relative importance of the stratospheric process in Eurasian cooling induced by BKS sea ice loss (Ding et al., 2023;Peings, 2019;Xu et al., 2021Xu et al., , 2023P. Zhang, Wu, Simpson, et al., 2018). ...
Plain Language Summary
Stratospheric polar vortex (SPV) stretching events are characterized by a zonally asymmetric pattern of lower stratospheric variability with positive height anomalies over northern Eurasia and negative anomalies over Canada. In this study, we mainly focus on the SPV stretching events that occurred from October to November, and this is because the surface cooling over high‐latitude Eurasia following these events is associated with the stratospheric pathway. Here, the autumn events can be categorized into lower (LoSIC/BKSIC) and higher (HiSIC/Ctrl) sea ice groups based on observations and simulations. We further demonstrate that lower BKS sea ice conditions are favorable for intensified upward propagation of waves into the stratosphere in the Euro‐Siberian sector prior to the onset of SPV stretching events, and subsequently affect the geometry of the SPV (SPV stretching pattern). The stratospheric wave‐2 ridge anomalies migrate downward to the mid‐troposphere 30–40 days following SPV stretching onset, leading to the enhancement of the Arctic‐North European high and attendant colder Eurasia. These results emphasize the critical modulating role of Arctic sea ice in the Eurasian cooling response to SPV stretching events.
... The increasing frequency of weak SPV states can account for nearly 60% of the cooling region over midlatitude Eurasia; this proportion rises to about 80% when El Niño-Southern Oscillation variability is included, indicating that persistent weak SPV and tropical variability together contribute to Eurasian cooling (Kretschmer et al. 2018a). Besides the SPV stretching events, an empirical orthogonal function method is applied to extract the planetary wave-1 pattern associated with stratospheric variability, which features a high surface pressure anomaly over Alaska and a low anomaly over eastern North America, thus favoring cold surges over North America via stratosphere-troposphere interactions (Ding et al. 2022(Ding et al. , 2023. ...
... With the increasing importance of weak SPV, most existing literature appears to emphasize the crucial role of SSWs in Eurasian and North American coldness (e.g., Hall et al. 2021;Kodera et al. 2016;Xu et al. 2022;Zhang et al. 2020), the SPV stretching events and related surface responses are only marginally mentioned in several studies (Cohen et al. , 2022Ding et al. 2022Ding et al. , 2023Kretschmer et al. 2018b;Liang et al. 2023;Matthias and Kretschmer 2020;Messori et al. 2022;Shen et al. 2023). However, the occurrence probability of SSWs is less than that of SPV stretching events Liang et al. 2023). ...
... For autumn SPV stretching events, the downward extension of stratospheric signals during the early stage directly results in negative AO/NAO-like anomalies and thereby the first cold anomalies in northern Eurasia. This pattern is broadly consistent with the lagged effect of the planetary wave-1 pattern (Ding et al. 2022(Ding et al. , 2023 and the features of the Eurasia-weakened pattern notified by Liang et al. (2023). The deepening of the East Asian trough and the increase of the Ural high associated with stratospheric wave activity during the late stage are favorable for the second cold surge in high-latitude Eurasia and mid-latitude East Asia respectively. ...
The weak stratospheric polar vortex (SPV) is usually linked to Northern Hemisphere cold spells. Based on the fifth generation of ECMWF atmospheric reanalysis and WACCM model experiments, we use K-means cluster analysis to extract the zonally asymmetric pattern of October-February stratospheric variability, which involves a stretched SPV and hence leads to cold surges in Northern Hemispheric mid-latitudes. There are contrasting effects and mechanisms between autumn (October–November) and late winter (February) SPV stretching events. In October, anomalies in the stratospheric circulation affect the near-surface 16–20 days after the weakening of the SPV. This contributes to a shift in the North Atlantic Oscillation (NAO) towards its negative phase, leading to cold anomalies over northern Eurasia. Together with the weakening of planetary wave-1 during days 31–40, the second stratosphere–troposphere coupling strengthens the East Asian trough and the Siberian high, resulting in Eurasian high-latitude cooling. For November, the suppressed upward propagation of wave-1 during days 11–15 is conducive to anomalous high pressure over northern Europe and thereby European cooling through a stratospheric pathway, while for days 21–30, the weakening of propagating wave-2 over Eastern Europe intensifies the mid-latitude wave train through a tropospheric pathway, favorable for cold temperatures in mid-latitude East Asia. In contrast, the late winter SPV stretching events and the attendant Eurasian coldness during 11–25 days are likely to have been simultaneously driven by the long-lived European high anomaly, which enhanced the upward-propagating tropospheric waves into the stratosphere and thus favored SPV stretching. It indicates that the tropospheric pathway, rather than the stratospheric pathway, plays a dominant role in cold Eurasia following February SPV stretching events.
... They argue that the tropospheric circulation regime associated with NA cold spells only acts as a precursor for the suppressed planetary wave-1 (a wave structure with one crest and one trough at a latitude circle) activity in the stratosphere. Ding et al. (2022Ding et al. ( , 2023, in contrast, show that strong stratospheric wave activity precedes positive NAO-like NA cooling at the surface with a 10-day lag. The divergent conclusions underscore that the surface impacts of stratospheric waves remain an area of active research. ...
... Ding et al. (2022) measure stratospheric wave activity with a simple metric based on the EOF analysis of the zonally asymmetric component of 10 hPa geopotential height, which can be readily applied to climate models. Ding et al. (2023) further show the impact of extreme stratospheric wave events on NA cold spells consistently across reanalysis and Coupled Model Intercomparison Project Phase 6 (CMIP6) ensemble means. The present study will assess extreme stratospheric wave activity in CMIP6 models. ...
... The subseasonal swing in temperature also resembles the surface fingerprints of the stratosphere-troposphere oscillation identified by Shen et al. (2022). These are also in line with other recent studies that suggest a stratospheric source of subseasonal predictability for NA surface temperature variability (e.g., Ding et al., 2022Ding et al., , 2023Messori et al., 2022). ...
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 United States. 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.
Midlatitude weather extremes such as blocking events and Rossby wave breaking are often related to large meridional shifts in the westerly jet stream. Numerous diagnostic methods have been developed to characterize these weather events, each emphasizing different yet interrelated aspects of circulation waviness, including identifying large-amplitude ridges or persistent anomalies in geopotential height. In this study, we introduce a new metric to quantify the circulation waviness in terms of effective time scale. This is based on the Rossby wave packet from the one-point correlation map of anomalous meridional wind, applicable to jet waviness involving multiple wavenumbers. Specifically, we estimate the intrinsic frequency of Rossby waves and decay time scale of wave amplitude in the reference frame moving at the local time mean zonal wind. The resulting effective time scale, derived from linear theory, serves as a proxy for the eddy mixing time scale in jet meandering. Remarkably, its spatial distribution roughly resembles that of circulation waviness in the Northern Hemisphere winter as depicted by local wave activity (LWA). In the high-latitude regions characterized by weak zonal winds, the long time scale in waviness aligns with large values in LWA. By contrast, short waviness time scales in subtropical jet regions correspond to the suppressed amplitude in waviness despite large values in eddy kinetic energy (EKE). Furthermore, the effective time scale in waviness largely captures the interannual variability of LWA in observations and its projected future changes in climate model simulations. Thus, this relation between the waviness time scale and zonal wind provides a physical mechanism for understanding how zonal wind changes impact regional weather patterns in a changing climate.
Significance Statement
The purpose of this study is to better understand what controls weather extremes in midlatitude regions such as blocking events and Rossby wave breaking. We introduce a novel concept, the effective time scale of jet stream meandering, which sheds light on these phenomena. Through analyzing Rossby waves in the reference frame moving at the local time mean zonal wind, we derive a scaling relation between circulation waviness and eddy mixing time scale. Our findings reveal that this time scale closely mirrors the spatial distribution of circulation waviness in the Northern Hemisphere winter. Importantly, it captures interannual variability and climate change responses. These insights provide a physical basis for understanding how changes in zonal wind impact regional weather patterns in observations and climate models.
It is widely accepted that Arctic amplification (AA)—enhanced Arctic warming relative to global warming—will increasingly moderate cold-air outbreaks (CAOs) to the midlatitudes. Yet, some recent studies also argue that AA over the last three decades to the rest of the present century may contribute to more frequent severe winter weather including disruptive cold spells. To prepare society for future extremes, it is necessary to resolve whether AA and severe midlatitude winter weather are coincidental or physically linked. Severe winter weather events in the northern continents are often related to a range of stratospheric polar vortex (SPV) configurations and atmospheric blocking, but these dynamical drivers are complex and still not fully understood. Here we review recent research advances and paradigms including a nonlinear theory of atmospheric blocking that helps to explain the location, timing and duration of AA/midlatitude weather connections, studies of the polar vortex’s zonal asymmetric and intra-seasonal variations, its southward migration over continents, and its surface impacts. We highlight novel understanding of SPV variability—polar vortex stretching and a stratosphere–troposphere oscillation—that have remained mostly hidden in the predominant research focus on sudden stratospheric warmings. A physical explanation of the two-way vertical coupling process between the polar vortex and blocking highs, taking into account local surface conditions, remains elusive. We conclude that evidence exists for tropical preconditioning of Arctic-midlatitude climate linkages. Recent research using very large-ensemble climate modelling provides an emerging opportunity to robustly quantify internal atmospheric variability when studying the potential response of midlatitude CAOs to AA and sea-ice loss.
The stratosphere-troposphere and the tropics-Arctic couplings were intermittently enhanced in the 2023/24 winter. Here we used ERA5 reanalysis data and found that due to the amplification of planetary wavenumber 1 and 2 pulses, three displacement-type sudden stratospheric warming events occurred in one winter under the background conditions of warming equatorial middle and east Pacific, active equatorial convections, and easterly stratospheric equatorial winds. During the sudden stratospheric warming events, the stratospheric disturbances propagated downward to the surface, followed by continental cold surges. The residual meridional circulation was strengthened across the tropics and Arctic, anomalously more water vapor was transported into the stratosphere in tropics, while ozone content diminished in the lower stratosphere and grew in the upper stratosphere over the tropics. Meanwhile, water vapor and ozone over the Arctic exhibited a dipping pattern from the upper to the lower stratosphere.
Extreme stratospheric wave activity has been linked to surface cold extremes over North America, but little is known whether the Quasi-biennial Oscillation (QBO) plays a role in this linkage. Here, by comparing strong stratospheric wave events during the westerly phase (wQBO) with those during the easterly phase (eQBO), we show that the cooling signature following strong wave events depends on the QBO phase in observations. During wQBO, strong wave events are followed by an increased risk of North American cold extremes and a vertical structure shift from a westward phase tilt to an eastward tilt. However, strong wave events under eQBO do not change the cold risk nor alter the vertical tilt. We further examine this dependence on QBO in QBO-resolving climate models, finding that the cooling signature of strong wave events in models is largely insensitive to QBO phases. This insensitivity is suggested to be linked to model biases in the stratospheric wave representation.
