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Composites of the 60 days after historical SSWs in the JRA-55 reanalysis for (a) mean sea level pressure anomalies [hPa]; (b) surface temperature anomalies [K]; and (c) precipitation anomalies [mm]. The stippling indicates regions that are significantly different at the 95% level from the climatology.  

Composites of the 60 days after historical SSWs in the JRA-55 reanalysis for (a) mean sea level pressure anomalies [hPa]; (b) surface temperature anomalies [K]; and (c) precipitation anomalies [mm]. The stippling indicates regions that are significantly different at the 95% level from the climatology.  

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... If the air from stratosphere descended over Central Europe and the British Isles, it may have pushed lower altitude air, containing high concentrations but relatively low 22 Na/ 7 Be ratio, into Finland. Butler et al. (2017) lists Northern Hemisphere SSWs that have been detected in the reanalysis since 1958 until 2010. From 2000 to 2010 there were several SSWs that might have caused an increase in the cosmgenic isotope concentrations. ...
... The increase in both 7 Be and 22 Na concentrations was clear by a factor of 2.5-2.6 but there was no clear increase in the 22 Na/ 7 Be ratio which remained rather steady. The panel C shows the Kotka data from the winter of 2010 when two SSW events occurred during the same winter which central dates are marked by the dashed lines (Butler et al., 2017). The first and possibly the stronger one had a central date on February 9, 2010, and the second, and possibly the weaker one, had a central date on March 24, 2010. ...
... In the month following an SSW there is generally anomalous warmth over Northern Canada and Alaska, the Middle East, and Central Asia, anomalous cold over Siberia and Northern Europe 23 , and cold air outbreaks in Europe and the United States [23][24][25][26] . These patterns reflect the surface manifestation of the negative phase of the Northern Annular Mode (NAM) 27 , which characterizes the latitudinal position of the midlatitude jet stream and appears to descend from the stratosphere to the troposphere during and after an SSW 28 . ...
... In the month following an SSW there is generally anomalous warmth over Northern Canada and Alaska, the Middle East, and Central Asia, anomalous cold over Siberia and Northern Europe 23 , and cold air outbreaks in Europe and the United States [23][24][25][26] . These patterns reflect the surface manifestation of the negative phase of the Northern Annular Mode (NAM) 27 , which characterizes the latitudinal position of the midlatitude jet stream and appears to descend from the stratosphere to the troposphere during and after an SSW 28 . ...
... These patterns reflect the surface manifestation of the negative phase of the Northern Annular Mode (NAM) 27 , which characterizes the latitudinal position of the midlatitude jet stream and appears to descend from the stratosphere to the troposphere during and after an SSW 28 . Surface predictability is enhanced in Asia, the Central United States, and the Middle East in the "window of opportunity" following an SSW, but it is degraded in Europe 23,29,30 . ...
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... However, SSW events are very rare in the SH due to the smaller planetary wave amplitude (Van Loon et al. 1973). The exception is the unique and remarkable case of September 2002, the only SSW event detected in the SH since the satellite observations began in 1979 (Butler et al. 2017). Despite the evidence of the importance of the SSW in positioning the main mid-latitude storms tracks, it is unclear what the influence of SSWs are in the subtropics. ...
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Four recent reanalysis products ERA-Interim, NCEP-CFSR, MERRA-2 and JRA-55 are evaluated and compared to an older reanalysis JRA-25, to quantify their confidence in representing Cut-off lows (COLs) in the Southern Hemisphere. The climatology of COLs based on the minima of 300-hPa vorticity (ξ300) and 300-hPa geopotential (Z300) provides different perspectives of COLs and contributes to the understanding of the discrepancies observed in the literature regarding their numbers and seasonality. The COLs compare better among the newest reanalyses than compared to the older reanalysis JRA-25. The difference in number between the latest reanalyses are generally small for both ξ300 and, with more COLs identified in ξ300 than in Z300 for all reanalyses. The spatial differences observed between the newest reanalyses are mainly due to differences in the track lengths, which is larger in ERA-Interim and JRA-55 than in NCEP-CFSR and MERRA-2, resulting in disparities in the track density. This is likely due to the difference in the assimilation data system used in each reanalysis product. The largest differences in intensities occur in the ξ300, because this field is very sensitive to the reanalysis resolution. The mean separation distance of the COLs that match between the latest reanalyses are generally small, while the older JRA-25 has a broader distribution and larger number of matches with relatively large distances, indicating larger uncertainties in location of COLs. The results show significant improvements for the most recent reanalyses compared to the older JRA-25 reanalysis, indicating a progress in representing the COL properties.
... The nonlinear interaction between storm systems and planetary-scale waves contributes to changes in the atmospheric circulation, which can constructively or destructively interfere with the large climatological standing waves; enhancement (destruction) of these waves can increase (decrease) upward propagation of energy in early to mid-winter that weakens (strengthens) the stratospheric polar vortex 77,87,88 . The tropospheric response to either a weakened or strengthened polar vortex is hemispheric in scale and most closely resembles the negative or positive Arctic Oscillation (AO), respectively 15,89,90 . ...
... In contrast, the observed temperature trends coupled with observational studies suggest that AA favours the increase of the meridional exchange of air masses between the Arctic and the midlatitudes, resulting in the NH midlatitude continents cooling relative to the whole NH as Arctic warming accelerates. This asymmetric distribution of observed NH warming is consistent with the surface temperature anomaly pattern following polar vortex disruptions 90 . ...
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The Arctic has warmed more than twice as fast as the global average since the late twentieth century, a phenomenon known as Arctic amplification (AA). Recently, there have been considerable advances in understanding the physical contributions to AA, and progress has been made in understanding the mechanisms that link it to midlatitude weather variability. Observational studies overwhelmingly support that AA is contributing to winter continental cooling. Although some model experiments support the observational evidence, most modelling results show little connection between AA and severe midlatitude weather or suggest the export of excess heating from the Arctic to lower latitudes. Divergent conclusions between model and observational studies, and even intramodel studies, continue to obfuscate a clear understanding of how AA is influencing midlatitude weather. Amplified warming in the Arctic has been linked to weather variability in the midlatitudes. This Review considers the evidence from both observations and modelling studies on this link for increasing severe winter weather, including cold temperatures and heavy snowfalls.
... The nonlinear interaction between storm systems and planetary-scale waves contributes to changes in the atmospheric circulation, which can constructively or destructively interfere with the large climatological standing waves; enhancement (destruction) of these waves can increase (decrease) upward propagation of energy in early to mid-winter that weakens (strengthens) the stratospheric polar vortex 77,87,88 . The tropospheric response to either a weakened or strengthened polar vortex is hemispheric in scale and most closely resembles the negative or positive Arctic Oscillation (AO), respectively 15,89,90 . ...
... In contrast, the observed temperature trends coupled with observational studies suggest that AA favours the increase of the meridional exchange of air masses between the Arctic and the midlatitudes, resulting in the NH midlatitude continents cooling relative to the whole NH as Arctic warming accelerates. This asymmetric distribution of observed NH warming is consistent with the surface temperature anomaly pattern following polar vortex disruptions 90 . ...
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... In this paper we are going to focus on the relations between the atmospheric and geophysical parameters during three winters: 2012-2013, 2013-2014 and 2014-2015 (hereafter, eQBO/SSW winter, wQBO/noSSW winter and eQBO/noSSW winter, respectively, see also Fig. S2 in the Supplementary Material). The first and third winters are characterized by the easterly QBO phase, however, a strong SSW event was observed only during the first one, in the beginning of January 2013 (January 6-7, see Butler et al., 2017). During the 2014-2015 winter two week SSWs were observed in the polar stratosphere, but there was no significant change of the stratospheric zonal wind at 60N, and the undisrupted polar vortex was observed until the end on winter (end of March -beginning of April 2015), see Manney et al. (2015) and Figure S3 in the Supplementary Material with plots of the zonal averaged temperature and geopotential height anomalies observed in 2012-2015 in the zone 60-90N. ...
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... 6 presents the periods associated with the states of the clusters 1-3 and concurrently the known SSW events identified by experts. Central dates of the "split" and "displacement" SSWs obtained by Charlton and Polvani for period 1958 -2002 [49], and non-classified SSWs from [50] for period 2003 -2013 are shown. Almost all expert-defined SSW events are clearly colocated in time with the segments of clusters 1-3. ...
... (a) Histogram of occurrence of PV states associated with each cluster; (b) zonal mean zonal wind along 60°N lat Diagram of occurrences of SSW events based on clustering result (colored segments) and their correspondence to the known SSW events[49,50]. ...
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A profound understanding of the stratospheric wintertime dynamics and its climate changes are important for improving seasonal forecast skill. The primary goal of the research of the wintertime Arctic stratospheric polar vortex (PV hereafter) is defining its states and their clustering. Manual classification is a highly time-consuming task suffering of researcher subjectivity. We apply deep learning methods that let us cluster the PV states based on their spatial structure. We designed the particular kind of neural networks called variational convolutional autoencoder with the sparsity constraint (SpCVAE). We applied the hierarchical agglomerative clustering algorithm to the states pf PV described by their embedded representation generated by SpCVAE. 96-dimensional embedded representation was found to be optimal with high samples reconstruction quality. The best number of clusters was chosen based on "elbow rule" and topic-specific reasoning. The approach applied let us automatically distinguish weak PVs of "displacement" and "split" types, as well as to isolate several strong vortex states of different shift directions. These results are only obtainable when one considers the spatial structure of the PV. We have constructed the calendar of the PV states based on the clustering result. Clustered events of weak PVs were examined and demonstrated good correspondence with the calendar of sudden stratospheric warmings that have been built manually. This result is now the basis for the research of the stratosphere-troposphere interaction for existing and future climate scenarios.
... The SSW onset dates in this study are also compared with previous studies [9,20,53,[60][61][62], and they are generally consistent. There are 35 SSW events in the JRA55 reanalysis and 38 SSW events in the NCEP/NCAR reanalysis, so the average frequency is 5-6× per decade. ...
... Only two reanalyses are used in this study, but the SSW events especially in the satellite era, have been shown to be fairly consistent between several reanalyses [61]. Furthermore, comparisons between CESM1-WACCM and other models are still lack, although the SSW modeling in MPI-ESM-LR is also reported [70]. ...
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Using the historical simulation from the CESM1-WACCM coupled model and based on the JRA55 and NCEP/NCAR reanalyses, the general statistical characteristics of the major sudden stratospheric warmings (SSWs) in this stratosphere-resolving model are assessed. The statistical and diagnostic results show that CESM1-WACCM can successfully reproduce the frequency of SSW events. As in the JRA55 and NCEP/NCAR reanalyses, five or six SSW events, on average, occur in a model decade. The seasonal distribution of SSWs is also well simulated with the highest frequency in January (35%). The unprecedented low SSW frequency observed in 1990s from the two reanalyses is also identified in a model decade (1930s). In addition, the overestimated duration of SSW events in the earlier WACCM version is not identified in CESM1-WACCM when compared with the two reanalyses. The model can well reproduce the downward propagation of the stratospheric anomaly signals (i.e., zonal wind, height, temperature) following SSWs. Both the modelling and observational evidences indicate that SSWs are proceeded by the positive Pacific-North America (PNA) and negative Western Pacific (WP) pattern. The negative North Atlantic Oscillation (NAO) develops throughout the SSW life cycle, which is successfully modeled. A cold Eurasian continent-warm North American continent pattern is observed before SSWs at 850 h Pa, while the two continents are anomalously cold after SSWs in both the reanalyses and the model.
... They considered separately vortex-displacement and vortex-splitting events, and documented the differences between their evolving vertical structures. Butler et al. (2017) used data from six different reanalysis products to produce a sudden stratospheric warming compendium for a region that extends from the surface to the stratosphere, as the importance of stratospheric-tropospheric connections have become increasingly apparent. Evidence from 3D numerical simulations with high resolution points to the existence of complex flow structures during the warmings. ...
Article
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This two-part paper aims to provide a Lagrangian perspective of the final southern warming in spring of 2002, during which the stratospheric polar vortex (SPV) experienced a unique splitting. We approach the subject from a dynamical systems viewpoint and search for Lagrangian coherent structures using a Lagrangian descriptor that is applied to reanalysis data. Part I presents our methodology and focuses by means of a kinematic model, on the understanding of fundamental processes for filamentation and ultimately for vortex splitting on an isentropic surface in the middle stratosphere. The present Part II discusses the three dimensional evolution of the flow during the selected event. For this, we apply the definition of vortex boundary developed in Part I for guidance in the selection of trajectories to illuminate the evolving flow structures, and invoke a criterion that allows to justify why at an isentropic level a pinched vortex will split in later times. Lagrangian structures identified include surfaces that are several kilometers deep, and which a particle trajectory analysis confirms as barriers to the flow. The role of Lagrangian structures in determining the fate of particles during the SPV splitting is discussed.