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Impact of Planetary Wave Reflection on Tropospheric Blocking over the Urals-Siberia Region in January 2008


doi: 10.1007/s00376-015-5052-4

  • Planetary wave reflection from the stratosphere played a significant role in changing the tropospheric circulation pattern over Eurasia in mid-January 2008. We studied the 2008 event and compared with composite analysis (winters of 2002/2003, 2004/2005, 2006/2007, 2007/2008, 2010/2011 and 2011/2012), when the downward coupling was stronger, by employing time-lagged singular value decomposition analysis on the geopotential height field. In the Northern Hemisphere, the geopotential fields were decomposed into zonal mean and wave components to compare the relative covariance patterns. It was found that the wavenumber 1 (WN1) component was dominant compared with the wavenumber 2 (WN2) component and zonal mean process. For the WN1 field, the covariance was much higher (lower) for the negative (positive) lag, with a prominent peak around +15 days when the leading stratosphere coupled strongly with the troposphere. It contributed to the downward coupling due to reflection, when the stratosphere exhibited a partially reflective background state. We also analyzed the evolution of the WN1 anomaly and heat flux anomaly, both in the troposphere and stratosphere, during January-March 2008. The amplitude of the tropospheric WN1 pattern reached a maximum and was consistent with a downward wave coupling event influenced by the stratospheric WN1 anomaly at 10 hPa. This was consistent with the reflection of the WN1 component over Eurasia, which triggered an anomalous blocking high in the Urals-Siberia region. We further clarified the impact of reflection on the tropospheric WN1 field and hence the tropospheric circulation pattern by changing the propagation direction during and after the event.
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  • Baldwin M. P., T. J. Dunkerton, 1999: Propagation of the Arctic Oscillation from the stratosphere to the troposphere. J. Geophys. Res., 104, 30 937- 30 946.10.1029/1999JD9004455cb2458d112b564926ed45f3ba29c8fchttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F1999JD900445%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/1999JD900445/fullGeopotential anomalies ranging from the Earth's surface to the middle stratosphere in the northern hemisphere are dominated by a mode of variability known as the Arctic Oscillation (AO). The AO is represented herein by the leading mode (the first empirical orthogonal function) of low-frequency variability of wintertime geopotential between 1000 and 10 hPa. In the middle stratosphere the signature of the AO is a nearly zonally symmetric pattern representing a strong or weak polar vortex. At 1000 hPa the AO is similar to the North Atlantic Oscillation, but with more zonal symmetry, especially at high latitudes. In zonal-mean zonal wind the AO is seen as a north-south dipole centered on 40°–45°N; in zonal-mean temperature it is seen as a deep warm or cold polar anomaly from the upper troposphere to 6510 hPa. The association of the AO pattern in the troposphere with modulation of the strength of the stratospheric polar vortex provides perhaps the best measure of coupling between the stratosphere and the troposphere. By examining separately time series of AO signatures at tropospheric and stratospheric levels, it is shown that AO anomalies typically appear first in the stratosphere and propagate downward. The midwinter correlation between the 90-day low-pass-filtered 10-hPa anomaly and the 1000-hPa anomaly exceeds 0.65 when the surface anomaly time series is lagged by about three weeks. The tropospheric signature of the AO anomaly is characterized by substantial changes to the storm tracks and strength of the midtropospheric flow, especially over the North Atlantic and Europe. The implications of large stratospheric anomalies as precursors to changes in tropospheric weather patterns are discussed.
    Baldwin M. P., T. J. Dunkerton, 2001: Stratospheric harbingers of anomalous weather regimes. Science, 294, 581- 584.10.1126/science.106331511641495eef35230b9a42dc2ec1960dde3dbee70http%3A%2F%2Fmed.wanfangdata.com.cn%2FPaper%2FDetail%2FPeriodicalPaper_PM11641495http://med.wanfangdata.com.cn/Paper/Detail/PeriodicalPaper_PM11641495Observations show that large variations in the strength of the stratospheric circulation, appearing first above approximately 50 kilometers, descend to the lowermost stratosphere and are followed by anomalous tropospheric weather regimes. During the 60 days after the onset of these events, average surface pressure maps resemble closely the Arctic Oscillation pattern. These stratospheric events also precede shifts in the probability distributions of extreme values of the Arctic and North Atlantic Oscillations, the location of storm tracks, and the local likelihood of mid-latitude storms. Our observations suggest that these stratospheric harbingers may be used as a predictor of tropospheric weather regimes.
    Barriopedro D., R. Garcia-Herrera, A. R. Lupo, and E. Hernãndez, 2006: A climatology of Northern Hemisphere Blocking. J.Climate, 19, 1042- 1063.10.1175/JCLI3678.10c4a80e05bcaa37ed0edbcc505a41ecbhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F233729991_A_Climatology_of_Northern_Hemisphere_Blockinghttp://www.researchgate.net/publication/233729991_A_Climatology_of_Northern_Hemisphere_BlockingAbstract In this paper a 55-yr (1948-2002) Northern Hemisphere blocking climatology is presented. Traditional blocking indices and methodologies are revised and a new blocking detection method is designed. This algorithm detects blocked flows and provides for a better characterization of blocking events with additional information on blocking parameters such as the location of the blocking center, the intensity, and extension. Additionally, a new tracking procedure has been incorporated following simultaneously the individual evolution of blocked flows and identifying coherently persistent blocked patterns. Using this method, the longest known Northern Hemisphere blocking climatology is obtained and compared with previous studies. A new regional classification into four independent blocking sectors has been obtained based on the seasonally preferred regions of blocking formation: Atlantic (ATL), European (EUR), West Pacific (WPA), and East Pacific (EPA). Global and regional blocking characteristics have been described, examining their variability from the seasonal to interdecadal scales. The global long-term blocking series in the North Hemisphere showed a significant trend toward weaker and less persistent events, as well as regional increases (decreases) in blocking frequency over the WPA (ATL and EUR) sector. The influence of teleconnection patterns (TCPs) on blocking parameters is also explored, being confined essentially to wintertime, except in the WPA sector. Additionally, regional blocking parameters, especially frequency and duration, are sensitive to regional TCPs, supporting the regional classification obtained in this paper. The ENSO-related blocking variability is evident in blocking intensities and preferred locations but not in frequency. Finally, the dynamical connection between blocking occurrence and regional TCPs is examined through the conceptual model proposed by Charney and DeVore. Observational evidence of a dynamical link between the asymmetrical temperature distributions induced by TCPs and blocking variability is provided with a distinctive contrast arm ocean/cold land pattern favoring the blocking occurrence in winter. However, the conceptual model is not coherent in the WPA sector, suggesting different blocking mechanisms operating in this sector.
    Christiansen B., 2000: A model study of the dynamical connection between the Arctic Oscillation and stratospheric vacillations. J. Geophys. Res., 105, 29 461- 29 474.10.1029/2000JD900542a266ee064c7a9c278356786877c92afchttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2000JD900542%2Fcitedbyhttp://onlinelibrary.wiley.com/doi/10.1029/2000JD900542/citedbyABSTRACT The dynamical connection between the stratosphere and the troposphere in the Northern Hemisphere winter is investigated with general circulation model (GCM) simulations under perpetual January conditions. The variability in the stratosphere is strongly dominated by vacillations on a timescale of 100 days. One-point correlation maps of the zonal mean zonal wind reveal the characteristic downward propagation of the stratospheric disturbances. The meridional structure of the stratospheric vacillations is well described by a few empirical orthogonal functions (EOFs) of geopotential height. The leading EOF describes a standing oscillation of the stratosphere, while the two next describe the vertical and horizontal propagation. In the troposphere the leading EOF of the surface pressure shows the characteristic circumpolar structure of the Arctic Oscillation. The covariance between the leading EOF of the surface pressure and the zonal mean zonal wind reveals a vertical structure similar to the observed below 10 hPa, while the sign of the covariance changes above 10 hPa where observations are scarce. The downward propagating stratospheric modes play a prominent role in the vertical coupling. Singular value decomposition of the cross-covariance matrix between the stratospheric zonal mean zonal wind and the surface pressure shows that a significant part of the tropospheric variability can be related to the downward propagating anomalies. The leading pattern of surface pressure anomalies found in this way closely resembles the pattern of the Arctic Oscillation and describes 20-30% of the total variance. This result is confirmed by studying the covariance between the geopotential height fields and the stratospheric principal components.
    Christiansen B., 2001: Downward propagation of zonal mean zonal wind anomalies from the stratosphere to the troposphere: Model and reanalysis. J. Geophys. Res., 106, 27 307- 27 322.10.1029/2000JD000214c783b03c5a5d38dcc5b8b3f21c12683dhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2000JD000214%2Fcitedbyhttp://onlinelibrary.wiley.com/doi/10.1029/2000JD000214/citedbyABSTRACT The connection between the Arctic Oscillation and the stratosphere is investigated on intra-annual timescales. Both the National Centers for Environmental Prediction reanalysis data and a general circulation model simulation are used. In the winter half year November-April the dominant variability in the stratosphere in middle and high latitudes has the form of downward propagation of zonal mean zonal wind anomalies. The strength of the anomalies decays below 10 hPa, but often the anomalies reach the surface. The time for the propagation from 10 hPa to the surface is ~15 days. When positive anomalies reach the surface, the phase of the Arctic Oscillation tends to be positive. The stratospheric variability and the downward propagation is found to be driven by the vertical component of the Eliassen-Palm flux. This flux propagates from the lower troposphere to the tropopause on a time scale of 5 days. Model and reanalysis compare well in the structure of the stratospheric variability and the connection between the stratosphere and troposphere. However, the strength of the stratospheric variability is ~25% weaker in the model.
    Coughlin K., K. K. Tung, 2005: Tropospheric wave response to decelerated stratosphere seen as downward propagation in northern annular mode. J. Geophys. Res. , 110,D01103, doi:10.1029/2004JD004661.10.1029/2004JD0046618254fe9f7e63cfde9374eb20aabdd141http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2004JD004661%2Fcitedbyhttp://onlinelibrary.wiley.com/doi/10.1029/2004JD004661/citedby[1] Baldwin and Dunkerton [1999] found that negative northern annular mode (NAM) anomalies sometimes descend all the way from the stratosphere into the lower troposphere. However, no viable mechanism has been proposed so far to account for the magnitude of the anomalies in the denser troposphere. Further, analysis shows that the character of the anomaly changes across the tropopause. Above the tropopause the NAM pattern is approximately zonal, and its descent represents the descent of decelerated zonal mean winds. This stratospheric change is explainable using theories similar to those for the descent of the zero-wind line associated with a major stratospheric sudden warming. However, such a reversal in the zonal mean wind rarely reaches the denser troposphere. The descent of the NAM anomalies into the troposphere may be implying a different relationship between the stratosphere and the troposphere. We note that in the troposphere the structure of the NAM has a large wave component. In some cases, this wave component appears to react to the decelerated wind configuration aloft. Here we show observations of the wave component and the zonal mean component in comparison to corresponding NAM events to show that the wave response is a sizable component of the NAM anomaly in the troposphere. We will also present a simple model calculation to show that tropospheric waves forced by topography can react to changing stratospheric winds. These tropospheric waves can project directly onto the tropospheric NAM patterns and produce anomalies in the index which appear to be connected to the negative NAM anomalies in the stratosphere.
    Dee, D. P., Coauthors, 2011: The ERA-Interim reanalysis: configuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553- 597.10.1002/qj.828b8698c40-b145-4364-9b39-4e603f942b9ff1b44af3b88d0a7be27ce2739fe46ee7http://onlinelibrary.wiley.com/doi/10.1002/qj.828/pdfhttp://onlinelibrary.wiley.com/doi/10.1002/qj.828/pdfABSTRACT ERA-Interim is the latest global atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). The ERA-Interim project was conducted in part to prepare for a new atmospheric reanalysis to replace ERA-40, which will extend back to the early part of the twentieth century. This article describes the forecast model, data assimilation method, and input datasets used to produce ERA-Interim, and discusses the performance of the system. Special emphasis is placed on various difficulties encountered in the production of ERA-40, including the representation of the hydrological cycle, the quality of the stratospheric circulation, and the consistency in time of the reanalysed fields. We provide evidence for substantial improvements in each of these aspects. We also identify areas where further work is needed and describe opportunities and objectives for future reanalysis projects at ECMWF. Copyright 2011 Royal Meteorological Society
    Dunn-Sigouin E., T. A. Shaw, 2015: Comparing and contrasting extreme stratospheric events,including their coupling to the tropospheric circulation. J. Geophys. Res.: Atmos., 120, 1374-1390, doi: 10.1002/2014JD022116.7b2c6d11-941e-489e-8b90-a2d2af18105acf40198a3edd6cbc185083d691bc1bf5http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2014JD022116%2Fabstractrefpaperuri:(a1b4f5b49fa5229cfbf539ee309cf851)/s?wd=paperuri%3A%28a1b4f5b49fa5229cfbf539ee309cf851%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2014jd022116%2Fabstract&ie=utf-8
    Geller M. A., J. C. Alpert, 1980: Planetary wave coupling between the troposphere and the middle atmosphere as a possible sun-weather mechanism. J. Atmos. Sci., 37, 1197- 1214.10.1175/1520-0469(1980)0372.0.CO;2294a4933d4f935efaa6a1261dcf55debhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F4691689_Planetary_wave_coupling_between_the_troposphere_and_the_middle_atmosphere_as_a_possible_sun-weather_mechanism%3Fev%3Dprf_cithttp://www.researchgate.net/publication/4691689_Planetary_wave_coupling_between_the_troposphere_and_the_middle_atmosphere_as_a_possible_sun-weather_mechanism?ev=prf_citAbstract The possibility of planetary wave coupling between the troposphere and solar-induced alterations in the upper atmosphere providing a viable mechanism for giving rise to sun-weather relationships is investigated. Some of the observational evidence for solar-activity-induced effects on levels of the upper atmosphere ranging from the thermosphere down to the lower stratosphere are reviewed. It is concluded that there is evidence for such effects extending down to the middle stratosphere and below. Evidence is also reviewed that these effects are due to changes in solar ultraviolet emission during disturbed solar conditions. A theoretical planetary wave model is then used to see at what levels in the upper atmosphere moderate changes in the mean zonal wind state would result in tropospheric changes. It is concluded that changes in the mean zonal flow of 20% at levels in the vicinity of 35 km or below would give rise to changes in the tropospheric planetary wave pattern that are less than but on the same order as the observed interannual variability in the tropospheric wave pattern at middle and high latitudes. Thus, planetary wave coupling between the troposphere and the upper atmosphere appears to be a plausible mechanism to give a tropospheric response to solar activity. This mechanism is not viable, however, to provide for short-period changes such as the suggested solar sector boundary vorticity index relation, but rather is applicable to changes of longer period such as the 11- or 22-year solar cycles.
    Hines C. O., 1974: A possible mechanism for the production of sun-weather correlations. J. Atmos. Sci., 31, 589- 591.10.1175/1520-0469(1974)031<0589:APMFTP>2.0.CO;240c5d4fa9e2f32af47fc41ecc2b7718ehttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F234226259_A_Possible_Mechanism_for_the_Production_of_Sun-Weather_Correlationshttp://www.researchgate.net/publication/234226259_A_Possible_Mechanism_for_the_Production_of_Sun-Weather_CorrelationsAbstract If, as has been alleged, variations in the outflow of solar plasma have some effect on our weather, then the relevant coupling mechanism must be sought. It is suggested here that planetary waves, which may be subjected to variable reflection in the upper atmosphere and so may induce variable interference patterns in the lower atmosphere, constitute a potential candidate.
    Hui G., 2009: China's snow disaster in 2008,who is the principal player? International Journal of Climatology, 29, 2191-2196, doi: 10.1002/joc.1859.10.1002/joc.1859b41dfdf0-aa39-4af8-bfea-e3e68fd96cfa8da3b8274a768873f8d09a392c161cdchttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fjoc.1859%2Fabstractrefpaperuri:(f1c9a07f0a6ab8b238e8904e10e9d44c)http://onlinelibrary.wiley.com/doi/10.1002/joc.1859/abstractAbstract The unprecedented snow disaster in January 2008 brought serious human and economic losses to China. It has been suggested that the La Nina event is the principal cause. But analysis indicates that in December 2007, the circulation patterns in the tropical regions are quite similar with those in January 2008. In contrast large differences existed at high latitudes, especially the Siberia high (SH) and the north polar vortex (NPV). The differences can also be found between other extreme heavy and light snow years. In the extreme heavy (light) snow years, the SH is stronger (weaker) and the NPV is deeper (shallower). But these extreme snow events don't correspond to ENSO events well. Statistical results also indicate that both the SH and the NPV are independent of ENSO. So, rather than the La Nina event, the abnormal circulations at the high latitudes may play a more crucial role in making this snow disaster. Copyright 2009 Royal Meteorological Society
    Kodera K., M. Chiba, 1995: Tropospheric circulation changes associated with stratospheric sudden warmings: A case study. J. Geophys. Res., 100, 11 055- 11 068.10.1029/95JD007716888aa20213f26675fb2659b37fd4557http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F95JD00771%2Fcitedbyhttp://onlinelibrary.wiley.com/doi/10.1029/95JD00771/citedbyIt was theoretically demonstrated by Matsuno that stratospheric warmings are caused by an intensified vertical propagation of tropospheric planetary waves. However, the question of how the resultant changes in the stratospheric circulation affect the troposphere in return is left unanswered. In the present study, a case study on the 1984&ndash;1985 stratospheric warming event is conducted to clarify the changes in the tropospheric circulation associated with stratospheric sudden warmings. The results of the present study indicate that during stratospheric warmings, not only the intensification of the upward propagation of planetary waves is found in the stratosphere, but also changes in the direction of the meridional propagation of waves occur in the troposphere as well as in the stratosphere. Changes in the meridional phase structure of tropospheric planetary waves produce enhanced cold surges over the oceans, which in turn generate intense synoptic eddies. Further disturbances, such as blockings, can be produced through interactions between the planetary waves and synoptic eddies, but this may be only indirectly related with the stratospheric warmings. Comparisons between the observed changes in circulation and results of numerical model experiments suggest a potential role of the stratosphere in the tropospheric circulation through changes in meridional propagation of planetary waves.
    Kuroda Y., K. Kodera, 1999: Role of planetary waves in the stratosphere-troposphere coupled variability in the Northern Hemisphere winter. Geophys. Res. Lett., 26, 2375- 2378.10.1029/1999GL900507189dbecbff624196e9207f6ed8a34dd2http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F1999GL900507%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/1999GL900507/fullABSTRACT The role of planetary waves in stratosphere-troposphere coupled variability is investigated using an extended singular value decomposition analysis of zonal-mean zonal wind and the vertical component of the Eliassen-Palm (E-P) flux for the winters from 1979/80 to 1995/96. The results suggest a close relationship between anomalies of zonal-mean zonal wind and the convergence zone of E-P flux, which together shift poleward and downward from the stratosphere to the troposphere as time advances. Following enhanced vertical propagation of waves into the stratosphere, the Arctic Oscillation (AO) pattern is seen in the 500 hPa geopotential height field in association with an increased poleward propagation of tropospheric waves.
    Kodera K., H. Mukougawa, and S. Itoh, 2008: Tropospheric impact of reflected planetary waves from the stratosphere. Geophys. Res. Lett., 35,L16806, doi: 10.1029/2008GL034575.10.1029/2008GL034575017099393141c4ed78e8fbe946b898cfhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2008GL034575%2Fabstracthttp://onlinelibrary.wiley.com/doi/10.1029/2008GL034575/abstract[1] A reflection of stratospheric planetary waves and its impact on the troposphere during a stratospheric sudden warming of March 2007 are investigated. Zonal propagation and reflection of the planetary waves is clearly seen in the longitude-height sections of the eddy geopotential height and the vertical and zonal component of the three-dimensional wave activity flux. A wave packet propagating upward and eastward from Eurasian continent was reflected by a negative wind shear in the upper stratospheric westerly jet caused by stratospheric warming. Waves then propagated downward to the American-Atlantic sector of the troposphere, which led to the formation of a deep trough over the Atlantic and brought cold weather to the northeastern part of the American continent.
    Kodera K., H. Mukougawa, and A. Fujji, 2013: Influence of the vertical and zonal propagation of stratospheric planetary waves on tropospheric blockings. J. Geophys. Res.: Atmos., 118, 8333- 8345.10.1002/jgrd.50650d46d36cc71e50f55d3171ea554040193http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fjgrd.50650%2Fabstracthttp://onlinelibrary.wiley.com/doi/10.1002/jgrd.50650/abstractAbstract [1] Case studies are used to elucidate the relationship between stratospheric planetary wave reflection and blocking formation in the troposphere. The enhanced upward propagation of a planetary-scale wave packet from the Eurasian sector, involving a Euro-Atlantic blocking, leads to a stratospheric sudden warming (SSW). Following the weakening of the stratospheric westerly jet due to polar warming, the stratospheric planetary wave packet then propagates downward over the American sector, inducing a ridge over the North Pacific as well as a trough over eastern Canada in the upper troposphere. The ridge promotes the formation of a Pacific blocking. This result explains why Pacific blockings tend to form after SSW, and why they are associated with suppressed upward propagation of planetary waves.
    Kodera K., K. Yamazaki, M. Chiba, and K. Shibata, 1990: Downward propagation of upper stratospheric mean zonal wind perturbation to the troposphere. Geophys. Res. Lett., 17, 1263- 1266.10.1029/GL017i009p01263f77afeadf155e25a75b56d17b4ef1152http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2FGL017i009p01263%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/GL017i009p01263/fullAn investigation is conducted to determine the influence of changes in the upper stratospheric mean zonal wind on the circulation of the lower atmosphere. In addition to observed data, results of numerical experiments with a general circulation model are used, in which the solar ultraviolet heating rate is varied to force changes in the mean zonal wind in the upper stratosphere. It is found that when the upper stratospheric mid-latitude westerlies are strong during December, lower stratospheric polar night jet is persistent and the westerlies in the polar region of the troposphere become stronger in the following February. These results are common to both the observations and the numerical experiments.
    Martius O., L. M. Polvani, and H. C. Davies, 2009: Blocking precursors to stratospheric sudden warming events.Geophys. Res. Lett., 36, L14806, doi: 10.1029/2009GL038776.10.1029/2009GL038776a3fde84e6085c650d1b511727425e3a3http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2009GL038776%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2009GL038776/fullAbstract Top of page Abstract 1.Introduction 2.Data and Methodology 3.Results 4.Discussion Acknowledgments References Supporting Information [1] The primary causes for the onset of major, midwinter, stratospheric sudden warming events remain unclear. In this paper, we report that 25 of the 27 events objectively identified in the ERA-40 dataset for the period 1957-2001 are preceded by blocking patterns in the troposphere. The spatial characteristics of tropospheric blocks prior to sudden warming events are strongly correlated with the type of sudden warming event that follows. Vortex displacement events are nearly always preceded by blocking over the Atlantic basin only, whereas vortex splitting events are preceded by blocking events occurring in the Pacific basin or in both basins contemporaneously. The differences in the geographical blocking distribution prior to sudden warming events are mirrored in the patterns of planetary waves that are responsible for producing events of either type. The evidence presented here, suggests that tropospheric blocking plays an important role in determining the onset and the type of warmings.
    Namias J., P. F. Clapp, 1951: Observational studies of general circulation patterns. Compendium of Meteorology, T. F. Malone, Ed., Amer. Meteor. Soc., 551- 568.f85021c9-5d91-4aa8-99b3-8131aeccf733b900702203b369f2f7c3f1844d987f24http%3A%2F%2Fagris.fao.org%2Fagris-search%2Fsearch.do%3FrecordID%3DUS201300397210refpaperuri:(1dabc331e261075c6f6f8204f54e532a)/s?wd=paperuri%3A%281dabc331e261075c6f6f8204f54e532a%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fagris.fao.org%2Fagris-search%2Fsearch.do%3FrecordID%3DUS201300397210&ie=utf-8
    Nath D., S. Sridharan, S. Sathishkumar, S. Gurubaran, and W. Chen, 2013: Lower stratospheric gravity wave activity over Gadanki (13.5\circN, 79.2\circE) during the stratospheric sudden warming of 2009: Link with potential vorticity intrusion near Indian sector. Journal of Atmospheric and Solar-Terrestrial Physics, 94, 54- 64.
    Nath D., W. Chen, L. Wang, and Y. Ma, 2014: Planetary wave reflection and its impact on tropospheric cold weather over Asia during January 2008. Adv. Atmos. Sci.,31, 851-862, doi: 10.1007/s00376-013-3195-8.10.1007/s00376-013-3195-877d600a26aed8ba054051d3b5e8bd7ebhttp%3A%2F%2Fd.wanfangdata.com.cn%2FPeriodical_dqkxjz-e201404011.aspxhttp://d.wanfangdata.com.cn/Periodical_dqkxjz-e201404011.aspxReflection of stratospheric planetary waves and its impact on tropospheric cold weather over Asia during January 2008 were investigated by applying two dimensional Eliassen–Palm(EP) flux and three-dimensional Plumb wave activity fluxes. The planetary wave propagation can clearly be seen in the longitude–height and latitude–height sections of the Plumb wave activity flux and EP flux, respectively, when the stratospheric basic state is partially reflective. Primarily, a wave packet emanating from Baffin Island/coast of Labrador propagated eastward, equatorward and was reflected over Central Eurasia and parts of China, which in turn triggered the advection of cold wind from the northern part of the boreal forest regions and Siberia to the subtropics. The wide region of Central Eurasia and China experienced extreme cold weather during the second ten days of January 2008, whereas the extraordinary persistence of the event might have occurred due to an anomalous blocking high in the Urals–Siberia region.
    Perlwitz J., H. F. Graf, 2001: Troposphere-stratosphere dynamic coupling under strong and weak polar vortex conditions. Geophys. Res. Lett., 28, 271- 274.10.1029/2000GL012405fbf1bb5f7675a213caa08ea80261ec48http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2000GL012405%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2000GL012405/fullThe relationship between Northern Hemisphere (NH) tropospheric and stratospheric wave-like anomalies of spherical zonal wave number (ZWN) 1 is studied by applying Canonical Correlation Analysis (CCA). A lag-correlation technique is used with 10-day lowpass filtered daily time series of 50- and 500-hPa geopotential heights. Generally stratospheric circulation is determined by ultralong tropospheric planetary waves. During winter seasons characterized either by an anomalously strong or weak polar winter vortex different propagation characteristics for waves of ZWN 1 are observed. The non-linear perspective of the results have implications for medium range weather forecast and climate sensitivity experiments.
    Perlwitz J., N. Harnik, 2003: Observational evidence of a stratospheric influence on the troposphere by planetary wave reflection. J.Climate, 16, 3011- 3026.10.1175/1520-0442(2003)016<3011:OEOASI>2.0.CO;2e8127032de2e1ad42d5481b2a6f2307dhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F253549403_Observational_Evidence_of_a_Stratospheric_Influence_on_the_Troposphere_by_Planetary_Wave_Reflectionhttp://www.researchgate.net/publication/253549403_Observational_Evidence_of_a_Stratospheric_Influence_on_the_Troposphere_by_Planetary_Wave_ReflectionRecent studies have pointed out the impact of the stratosphere on the troposphere by dynamic coupling. In the present paper, observational evidence for an effect of downward planetary wave reflection in the stratosphere on Northern Hemisphere tropospheric waves is given by combining statistical and dynamical diagnostics. A time-lagged singular value decomposition analysis is applied to daily tropospheric and stratospheric height fields recomposed for a single zonal wavenumber. A wave geometry diagnostic for wave propagation characteristics that separates the index of refraction into vertical and meridional components is used to diagnose the occurrence of reflecting surfaces. For zonal wavenumber 1, this study suggests that there is one characteristic configuration of the stratospheric jet that reflects waves back into the troposphere when the polar night jet peaks in the high-latitude midstratosphere. This configuration is related to the formation of a reflecting surface for vertical propagation at around 5 hPa as a result of the vertical curvature of the zonal-mean wind and a clear meridional waveguide in the lower to middle stratosphere that channels the reflected wave activity to the high-latitude troposphere.
    Perlwitz J., N. Harnik, 2004: Downward coupling between the stratosphere and troposphere: The relative roles of wave and zonal mean processes. J.Climate, 17, 4902- 4909.10.1175/JCLI-3247.15e6f2f86b13c81466b2db91845ac28dbhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F228421473_Downward_coupling_between_the_stratosphere_and_troposphere_The_relative_roles_of_wave_and_zonal_mean_processeshttp://www.researchgate.net/publication/228421473_Downward_coupling_between_the_stratosphere_and_troposphere_The_relative_roles_of_wave_and_zonal_mean_processesWave and zonal mean features of the downward dynamic coupling between the stratosphere and troposphere are compared by applying a time-lagged singular value decomposition analysis to Northern Hemisphere height fields decomposed into zonal mean and its deviations. It is found that both zonal and wave components contribute to the downward interaction, with zonal wave 1 (due to reflection) dominating on the short time scale (up to 12 days) and the zonal mean (due to waveean-flow interaction) dominating on the longer time scale. It is further shown that the two processes dominate during different years, depending on the state of the stratosphere. Winters characterized by a basic state that is reflective for wave 1 show a strong relationship between stratospheric and tropospheric wave-1 fields when the stratosphere is leading and show no significant correlations in the zonal mean fields. On the other hand, winters characterized by a stratospheric state that does not reflect waves show a strong relationship only between stratospheric and tropospheric zonal mean fields. This study suggests that there are two types of stratospheric winter states, characterized by different downward dynamic interaction. In one state, most of the wave activity gets deposited in the stratosphere, resulting in strong wave ean-flow interaction, while in the other state, wave activity is reflected back down to the troposphere, primarily affecting the structure of tropospheric planetary waves.
    Plumb R. A., 1985: On the three-dimensional propagation of stationary waves. J. Atmos. Sci., 42, 217- 229.10.1175/1520-0469(1985)042<0217:OTTDPO>2.0.CO;2ccdb9bc2c2853e3ba3d7632e5f9db2c5http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10013124971%2Fhttp://ci.nii.ac.jp/naid/10013124971/On the three-dimensional propagation of stationary waves. PLUMB R. A. J. Atmos. Sci. 42, 217-229, 1985
    Shaw T. A., J. Perlwitz, 2013: The life cycle of Northern Hemisphere downward wave coupling between the stratosphere and troposphere, J.Climate, 26, 1745- 1763.10.1175/JCLI-D-12-00251.1cf850a90-39ec-4ae6-9f45-4a58b998f54fe75651b021f3b8a0dba77c52c8ea1a84http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F258795490_The_Life_Cycle_of_Northern_Hemisphere_Downward_Wave_Coupling_between_the_Stratosphere_and_Troposphererefpaperuri:(428500ae0defc9db71b5836752217110)http://www.researchgate.net/publication/258795490_The_Life_Cycle_of_Northern_Hemisphere_Downward_Wave_Coupling_between_the_Stratosphere_and_TroposphereAbstract The life cycle of Northern Hemisphere downward wave coupling between the stratosphere and troposphere via wave reflection is analyzed. Downward wave coupling events are defined by extreme negative values of a wave coupling index based on the leading principal component of the daily wave-1 heat flux at 30 hPa. The life cycle occurs over a 28-day period. In the stratosphere there is a transition from positive to negative total wave-1 heat flux and westward to eastward phase tilt with height of the wave-1 geopotential height field. In addition, the zonal-mean zonal wind in the upper stratosphere weakens leading to negative vertical shear. Following the evolution in the stratosphere there is a shift toward the positive phase of the North Atlantic Oscillation (NAO) in the troposphere. The pattern develops from a large westward-propagating wave-1 anomaly in the high-latitude North Atlantic sector. The subsequent equatorward propagation leads to a positive anomaly in midlatitudes. The near-surface temperature and circulation anomalies are consistent with a positive NAO phase. The results suggest that wave reflection events can directly influence tropospheric weather. Finally, winter seasons dominated by extreme wave coupling and stratospheric vortex events are compared. The largest impacts in the troposphere occur during the extreme negative seasons for both indices, namely seasons with multiple wave reflection events leading to a positive NAO phase or seasons with major sudden stratospheric warmings (weak vortex) leading to a negative NAO phase. The results reveal that the dynamical coupling between the stratosphere and NAO involves distinct dynamical mechanisms that can only be characterized by separate wave coupling and vortex indices.
    Shaw T. A., J. Perlwitz, and N. Harnik, 2010: Downward wave coupling between the stratosphere and troposphere: The importance of meridional wave guiding and comparison with zonal-mean coupling. J.Climate, 23, 6365- 6381.10.1175/2010JCLI3804.1c1f7d0f81188ad452187f66a14aa400dhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FADS%3Fid%3D2010JCli...23.6365Shttp://onlinelibrary.wiley.com/resolve/reference/ADS?id=2010JCli...23.6365SAbstract The nature of downward wave coupling between the stratosphere and troposphere in both hemispheres is analyzed using the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) dataset. Downward wave coupling occurs when planetary waves reflected in the stratosphere impact the troposphere, and it is distinct from zonal-mean coupling, which results from wave dissipation and its subsequent impact on the zonal-mean flow. Cross-spectral correlation analysis and wave geometry diagnostics reveal that downward wave-1 coupling occurs in the presence of both a vertical reflecting surface in the mid-to-upper stratosphere and a high-latitude meridional waveguide in the lower stratosphere. In the Southern Hemisphere, downward wave coupling occurs from September to December, whereas in the Northern Hemisphere it occurs from January to March. A vertical reflecting surface is also present in the stratosphere during early winter in both hemispheres; however, it forms at the poleward edge of the meridional waveguide, which is not confined to high latitudes. The absence of a high-latitude waveguide allows meridional wave propagation into the subtropics and decreases the likelihood of downward wave coupling. The results highlight the importance of distinguishing between wave reflection in general, which requires a vertical reflecting surface, and downward wave coupling between the stratosphere and troposphere, which requires both a vertical reflecting surface and a high-latitude meridional waveguide. The relative roles of downward wave and zonal-mean coupling in the Southern and Northern Hemispheres are subsequently compared. In the Southern Hemisphere, downward wave-1 coupling dominates, whereas in the Northern Hemisphere downward wave-1 coupling and zonal-mean coupling are found to be equally important from winter to early spring. The results suggest that an accurate representation of the seasonal cycle of the wave geometry is necessary for the proper representation of downward wave coupling between the stratosphere and troposphere.
    Shaw T. A., J. Perlwitz, and O. Weiner, 2014: Troposphere-stratosphere coupling: Links to North Atlantic weather and climate, including their representation in CMIP5 models. J. Geophys. Res.:Atmos., 119, 5864- 5880.10.1002/2013JD02119171133adda40a21b33a3c996ae215d54fhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2013JD021191%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1002/2013JD021191/pdfAbstract A new dynamical metric of troposphere-stratosphere coupling is established based on extreme stratospheric planetary-scale wave heat flux events, defined as the 10th and 90th percentile of the daily high-latitude averaged heat flux distribution at 50 Pa using ERA-Interim reanalysis data. The stratospheric heat flux extremes are linked instantaneously to high-latitude planetary-scale wave patterns in the troposphere and zonal wind, temperature and mean sea level pressure anomalies in the Atlantic basin. The impacts are reminiscent of different phases of the North Atlantic Oscillation. In particular extreme positive (negative) heat flux events in the stratosphere are associated with an equatorward (poleward) jet shift in the North Atlantic basin. The metric is used to evaluate troposphere-stratosphere coupling in models participating in the Coupled Model Intercomparison Project Phase 5. The results show that models with a degraded representation of stratospheric extremes exhibit robust biases in the troposphere relative to ERA-Interim. In particular, models with biased stratospheric extremes exhibit a biased climatological stationary wave pattern and Atlantic jet stream position in the troposphere. In addition these models exhibit biases in geopotential height and zonal wind extremes in the North Atlantic region. The stratospheric biases are connected to model lid height but it is not sufficient for assessing the tropospheric impacts. Our analysis reveals that the mean bias of the stratospheric heat flux is also not sufficient for assessing the representation of troposphere-stratosphere coupling. Overall the results suggest that a metric based on stratospheric heat flux extremes should be used in conjunction with metrics based on extreme polar vortex events in multi-model assessments of troposphere-stratosphere coupling.
    Tibaldi S., F. Molteni, 1990: On the operational predictability of blocking. Tellus A, 42, 343- 365.10.1034/j.1600-0870.1990.t01-2-00003.x14e00c4593f3fca22fd1d7852e693d15http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1034%2Fj.1600-0870.1990.t01-2-00003.x%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1034/j.1600-0870.1990.t01-2-00003.x/pdfABSTRACT The entire 7-year archive of ECMWF operational analysis and forecast data is used to assess the skill of the Centre's model in short- and medium-range forecasting of atmospheric blocking. The assessment covers 7100-day periods, from 1 December to 10 March of all winters from 1980-81 to 1986-87, inclusive. A slightly modified version of the Legen&auml;s and &Oslash;kland objective zonal index is used to quantify both observed and forecast occurrence of blocking. The study is performed on 500 hPa geopotential height and on Euro-Atlantic and Pacific blocking separately. It is found that blocking frequency is severely underestimated in medium-range forecasts; the model is, on average, reasonably skilful if the initial conditions are blocked, but blocking onset is poorly represented if it occurs more than a few days into the forecast. This inability in entering the blocking regime has a substantial impact on the systematic error of the model.
    Treidl R. A., E. C. Birch, and P. Sajecki, 1981: Blocking action in the Northern Hemisphere: A climatological study. Atmos.-Ocean, 19, 1- 23.10.1080/07055900.1981.96490964f2e4ec18742f6d6f3cb735abff60925http%3A%2F%2Fwww.tandfonline.com%2Fdoi%2Fabs%2F10.1080%2F07055900.1981.9649096http://www.tandfonline.com/doi/abs/10.1080/07055900.1981.9649096Using criteria developed from scientific studies, the blocking situations observed in the Northern Hemisphere during the period 1945–1977 are subjectively assessed and statistically analysed. Earlier findings are largely confirmed while new results are presented on seasonal and secular blocking trends, and the probability of multiple blocking. A catalogue of'664 blocking cases was prepared but is not included here because of its size; however, copies of the catalogue may be obtained from the authors.
    Uppala S. M., D. Dee, S. Kobayashi, P. Berrisford, and A. Simmons, 2008: Towards a climate data assimilation system: Status update of ERA-Interim. ECMWF Newsletter, 115, 12- 18.b049b4451ca6109dc3d45d5791497345http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F284038917_Towards_a_climate_data_assimilation_system_Status_update_of_ERA-Interim%3Fev%3Dauth_pubhttp://www.researchgate.net/publication/284038917_Towards_a_climate_data_assimilation_system_Status_update_of_ERA-Interim?ev=auth_pub
    Zhou W., J. C. L. Chan, W. Chen, J. Ling, J. G. Pinto, and Y. Shao, 2009: Synoptic-scale controls of persistent low temperature and icy weather over Southern China in January 2008. Mon. Wea. Rev., 137, 3978- 3991.10.1175/2009MWR2952.1d9b407fb4b1b62b6ff39473041586094http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F249621701_Synoptic-Scale_Controls_of_Persistent_Low_Temperature_and_Icy_Weather_over_Southern_China_in_January_2008http://www.researchgate.net/publication/249621701_Synoptic-Scale_Controls_of_Persistent_Low_Temperature_and_Icy_Weather_over_Southern_China_in_January_2008In January 2008, central and southern China experienced persistent low temperatures, freezing rain, and snow. The large-scale conditions associated with the occurrence and development of these snowstorms are examined in order to identify the key synoptic controls leading to this event. Three main factors are identified: 1) the persistent blocking high over Siberia, which remained quasi-stationary around 65E for 3 weeks, led to advection of dry and cold Siberian air down to central and southern China; 2) a strong persistent southwesterly flow associated with the western Pacific subtropical high led to enhanced moisture advection from the Bay of Bengal into central and southern China; and 3) the deep inversion layer in the lower troposphere associated with the extended snow cover over most of central and southern China. The combination of these three factors is likely responsible for the unusual severity of the event, and hence a long return period.
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Manuscript received: 24 March 2015
Manuscript revised: 28 July 2015
Manuscript accepted: 20 August 2015
通讯作者: 陈斌, bchen63@163.com
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Impact of Planetary Wave Reflection on Tropospheric Blocking over the Urals-Siberia Region in January 2008

  • 1. Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100190

Abstract: Planetary wave reflection from the stratosphere played a significant role in changing the tropospheric circulation pattern over Eurasia in mid-January 2008. We studied the 2008 event and compared with composite analysis (winters of 2002/2003, 2004/2005, 2006/2007, 2007/2008, 2010/2011 and 2011/2012), when the downward coupling was stronger, by employing time-lagged singular value decomposition analysis on the geopotential height field. In the Northern Hemisphere, the geopotential fields were decomposed into zonal mean and wave components to compare the relative covariance patterns. It was found that the wavenumber 1 (WN1) component was dominant compared with the wavenumber 2 (WN2) component and zonal mean process. For the WN1 field, the covariance was much higher (lower) for the negative (positive) lag, with a prominent peak around +15 days when the leading stratosphere coupled strongly with the troposphere. It contributed to the downward coupling due to reflection, when the stratosphere exhibited a partially reflective background state. We also analyzed the evolution of the WN1 anomaly and heat flux anomaly, both in the troposphere and stratosphere, during January-March 2008. The amplitude of the tropospheric WN1 pattern reached a maximum and was consistent with a downward wave coupling event influenced by the stratospheric WN1 anomaly at 10 hPa. This was consistent with the reflection of the WN1 component over Eurasia, which triggered an anomalous blocking high in the Urals-Siberia region. We further clarified the impact of reflection on the tropospheric WN1 field and hence the tropospheric circulation pattern by changing the propagation direction during and after the event.

1. Introduction
  • In recent decades, several studies have related tropospheric variability with the downward propagation of stratospheric anomalies through planetary wave reflection (Kodera et al., 2008). Due to the gradual increase in atmospheric pressure downward, the reflected component gets attenuated faster and its impact is considered to be minimal on the tropospheric regime. The theory of planetary wave reflection on tropospheric fields was initially proposed by (Hines, 1974) and (Geller and Alpert, 1980). Later, several authors discussed the coupling processes and downward propagation in the light of Northern Annular modes (Baldwin and Dunkerton, 1999, 2001), wave-mean flow interaction (Baldwin and Dunkerton, 2001; Christiansen, 2001) and anomalous propagation of the mean zonal wind field (Kodera et al., 1990; Kuroda and Kodera, 1999; Christiansen, 2000) down to the troposphere. (Perlwitz and Graf, 2001) and Perlwitz and Harnik (2003, 2004) statistically described the vertical coupling of the wavenumber 1 (WN1) and wavenumber 2 (WN2) components downward with the tropospheric height fields. In winter the stratosphere is either reflective or non-reflective based on the strength of the polar vortex (Perlwitz and Harnik, 2004). The WN1 and zonal mean component play a major role when the background is reflective. Furthermore, employing time-lagged correlation analysis, (Perlwitz and Harnik, 2004) explained the gradual tilt in phase of the WN1 regression pattern (leading mode) vertically upward. With negative time lag the tilt is westward, but with positive lag it is eastward, and this feature is consistent with the downward propagation of planetary waves due to reflection.

    (Shaw et al., 2010) described the characteristics of downward wave coupling between the stratosphere and troposphere, using the 40-year ECMWF (European Centre for Medium-Range Weather Forecasts) reanalysis dataset. Employing cross-spectral correlation analysis and wave geometry diagnostics, they found that the downward WN1 coupling occurs both in the presence of a vertical reflecting surface in the mid-to-upper stratosphere and a high-latitude meridional waveguide in the lower stratosphere. They also discussed the importance of the seasonal cycle of the wave geometry for the proper representation of downward wave coupling between the stratosphere and troposphere, both in the Northern and Southern Hemisphere. In a separate study, (Shaw and Perlwitz, 2013) statistically investigated the life cycle of Northern Hemisphere wave coupling events and found that it occurs over a period of 28 days. Furthermore, they showed that during the downward coupling process, there is a transition in stratospheric WN1 heat flux, from positive to negative, and the WN1 phase tilts from westward to eastward. (Shaw et al., 2014) established a new dynamical metric of troposphere-stratosphere coupling, based on extreme stratospheric planetary-scale wave heat flux events.

    (Coughlin and Tung, 2005) demonstrated the possibility of wave reflection in the context of major sudden stratospheric warming (SSW) events and discussed its impact on the tropospheric weather regime (Nath et al., 2013). They illustrated the changes in the tropospheric WN1 field in response to the reflected component from the stratosphere. Separately, (Kodera et al., 2008) related the occurrence of an extreme cold event in March 2007 over the northeast coast of the North American continent, to the upward and reflected component of planetary waves over Eurasia and the North American sector, respectively. During an SSW event in 1984-85, (Kodera and Chiba, 1995) investigated the changes in circulation pattern due to downward and equatorward propagation of midlatitude planetary waves to the troposphere. Geopotential anomalies that propagate downward to the troposphere have a significant impact on the tropospheric weather regime, particularly in non-reflective years (Perlwitz and Harnik, 2004). (Perlwitz and Harnik, 2004) categorized the reflective and non-reflective basic states for planetary WN1 reflection based on the zonal-mean zonal wind difference between 2 and 10 hPa, averaged over 58°-74°N and over time. The reflective basic state corresponds to a negative index with the polar night jet peaking in the mid-stratosphere; whereas, for the non-reflective state, the zonal wind increases with increasing height. The planetary waves propagate upward along the stratospheric westerly jet, weakening the polar night jet in the upper stratosphere. This inhibits further propagation of the planetary waves high up in the stratosphere and it reflects back to the troposphere. During reflective winters the stratospheric signals are weak and get attenuated above the tropopause (Perlwitz and Harnik, 2004). Moreover, in strong polar vortex winters, the WN1 reflection pattern is more prominent (Perlwitz and Graf, 2001); whereas, in weak vortex years, stratosphere-troposphere coupling is relatively strong (Baldwin and Dunkerton, 1999, 2001).

    Here, we considered a specific case in the pre-warming phase of a major SSW event in January 2008. As reported previously (Hui, 2009; Zhou et al., 2009; Nath et al., 2014), in January and early February 2008, parts of Eurasia and China experienced extreme cold events, snowfall and freezing rain, particularly in the southern part of China. These phenomena caused excessive damage, disruption and major infrastructure loss, resulting in broken power transmission lines and chaotic traffic conditions (Zhou et al., 2009). China experienced substantial economic losses of 53.8 billion RMB due to freezing rain alone. Between 10 January and 2 February 2008 there were four episodes of severe and persistent snow over the Yangtze River basin, South China, and Southwest China. The 2008 event was the coldest event since at least 1979, bringing about 107 casualties, according to the Ministry of Civil Affairs.

    (Zhou et al., 2009) indicated the key factors as the occurrence of a persistent blocking high over Siberia, as well as strong and persistent southwesterly flow, which triggered moisture advection from the Bay of Bengal to southern-central China, and the formation of a deep inversion layer in the lower troposphere. Furthermore, (Hui, 2009) attributed these adverse meteorological conditions with abnormal circulation anomalies at high latitudes. In a separate study, (Nath et al., 2014) demonstrated that when the stratospheric basic state is partially reflective, a wave packet emanating from Baffin Island/the coast of Labrador propagates eastward, equatorward and reflects back over central Eurasia and parts of China, which in turn triggers the advection of cold wind from the northern part of the boreal forest region and Siberia to the subtropics. The extraordinary persistence of this particular cold event has been linked with anomalous blocking high over the Urals-Siberia region.

    Despite many previous studies having investigated the key factors (like tropospheric blocking) that triggered the extreme cold event in January 2008, none explored the causative mechanism underpinning the occurrence of the anomalous blocking high over the Urals-Siberia region. In the present study, we performed lagged correlation analysis to understand the respective contribution of the wave (WN1 and WN2) and zonal mean flow in conjunction with stratosphere-troposphere coupling processes. By analyzing the squared covariance between the stratospheric and tropospheric height field, we identified the key dates in January 2008 when the impact of the stratospheric basic state on the troposphere was at a maximum (or vice versa). We also compared the singular value decomposition (SVD) pattern of the 2008 event with the composite mean pattern, which included several winters for which downward wave coupling has been reported. Based on (Shaw and Perlwitz, 2013), (Kodera et al., 2013), and (Dunn-Sigouin and Shaw, 2015), we chose the winters of 2003, 2005, 2007, 2008, 2011 and 2012 for the composite analysis. We also estimated the changes in the tropospheric WN1 field and hence the tropospheric circulation, during and after the event. Furthermore, we clarified the role of planetary wave reflection on the tropospheric circulation pattern and formation of strong Urals-Siberia blocks.

2. Data and methodology
  • Daily mean ECMWF Interim Reanalysis (ERA-Interim) data (Uppala et al., 2008; Dee et al., 2011) were used for potential vorticity, geopotential height, zonal wind, meridional wind, and temperature. The individual parameters were archived from December to April from 2002/2003 to 2011/ 2012. The ERA-Interim data are available at 37 pressure levels from 1000 hPa to 1 hPa, with a horizontal resolution of 1.5°× 1.5°.

  • 2.2.1. Time-lagged SVD

    Time-lagged SVD analysis was used to establish the dynamical connection between the geopotential height fields in the stratosphere and troposphere (Perlwitz and Harnik, 2003, 2004). The leading coupled modes were extracted from the spatiotemporal structures of the geopotential height perturbations. We performed the analysis and estimated the covariance with the temporal series of two height fields at individual time lags separately. The height perturbations were arranged such that each column was the time series for a given location. Based on Perlwitz and Harnik (2003, 2004), the geopotential height fields could be expanded orthogonally, which can be expressed as \begin{eqnarray} H_{1}(x,t)&=&\sum_{n=1}^Nu_n(x)a_n(t) ,(1)\\ H_{2}(x,t+\tau)&=&\sum_{n=1}^Nv_n(x)b_n(t+\tau) , (2)\end{eqnarray} where H1 and H2 are the geopotential height fields at time t and t+τ, un and vn are the singular matrices, N is the number of modes, and sn2 is the square of nth singular value of the covariance matrix between H1 and H2-constructed by taking the covariance between the two expansion coefficients a(t) and b(t+τ). The coupled modes are arranged with increasing n and decreasing covariance. S is the total squared covariance between the two coupled fields: \begin{equation} S=\sum_{n=1}^Ns_n^2. (3)\end{equation} In our analysis, the reference height was fixed at 10 hPa and the SVD analysis was performed with the levels descending downward from 10 hPa to 1000 hPa at different lags (τ) in time lags from -30 to +30 days, i.e., 61 time lags. At the reference height, the time span from 1 January to 30 March, i.e., 90 days, remained fixed; whereas, the other levels were shifted temporally by -30 to 30 days with an interval of 1 day. Thus, positive lags indicated that the stratosphere was leading and the troposphere was lagging, and vice versa for negative lags. In order to detect the time lag at which the dynamical relation between H1 and H2 was maximal, correlation coefficients between the leading coupled modes, a1 and b1, were computed for each of the 61 SVD analyses.

    To understand the contribution of wave processes and the zonal mean field, SVD analysis was performed between zonal mean fields, the eddy field (i.e., deviation from the zonal mean), and the WN1 and WN2 height fields, separately. Prior to the SVD analysis, we removed the mean seasonal cycle and multiplied the data by the square root of the density and the cosine of latitude along the altitude and latitude, respectively. In order to concentrate on the intra-annual variability and exclude the influence of a trend in the covariance, the annual mean averages of the geopotential height fields were removed. Although we did not use any temporal filtering, strong spatial filtering was applied in the wavenumber domain, both for the H1 and H2 zonal mean fields. To extract the WN1 and WN2 components from the geopotential height perturbations, we applied the least squares fitting (LSF) method for spectral analysis. This method was used to fit a set of zero mean observations, yi, at times i=1,2,…,N, to the equation given by \begin{equation} y_{i}=(A+B)\cos(2\pi w\lambda_{i}) , (4)\end{equation} where w is the wavenumber, Λi represents the longitudes, and A and B are the coefficients to be fitted. The individual wave components were then computed using the empirical relation \begin{equation} Y_{s}=A\sin(w\lambda_{i}+\phi) , (5)\end{equation} where A is the amplitude and φ is the phase, estimated by means of LSF analysis.

    In order to estimate a grid size independent measure of S, the mean squared covariance, C, between two grid points of the H1 and H2 fields can be defined as \begin{equation} C=\sqrt{\dfrac{S}{m_1m_2}} , (6)\end{equation} where m1 and m2 are the grid points of H1 and H2 fields. Here, we interpolated the height fields to 4.5° in longitude and 3° in latitude and performed the SVD analysis between 30° and 85° N. Hence, m1=m2=1600, corresponding to 80 and 20 longitudinal and latitudinal grid points, respectively.

    Figure 1.  The covariance (units: gpm$^2$) between the geopotential height fields at 10 hPa and all pressure levels between 1000 and 10 hPa, for time lags ranging from $-30$ to 30 days: (a-d) the covariance of the zonal mean, deviations from the zonal mean, WN1 height, and WN2 height, respectively, for the composite winter of 2002/2003, 2004/2005, 2006/2007, 2007/2008, 2010/2011 and 2011/2012; (e-h) the same, but for 2008 case. A positive time lag indicates that the stratospheric field is leading. The left and right axes represent the height (units: km) and pressure (units: hPa), respectively.

    2.2.2. Blocking index

    Midlatitude blocking is characterized by local formation of anomalous easterly flow due to the blocking of the westerly jet and mass transfer from high- to midlatitudes (Namias and Clapp, 1951; Treidl et al., 1981; Barriopedro et al., 2006). In general, the blockings are quasi-stationary patterns that persist for several weeks and have a significant impact on rainfall redistribution and the occurrence of extreme weather events at regional scales. A persistent blocking pattern also induces strong advection of polar air, southward, leading to extreme cold weather in boreal winter months (Nath et al., 2014). For January 2008, (Zhou et al., 2009) and (Nath et al., 2014) reported an anomalous and persistent blocking pattern in the Urals-Siberia region. The frequency of blocking exceeded the climatological high over 55°-70°E. We computed the blocking index from (Tibaldi and Molteni, 1990), with the additional criteria proposed by (Barriopedro et al., 2006). The 500 hPa geopotential height gradients in the north and south (GHGN and GHGS) (units: gpm/latitude) were simultaneously computed using the following expressions: \begin{align} { GHGN}&=\dfrac{H(\lambda,\theta_N)-H(\lambda,\theta_0)}{\theta_N-\theta_0} ,(7a)\\ { GHGS}&=\dfrac{H(\lambda,\theta_0)-H(\lambda,\theta_S)}{\theta_0-\theta_S} , (7b)\end{align} \begin{align} \theta_{ N}=78.0+\delta,\\ \theta_0&=&60.0+\delta,\\ \theta_{ S}&=&39.5+\delta , \end{align} $$ \delta=-4.5,-3.0,-1.5,0.0,1.5,3.0,4.5. (7c) $$ where H(Λ,θ) is the 500 hPa geopotential height, δ is the shift in latitude, GHGS is the measure of the zonal geostrophic wind component, and GHGN is imposed to exclude the non-blocked flows (Barriopedro et al., 2006). An arbitrary longitude was considered to be blocked if the following conditions were satisfied: \begin{eqnarray*} &&{ GHGN}<-10\%\;{ gpm}(^\circ)^{-1}\\ &&{ GHGS}>0 \end{eqnarray*}

    \(H(\lambda,\theta_0)-\overline {H(\lambda,\theta_0)}>0 .\) (7d)

    To identify the potential blocks, a three-day running mean filter was applied at each longitude.

3. Results and discussion
  • First, we compared the relative dominance of the zonal mean and the height wave fields for January-February-March (JFM), as obtained from the SVD analysis. The squared covariance between the 10 hPa and various pressure levels (10 to 1000 hPa) at different time lags (-30 to 30 days), for the zonal mean, deviation from the zonal mean, WN1 height, and WN2 height are shown in Fig. 1. The composite mean patterns for six winters (2002/2003, 2004/2005, 2006/2007, 2007/2008, 2010/2011 and 2011/2012) when the downward coupling was stronger is shown in the upper panels, Figs. 1a-d. The lower panels, Figs. 1e-h, exhibit the SVD patterns for the 2008 event, in order to compare the consistency with the composite mean pattern. For the zonal mean field (Figs. 1a and e), in the positive time lag (stratosphere leads), the covariance is stronger and extended (longer time scale) in the lower stratospheric heights. Meanwhile, in the negative time lag (troposphere leads), the covariance is relatively weaker and less persistent below 20 km. In the present analysis, the covariability is maximum and dominant for the leading coupled mode (first), because it explains around 80% of the squared covariance in all height regions.

    Figures 1b and f depict the covariability of the deviation from the zonal mean field for the composite and 2008 case, respectively. Unlike the zonal mean field, the covariance is relatively stronger for the negative and positive time lag in the mid-tropospheric (3-11 km) and lower stratospheric (>16 km) heights, respectively. The mid-tropospheric covariance is well extended over all time lags (-30 to 30 days), with a maximum around 15 days; whereas, in the lower stratospheric heights, the covariance is biased towards the positive side, with peaks around 0-5 day (>20 km) both for the composite and 2008 case. The zonal deviation field includes the contribution of various wave processes, and at given latitudes the WN1 and WN2 components were separated out using the LSF method, described in section 2.2.1.

    As is clear from Fig. 1c, the WN1 covariance (300 gpm2) for the composite case compares well with the deviation from the zonal mean field (400 gpm2). We can see the humps with larger covariance at the lags of -3 days (troposphere leads) and +15 days (stratosphere leads) in the WN1 field. The features are quite consistent with the 2008 case (Fig. 1g), at least on the positive side, with stronger downward coupling at +15 days' lag. Another noticeable feature is the intense downward coupling due to the WN1 field, with a persistent covariance pattern down to the surface at +15 days' lag. But, for the WN2 field, both the composite and 2008 event (Figs. 1d and h) exhibit much weaker covariance (100 gpm2) throughout the height range. The WN2 covariance, meanwhile, although weaker than WN1, exhibits a dominant peak at around +5 days' lag (stratosphere leads) and 400 hPa, both for the composite and 2008 event. (Perlwitz and Harnik, 2004) linked the planetary wave reflection with the humps in the positive lags when the basic state of the stratosphere was reflective. In (Nath et al., 2014), it was shown that, apart from the zonal mean reflective index, the longitudinal variation too has a severe impact on regional weather extremes, and the concept of a partially reflective stratospheric background state was introduced. All six winters chosen for the composite analysis - based on Shaw and Perlwitz (2013), Kodera et al. (2013) and Dunn-Sigouin and Shaw (2015) - except 2006/2007, exhibit a partially reflective stratospheric background state. Therefore, during these years, when the downward coupling was stronger, the relative dominance of the WN1 covariance from the stratosphere should have had a significant impact on the tropospheric circulation pattern.

    To further elucidate the relative contribution of the zonal mean and the WN1 field, we plotted the covariance for the zonal mean (850 hPa) and WN1 (400 hPa) field, both for the composite (Fig. 2a) and 2008 event (Fig. 2b). As can be seen, for the WN1 field, the covariance is much higher (lower) for the negative (positive) lag, with a prominent peak at around +15 days when the leading stratosphere coupled strongly with the troposphere. Meanwhile, in the negative lag (troposphere leads), the peaks are prominent at lags of -4 days and -10 days for the composite and 2008 case, respectively. This difference is obvious and can be attributed to the difference in upward wave propagation during the six winters included in the composite analysis. The winters were chosen based on (Shaw and Perlwitz, 2013), (Kodera et al., 2013) and (Dunn-Sigouin and Shaw, 2015), when downward coupling was prominent, irrespective of any precursory upward wave propagation events. For example, according to Dunn-Sigouin and Shaw (2015, Table 1), there were three downward propagating events, on 6 February, 25 February and 31 March 2003, but the upward propagation occurred long before the downward coupling events on 14 January 2003. Similarly, for the 2008 case, there is no upward propagation prior to the wave reflection event. In the zonal mean field, the covariance is much lower compared with the WN1 field, and the absence of any significant maxima, either in the positive or negative time lag, is prominent.

    Figure 2.  The covariance [gpm$^2$] between the tropospheric and stratospheric height fields at 30$^\circ$-85$^\circ$N for time lags between $-30$ and 30 days for the composite winters (upper panel) and 2008 case (bottom panel) between the leading coupled mode. Blue line: covariability between 10 hPa and 850 hPa zonal mean fields; Black line: covariability between 10 and 400 hPa height WN1 fields. A positive time lag indicates that the stratospheric field is leading. The maxima are significant at least to the 99% confidence level.

    Figure 3.  (a) Evolution of the total 400 hPa (black contours) and 10 hPa (color shading) WN1 pattern averaged from 60$^\circ$ to 80$^\circ$N for the 2008 event as a function of time from $-20$ to 20 days and longitude. The contour interval is 10 m, and the ranges are $-100$ to 100 m and $-75$ to 75 m for the black contours and color shading, respectively. (b) As in (a) but for the WN1 heat flux anomaly. The contour interval is 0.01 m$^2$ s$^-2$ and 0.003 m$^2$ s$^-2$, and the ranges are from $-0.1$ to 0.1 m$^2$ s$^-2$ and $-0.018$ to 0.018 m$^2$ s$^-2$ for the black contour and color shading, respectively. The bold dotted and light normal black contours in (a, b) represent the negative and positive anomalies, respectively. The longitude-time section of the blocking index (gray contour lines) is overplotted in both (a) and (b).

  • Based on the SVD timed lagged analysis, we identified that——both in the composite and 2008 case——downward coupling due to reflection was strongest at +15 days' lag. This corresponds to 15 of January as the key date for the 2008 case study. The evolution of the high-latitude WN1 pattern can be illustrated using a Hovmöller plot. Figure 3a shows the total WN1 pattern averaged between 60° and 80°N at 400 hPa (black contours) and 10 hPa (coloring) as a function of longitude and time from -20 to +20 days (15 January as the start date). Downward WN1 coupling events clearly coincide with changes in the tropospheric wave pattern. In the first stage (-20 to -10 days), the 400 hPa WN1 pattern over the Urals-Siberia region is very weak. During the second stage (-10 to 0 days), the amplitude of the 10 hPa high-latitude WN1 pattern reaches a maximum and precedes the maximum amplitude at 400 hPa, which occurs during stage three (0 to10 days). The features are consistent with Shaw and Perlwitz (2013, Fig. 5). The amplitude of the 400 hPa WN1 pattern reaches a maximum during the third stage and at the same time the pattern continues to move westward. Finally, in the fourth stage (10 to 20 days), the amplitude of the WN1 pattern decreases significantly and, overall, the wave pattern evolution is very consistent with a downward wave coupling event: a stratospheric WN1 anomaly at 10 hPa precedes a tropospheric WN1 anomaly at 400 hPa. All the features are highly consistent with Shaw and Perlwitz (2013, Fig. 5).

    Figure 4.  Heterogeneous regression pattern (units: gpm) of the leading coupled mode of the (a, b) 10 hPa and (c, d) 400 hPa WN1 fields at time lags of (a, c) $-10$ days and (b, d) $+15$ days for the 2008 case. The color shading varies from $-400$ to 400 gpm in (a, b) and from $-40$ to 40 gpm in (c, d), with an interval of 40 gpm and 4 gpm, respectively. These maps were constructed by regressing the time series of the 10 hPa (400 hPa) WN1 fields onto the temporal expansion coefficients of the leading mode of 400 hPa (10 hPa). The percentage in the title of the individual subplots indicates the variance accounting for the leading coupled mode.

    Figure 5.  Phase difference (degrees) between the associated WN1 regression patterns at 10 hPa and 400 hPa, averaged over the latitudinal band of 30$^\circ$-85$^\circ$N, as a function of time lags. Negative and positive values indicate westward and eastward phase shifts with heights, respectively. The red line indicates zero phase difference.

    Figure 6.  Daily 500 hPa geopotential height maps from (a-l) 10-21 January 2008, respectively. The contours are from 5100 to 5900 gpm, with an interval of 60 gpm. Heights between 5100 to 5160 gpm are marked with red contours.

    A downward coupling event due to reflection is linked with the transition of the heat flux anomaly (product of meridional wind and temperature anomaly) from positive to negative in the stratosphere (Shaw and Perlwitz, 2013). Therefore, we also plotted the evolution of the WN1 heat flux anomaly for the 2008 case (Fig. 3b). Like the WN1 anomaly, the heat flux pattern averaged between 60°N to 80°N at 400 hPa (black contours) and 10 hPa (coloring) as a function of longitude and time from -20 to +20 days is shown. In the first stage (-20 to -10 days), the heat flux anomaly in the stratosphere is strongly positive, particularly over the Urals-Siberia region, which is indicative of an upward wave coupling precursor. Meanwhile, in the troposphere, there is no significant heat flux anomaly during this stage. In the second stage (-10 to 0 days), the heat flux anomaly in the stratosphere changes sign from positive to negative. In addition, a positive heat flux anomaly starts to develop in the troposphere. In the third stage (0 to 10 days), the negative anomaly in the stratosphere attains its maximum, with subsequent development of a strong positive heat flux anomaly in the troposphere. The tropospheric maxima clearly lag the minima in the stratosphere. Finally, in the fourth stage (10-20 days), the heat flux anomaly weakens, but remains negative in the stratosphere; while in the troposphere, it fades out completely. The features are highly consistent with (Shaw and Perlwitz, 2013), completely describing the evolution of the WN1 anomaly during the downward wave coupling events.

  • To illustrate the zonal propagation of planetary waves, (Nath et al., 2014) computed the eddy component of Plumb fluxes (Plumb, 1985) in 3D space. From the vertical component of wave activity flux at 200 hPa, they showed that the reflection phenomena were more prominent from 10 to 19 January 2008. Furthermore, the upward propagation was stronger over the Labrador coast and Baffin Island. Whereas, the reflected components were prominent over the Eurasian continent and eastern parts of China. Upward and reflected fluxes were prominent in the upstream and downstream regions of the reflecting surfaces (Nath et al., 2014), respectively, indicating the impact of the polar jet stream, which preferentially guided the planetary waves (WN1) eastward and downward of the source region (Nath et al., 2014).

    Several authors have delineated the role of blocking as a precursor to SSW events (Martius et al., 2009). Recently, (Kodera et al., 2013) elucidated the relationship between stratospheric planetary wave reflection and blocking formation in the troposphere during SSW events. The upward propagation of the planetary waves in the pre-warming stage involves a Euro-Atlantic block; whereas, the downward propagation promotes the formation of Pacific blocking during the warming event. The longitude-time section of the blocking index were overplotted (gray contour lines) in both Figs. 3a and b to address the coincidence of the blocking event with tropospheric WN1 evolution due to reflection. The blocking index (dimensionless; contour lines) represents the zonal and temporal spread over which the mid-tropospheric flow is blocked, i.e., for which all three criteria [Eqs. (7a-c)] are satisfied simultaneously. A strong Urals-Siberia blocking is predominant in the third stage, when the amplitude of the 400 hPa WN1 pattern reaches its maximum value and the pattern continues to move westward. Consistently, the negative heat flux anomaly in the stratosphere attains its maximum with subsequent development of a strong positive anomaly in the troposphere. This feature is analogous to the formation of an anomalous Urals-Siberia blocking high, downstream of the reflected fluxes (Nath et al., 2014) from 15 to 25 January. The blocking index was calculated using Eqs. (7a-d). Furthermore, it is clear from the Eliassen-Palm (EP) flux vector (Nath et al., 2014) that, around mid-January, the upward component of the high latitude wave guide was very weak; it was only the downward component that could have contributed to the development of the blocking high over the Urals-Siberia region.

  • (Perlwitz and Graf, 2001) showed that the stratospheric WN1 field (50 hPa) leads the tropospheric WN1 field (500 hPa) by 6 days; whereas, using time-lagged SVD analysis, (Perlwitz and Harnik, 2004) investigated the close relationship between the 500 hPa (tropospheric field) and 50 hPa, 30 hPa and 10 hPa (stratospheric field) levels, individually, in composites of all winter seasons. Again, (Kodera and Chiba, 1995) showed that, in the troposphere, the circulation pattern changed significantly in relation to SSW events during 1984-85. They also linked the generation of anomalous cold surges and synoptic-scale eddies due to changes in the meridional propagation of tropospheric waves around the 500 hPa level. They further suggested that the changes in planetary wave structure could trigger enhanced baroclinic waves in the troposphere.

    In the present study, we expected the heterogeneous regression pattern at dominant positive and negative time lags to exhibit gradual eastward and westward shift, in phase, along the vertical direction, respectively. As discussed in section 3.1, strong downward coupling due to the WN1 field with a persistent covariance (between 10 hPa and 400 hPa) pattern down to the surface at lags of -10 days (troposphere leads) and +15 days (stratosphere leads) is noticeable in Fig. 2a. The two maxima exceed at least the 99% confidence level. The strength of coupling between the modes at different time lags is well illustrated by the heterogeneous regression patterns of the WN1 field at 400 hPa and 10 hPa, constructed when the 400 hPa (10 hPa) field leads the 10 hPa (400 hPa) at a time lag of -10 days (+15 days). The regression patterns in the lag of -10 days (+15 days), both for 10 hPa and 400 hPa, are plotted in Figs. 4a and b (4c and d), respectively, to understand the evolution of the entire process. The regression patterns associated in the negative and positive time lags exhibit completely different structure. At -10 days' lag, the ridge of the WN1 field at 10 hPa shifts westward with respect to the 400 hPa level; whereas, at +15 days' lag, the regression pattern at 400 hPa level shifts eastward relative to the 10 hPa level. This eastward shift in phase of the WN1 field at 400 hPa is consistent with the downward reflected wave. Since the square of the correlations represents the variance explained locally, the leading coupled mode accounts for up to 70% and more of the variance in the region of large amplitude. The variance, in terms of percentage, is noted in the titles of the individual subplots. In addition, at 400 hPa and for positive time lags, the phase tilts largely eastward with increasing latitude. We further compared the phase shift (degrees) between the WN1 field at the 10 hPa level and 400 hPa level; the differences at different time lags are plotted in Fig. 5. A continuous westward phase shift (negative) of the WN1 ridge, averaged over the latitude band of 30°-85°N, is clearly visible for all negative time lags; whereas, after +10 days' lag, the phase shift tends towards the zero mark, becomes eastward (positive) after +15 days' lag, and westward again from +22 days' lag.

    To further establish the link between anomalous blocking patterns in the Urals-Siberia region and the tropospheric WN1 field due to changes in circulation pattern, the daily march of the 500 hPa geopotential height fields from 10 to 21 January are plotted in Fig. 6. The contours from 5100 to 5900 gpm, with an interval of 60 gpm, are shown, and the geopotential height between 5100 and 5160 gpm is marked in red. Up until 12 January, the WN1 pattern in the troposphere is not very clear and is mainly concentrated at high latitudes. But, from 13 January onward, the WN1 pattern starts to develop slowly and exhibit a meridionally elongated pattern stretching from the Bering Sea to the North European plains. The pattern becomes clearer and is fully developed by 15 January. From 16 January, the trough starts to propagate equatorward and eastward over the Urals-Siberia region; and by 20 January, it reaches the Asian landmass, close to the Tibetan plateau. Moreover, it is clear from the EP flux (Nath et al., 2014) that, around mid-January, the upward component of the high latitude wave guide was very weak; it was only the downward component that could contribute to the development of the blocking high over the Urals-Siberia region. Furthermore, this development of the tropospheric WN1 pattern is consistent with the wave reflection from the stratosphere since 11 January. Moreover, the southeastward propagation and the intensification of the trough is consistent with the development of the strong blocking pattern in the Urals-Siberia region from 16 January onward. Although the reflection of the planetary waves ceases by 19 January, the blocking event persists until the last week of January with gradual eastward propagation. This extraordinary persistence had a significant impact on the extreme cold event over Eurasia and parts of China (Nath et al., 2014).

4. Summary and discussion
  • Time-lagged SVD analysis was performed to compare the covariance and correlation coefficients for the zonal mean and wave processes in the latitude band of 30°-85°N. We compared the 2008 winter with the composite mean pattern of six winters (2002/2003, 2004/2005, 2006/2007, 2007/2008, 2010/2011 and 2011/2012) when the downward coupling was stronger. The features were consistent in the zonal mean field, with stronger and extended (longer time scale) covariance in the lower stratospheric heights (positive lag side). For the WN1 field, the covariance was much higher (lower) for the negative (positive) lag, with a prominent peak around +15 days when the leading stratosphere coupled strongly with the troposphere. In all six winters except 2007, the basic state was partially reflective of the WN1 field; and during 2008, the reflective index was strongly negative over the Atlantic Ocean and Eurasian continent (Nath et al., 2014), favorable for the propagation of planetary waves down to the troposphere. We also analyzed the evolution of the WN1 anomaly and heat flux, both in the troposphere and stratosphere, during JFM 2008. The amplitude of the tropospheric WN1 pattern reached a maximum and was consistent with a downward wave coupling event. A stratospheric WN1 anomaly at 10 hPa preceded a tropospheric WN1 anomaly at 400 hPa. Similarly, the negative heat flux anomaly in the stratosphere attained its maximum, with subsequent development of a strong positive heat flux anomaly in the troposphere. The tropospheric maxima clearly lagged the minima in the stratosphere.

    To interpret the occurrence of the anomalous blocking high over the Urals-Siberia region due to wave reflection, we focused on the period 10-21 January 2008. The blocking anomaly developed strongly when the amplitude of the 400 hPa WN1 pattern reached its maximum value, and the pattern continued to move westward in response to the reflection of planetary waves down to the troposphere. From the zonal-mean EP flux vectors (Nath et al., 2014), on 10 January, the high-latitude wave guide pointed vertically upward. On 13 January, the upward fluxes exhibited a gradual weakening trend, while a strengthening in the downward component was evident. This reflection/overturning was stronger on 16 January and continued until 19 January. In 3D space, wave fluxes propagated upward from the Labrador coast and reflected back to the Eurasian continent.

    To illustrate the strength of the coupling between 10 hPa and the 400 hPa WN1 fields, we plotted the heterogeneous regression pattern at negative and positive time lags. The associated regression patterns for the 10 hPa and 400 hPa levels at negative and positive time lags shifted westward and eastward relative to the 400 hPa and 10 hPa levels, respectively. This eastward phase shift of the WN1 ridge at 400 hPa was consistent with the reflection of the WN1 field from the stratosphere. The relationship between the tropospheric WN1 field and the anomalous blocking high in the Urals-Siberia region was further established based on the daily march of the 500 hPa geopotential height fields. From the EP flux it was quite clear that, around mid-January, the upward component of the high-latitude wave guide was very weak; it was only the downward component that could have contributed to the development of the blocking high over the Urals-Siberia region. Moreover, we found that, from 13 January onward, the WN1 pattern started to develop slowly and exhibit a meridionally elongated pattern stretching from the Bering Sea to the North European plains. The pattern developed fully by 15 January and, from 16 January onward, the trough started to propagate equatorward and eastward over the Urals-Siberia region. And by 20 January, it reached the Asian landmass, close to the Tibetan Plateau. This development of the tropospheric WN1 pattern was due to the reflection of the WN1 field from the stratosphere from 11 January onwards.

Reference

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