Overview of the Major 2012-2013 Northern Hemisphere Stratospheric Sudden Warming

LIU Yi(刘毅)and ZHANG Yuli,2∗(张玉李)

1 Key Laboratory of Middle Atmosphere and Global Environment Observation,Institute of Atmospheric Physics, Chinese Academy of Sciences,Beijing 100029

2 University of Chinese Academy of Sciences,Beijing 100049

Overview of the Major 2012-2013 Northern Hemisphere Stratospheric Sudden Warming:Evolution and Its Association with Surface Weather

LIU Yi1(刘毅)and ZHANG Yuli1,2∗(张玉李)

1 Key Laboratory of Middle Atmosphere and Global Environment Observation,Institute of Atmospheric Physics, Chinese Academy of Sciences,Beijing 100029

2 University of Chinese Academy of Sciences,Beijing 100049

In this study,we analyzed the dynamical evolution of the major 2012-2013 Northern Hemisphere(NH) stratospheric sudden warming(SSW)on the basis of ERA-Interim reanalysis data provided by the ECMWF. The intermittent upward-propagating planetary wave activities beginning in late November 2012 led to a prominent wavenumber-2 disturbance of the polar vortex in early December 2012.However,no major SSW occurred.In mid December 2012,when the polar vortex had not fully recovered,a mixture of persistent wavenumber-1 and-2 planetary waves led to gradual weakening of the polar vortex before the vortex split on 7 January 2013.Evolution of the geopotential height and Eliassen-Palm flux between 500 and 5 hPa indicates that the frequent occurrence of tropospheric ridges over North Pacific and the west coast of North America contributed to the pronounced upward planetary wave activities throughout the troposphere and stratosphere.After mid January 2013,the wavenumber-2 planetary waves became enhanced again within the troposphere,with a deepened trough over East Asia and North America and two ridges between the troughs.The enhanced tropospheric planetary waves may contribute to the long-lasting splitting of the polar vortex in the lower stratosphere.The 2012-2013 SSW shows combined features of both vortex displacement and vortex splitting.Therefore,the anomalies of tropospheric circulation and surface temperature after the 2012-2013 SSW resemble neither vortex-displaced nor vortex-split SSWs,but the combination of all SSWs. The remarkable tropospheric ridge extending from the Bering Sea into the Arctic Ocean together with the resulting deepened East Asian trough may play important roles in bringing cold air from the high Arctic to central North America and northern Eurasia at the surface.

stratospheric sudden warming,polar vortex splitting,planetary wave,Eliassen-Palm flux, cold wave

1.Introduction

Stratospheric sudden warming(SSW)dominates the Northern Hemisphere(NH)wintertime variation of the general circulation in the middle atmosphere (e.g.,Andrews et al.,1987).When an SSW event occurs,the stratospheric polar night jet circling the wintertime polar vortex slows down or even reverses direction within a few days,accompanied by a rise of stratospheric temperature by several tens of Kelvins in the polar region.Any SSW event can be regarded as a“major warming”if the circulation reversal is observed at 10 hPa(approximately 32 km)or below(e.g.,McInturff,1978).The occurrence of SSW is affected by various external factors,such as solar activity,sea surface temperature,and quasi-biennial oscillation(e.g.,Labitzke and van Loon,2000;Baldwin et al.,2001;Liu and Lu,2010).In recent decades,scientific interest

in major SSW has increased due to its influence on atmospheric transport and chemistry(e.g.,Manney et al.,2008,2009;Liu C.X.et al.,2009,2010;Liu Y.et al.,2009,2011)and implications for extended-range weather forecasting at the midlatitude surface(e.g., Baldwin and Dunkerton,2001;Baldwin et al.,2003; Sigmond et al.,2013).

Supported by the National(Key)Basic Research and Development(973)Program of China(2010CB428604),National Natural Science Foundation of China(41105025),and the Dragon Three Program(10577).

∗Corresponding author:zhangyuli@mail.iap.ac.cn.

©The Chinese Meteorological Society and Springer-Verlag Berlin Heidelberg 2014

During the development of major SSW,the cold stratospheric polar vortex becomes either displaced from the pole or splits(Charlton and Polvani,2007) as a result of the enhanced upward planetary waves (Matsuno,1971).Nakagawa and Yamazaki(2006) classified SSW events according to their downward propagation into the troposphere and showed that the upward wavenumber-2 is more likely to result in vortex splitting and downward propagation of SSW. Many studies(e.g.,Baldwin and Dunkerton,1999) have indicated the possibility that SSW can significantly influence tropospheric circulation(e.g.,Julian and Labitzke,1965;Kodera et al.,2011)and surface climate.Scaife and Knight(2008)showed that much of the cold anomaly over Europe in the 2005-2006 winter is likely attributable to the occurrence of SSW.A recent study by Mitchell et al.(2013) also suggests that the displacement or splitting of the stratospheric polar vortex during a major SSW may lead to different patterns of surface weather and climate anomalies. Yi et al. (2013)investigated the relationship between weak polar vortex and surface temperature over East Asia,suggesting the potential linkage between high-level air with high potential vorticity and cold weather at the surface. In the 2012-2013 winter,many countries across the high-and mid-latitude Eurasian Continent experienced extremely cold episodes.China also witnessed the coldest winter throughout the past few decades (http://www.usatoday.com/story/news/world/2013/ 01/08/china-cold/1817271/).Moreover,an additional major SSW event occurred in early January 2013, which led to a long-lasting polar vortex splitting deep into the upper troposphere.

In this study,we investigate the evolution of the major 2012-2013 NH SSW and its potential linkage to the wintertime surface temperature over North China. The rest of the paper is organized as follows.Section 2 describes the data and methods used.Section 3 examines the dynamical evolution of the warming event in terms of temperature,zonal wind,geopotential height,and planetary wave activity.Section 4 shows the relationship between SSW and surface temperature.Finally,the main conclusions are summarized and discussed in Section 5.

2.Data and methodology

In this study,the daily mean horizontal wind, temperature,and geopotential height are derived from the ECMWF Reanalysis Interim(ERA-Interim)data (e.g.,Simmons et al.,2007a,b;Dee et al.,2011)with a horizontal resolution of 1.5°× 1.5°. Cohen and Jones(2011)summarized the central dates of major SSW events occurring during 1948-2010 on the basis of the NCEP/NCAR reanalysis,during which 18 vortexdisplacement events and 17 vortex-splitting events occurred,respectively.In this study,the surface temperature anomaly and 500-hPa geopotential height after the vortex-splitting in early January 2013 are compared with the composite anomalies following various types of SSWs during 1979-2013.

The northern annular mode(NAM)is known as the leading mode of NH circulation and is defined as the leading empirical orthogonal function(EOF) of daily wintertime geopotential anomalies north of 20°N(Baldwin and Dunkerton,2001).The NAM index(NAMI),defined as the time series of the leading EOF,is calculated in this study to provide a measure of the vertical coupling and the influence of SSW on surface temperature.

The Eliassen-Palm(E-P)flux(Eliassen and Palm, 1961)and its divergence(e.g.,Edmon et al.,1980), which depict planetary wave activity and eddy forcing on zonal mean flow,respectively,are examined to characterize the feature of the planetary wave in the 2012-2013 winter.The components of the E-P flux, F≡(O,F(λ),F(z)),are defined as

where θ refers to potential temperature.The divergence of the E-P flux is given by

Due to differences in magnitude through the troposphere and stratosphere,the E-P flux in this study is scaled by 1.0 below 100 hPa,2.0 between 100 and 10 hPa,and 0.4 above 10 hPa.Because meridional heat flux is proportional to the vertical component of the E-P flux,the 100-hPa zonal mean eddy heat fluxes of the first three zonal wavenumber components averaged over 45°-75°N are computed to quantify the activity of upward-propagating wavenumber-1,-2,and-3 planetary waves(Fig.1).

3.Dynamical evolution

Fig.1.Evolution of the daily values from 1 November 2012 to 1 April 2013 of(a)area-weighted mean temperature (shading;K)poleward of 60°N and zonal-mean zonal wind(westerly:blue solid contour;easterly:red dashed contour; zero:white solid contour;m s-1;interval 5 m s-1)at 64.5°N between 300 and 1 hPa;(b)10-hPa temperature(red solid line;K)over the North Pole and 10-hPa zonal-mean zonal wind(blue dashed line;m s-1)at 64.5°N;(c)100-hPa eddy-heat flux averaged between 45°and 75°N for zonal wavenumber-1(blue dashed line),wavenumber-2(red dotted line),and sum of the first three wavenumbers(black solid line).The dashed vertical line indicates the peak date of the 10-hPa temperature at the North Pole on 7 January 2013.

Figure 1a shows the temporal evolution of vertical profiles of Arctic air temperature(60°-90°N)and zonal-mean zonal wind along 64.5°N from the upper troposphere(500 hPa)to the upper stratosphere (1 hPa).It is shown that the warming and easterly winds occurred at 1 hPa at the beginning of January 2013 and propagated downward into the upper troposphere and lower stratosphere(UTLS)within approximately two weeks(6-20 January 2013).Afterward,the upper and mid stratosphere cooled down at the end of January.The warming signal persisted in the UTLS region until late February and early March. Furthermore,a good correlation appeared between the warming signals in the mid stratosphere including the increased temperature and decreased zonal wind at 10 hPa(Fig.1b)and the zonal-mean eddy heat flux at 100 hPa(Fig.1c),suggesting the importance of the planetary waves upward-propagating from the troposphere to the initiation and development of the warming event.For example,enhanced activity of the wavenumber-2 planetary wave began in late November and early December 2012(Fig.1c),leading to a pronounced disturbance in the stratospheric polar vortex

(Figs.2a-7a).Although the planetary wave activity was not strong enough to cause a major warming event,it was followed by a pronounced temperature increase(up to 15 K or so)at 10 hPa over the North Pole and a pronounced decrease(approximately from 40 to 10 m s-1)in the 10-hPa zonal-mean zonal wind at 64.5°N(Fig.1b).Before the full recovery of the stratospheric polar vortex,an additional burst of the upward-propagating wavenumber-1 planetary wave began in mid December and peaked on 23 December 2012(Fig.1c).Shortly after this peak,a third burst of planetary wave began on 28 December 2012 and peaked on 5 January 2013.Different from the previous two bursts of upward planetary waves beginning in late November and mid December,the third burst of planetary wave was composed of mixed wavenumber-1 and-2 components.The highly active planetary waves from mid December 2012 to early January 2013 led to a splitting of the stratospheric polar vortex on 7 January 2013,with a 10-hPa temperature over the North Pole rising from 200 K to above 240 K within 10 days,from 30 December 2012 to 7 January 2013. After mid January 2013,the fourth planetary wave burst,which was dominated by a wavenumber-2 component,leading to a persistent disturbance of the lower stratosphere(Fig.1a).

To illustrate the relationship between the tropospheric planetary wave and the horizontal distribution of the stratospheric polar vortex,Figs.2-7 show the evolution of the daily mean E-P fluxes of wavenumbers-1 and-2,all wavenumbers,and their divergences in both the troposphere and stratosphere, as well as geopotential height in the mid troposphere (500 hPa),UTLS(100 and 50 hPa),mid stratosphere (10 hPa),and upper stratosphere(5 hPa).The plot of E-P flux at 1 hPa was omitted,given its similarity to that at 5 hPa.

As shown in Fig. 2c,on 1 December 2012, upward-propagating E-P flux occurred above the mid troposphere(approximately 500 hPa)between 45°and 60°N.The E-P flux of the wavenumber-2 planetary wave(Fig.2b)was greater than that of wavenumber-1(Fig.2a),suggesting that the wavenumber-2 was more active at this time.The upward E-P flux of the wavenumber-2 planetary wave showed a slight poleward propagation between the lower and mid stratosphere,leading to a pronounced wavenumber-2 disturbance in the lower stratospheric polar vortex(Figs. 2d-f).The E-P flux turned equatorward as soon as it arrived at 30 hPa.As a result,the disturbance of the mid and upper stratospheric polar vortex was much weaker than that in the lower stratosphere,as revealed by a comparison of Figs.2g-h and Figs.2e-f.The area of negative E-P flux divergence between 70 and 10 hPa was confined to the region south of 75°N,which was still far from the core of the polar vortex.Therefore,as shown in Figs.1a and 1b,the wavenumber-2 disturbance did not cause a major SSW event.

On 21 December 2012,the upward E-P flux in the troposphere became stronger(Fig.3c),leading to large areas of negative E-P flux divergence between 45°and 75°N in the mid and upper troposphere(500-200 hPa).The main contributor was the E-P flux of the wavenumber-1 planetary wave in the stratosphere,as revealed by a comparison of Figs.3a and 3b.The upward-propagating wavenumber-1 planetary wave(Figs. 3a and 1c)led to a similar circulation pattern throughout the stratosphere(100-5 hPa),with the stratospheric polar vortex over North Europe and high geopotential height over the Aleutian Islands(Figs.3e-h).In the troposphere,the active wavenumber-1 planetary wave was related to a deep trough over East Asia.Moreover,strong tropospheric wavenumber-2 activity also occurred(Fig.3b), which disturbed the wavenumber-1 pattern by producing multiple ridges along the latitude.The strongest ridge in the troposphere was located to the east of the Date Line,extending from North Pacific to the Aleutian Islands and the Bering Sea(Fig.3d).

Fig.2.Eliassen-Palm(E-P)flux(vector;kg s-2)and its divergence(negative contour;kg m-1s-2)of(a)wavenumber-1,(b)wavenumber-2,and(c)all wavenumbers for the troposphere and stratosphere on 1 December 2012,and the corresponding geopotential height(×103gpm)at(d)500,(e)100,(f)50,(g)10,and(h)5 hPa.

Fig.3.As in Fig.2,but for 21 December 2012.

Fig.4.As in Fig.2,but for 30 December 2012.

In late December 2012,the upward E-P flux persisted throughout the troposphere and stratosphere, producing an increased area of negative E-P flux divergence in the upper stratosphere above 10 hPa(e.g., Fig.4c). However,the area with E-P flux convergence in the upper stratosphere was confined equatorward of 60°N,suggesting that the polar vortex was weak at those altitudes. The E-P flux of the wavenumber-1 planetary wave(Fig.4a)was greater

than that of wavenumber-2(Fig.4b).As a result, the distribution of geopotential height shows that the wavenumber-1 pattern of disturbance can be attributed to a tropospheric ridge extending from the subtropical eastern Pacific northward toward the west coast of North America(Fig.4d).As a result,similar ridges appeared over the same region throughout the UTLS(Figs.4e-f).The poleward intrusion from midlatitude was also shown by the poleward propagation of E-P flux between 100 and 30 hPa,as revealed by comparison between Fig.4c and Figs.4e-f.In contrast,the upper stratospheric ridges over East Asia (e.g.,10-5 hPa)indicated a pronounced westward tilt of the planetary wave with height(Figs.4g-h).The lag of ridge between the upper and lower stratosphere further indicates the upward influence of the planetary wave prior to the onset of the 2012-2013 major SSW.

With the propagation and development of the upward planetary waves,the Aleutian high intensified on 5 January 2013(Figs.5g-h).As a result,poleward propagation of the E-P flux was enhanced in the upper stratosphere,leading to large areas of negative EP flux divergence within the Arctic region(Fig.5c). Different from the wavenumber-1 pattern in the upper stratosphere(Figs.5g-h,5a,and 1c),an additional tropospheric ridge developed quickly over the North Atlantic/Europe sectors,producing the pronounced wavenumber-2 patterns just prior to the major SSW throughout the mid troposphere and the UTLS(Figs. 5d-f,5b,and 1c).

With the upward propagation of the planetary wave related to the ridge over the North Atlantic/Europe sectors,the wavenumber-2 pattern became clear in the mid and upper stratosphere on 7 January 2013(Figs.6b and 6g-h).As a result,the EP fluxes of both wavenumber-1 and-2 planetary waves were enhanced poleward and converged between 70 and 10 hPa(Figs.6a-c),causing the occurrence of easterly winds at high latitudes(Fig.1a).The easterly winds in the mid and upper stratosphere prohibited the upward propagation of the planetary wave beyond the mid stratosphere,leading to the subsequent splitting of the polar vortex in the lower stratosphere (Figs.6f and 7e).

On January 20,two weeks after the central date of the SSW,the upper stratospheric polar vortex remained completely broken up(Figs.7g-h),and the lower stratospheric polar vortex remained split(Figs. 7e-f)due to the wavenumber-2 activity at those levels(Fig.7b).In the troposphere,an additional ridge that developed over the west coast of North America triggered a fourth planetary wave burst after mid January 2013(Fig.1c).However,with the prevalence of easterlies in high latitudes throughout the stratosphere (Figs.7e-h),the tropospheric disturbances did not propagate further upward(Fig.7c).The negative EP flux divergence of the wavenumber-2 planetary wave below 200 hPa(Fig.7b)suggests that the interactions between the planetary wave and zonal flow remained strong in the upper troposphere.These interactions may also be important in the persistence of the polar vortex splitting in the UTLS.At the end of January 2013,the polar vortex began to recover gradually.

4.Surface temperature

As shown by recent studies(e.g.,Thompson et al.,2002;Baldwin et al.,2003),stratospheric circulation anomalies associated with major SSW events may lead to large-scale anomalous weather patterns at the surface.As shown in Fig.1a,a long-lasting disturbance in the polar vortex in the UTLS region propagated downward and may have influenced the troposphere and the surface.Figures 8a,8c,8e,and 8g show the time-height evolution of the composite NAMI throughout the winters(from November to April)during the vortex displacement,vortex splitting,all SSW events with no separated vortex displacement and vortex splitting,and the 2012-2013 SSW,respectively. The positive NAMI before the vortex-split SSW(Fig. 8c)was stronger than that before the vortex-displaced SSW(Fig.8a).Strong negative NAMI occurred along with the SSW events(Figs.8a,8c,8e,and 8g),suggesting strong disturbances of the polar vortex.As the disturbance propagated downward,this negative NAMI gradually descended to the troposphere,likely resulting in the anomalous tropospheric circulation and surface temperature.

Fig.5.As in Fig.2,but for 5 January 2013.

Fig.6.As in Fig.2,but for 7 January 2013.

Fig.7.As in Fig.2,but for 20 January 2013.

Fig.8.(a,c,e,g)Composites of 1979-2013 time-height evolution of the northern annular mode index(NAMI),and (b,d,f,h)30-day average 500-hPa geopotential height(contour;m)and surface temperature anomalies(shading;K) after the central dates of the 2012-2013 SSW during(a,b)vortex-displacement SSW events;(c,d)vortex-split SSW events;(e,f)all SSW events;and(g,h)the 2012-2013 SSW.Lag 0 in(a,c,e)indicates the central dates of SSW events. Positive and negative values in(b,d,f,h)are indicated by solid and dashed contours,respectively,with an interval of 30 gpm.The color-shaded(blue for negative;red for positive)anomalies in(b,d,f)are statistically significant at the 95%confidence level based on Student’s t-test.

Figures 8b,8d,8f,and 8h show a comparison of the 30-day mean surface temperature anomaly and the 500-hPa geopotential height anomaly following the central date of the major 2012-2013 SSW for the vortex displacement,vortex splitting,and all SSW events during 1979-2013.As shown in Figs.8b and 8d,after the vortex displacement and vortex splitting events, large-scale surface temperature anomalies occurred. After the vortex-split SSW events,the two split polar vortex components tended to move toward East Asia and North America separately,leading to the deepening of a tropospheric trough over each region.As a result,negative surface temperature anomalies occurred over the northern Eurasian continent and southern United States(Fig.8d).

Similar to a previous study by Mitchell et al.(2013),the temperature anomalies after vortexdisplaced SSWs were significantly weaker,as revealed by a comparison of Figs. 8b and 8d. However, the pattern of surface temperature anomaly after the 2012-2013 SSW event did not resemble the composites of either vortex-displaced or vortex-split SSW events.This result likely occurred because the 2012-2013 SSW shows features of both vortex-displaced and vortex-split SSW events,which were driven by both wavenumber-1 and-2 planetary waves.Therefore,the 2012-2013 SSW resembles the composite pattern of all SSW events(Figs. 8e and 8f). After the major 2012-2013 SSW,the 500-hPa geopotential height was characterized by deep troughs over East Asia and North America and strong ridges over North Pacific and North Atlantic.This remarkably strong wavenumber-2 pattern likely contributed to the longlasting disturbances throughout the UTLS(Fig.1a). As a result of the persistent trough over East Asia,the surface temperature over northern Eurasia showed a pronounced negative anomaly,which is highly similar to the composite circulation anomaly occurring after vortex-split SSWs(Figs.8d vs.8h).However,due to the strong ridge developing from the Bering Sea to the Arctic Ocean,an additional negative surface temperature anomaly occurred over central North America that differed from the composite of either vortexdisplacement or vortex-split SSW events,as revealed by a comparison of Fig.8h to Figs.8a and 8d.

5.Summary

In this study,we analyzed the dynamical evolution of SSW in the 2012-2013 boreal winter and its relation to the wintertime surface temperature over the NH.It is shown that four bursts of intermittent planetary waves occurred from the troposphere into the polar stratosphere,including three prior to the polar vortex splitting and one following.From late November to early December 2012,the wavenumber-2 planetary wave led to a pronounced disturbance of the stratospheric polar vortex,although no major SSW occurred.Mixed wavenumber-1 and-2 planetary waves from mid December 2012 until the SSW onset on 7 January 2013,led to strong upward E-P flux and convergence throughout the stratosphere at high latitudes.These disturbances were crucial for the weakening and splitting of the stratospheric polar vortex.

Evolution of geopotential height and the E-P flux diagnostics at multiple pressure levels(500-5 hPa)indicate that the frequent occurrence of tropospheric ridges from North Pacific to the west coast of North America can be the triggering mechanism of the upward planetary waves that led to similar circulation patterns throughout the UTLS and the midstratosphere.The lag of geopotential pattern in the upper stratosphere also indicates the upward influence of the planetary wave.After the warming signal arrived in the lower stratosphere,the tropospheric wavenumber-2 planetary wave increased again after mid January 2013.

Due to the prevalence of easterlies in high latitudes throughout the stratosphere,the tropospheric disturbances were unable to propagate further upward, although the interactions between the planetary wave and zonal flow in the upper troposphere were still important to the persistence of the polar vortex splitting in the UTLS.With the persistent warming in the lower stratosphere,the upper stratosphere began to cool down and recover at the end of January 2013. These results are consistent with those of our previous

study(e.g.,Liu Y.et al.,2009),which revealed that the upward tropospheric planetary waves could hardly penetrate the disturbed lower stratosphere,as shown in Fig.1a.

The strong downward negative NAMI after SSW, which indicates the downward propagation of the disturbance of the polar vortex,likely resulted in the anomalies of 500-hPa circulation and surface temperature.

The 2012-2013 SSW shows combined features of both vortex-displaced and vortex-split SSWs.Therefore,the anomalies of 500-hPa geopotential height and surface temperature after the 2012-2013 SSW resemble the compositions of all SSW events with no separated vortex displacement and vortex splitting.It was shown that the remarkable ridge extending from the Bering Sea to the Arctic Ocean together with the resulting deepened East Asian trough play important roles in bringing cold air from the high Arctic to central North America and northern Eurasia.

In this study,we compared the tropospheric circulation and surface temperature anomalies related to vortex-displaced SSW and vortex-split SSW with those of the 2012-2013 major SSW and discovered a statistic connection between the SSW and tropospheric anomalies. However,the dynamical explanation for the observed connection is still missing. Further studies are required to examine the roles of both the troposphere and stratosphere in creating and maintaining the large-scale circulation anomalies months after the central dates of SSWs.

Acknowledgments.We thank Dr.Chuanxi Liu for his helpful comments and discussion. The ERA-Interim reanalysis data were kindly provided by the ECMWF at http://www.ecmwf.int/products/ data/.

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(Received September 9,2013;in final form April 30,2014)