Interannual Variability of the Wintertime Northern Branch High Ridge in the Subtropical Westerlies and Its Relationship with Winter Climate in China

FAN Guangzhou（范广洲），ZHANG Yongi（张永莉），WANG Bingyun（王炳赟），HUA Wei（华维），and WANG Yongi（王永立）

1 College of Atmospheric Sciences，Chengdu University of Information Technology/Plateau Atmosphere and Environment Key Laboratory of Sichuan Province，Chengdu 610225

2 Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters，Nanjing University of Information Science&Technology，Nanjing 210044

3 School of Atmospheric Physics，Nanjing University of Information Science&Technology，Nanjing 210044

4 Key Laboratory of Regional Climate-Environment for East Asia，Institute of Atmospheric Physics，Chinese Academy of Sciences，Beijing 100029

Interannual Variability of the Wintertime Northern Branch High Ridge in the Subtropical Westerlies and Its Relationship with Winter Climate in China

FAN Guangzhou1,2∗（范广洲），ZHANG Yongli1（张永莉），WANG Bingyun1,3（王炳赟），HUA Wei1（华维），and WANG Yongli4（王永立）

1 College of Atmospheric Sciences，Chengdu University of Information Technology/Plateau Atmosphere and Environment Key Laboratory of Sichuan Province，Chengdu 610225

2 Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters，Nanjing University of Information Science&Technology，Nanjing 210044

3 School of Atmospheric Physics，Nanjing University of Information Science&Technology，Nanjing 210044

4 Key Laboratory of Regional Climate-Environment for East Asia，Institute of Atmospheric Physics，Chinese Academy of Sciences，Beijing 100029

The high ridge in the northern branch of the subtropical westerly（hereafter referred to as northern branch ridge）extends from the north of the Tibetan Plateau to the north of Barr Kashmir Lake（32.5°-47.5°N，70°-95°E）during wintertime.The intensity index and zonal position index of the wintertime northern branch ridge（WNBR）are defined in this study by using the NCEP-NCAR reanalysis data and precipitation and temperature observations at 160 stations in China.Interannual variation features of the WNBR and its relation with precipitation and surface air temperature in China are discussed based on wavelet analysis，correlation analysis，and composite analysis.The results indicate that the intensity and zonal position of the WNBR exhibit significant interannual variability.The intensity of the WNBR gradually weakens and varies in periodic cycles of 4-6，quasi-2，and quasi-16 yr.Its zonal position shifts westward slightly and varies in periodic cycles of quasi-8 and quasi-16 yr.The WNBR is highly correlated with precipitation and surface air temperature in China.When the WNBR is strong（weak）and its zonal position shifts eastward（westward），winter precipitation in Northeast China and the northern region of Northwest China increases（decreases），whereas precipitation decreases in central China，South China，and eastern regions of Northwest and Southwest China.Meanwhile，surface air temperatures in most areas of China are higher（lower）than normal.Further investigation has revealed that the two indexes are closely related to variations in general atmospheric circulation，which explains why there exists a close linkage between the variation of the WNBR and climate in China.It is believed that the WNBR is also tied to the Rossby wave，the North Atlantic Oscillation，the East Asian trough，and the North Pacific jet stream.

wintertime northern branch ridge，interannual variability，precipitation，temperature，atmospheric circulation

1.Introduction

The upper-level westerly circulation system serves as a bridge between the polar region，the North Atlantic Ocean region，and the East Asian monsoon region.It is also an important component of the atmospheric circulation system in the Northern Hemisphere（Stephen and An，1995；Qu et al.，2004）.Due to the dynamic forcing of the Tibetan Plateau（TP），the two bypassing flows to the north and south sides of theTP are quite different.Specifically，anticyclonic flow can be found on the northern side of the TP，while cyclonic curvatures appear simultaneously on the southern side.This presents a typical circulation pattern characterized by a ridge to the north and a trough to the south of the TP.Yeh（1950）first revealed this phenomenon in as early as 1950.Gu（1951）described the TP as a barrier for atmospheric circulation，suggesting that the TP forcing can block atmospheric flows and split the low-and mid-level westerlies into the northern and southern branches.As a result，high-pressure ridges form on the northern side of the TP while low pressure troughs form on the southern side（Chaudhury，1950；Yang，1954；Newton，1971）.However，the splitting of westerlies caused by the dynamic forcing of the TP is not limited to the lower atmosphere and can reach the upper troposphere（Yeh and Gu，1955）. In previous studies，the high-pressure ridge formed to the north of the TP was defined as a blocking high，which is also regarded as a component of the climatological“three-trough and three-ridge”pattern of the troposphere in wintertime of the Northern Hemisphere（Rex，1950；Wu，1957）.Some scholars also considered this ridge as a semi-permanent high（Yeh，1952）.

Supported by the National Natural Science Foundation of China（41275079，41305077，41405069，91537214，and 41505078）and Scientific Research Fund for the Young Academic Leaders，Chengdu University of Information Technology（J201518 and J201516）.

∗Corresponding author：fgz@cuit.edu.cn.

©The Chinese Meteorological Society and Springer-Verlag Berlin Heidelberg 2015

By combining satellite cloud images and weather maps at 500 hPa，researchers also found that a closed high-pressure center frequently appears in Xinjiang，which is located to the north of the TP，when the high-pressure ridge is stable.This closed high-pressure center is referred to as the high-pressure ridge in the northern branch of the westerlies（Li，1977；Shi et al.，1981；Huang et al.，2013）and is abbreviated as the northern branch ridge（NBR）in this study.According to numerical simulation results，solely under the dynamic forcing of the TP，bypassing of the westerly airflow is more distinct in the middle and lower troposphere than in the upper troposphere.This phenomenon is particularly significant in midlatitude regions to the north of 40°N（Wang et al.，1984）.Furthermore，the TP can also significantly affect atmospheric circulation to the north of the plateau（Yang et al.，1959）.The coupled northern ridge-southern trough pattern is largely responsible for disastrous weather of freezing rain and snow that is widespread in southern China（Wang et al.，2011）.

Many previousstudiesfocused on thelowpressure trough in the southern branch of the TP（Ramaswamy，1956；Yanai et al.，1992；Suo and Ding，2009；Zhang et al.，2012，2014），while few studies have been conducted on analysis of characteristic variations of the high ridge in the northern branch of the westerlies and its related effects on weather and climate in China.This article investigates characteristics of the interannual variation of the wintertime northern branch ridge（WNBR），and analyzes its relationship with wintertime climate in China.Physical mechanisms behind the interaction of WNBR and wintertime climate in China have also been explored in this study.

2.Data and methodology

The data used in this study include monthly precipitation and surface air temperature observations at 160 meteorological stations in China.The observations are provided by the National Climate Center of the China Meteorological Administration.Monthly average reanalysis data，such as geopotential height，vertical velocity，meridional and zonal winds，etc.，with a horizontal resolution of 2.5°×2.5°，are extracted from the NCEP-NCAR Reanalysis Project（NNRP）. Considering the fact that NNRP reanalysis is of higher reliability since 1979，the data from 1979 to 2015 are selected for the present study（Xu et al.，2001；Yang et al.，2002）.Generally，winter refers to December of the present year and January and February of the next year.The circulation pattern in January is similar to that in the mean field of the three months（figure omitted）.Therefore，in this study，the data in January are used to represent winter for analysis.Linear trend estimation，wavelet analysis，correlation analysis，and composite analysis are used in this study（Shi and Gu，1993；Torrence and Compo，1998）.

3.Definition and interannual evolution characteristics of the WNBR

3.1 Definition of the indexes

Figure 1 illustrates the average geopotential heig-ht vorticity fields at 500 hPa in January over the study period.It shows clearly a“two-trough-one-ridge”pattern that appears in the mid-and high-latitude regions in Europe and Asia in January.The planetary trough covers the region from Europe to West Asia，and the East Asian deep trough dominates East Asia. The ridge extends from northern TP to the region near Lake Baikal.Two strong negative-vorticity centers can also be found，which correspond to the high-pressure ridge surrounding Lake Baikal（Xu et al.，2011；Chen et al.，2013）and the northern branch ridge extending from the northern TP to the region north of Balkhash Lake respectively（Li，1977；Shi et al.，1981）.Furthermore，the absolute value of the negative vorticity is the largest in the region of the northern TP（32.5°-47.5°N，70°-95°E）with the latitude ranging from 30°to 50°N，and the anticyclonic curvature is obvious in the contour lines.Accordingly，this region is often regarded as the main region influenced by activities of the WNBR.It can be concluded that the NBR is a topographic ridge formed in the northern TP because of the plateau's blocking effects on the subtropical westerly jet stream.

A high-pressure ridge connects the points at which anticyclonic curvatures of the isobars（pressure contour lines）are the largest and the air pressure（geopotential height）is the greatest along the latitude circle.The ridgelines，although bent to some degree，are close to a certain longitude.Moreover，the air pressure in the middle of a ridge is higher than that on both sides of the ridge.Based on methods to determine the intensity and position of the East Asian deep trough and the south westerly trough proposed by Gilles（1983）and Zhang et al.（2014），the intensity index of the WNBR，IWI，can be defined as：

Fig. 1. Distribution of climatological average geopotential height（gpm；contours）and vorticity（10-5s-1；shaded）at 500 hPa in January of 1979-2015.Thick solid line denotes topography of 3000 m and dotted rectangle denotes the northern branch high ridge area（32.5°-47.5°N，70°-95°E）.

where H500denotes the geopotential height at 500 hPa；λidenotes the region with the longitudes ranging from 70°to 95°E（i.e.，i from 1 to 11）；ϕjdenotes the region with the latitudes ranging from 32.5°to 47.5°N（i.e.，j from 1 to 7）；tkdenotes the period from 1979 to 2014（i.e.，k from 1 to 36）；and N=7.This means that，within the area influenced by the WNBR，the 500-hPa height fields in the region from 32.5°to 47.5°N are averaged for January，and then 11 average values of the geopotential height in the region from 70°to 95°E can be obtained（the average values are at a resolution of 2.5 longitude degrees）.The maximum IWIis obtained from the 11 values.The corresponding longitude of IWIis then defined as the zonal position index IWP（WNBR zonal position index）.The greater the value of IWI，the stronger the WNBR，and vice versa.As to IWP，a relatively large value indicates eastward shift of the WNBR.Conversely，if the value is small，the WNBR shifts westward.The two indexes，IWIand IWP，not only reflect the intensity of the WNBR，but also describe changes in position of the WNBR along the east-west direction.

3.2 Interannual variations

As stated previously，the NBR forms due to the TP's blocking effects on the subtropical westerly jet（Chaudhury，1950；Yang，1954；Newton，1971）；therefore，its variations are inevitably related to the upstream westerlies.In the present study，the average zonal wind speed in the midlatitude region of the TP（30°-50°N，40°-70°E）was adopted as the intensity index of the westerlies，and the relationship between the intensity and zonal position of the WNBR with thewesterlies from the upstream of the TP was investigated.Figure 2 illustrates time series of the normalized anomalies of IWI，IWP，and the intensity index of the westerlies.In the last 37 years（1979-2015），the WNBR varied significantly in intensity and tended to be weakening overall，with a linear trend coefficient of only-0.01（10 yr）-1.The intensity index of the midlatitude westerlies also weakened slightly，with a linear trend coefficient of-0.14（10 yr）-1.Moreover，the correlation coefficient between IWIand the intensity index of the westerlies is 0.43，which is statistically significant at the confidence level of 99%.From the variations of IWP，it is found that the NBR has been located in the region of 80°-85°E during the past 37 years and moved in a slightly westerly direction，with the linear trend coefficient of only-0.09（10 yr）-1.The correlation coefficient between IWPand the intensity index of the midlatitude westerlies is 0.65，which is statistically significant at the conficence level of 99.9%. These results indicate that the changes of NBR are closely correlated with the midlatitude westerlies in the upstream of the TP.

Standardized anomalies are calculated for the two indexes，and the critical values of±1 are determined for IWI，while the critical values of±0.8 are determined for IWP（since the number of samples of the IWPexceeding±1 is small）.Next，7 yr of strong WNBR and 9 yr of eastward-shifted WNBR are identified；5 yr of weak WNBR and 5 yr of westward-shifted WNBR are also identified.The results are listed in Table 1. The conditions in these years are used for analysis of circulation characteristics in the following paragraphs.

Fig.2.Time series of（a）normalized IWI，（b）IWP，superposed with the westerly intensity index（dashed line with open circles）and its linear trend（dashed line）.

Table 1.Typical years of strong/weak and eastward/ westward shift of the WNBR

By means of wavelet analysis，the periodic variation characteristics of the WNBR are investigated. As shown in Fig.3，on long timescales from 11 to 18 yr，both the intensity and the position of the WNBR exhibit a quasi-16-yr cycling period.Meanwhile，the intensity and position also exhibit a short cycling period of 3-10 yr.The corresponding wavelet variance contribution shows that IWIhas a cycling period of 4-6 yr and a quasi-cycling period of 2 yr.To be specific，during the period from 1979 to 1993，IWIexhibits an obvious quasi-cycling period of 5 yr；from 1994 to 2005，the quasi-cycling period of 4 yr becomes dominant.Additionally，IWIhas quasi-cycling periods of 2 and 6 yr since 2006.For IWP，there are two primary periods：quasi-5 and quasi-8 yr，where the latter period is fairly remarkable.

4.Relationship between the WNBR and climate in China

4.1 Relationship between IWIand climate in China

To investigate therelationship between the WNBR and climate in China，the correlation coefficients between IWIand temperature and precipitation data are calculated.Temperature and precipitation data were collected at 160 weather stations in China.As shown in Fig.4a，in the regions of Northwest China，east of Southwest China，North China，central China，and South China，IWIexhibits a significant negative correlation with precipitation in winter，and the correlation coefficients in most regions are statistically significant at the confidence level of 95%.The results indicate that a larger value of IWIis indicative of a stronger WNBR and thus lower precipitation in the regions；on the contrary，a smaller IWIindicates a weaker WNBR and more precipitation in the regions.The regions with positive correlations are primarily located in the east of Northwest China，Southwest China，north of South China，and Northwest China.However，there are only a few regions in which the correlation coefficients are statistically significant at the confidence level of 95%.Correspondingly，these regions are abundant in precipitation.

IWIdemonstrates a positive correlation with the air temperature in most parts of China（see Fig.4b），and the regions of significant correlations are mainly located in Northwest China，North China，central China，South China，and the east of Southwest China，where the correlation coefficients are statistically significant at the 0.01 level.The results indicate that，for a greater value of IWI，the WNBR is relatively stronger，and thus，the air temperatures are higher in most of China，while the air temperatures in a fewregions in the south of Southwest China with negative correlations are comparatively lower.Conversely，for a smaller value of IWI，the WNBR and the air temperatures present completely opposite variation tendencies.

Fig.3.The wavelet coefficient for（a）IWIand（b）IWP.

Fig.4.Simultaneous correlation between IWIand（a）winter precipitation and（b）temperature in China.Correlations significant at the 0.01 and 0.05 significance levels are shaded with light and dark grey.

4.2 Relationship between IWPand climate in China

Figure 5a illustrates distribution of the correlation between IWPand winter precipitation collected at 160 weather stations in China.In the northern parts of Northwest China，Northeast China，North China，East China，and most parts of Southwest China and South China，the correlation coefficients are positive，suggesting that when the value of IWPis larger than normal，the WNBR shifts eastward，and the precipitation in these regions are less than normal.Meanwhile，in Northwest China，eastern Southwest China，and some parts of central China and South China，the correlation coefficients are negative and the corresponding precipitation is higher than normal.Figure 5b shows the correlation distribution of IWPand air temperature.It is clear that the correlations are positive in most of China.In particular，the correlations are statistically significant at the confidence level of 99%in the north and east of Northwest China，east of Southwest China，north of North China，and most parts of central China and South China，where IWPis relatively large.When the WNBR shifts eastward，a significant increase in temperature can be found in these regions，while in other regions in the south of Southwest China，the correlations are negative and the air temperatures decrease.

Fig.5.As in Fig.4，but for IWP.

5.Possible mechanisms behind the high correlation between WNBR and climate in China

As described above，both the intensity and position of the WNBR are significantly correlated with regional anomalies in precipitation and air temperature in China.In this section，by analyzing the differences in 850-hPa wind field，500-hPa vertical velocity，and 200-hPa wind field between the years with strong and weak WNBR，and between the years of eastward-shifted WNBR and those of more western WNBR，possible mechanisms behind the high correlations between the WNBR and climate of China are further explored.Anomalies in circulation patterns corresponding to these different years are also analyzed.

5.1 Characteristic circulations in typical years of strong/weak WNBR

Figures 6a and 6b display composite wind fields at 850 hPa in relatively strong and weak WNBR years，respectively.In the years of strong WNBR，the polar vortex is primarily located in eastern Europe，and the southward movements of cold air mass are blocked by the abnormally strong anticyclonic circulation in northern Asia，giving rise to unusually high air temperatures in most parts of China.Extremely strong southwesterly winds in the Caspian Sea and Arabian Sea contribute to the transport of water vapor from tropical oceans to northwestern China.Meanwhile，the southerly winds to the west of the North Pacific subtropical high pull water vapor to northeastern China，bringing about significant rainfall to the region. The abnormal northerly winds in low-latitude regions inhibit the formation of precipitation.By contrast，the circulation in the years of weak WNBR is different. The anomalous anticyclonic circulation is dominant in the mid-and high-latitude regions of Eurasia.The northerly winds are strong in northern Asia，and the cold air mass from the polar region moves southward along the east side of the high pressure system，giving rise to relatively low temperatures in China（Shen et al.，2015）.The abnormal westerly winds in midand low-latitude regions of Asia transport water vapor from the Bay of Bengal and the South China Sea to most regions of China.

Fig.6.Composite anomalous（a，b）850-hPa wind（m s-1），（c，d）500-hPa vertical velocity（hPa s-1），（e，f）200-hPa wind（m s-1）in winter for（a，c，e）strong and（b，d，f）weak WNBR cases.

Figure 6c shows the 500-hPa vertical velocity field in the years of strong WNBR.It shows clearly that the vertical velocity in Northwest China and Northeast China is characterized by a negative anomaly，suggesting that ascending atmospheric motions are intense in these regions，which possibly contributes to increase in precipitation.In other regions，the vertical velocity field presents an obvious positive anomaly，suggesting intense descending atmospheric motions.The descending motions are not favorable for the development and maintenance of convective activities，and lead to high air temperatures and less precipitation. Figure 6d shows the 500-hPa vertical velocity field in the years of weak WNBR.In these weak WNBR years，the vertical velocity in a majority of places of China demonstrates a negative anomaly，i.e.，the ascending atmospheric motions are intense and conducive to the development of convective activities and thus formation of precipitation.Therefore，the air temperatures are relatively low.In Northwest China，Northeast China，and the southeast of Southwest China，the vertical velocity field is characterized by a positive anomaly，i.e.，the descending atmospheric motions are intense and may possibly prohibit the formation of precipitation.

Figures 6e and 6f illustrate the 200-hPa composite wind fields in the years of different intensities of WNBR.In the years of strong WNBR，the extraordinarily strong polar vortex is located in western Europe，and the anticyclonic circulation that covers a large area from the north of Asia to the vicinity of the TP is fairly intense.The South Asian high is located above the TP，preventing precipitation from developing in the area between the Yangtze River and the Huaihe River.The abnormal cyclonic circulation is also quite strong in the Arabian region and eastern China.In the years of weak WNBR，the abnormal anticyclonic circulation is dominant in western Europe，and the intense cyclonic circulation prevails over the TP.The strong South Asian high above East China is conductive to the formation of precipitation in the area between the Yangtze River and the Huaihe River（Zhang and Wu，2001，Hu et al.，2010）.

5.2 Circulation features in the years of eastward/westward shifted WNBR

As shown in Fig.7a，when the WNBR shifts abnormally towards the east，the 850-hPa wind field in the mid-and high-latitude Eurasia presents an inverted-pattern.In western Europe，the abnormal anticyclonic circulation plays a dominant role，and the polar vortex is located around the Ural Mountain.The abnormal southerly winds in northern Asia block the southward movement of cold air，and the abnormal westerly winds transport water vapor to Northwest China. The Pacific subtropical high moves northward，and the easterly winds on the southwest side of the subtropical high deliver water vapor to Northeast，North，and East China，giving rise to significant precipitation in these regions.Figure 7b shows that，when the WNBR shifts abnormally westward，the Ural Mountain-centered high-pressure circulation is quite remarkable and the abnormal northerly winds are dominant in the east of North Asia.The cold air mass directly affects China from the north，and thus the air temperatures are overalllower than normal.Under the influence of the abnormal easterly winds in the East China Sea，water vapor is transported to central China and Northwest China，and interacts with the southbound cold air mass，resulting in large amounts of precipitation.

Figure 7c shows the 500-hPa vertical velocity fieldin the years of eastward shifted WNBR.In northern area of Northwest，North，East China，and southern area of South China，negative anomalies occur in the vertical velocity field，indicating intense ascending atmospheric motions.In other regions of China，the vertical velocities are characterized by positive anomalies，i.e.，the descending atmospheric motions are dominant，which blocks the formation of precipitation and leads to higher than normal temperatures.In the years of more western WNBR（Fig.7d），the vertical velocity field in Northwest China，the eastern area of Southwest China，and the mid-lower reaches of the Yangtze River presents a negative anomaly，i.e.，the ascending motions are significant and favorable for precipitation formation.In other regions，vertical velocity is characterized by a positive anomaly，i.e.，descending atmospheric motions are remarkable，which possibly inhibits the formation of precipitation.

Fig.7.As in Fig.6，but for（a，c，e）eastward and（b，d，f）westward shifted WNBR cases.

Figures 7e and 7f present the 200-hPa composite wind fields in the years when the WNBR shifts eastward and westward，respectively.Opposite circulation patterns can be found in Figs.7e and 7f.In the years of more eastern WNBR（Fig.7e），abnormal anticyclonic circulation is significant in mid-and high-latitude western Europe，and the South Asian high shifts northward.The cyclonic circulation prevails over east of the Ural Mountain while an abnormal cyclone prevails over South Asia.Comparatively，in the years of more western WNBR（Fig.7f），anticyclonic circulation is extremely strong in the midand high-latitude regions surrounding the Ural Mountain.Cyclonic circulation is dominant from the northof Asia to the vicinity of the TP，and the South Asian high is located above eastern China，which possibly contributes to the formation of precipitation in the area between the Yangtze River and the Huaihe River（Zhang and Wu，2001；Hu et al.，2010）.

5.3 Anomalous circulation related to variation in the WNBR intensity and the precursor circulation features

Figure 8 shows the height anomaly field of strong and weak WNBR years.In the years of strong WNBR，the Arctic Oscillation（AO）in its positive phase is strong and the anticyclonic circulation around Iceland is extraordinarily intense.Anomalously low pressure prevails in western Atlantic Ocean and the polar vortex extends broadly over the Eurasian continent.The Siberian high is relatively weak，and the zonal anticyclonic circulation extends from the northern TP to the east coast of Asia.Additionally，the anomalous cyclonic circulation is dominant around the Aleutian Islands，with relatively weak East Asian deep trough and intense North Pacific jet streams（Wu and Wang，2002；Yang and Li，2008；Li and Zhang，2015）.In the years of weak WNBR，the polar vortex is strong and mainly dominates the Western Hemisphere.Both the North Atlantic Oscillation（NAO）and the North Pacific Oscillation（NPO）are significant，and the blocking high in the high-latitude region of Eurasia continent is strong（Luo et al.，2007；Cai and Diao，2011）. The anomalous cyclonic circulation prevails over the areas around the northern TP，while the East Asian trough is relatively weak.

In order to determine the anomalous precursor atmospheric circulation in different years when the NBR exhibits different intensities，the atmospheric circulation anomaly is analyzed from October to December of the previous year（Fig.9）.In October，the polar high is strong，and the Rossby wave in mid and high latitudes propagates across the eastern Pacific Ocean，through the North American and western European continents，and finally reaches the Ural Mountain（Blackmon et al.，1984）.The NAO is relatively weak，and the cyclonic circulation covers an area from the east coast of East Asia to the North Pacific Ocean.The cyclones above the West Pacific are migrating from Asia（Chang，2005），and the East Asian trough strengthens（Fig.9a）.In November，the polar high in the Northern Hemisphere becomes stronger，and extends southward to the Eurasian continent.Meanwhile，the NAO also strengthens，and jet streams above the East Asian trough and the North Pacific Ocean reach their strongest phase（Fig.9c）. In December，the polar high weakens and its intensity decreases as it moves towards the polar region.Thecyclonic circulation pattern is uniformly distributed in the region from Eurasia to the North Pacific Ocean，and anomalous high pressure system dominates the areas along the coast of East Asia，while the East Asian trough weakens（Fig.9e）.

Fig.8.Composite anomalous 500-hPa geopotential height（gpm）in January for the（a）strong and（b）weak WNBR cases.Shadings are the same as in Fig.4.

Fig.9.Composite anomalous 500-hPa geopotential height（gpm）in（a，b）October，（c，d）November，and（e，f）December of the previous year for（a，c，e）strong and（b，d，f）weak WNBR cases.Shadings are the same as in Fig.4.

Figure 9b shows the circulation anomaly in October of the previous year when the NBR is weak. It can be found that the AO is usually in its positive phase and the polar vortex is strong，while abnormal high pressure is dominant in the mid-and highlatitude regions.Specifically，the anticyclonic circulation is strongest in West Europe，the NPO is significant，and the Siberian high and the Aleutian low are relatively weak（Yang and Li，2008）.The polar vortex located to the south of the Ural Mountain reaches its maximum intensity in November，and intense Rossby wave in mid-and high-latitude regions propagates from the North Atlantic Ocean through Europe and finally reaches Asia（Chen et al.，2013）. Meanwhile，the anomalous cyclonic circulation covers a zonal belt extending from the coastal region of East Asia across the North Pacific Ocean and reaching the North American continent.The above results indicate that the East Asian trough and the jet streams above North Pacific Ocean have reached their strongest intensity（Fig.9d）. In December，the polar vortex weakens gradually and extends towards North America.The high pressure surrounding Lake Baikal and the East Asian deep trough tend to be stable（Fig.9f）. Apparently，the precursor circulation for a strong or weak WNRB year is most distinct in November.

5.4 Anomalous circulationsrelatedtoeastward/westward shiftedWNBRandthe precursor circulation features

Figure 10 shows the height anomaly in the years when the NBR shifts towards the west and east，respectively.In the years when the NBR shifts eastward，as shown in Fig.10a，the polar vortex is fairly intense，and extends southward to reach Canada and West Siberia.This suggests that when the Siberian high weaken，the AO is in its significantly positive phase in winter（Wu and Wang，2002）.The stationary Rossby wave propagates from the Ural Mountain to East Asia and North Pacific Ocean（Ding and Wang，2005；Shi et al.，2009），and both the East Asian deep trough and the North Pacific Ocean jet streams weaken.As shown in Fig.10b，in the years when the NBR shifts eastward，the polar high becomes stronger and extends to the west coast of the Atlantic Ocean and the Eurasian continent.A zonal belt of decrease in geopotential height extends from the TP to the coast of East Asia and reaches the West Pacific Ocean（Chang，2005）. The East Asian deep trough is relatively strong whilethe North Pacific jet streams weaken，corresponding to the positive anomaly of geopotential height in the Aleutian Islands（Chen et al.，2013）.

Fig.10.As in Fig.8，but for the（a）eastward and（b）westward shift of the WNBR cases in January.

Figure 11a shows the anomalous circulation field in the previous October of the years when the ridge shifts eastward.AO is in its positive phase，i.e.，an abnormally low vortex is located in the polar region. In mid-and high-latitude regions，anticyclonic circulation is dominant，and both the East Asian deep trough and the West Pacific Ocean jet stream are weak.In November，as shown in Fig.11c，the polar vortex becomes stronger and extends to Europe；the NAO is significant and imposes certain effects over the downstream of the West Pacific Ocean（Palmer，1988）.The Rossby wave in the mid-and high-latitude regions moves from the North Atlantic Ocean，crosses the Ural Mountain，and propagates to Siberia.When the blocking high around the Ural Mountain is strong，the East Asian trough and the North Pacific Ocean jet streams also reach their maximum intensity（Wang et al.，2010）.In December（Fig.11e），the AO is in its positive phases and the polar vortex weakens.So do the East Asian deep trough and the North Pacific Ocean jet streams（Fig.11e）.

In October of the previous years when the ridge shifts westward，the circulation is characterized by a dipole pattern，i.e.，two polar vortexes are located around Iceland and the Aleutian Islands，respectively.The NAO is relatively weak while the NPO is strong，suggesting that a seesaw-type oscillation between the Aleutian low and the Icelandic low is significant（Kutzbach，1970；Wallace and Gutzler，1981）. Meanwhile，the East Asian deep trough is weak（Fig. 11b）.In November，the dipole-pattern becomes more significant，and the polar vortexes are located over the North American and the Eurasian continents，respectively. When AO is in its positive phase，the Siberian high weakens（Wu and Wang，2002），while Rossby wave moves from the North Atlantic Ocean to East Asia and the North Pacific Ocean（Ding and Wang，2005；Shi et al.，2009）.This also corresponds to the time when the East Asian deep trough and the North Pacific Ocean jet stream are weakest（Fig. 11d）.In December，the polar vortex is located to the east of the North Pacific Ocean，and the Rossby wave moves from the Pacific Ocean，crosses the west coast of North America and the Atlantic Ocean，and finally reaches Europe（Blackmon，1984）.The East Asian deep trough intensifies significantly（Fig.11f）.

6.Summary

In this paper，characteristics of the interannual variation in the intensity and zonal positions of the WNBR and their relationship with China's climate are analyzed.Anomalies in atmospheric circulation during the same period are investigated to explore the physical mechanisms behind the internannual variation of the WNBR.Conclusions are given as follows.

（1）In winter，a high-pressure ridge exists within the region from the north of the TP to the north of Balkhash Lake（32.5°-47.5°N，70°-95°E）.The ridge is referred to as the northern branch ridge（NBR）.From the perspective of climatology，the intensity index IWIand the zonal position index IWPof the WNBR are defined. The variational tendencies of the WNBR during 1979-2015 are investigated.It is found that the WNBR has weakened gradually，and exhibited cycling periods of 4-6，quasi-2，and 16 yr.The NBR has shifted westward with quasi-cycling periods of 8 and 16 yr.Moreover，the ridge has been highly correlated with the upstream westerlies in the midlatitude regions.

（2）The intensity of the WNBR greatly affects China's precipitation and air temperature in winter.If the WNBR is relatively strong，precipitation in Northeast China and the north of Northwest China will increase，whereas precipitation in central China，North China，South China，and the eastern regions of Northwest and Southwest China will decrease. Average surface air temperature in most areas of China rises significantly during strong WNBR years.However，the variation tendencies are entirely different in weak WNBR years.In strong WNBR years，atmospheric circulation in Northwest and Northeast China is characterized by low-level convergence and high-level divergence，accompanied with significant ascending atmospheric motions.Such a circulation pattern favorsthe development of precipitation. In other regions of China，divergence occurs in the lower atmosphere while convergence occurs in the higher atmosphere，and descending atmospheric motions are distinct. Such a pattern inhibits the formation of precipitation and leads to high surface temperature.In strong WNBR years，the AO is strong and the planetary stationary wave moves eastward from North Atlantic. These precursor circulation features are most remarkable in the previous November of strong WNBR years.Meanwhile，the polar high，the East Asian deep trough，and North Pacific jet streams are moderately strong.In the years of weak WNBR，the variation tendencies are essentially opposite.

Fig.11.As in Fig.9，but for the（a，c，e）eastward and（b，d，f）westward shifted WNBR years during October-December of the previous year.

（3）The zonal position of the WNBR greatly affects precipitation and surface air temperature in China during wintertime. In the years when the WNBR shifts eastward，precipitation increases in the northern region of Northwest，Northeast，North China，and some regions of East China，whereas precipitation decreases in central China，the eastern regions of Northwest and Southwest China，and some regions in South China.Surface air temperature in most of China increases significantly.In the years when the WNBR shifts westward，precipitation and surface air temperature change with opposite signs.When the WNBR moves to the east，the atmospheric circulation is characterized with convergence at lower levels and divergence at upper levels in Northeast，Northwest，and East China.Correspondingly，ascending atmospheric motions are remarkable，which is favorable for the formation of precipitation.In other regions，the descending atmospheric motions are dominant，and precipitation decreases.The southward movements of cold air masses are blocked by the southerly winds in high-latitude regions of Asia.As a result，surface air temperatures in most areas of China are relatively higher.Furthermore，the AO is anomalously strong；the stationary wave moves eastward from the European continent；the NAO，the East Asian deep trough，and the North Pacific jet streams are all relatively strong.The opposite is true in the years when the WNBR shifts westward.

The NBR is located in northeastern area of the Northern Hemisphere. It is a system that largely exists on the northern side of the TP（Rex，1950；Wu，1957）.Questions remain unanswered regarding whether there exist any relations between the NBR and the southern branch trough in low-latitude regions and what relationship it is between the NBR and the thermal-dynamic forcing of TP as well as the TP monsoon and the Asian monsoon.These topics will be the focus for future studies.

Acknowledgments.The authors would like to thank the reviewers for their comments and suggestions.We also appreciate the editors for their efforts to help improve our paper.

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（Received January 25，2015；in final form August 1，2015）