J. Meteor. Res.  2015, Vol. 28 Issue (2): 228-236   PDF    
http://dx.doi.org/10.1007/s13351-015-4104-0
The Chinese Meteorological Society
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Article Information

SHEN Lelin, CHEN Longxun, JIN Qihua, ZHU Yanfeng. 2015.
A New Circulation Index to Describe Variations in Winter Temperature in Southwest China
J. Meteor. Res., 28(2): 228-236
http://dx.doi.org/10.1007/s13351-015-4104-0

Article History

Received April 30, 2014;
in final form September 28, 2014
A New Circulation Index to Describe Variations in Winter Temperature in Southwest China
SHEN Lelin1 , CHEN Longxun2, JIN Qihua2, ZHU Yanfeng3    
1 National Climate Center, Laboratory for Climate Studies, Beijing 100081;
2 Chinese Academy of Meteorological Sciences, Beijing 100081;
3 National Meteorological Information Center, Beijing 100081
ABSTRACT:A new circulation index (ISW) that can realistically describe winter temperature variations over Southwest China is defined based on analysis of the NCEP/NCAR reanalysis data (version 1) and the observations at 585 stations in China. The study period is from January 1961 to February 2011. The relationship between ISW and general circulation patterns in East Asia is also analyzed. Results show that ISW successfully captures the variations in winter temperature over Southwest China. High ISW values correspond to the intensified Mongolian high, the weakened Aleutian low, increases in the strength of the Middle East westerly jet stream over the south of the Tibetan Plateau (TP), and decreases in the strength of the subtropical westerly jet over the north of the TP. Meanwhile, the East Asian trough deepens and extends southwestward, making it easier for the cold air mass from the north to intrude Southwest China along the trough. These circulation patterns lead to a decrease in winter temperature over Southwest China (and vice versa). In addition to the East Asian winter monsoon, the two westerly jets that dominate the upper level circulation over East Asia also exert important influences on winter temperature in Southwest China, especially the Middle East westerly jet to the south of the TP.
Keywordswinter temperature     Southwest China     westerly jet     Tibetan Plateau (TP)    
1. Introduction

The worst blizzard in the past 50 years struckSouth China in the winter of 2008,devastating largeareas of this region. Great concerns have been raisedabout the mechanism for the anomalous winter climate and interannual climate variation,which poses achallenging research topic for government-sponsoredresearchers and meteorologists. The weather and climate in monsoon regions are directly affected byanomalous circulation patterns of East Asian wintermonsoon,which is the dominant weather system inthe Northern Hemisphere during the winter. Variousmonsoon indices have been defined to investigate characteristicsof the East Asian winter monsoon(EAWM) and their impacts on regional weather and climate.Most of these indices were defined based on certaincharacteristics of the EAWM circulation,such as sealevel pressure(Guo,1994; Shi,1996; Gong et al., 2001),the East Asian trough(Sun and Li, 1997),subtropicalsurface meridional wind( Ji and Sun,1997;Chen et al., 2000),the East Asian jet stream at 300hPa(Jhun and Lee, 2004), and zonal wind speed(u)(Wang and Jiang,2004). Gao(2007)summarized fourEAWM indices that can reflect the basic characteristicsof the EAWM circulation, and showed that allthe four indices can correctly describe anomalies ofthe EAWM circulation. Specifically,in strong(weak)EAWM years,both the Siberian high and the upperlevelsub-tropical westerly jet are stronger(weaker)than normal, and the Aleutian low and the East Asiantrough are deeper(shallower)than normal. Such ano-malous conditions are favorable(unfavorable)forstronger northwesterly winds and lower temperaturesover the subtropical region of East Asia.

Wang and Chen(2010)analyzed the definitionsof 18 existing EAWM strength indices and selectedsix among them for further analysis. Their resultsshowed that all the indices could describe the changesin the EAWM system components,but almost eachindividual index demonstrated a weak performance indescribing the surface air temperature over large areasto the east of the Sichuan Basin,which might berelated to topographic effects. Chen and Sun(2001) and Zhu(2008)also revealed the poor correlation betweenthe EAWM index and surface air temperaturein Southwest China; they attributed this poor correlationto complex terrain and large local-scale variationsin temperature in Southwest China. Jiang and Li(2010)also indicated that the current EAWM indicescannot well reflect the influence of cold air on the temperaturechanges in Southwest China. This might bebecause climate in Southwest China is highly variableas Southwest China covers a broad area of the TibetanPlateau(TP)as well as the vast area to the east of theTP. Since Southwest China is a region with the mostabundant water resources in China,the anomaous climatein this area affects water resources not only inlocal areas but also downstream areas(Zhou et al., 2009). Consequently,research on temperature changesin Southwest China is very important.

Several previous studies have investigated themechanisms of temperature changes in winter inSouthwest China, and found that there was a significantcorrelation between the Middle East jet stream and surface air temperatures in Southwest China(Wen et al., 2009; Ni et al., 2010a,b). Jiang and Li(2011)studied the temperature variations by dividingSouthwest China into eastern and western sub-regions.They found that the zonal migration of the westernPacific subtropical high,accompanied by anomalousvertical motion and regional winds,was closely relatedto the variability in surface air temperatures inSouthwest China. Anomalous snow cover on the TPmight also affect China’s winter climate(Chen et al., 1996; Dong and Yu, 1997). Although great progress has been achieved in studies of the EAWM index and its relationship with temperature over East China,anumber of questions about winter temperature change and variation in Southwest China are not addressedyet. Therefore,in this study,we define a circulationindex,ISW,to reflect winter temperature in SouthwestChina, and analyze its relationship with the backgroundcirculation in East Asia.

2. Data and methods

The data used in this study are long-term,highqualityobservations of monthly mean surface air temperatureat 585 meteorological stations,which are chosenfrom 740 meteorological stations in China. Thedata are provided by the National Meteorological InformationCenter of the China Meteorological Administration.The data cover the period 1961–2011.Datasets of the NCEP/NCAR reanalysis(version 1.0)for the period from January 1961 to February 2012are also used. The data have a horizontal resolutionof 2.5°×2.5°,including wind field,geopotential heightat 500 hPa, and sea level pressure. In this paper,winterrefers to the period from December of one specificyear to February of the next year. For example,thewinter of 1961 refers to the period from December 1961to February 1962. In this study,we focus on the period1961–2011(winter); and the mean climate stateis also for the period 1961–2011.

Several methods,including the empirical orthogonalfunction(EOF) and correlation analyses,are appliedin this study. A 9-yr Gaussian type filter is usedto retain the signals of 10 yr and longer timescales ininterdecadal variability analyses. The student t-test isused to assess the statistical significance of the resultsobtained. As shown in Fig. 1,the non-shaded areas inthe study domain(23°–39°N,90°–105°E)is locatedin Southwest China.

Fig. 1. Concurrent correlation coefficients of four indices and China’s winter temperature during 1961–2011:(a)IJi,(b)IGong,(c)IJhun, and (d)IZhu. Correlation coefficients at or below the 0.05 significance level are shaded.
3. The circulation index(ISW)representing thewinter temperature in Southwest China 3.1 The definition of ISW

Based on the existing EAWM indices,we are able to describe anomalous patterns of the EAWM circulations.We chose four indices and analyzed their relationshipwith winter temperatures in China,respectively(Fig. 1). The four indices include: the index oflow-level meridional wind in the east coast of Chinesemainl and (IJi),the index of l and -sea pressure(IGong),the index of the variation of the upper-level East Asianjet stream(IJhun), and the index of the variation ofwinter temperatures in most of East China(IZhu).

Figure 1 shows the simultaneous correlation coefficientsbetween these four indices and winter temperaturesin China at 585 stations. A common featureis that there are no significant relationships betweeneach index and temperature changes in SouthwestChina. This area has a complex terrain,with amaximum height above 4500 m and a minimum heightbelow 500 m,which is very different from the terrainin East China.

Ding(1990)pointed out the three major pathsof cold waves that influence China. According to theoperational forecast experiences,the circulation systemsinfluencing winter monsoon outbreaks often existin the middle troposphere. Accordingly,the simultaneouscorrelations between winter temperaturesover Southwest China and winter mean zonal windfields(10°–80°N,40°–160°E)at 500 hPa during 1961–2011 are calculated in this study(figure omitted). Theresults show a significant “negative-positive-negative”(“– + –”)correlation pattern corresponding to threesignificant correlation regions in high latitudes(65°–80°N),middle latitudes(39°–51°N), and low latitudes(15°–29°N),respectively. The absolute values of correlationsin middle and low latitudes are larger than 0.6,implying that the zonal wind speed in these regionsis closely related to winter temperatures in SouthwestChina.

According to the above analyses,we performedEOF analyses on anomalies of 500-hPa winter zonalwinds in the East Asian region(10°–80°N,40°–160°E) and found that the first two modes account for 22.8% and 19.2% of the total variation,respectively. FromFig. 2,we can see that the first mode also shows a clear “+ – +” pattern in high,middle, and low latitudes.Out-of-phase variation in zonal winds existsbetween south of 35°N and north of 35°N in East Asia.The correlation coefficient between the time series ofthe first mode and winter temperatures in SouthwestChina is –0.54,which passes the 0.001 significance test and indicates a strong relationship in-between. Therefore,we chose the first mode from the EOF analyses and discussed its relationship with winter temperaturesin Southwest China.

Fig. 2. The spatial pattern of the first EOF mode for the winter mean 500-hPa zonal wind field(10°–80°N,40°–160°E)during 1961–2011.

According to the first mode’s characteristics,wedefined a circulation index to reflect winter temperaturesover Southwest China. It is expressed as:

where U500(18°−29°N,80°−120°E)represents 500-hPazonal wind averaged over the region(18°–29°N,80°–120°E), and U500(39°−50°N,80°−120°E)represents thatover the region(39°–50°N,80°–120°E; the boxed regionsin Fig. 2). Figure 3 shows the temporal variationof the normalized index ISW during the period1961–2011 with a solid-square line. There is a simultaneousnegative correlation between ISW and wintertemperatures in Southwest China with the value of–0.78,which passes the 0.001 significance test. PositiveISW values correspond to a stronger zonal windshear in the middle troposphere between low and middlelatitudes in East Asia,a stronger zonal flow tothe south of 29°N, and lower winter temperatures inSouthwest China. Negative ISW values are accompaniedby higher winter temperatures in SouthwestChina. To compare the temporal variation in wintertemperature in Southwest China,Fig. 3 shows twocurves, and ISW is multiplied by –1.0. We can seefrom Fig. 3 that ISW exhibits an increasing trend afterthe late 1960s,reaches a peak value in the 1980s, and continuously decreases after the early 1990s. Theindex decrease occurs twice,one in the early 1980s and the other in the early 1990s, and the downward trendis more significant in the latter period. The nine-pointrunning mean of winter temperatures in Southwest China demonstrates two obvious warming periods afterthe 1980s, and the interannual variability shownby these two lines is well superposed.

Fig. 3. A normalized time series of winter temperatures in Southwest China(the hollow-circle and dashed lines showthe long-term tendency of the nine-point running mean) and negative ISW(the solid-square and solid lines show thelong-term tendency of the nine-point running mean).
3.2 Correlations between ISW and winter temperaturesin China

Figure 4 displays simultaneous correlation coefficientsof ISW values and winter temperatures at 585stations over China. Apparently significant correlationsoccur over most areas of Southwest China(includingthe eastern part of the TP,Yunnan Province, and some parts of the continent extending eastwardto 110°E) and north of HeilongIJiang Province. Comparedwith Fig. 1,ISW in this section can be used to reflect the winter temperatures in Southwest China and in several small areas of HeilongIJiang Province(these areas are not covered by the EAWM indices).The simultaneous correlation coeffcients of ISW and the four EAWM indices(IIJi,IGong,IJhun, and IZhu)mentioned above are –0.28,0.15,0.35, and 0.22,respectively.Only the correlation between ISW and IJhun is significant. As a result,it is concluded that,in addition to the East Asian winter monsoon,thereare other factors that have important effects on wintertemperatures in Southwest China; these factors arefurther discussed in the following sections.

Fig. 4. Concurrent correlation coefficients of ISW and winter temperatures at 585 stations in China(the shadingareas are significant at the 99.9% level).
4. Correlation between ISW and East Asianatmospheric circulation

Previous studies have revealed several circulationsystems that have significant effects on winter climatein East Asia,including the Siberian high(Gong and Wang, 2002),the Aleutian low(Yang et al., 2005),the 500-hPa trough(Chan and Li, 2004), and the subtropicalwesterly jet(Hanawa et al., 1988). In thispaper,the simultaneous correlations between ISW and sea level pressure,the geopotential height at 500 hPa,zonal winds at 200 hPa, and wind fields at 850 hPaare analyzed,respectively(Fig. 5).

Fig. 5. Concurrent correlation coefficients of ISW and (a)sea level pressure,(b)500-hPa geopotential height,(c)200-hPa zonal wind, and (d)850-hPa wind,respectively(correlations at the 0.05 significance level are shaded).

Figure 5a shows simultaneous correlation coefficientsbetween ISW and winter mean sea level pres sure. A positive high correlation center occurs in EastChina,where the Mongolian high-pressure system islocated. Another positive high correlation center is locatedat the equatorial zone in theWest Pacific. Thereare two negative high correlation centers: one is locatedin northwestern Europe and the other is overthe Northwest Pacific(the active position of the Aleutianlow). In high(low)ISW years,strong(weak)coldMongolian high pressures and weak(strong)Aleutianlow pressures lead to low(high)winter temperaturesin Southwest China. Similar results are found throughthe analysis of composite anomalies for strong and weak ISW values(figure omitted). It is inferred thatthe Mongolian high and Aleutian low are highly correlatedwith winter temperatures in Southwest China.

Figure 5b shows simultaneous correlation coefficientsbetween ISW and winter mean geopotentialheights at 500 hPa. The maximum positive correlationappears in the north of Mongolia with the center overBaikal,while the maximum negative correlation appearsin the south of the TP with a zonally elongatedshape that extends to Northwest Pacific. The otherarea of negative correlation is located in northwesternEurope. This correlation pattern implies that theanomalous winter temperatures in Southwest Chinacan result in a meridional circulation that prevails at500 hPa. The composite maps(figures omitted)of theanomalies of the 500-hPa geopotential height fields inhigh and low ISW years indicate that all of the anomalouspatterns in high ISW years are clearly differentfrom those in low ISW years. High ISW years are characterizedby a stronger anomalous trough in northwesternEurope,a stronger anomalous ridge in Baikal, and a deeper East Asian trough that extends southwestwardto reach northern Bay of Bengal. SouthwestChina is controlled by northwesterly flow behind thisdeepened trough,along which cold air mass can betransported from high latitude,leaving this area to beanomalously cold, and vice versa.

In the winter mean zonal wind field at 200 hPa,there are two westerly jets in East Asia: one in thevicinity of 20°N(relatively strong and stable) and theother near 50°N(Zhang and Wang, 1987). Figure 5c shows that ISW is positively correlated with the jet stream near 20°N in the south of the TP and negativelycorrelated with the jet stream near 50°N in thenorth of the TP. Some studies have shown that theposition and intensity of the Middle East jet stream,which is the source of the jet in the vicinity of 20°N,were closely related to winter temperatures in SouthwestChina(Wen et al., 2009; Ni et al., 2010a; Qu et al., 2012). The correlation coefficient between theindex of the Middle East jet stream defined by Yanget al.(2004) and ISW is 0.36,at the 0.05 significancelevel. These results imply that the 200-hPa jet streamsin both the north and the south of the TP have effectson winter temperatures over Southwest China.

Figure 5d shows simultaneous correlation coefficientsbetween ISW and the wind field at 850 hPa.Significant correlations occur in the region with ananomalous anticyclone centering on Baikal,in the regionof Siberia with anomalous westerly winds, and inthe West Pacific region with anomalous northeasterlywinds. Analyses of the composite anomalies of the850-hPa wind field for high and low ISW years(figureomitted)reveal that high ISW often corresponds toanomalous anticyclonic circulation near Baikal. Meanwhile,anomalous northerly winds emerge in the vicinityof Icel and , and anomalous northeasterly winds arefound in the area from eastern Baikal to SouthwestChina, and throughout Northeast and North China.Such circulation patterns cause cold winters in SouthwestChina, and vice versa. Significant correlationsalso exist in the areas between south of the TP and north of India. A previous study has revealed that thetrough of the southern TP was somewhat related totemperatures in Southwest China(Wen et al., 2009).

Based on the above analyses,high ISW values arecharacterized by a strong cold Mongolian high,a weakAleutian low,a strong Middle East jet stream in thesouth of the TP,a weak subtropical jet stream in thenorth of the TP, and a deepened East Asian trough.The atmospheric circulations corresponding to weakISW years are opposite to those in strong ISW years.

5. Summary

In this study,we defined a circulation index that successfully describes winter temperatures over SouthwestChina.

(1)Based on EOF analyses on the anomalies of500-hPa winter zonal winds in East Asian region(10°–80°N,40°–160°E),we defined a circulation index,ISW,which successfully reflects winter temperatures overSouthwest China. The correlation coefficient betweenthe index and surface temperature in Southwest Chinacan be up to –0.78. Interdecadal changes in ISW exhibitan increasing trend after the late 1960s,reachits peak value in the 1980s, and decrease continuouslysince the 1980s.

(2)ISW reveals that variations in winter temperaturesin Southwest China are closely related to theintensities of the Mongolian high and Aleutian low.Positive ISW values correspond to strong Mongolianhigh,weak Aleutian low, and low winter temperaturesover Southwest China, and vice versa.

(3)Two westerly jet streams at 200 hPa havecertain effects on winter temperatures in SouthwestChina. In high ISW years,the Middle East jet streamin the south of the TP is strong,whereas the subtropicaljet stream in the north of the TP is weak atthe upper level. At the lower level,the anomalousnortherly wind emerges in the vicinity of Icel and , and the anomalous northeasterly wind controls large areasfrom eastern Baikal to Southwest China and throughoutNortheast and North China. Such a circulationpattern favors the transport of cold air mass fromIcel and to Chinese mainl and . As the deepened EastAsian trough extends southward,cold air mass canreach south of the TP,leading to low temperatures inSouthwest China, and vice versa. In addition to theinfluence of the East Asian winter monsoon,the twowesterly jets over East Asia also exert important influenceson changes in winter temperatures over SouthwestChina. Effects of the westerly jet to the southof the TP are especially strong. Further study is requiredto determine how the two jet streams affectwinter temperature in detail and how they interactwith each other.

Acknowledgments. The authors would like tothank Dr. Gao Hui and Dr. Chen Junming for theirhelpful comments and suggestions.

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