Relationship analysis between September precipitation in western China and 700 hPa wind field in East Asia

ShaoYong Chen , Quan Xia , JunTin Guo , Shu Lin

1. Lanzhou Institute of Arid Meteorology, China Meteorological Administration; Key Laboratory of Arid Climatic Changing and Reducing Disaster of Gansu; Key Laboratory of Arid Climatic Changing and Reducing Disaster of CMA, Lanzhou, Gansu 730020, China

2. Baiyin Meteorological Bureau, Baiyin, Gansu 730900, China

Relationship analysis between September precipitation in western China and 700 hPa wind field in East Asia

ShaoYong Chen1,2*, Quan Xia2, JunTin Guo2, Shu Lin1

1. Lanzhou Institute of Arid Meteorology, China Meteorological Administration; Key Laboratory of Arid Climatic Changing and Reducing Disaster of Gansu; Key Laboratory of Arid Climatic Changing and Reducing Disaster of CMA, Lanzhou, Gansu 730020, China

2. Baiyin Meteorological Bureau, Baiyin, Gansu 730900, China

The regional wind index influencing September precipitation in western China has been defined using the 700 hPauandvcomponents of NCAR/NCEP reanalysis data of 1961 to 2006. There are three regional wind field indices: southwest, southeast, and north, and these indices reflect the change of East Asian monsoon. The relationship between the indices was studied, and results show that they not only have a close relationship, but also have independence. Moreover, there is an obvious relationship between the wind indices and the autumn in particular the September precipitation in western China. The effect of wind indices on rain occurrence is most different: the influenced area of the southwest wind index is larger than that of the southeast wind index, and the southwest wind index is a controlling factor on autumn precipitation in western China. The weakening of southwest wind is the main cause resulting in reduction of autumn precipitation on the east side of the Tibetan Plateau.

western China; September precipitation; 700 hPa wind field; regional wind index

1. Introduction

In western China, precipitation in autumn is more than in spring, and its abnormal phenomenon is most obvious in autumn than in the other three seasons. Decadal change trends of precipitation in the east part and the west part of western China are contrary. In autumn of El-Nino years,there is a strong Xinjiang ridge and a weak India-Burma trough, thus precipitation is reduced in the east part; in autumn of La-Nina years, precipitation in this part increases(Liet al., 2001; Zhanget al., 2003). For example, there was a decreasing trend of autumn precipitation and continuous wet weather in Qinghai Province from 1971 to 2005 (Ma,2008); a decreasing trend of autumn precipitation and rainy days in Gansu Province from 1960 to 2003 (Lin and Lu,2004); an increasing trend of autumn precipitation in middle and southern Tibetan Plateau, but a decreasing trend in western Sichuan Province from 1960 to 2000 (Huang and Zhang, 2007). The vapor source of autumn precipitation in western China changes every year. When Bengal Bay low pressure is active and strong, a low pressure cell not only directly impacts the southwestern China, but also can crosse the Tibetan Plateau and affect the east of northwestern China;when Subtropical High pressure is strong and lies in west,plenty of vapor can arrive in western China, and even typhoons from the South China Sea can bring about autumn rain in western China (Feng and Guo, 1983). All these are closely related to the evolution phase and strength of East Asian monsoon. There are numerous definitions of the East Asian monsoon index. For example, Guo (1983) calculated the strength of the East Asian summer monsoon by adding up the ≤-5 hPa sea level atmospheric pressure differential between 110°E to 160°E along every 10-degree latitude circle within 10°N-50°N; Zhao and Zhang (1994) estab-lished a winter monsoon strength index based on Guo’s method; Shi (1966) and Shiet al. (1996) defined the East Asian monsoon index as the standardized value of sum of sea level atmospheric pressure differential between 110°E to 160°E along every 5-degree latitude circle within 20°N-50°N so as to eliminate the impact induced by mean square deviation uneven of different lattice points; Tao and Chen (1988) took the average ratio of south-wind weight to north-wind weight at 850 hpa within a range of 0°-40°N and 90°E-150°E from June to August as the East Asian summer monsoon strength index. Lianget al. (1999), taking the South China Sea region of 5°N-20°N, 105°E-120°E as consideration, integrated the pentad average southwest wind weight and the OLR (outgoing longwave radiation) value into the southwest monsoon index; Fujikawa and Ishihara(1997) used four regional averaged OLR values in Southeast Asia to reflect the change of East Asian summer monsoon.Each of these monsoon index definitions has its advantages,and plays certain role in related research and professional work. But we here must take the following into consideration: (1) East Asian monsoon activities are influenced by both tropical and subtropical monsoons. These two different types of monsoon are either connected each other or being independent. It is quite obvious that a monsoon index could not describe their respective change characteristics. (2) The effect of middle and high latitudes should be considered in studying autumn precipitation of western China.

Qiaoet al.(2002) researched the relationship between the western Pacific subtropical anticyclone and flood season precipitation in China, in which they took a range of 20°N-40°N, 100°E-140°E at 850 hPa to define the area index and intensity index of north wind, southwest wind and southeast wind. This type of monsoon index reflects the change of middle-high latitude circulation, and distinguishes each type of monsoon. The altitude is higher in western China, the influenced region by 850 hPa East Asian monsoon is smaller. Therefore, we refer to Qiaoet al.’s method but at 700 hPa to analyze the relationship between September precipitation of western China and East Asian monsoon,and define the regional wind index to analyze its relationship with precipitation. This study aims at explaining the anomaly change of autumn rain in western China, and providing ideas for climate diagnosis and forecast.

2. Data and methods

Data used in this article include: NCEP/NCAR 700 hPa northern hemisphere reanalysis data (2.5°×2.5°) ofuandvcomponents; the monthly precipitation data (1961-2006)from 280 stations in western China that are compiled by the China Meteorological Administration.

The key areas in wind fields that influence autumn precipitation of western China are determined by synthetically analyzed the vector wind difference at 700 hPa between in more autumn precipitation years and less autumn precipitation years. The two-dimensional statistics and testing method is applied to test the significance of wind field difference (Shiet al., 2004).

Air flows which influence precipitation of western China mainly include southwest flow from the south Asian, southeast flow at south side of the western Pacific subtropical anticyclone, and north flow at middle-high latitude. To reflect changes of these three air flows, we compute each air flow index, and identify each lattice point’s wind direction in areas of western China. In this article, we refer to Qiaoet al.’s (2002)method to determine the wind direction of each lattice point with 700 hPa (2.5°×2.5°)uandvcomponents data: (1) ifv＜0,it is a north wind; (2) ifv＞0, a south wind. Whenv＞0, ifu＞0,it is a southwest wind; ifu＜0, a southeast wind.

3. Definition of regional wind index

3.1. Selection of range

In western China, September precipitation accounts for 54% of autumn precipitation, and even reaches up to 70% in the mid-west of Gansu Province. September precipitation reduces from southeast to northwest, and the high value appears in the southeastern side of the Tibetan Plateau. The 50 mm equivalent line runs in a southwest-northeast direction, which passes through mid-south of the Tibetan Plateau to Yangtze River and Yellow River source regions, to Qilian Mountains, to middle Gansu Province, to southern Ningxia,and to north Shanxi. This line divides the distribution of September precipitation in western China into a southeast rainy region and a northwest seldom rainy region. The distribution of autumn precipitation is the same. Therefore, we selected five years with more September precipitation (1964,1967, 1973, 1982, 1983) and five years with less September precipitation (1991, 1992, 1998, 2002, 2006) for study and synthesized 700 hPa vector wind. The anomaly values in these years are all more than 1 times of the mean square error. Because of the effect of precipitation in high value areas, the years that we selected from the mean precipitation sequence of western China mainly reflected the situation of rainy areas.

In more precipitation years, there is a large-scale vector wind anomaly anticyclone area from east of the Tibetan Plateau to South China Sea and East China Sea. The western China is controlled by the southwest wind anomaly in rear of the anticyclone, and the anticyclone center is in Taiwan Province of China, and the wind speed anomaly center is in Hunan Province. In less precipitation years, the western China is controlled by the north wind anomaly in rear of the cyclone. Figure 1a is the difference distribution of 700 hPa September vector wind anomaly in western China between in more and less rainy years, in which we can see the maximum difference value center is in south of China. Figure 1b presentsF-test wind anomaly values of Asia (10°N-60°N, 70°E-160°E). There are three regions which pass the significance test (F0.05=4.74,F0.01=9.55): (1)the first area is between lakes Balkhash and Baikal; (2) the second in south China, with the maximum difference value of 75; and (3) the third in Japan and its eastwards area in Pacific. We selected the maximum difference area(20°N-35°N, 100°E-120°E) as the key area (rectangular box in Figure 1b) influencing September precipitation of western China, and defined the local wind index of this area. It is worthwhile to mention that this area is in the monsoon region (20°N-40°N, 100°E-140°E) determined by Qiaoet al. (2002). It is evident that the box region basically includes the high correlation area that reflects each autumn monsoon flow activity in western China, and the change of wind direction and wind speed and its influence on September precipitation of western China may reflect the East Asian monsoon variety.

Figure 1 Difference composition (a, unit: m/s) and F-test (b) of 700 hPa September vector wind anomaly in western China between in more and less rain years

3.2. Determination of regional wind index

In the aforementioned box area, we selected every other 2.5° of latitude and longitude as grid points. There are 54 grid points in range of 20°N-35°N, 100°E-115°E. The regional wind indices are defined as follows in aspects of southwest wind (u＞0,v＞0), southeast wind (u＜0,v＞0) and north wind (v＜0) areas:

(1) Southwest wind area (ASW)

(2) Southeast wind area (ASE)

(3) North wind area (AN)

wherenSW,nSEandnNare the number of grid points of the three types of wind on 700 hPa.Nis the total number of grid points. According to the above three definitions of regional wind characteristic index, we got a time-sequence for every September in 46 years (1961-2006).

4. Relationship between regional wind indices

In order to understand the relationship between the three aforementioned characteristic indices, we first calculate the monthly anomaly of each index year by year in the 46 years(1961-2006), then calculate the correlation coefficients among them, with results listed in Table 1. From Table 1 it can be seen that:ANandASW,ANandASE, are negatively correlated with each other, and the correlation coefficients have passed theα=0.05 significance level test.ASEandASWhave weak negative correlation, butANandASWhave remarkable negative correlation. The results indicate that the ranges of southwest and southeast wind are small when north wind prevails, andvice versa. The distinct negative correlation betweenANandASWcan also be explained as:becauseANis adjacent toASWin the northern side of the westward subtropical anticyclone,ASWdiminishes certainly along withANincreases. The area between southwest wind and southeast wind is independent.

Table 1 Correlation coefficient between various wind indices in September

5. Temporal variation of regional wind index

Among the above three regional wind indices, southwest wind index is the largest, accounting for 45% of the entire area, north and southeast wind indices account for 27% and 28%, respectively. Southwest wind is related to a southern trough formed by westerly current flowing around the southern side of the Tibetan Plateau. The southwest wind can be seen all year round, shifting to south in winter, and to north in summer. Southeast wind originates mainly from the subtropical ocean area at south side of the western Pacific subtropical anticyclone (WPSA). Its range is much smaller than that of southwest wind, and the range diminishes with the WPSA moves back southward since early autumn. Thus,it may be taken as effective that the three wind area indices can well reflect the climate features of each air flow change in eastern Asia.

Figure 2 presents the interannual variation of regional wind in September over the past 46 years in western China.The range of north wind has significantly enlarged; southwest wind area has significantly decreased; and relatively little change has happened for southeast wind. The change tendency of September precipitation in western China shows that the reduced areas of precipitation are mainly situated in the eastern side of the Tibetan Plateau, which are in accord with the research results of Ma (2008), Lin and Lu (2004),and Huang and Zhang (2007). There are also several remarkably reduced areas of precipitation that situated in the western Sichuan Plateau, east of Gansu Province, southern Ningxia Province, north of Shanxi and west of Guizhou Province, mid-east of Sichuan Province, the border area of Hunan and Chongqing. Reduced precipitation in the eastern side of the Tibetan Plateau is related to southwest wind variety because southwest wind is one of the main vapor transport routes of precipitation in eastern Tibetan Plateau (Wanget al., 2004; Yu, 2004). When southeast wind trend tends to be stable, southwest wind shifts south along with north wind area enlarges, so the amount of vapor that reaches the eastern side of the Tibetan Plateau is reduced, resulting less precipitation.

Figure 2 The interannual variation of regional wind indices in September

6. Relationship between regional wind and September precipitation

6.1. Correlation coefficient

Figure 3 is the correlation coefficient distribution between the standardized anomaly of wind area indices (north wind, southeast wind and southwest wind) and the percent of precipitation anomaly in September from 1961 to 2006 in western China. Correlation between north wind index and September precipitation anomaly (Figure 3a) shows the negative correlated areas are in the middle of Gansu, Shanxi,eastern Sichuan, Guizhou, western Guangxi Province at east side of the Tibetan Plateau. Correlation between southwest wind index and September precipitation anomaly (Figure 3c)shows the main positive correlated areas are in east of Gansu,southern Ningxia, Shaanxi, northern Sichuan, eastern Sichuan, Guizhou, and western Guangxi Province; the higher correlated center is along the Weihe River. In addition, there are weak positive correlated areas in Xinjiang and eastern Qinghai Province, and weak negative correlated areas in Yunnan Province, mid-east of the Tibetan Plateau, west of the Qinghai Plateau. This relationship shows that, in September, when the range of southwest wind is larger and that of north wind is smaller in the key area, there are more precipitation in western China but less precipitation in the Tibetan Plateau, andvice versa. Figure 3b presents the correlation between southeast wind index and September precipitation anomaly, though the figure appears several negatively correlated areas, the significantly correlated area is small.Thus, we can say that September precipitation in western China is little related to southeast wind area index.

Figure 3 Distribution of correlation coefficient between regional wind index and September precipitation in western China during 1961-2006. Solid line is the positive correlation, dashed line is the negative. Three types of shaded area from light to dark are correlation coefficient r≥0.29, 0.38 and 0.47, respectively, equaling to the significant level at α=0.05, 0.01 and 0.001, respectively.

6.2. Composite analysis

Composite analysis is conducted in the eight years(1961, 1967, 1971, 1972, 1974, 1975, 1983, 1984) with larger southwest wind index in September and eight years(1977, 1986, 1987, 1990, 1991, 1996, 1998, 2002) with smaller southwest wind index in September (Figure 4). In larger index years (Figure 4a), precipitation is abnormally more in the eastern side of the Tibetan Plateau, west and north of Xinjiang, but less in the south and east of Xinjiang,most of the Tibetan Plateau, west of Inner Mongolian, and west of Gansu Province; In smaller index years (Figure 4b),precipitation distribution is generally opposite with the larger index years. Precipitation is more in area from middle Tibetan Plateau to Yunnan Province, northwest and west of Xinjiang, west of Inner Mongolian, whereas less in other areas of western China. The composite analysis results are consistent with that of correlation analysis, which further suggests that southwest wind anomaly can well reflect September precipitation anomaly in western China.

Figure 4 Composition of September precipitation in western China for larger (a) and smaller (b) 700 hPa southwest wind value years

7. Relationship between September precipitation and height field

In typical rainy years, there is negative anomaly in western China and positive anomaly in eastern China at 700 hPa height field. The India-Burma trough strengthens, southwest flow develops, there is a low trough from the Aral Sea to Balkhash Lake, and the western Pacific subtropical anticyclone extends westward. This "high in the east and low in the west" pattern is greatly beneficial to precipitation in western China. This can also explain the positive correlation between southwest wind area index and precipitation in most of western China and Xinjiang Province. The autumn rainfall in western China is a direct result of southwest wind,but the autumn rainfall in Xinjiang is often caused by the low trough in Balkhash Lake rather than directly by southwest wind.

In typical drought years, there is positive anomaly in western China and negative anomaly in eastern China at 700 hPa height field. The India-Burma trough weakens, the high pressure ridge over the Ural Mountain develops, the low trough in Balkhash Lake weakens, the north wind strengthens, and the western Pacific subtropical anticyclone extends eastward. The "negative in the east and positive in the west"is not beneficial to precipitation in western China (Liuet al.,2004).

Bai and Dong (2004) considered that, at 500 hPa height field, the high pressure ridge over the Ural Mountain and the western Pacific subtropical anticyclone are stronger in autumn rainy years in western China, and a broad low trough is easily formed in area from lakes Balkhash to Baigal. This is beneficial for moisture masses transported continuously from the Bengle Bay to western China by southwest and the Plateau monsoons. Thus, the Bengle Bay and South China Sea become the main vapor source of precipitation in northwestern China (Yu and Wang, 2003). Meanwhile, cold air continuously moves southeast from the low trough of Balkhash Lake, then it is blocked in the Sichuan Basin and its north and west areas. These make the cold and warm air interchange in western China, synthetically causing continual wet weather in this area.

8. Conclusions

(1) We define three regional wind characteristic indices(north, southwest and southeast wind) from aspect of circulation influencing the autumn precipitation in western China.These three indices have certain relations with each other,but are independent. This is a reflection of the change of East Asian monsoon.

(2) When north wind prevails over a large region, the range of southwest and southeast winds decreases, and the change of southwest and southeast wind area indices is relatively independent.

(3) Each wind index has a clear relation with September precipitation of western China, but each air flow has a different influenced area, that is, each monsoon index corresponds to a certain precipitation distribution. When the range of southwest wind is larger and north wind is smaller,more precipitation occurs in most western China, and less precipitation in the Plateau; andvice versa.

(4) Southwest wind is the main vapor channel of autumn precipitation in the eastern side of the Tibetan Plateau, the range of southwest wind decrease will cause a reduction of autumn precipitation.

This project is supported by the National Natural Sciences Foundation of China (Grant No. 40675066).

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10.3724/SP.J.1226.2011.00419

*Correspondence to: ShaoYong Chen, Senior Engineer of Lanzhou Institute of Arid Meteorology, China Meteorological Administration, Lanzhou, Gansu 730020, China. Email: csy505@tom.com

18 March 2011 Accepted: 10 July 2011