Analysis on the decadal scale variation of the dust storm in North China

2260 Science in China Ser. D Earth Sciences 2005 Vol.48 No.12 2260 2266 Analysis on the decadal scale variation of the dust storm in North China KANG Dujuan 1,2 & WANG Huijun 1 1. NZC/LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100871, China; 2. Environmental Fluid Mechanics Laboratory, Stanford University, Stanford CA 94305, USA Correspondence should be addressed to Kang Dujuan (email: kangdj@stanford.edu) Received June 20, 2003; revised July 18, 2005 Abstract In this paper, the temporal variation characteristics of the dust storm in North China are investigated. Based on power spectrum analysis and wavelet analysis, 1956-1970 and 1985-1999 are chosen as the high-frequency and low-frequency dust storm decades respectively. Analysis results clearly show that the spring and wintertime anomalous atmospheric circulation between these two decades are significantly different. Compared with the former decade, there are a strengthened polar vortex, enhanced westerlies near 50 N, and a weak East Asian major though in the winter of the latter decade. The north and center parts of the Siberian high and the Aleutian low become weak. The southerly and easterly wind anomalies appear over the north and east parts of China, which implies the weakening of East Asian winter monsoon. Meanwhile, northern China experiences warmer winters and wetter springs, which are in favor of the weakening of dust storm intensity in North China. There are significant out-of-phase relationships between dust frequency and wintertime westerly intensity, as well as between dust frequency and Arctic Oscillation. It is also found that the frequency of dust weather is strongly related to winter-springtime East Asian monsoon intensity. Keywords: dust storm, climate anomaly, decadal scale variability, East Asian monsoon, AO. DOI: 10.1360/03yd0255 Dust storm is a disastrous weather that can cause serious environmental consequences and hazards. It causes excessive soil mass and nutrient loss in source areas, and air pollution in deposition area, which result in bad influence on agriculture, industry, traffic, and peoples daily life [1-4]. Dust storm can also affect the thermal balance of planetary radiation and then lead to meso- to macroscale climatic modification [5-7]. Northwest China and North China are two high-frequency centers of dust storms, and the main areas in North China are influenced by dust weather. Furthermore, the dust even has been transferred to East Asian and Pacific areas and affects the climate, radiation, and environment quality [8-11]. In recent years, strong dust storms frequently happened in North China and caused huge damage to human life and property [12,13]. For example, in 2002 a very severe dust weather process, which lasted from March 18 to 20, influenced most of the northern and eastern areas of China and has drawn great attention of the government and the scientists. It has been very important and necessary to study the dust storm and solve the problems it causes. From the 1950s to 1990s, the frequency of dust weather has been decreasing, especially after the mid- Copyright by Science in China Press 2005

Analysis on the decadal scale variation of the dust storm in North China 2261 1970s [14,15]. Such climatic changes of dust storm are highly related to the climate and atmospheric circulation anomalies. The transition of the global atmospheric circulation appeared at the end of 1970s [16,17]. The weakening of East Asian monsoon circulation and the changes of temperature and precipitation after this transition can be clearly seen. Such changes can directly affect the occurrence and development of dust weather. So it will be of great importance to study the atmospheric circulation anomalies accompanying the dust weather. With the prediction of the latter, we can acquire a basic understanding and forecasting of the climatic situation in dust season, which may greatly benefit the economy and people s life. Our work in this paper is focused on solving such problems. We find the decadal scale variation of the dust storm in North China; then investigate the accompanying spring-wintertime anomalous atmospheric circulation and try to understand the relationship between them. accordance with the analysis results of other 5 stations by Zhou Zijiang [9]. Based on such characteristics, our study will focus on the circulation anomalies in the spring and the prior winter of each year. The datasets used in the analysis consist of (1) Monthly dust weather frequency of Beijing and Tianjin station (1954-1999). Here dust haze, blowing dust and dust storm are referred to as dust weather, (2) NCEP/NCAR monthly reanalysis global atmospheric circulation data (1948-1999), 2.5 2.5, 17 levels, from 1000 hpa to 10 hpa, (3) Monthly precipitation data (1951-1999) for 160 stations in China from China Meteorological Administration. 1 Temporal distribution of dust weather in North China The highest frequency of dust weather of a year is in spring, and the lowest is in autumn. In spring, the surface dust is loosen when it is warm enough for the ice to melt, at the same time the cold front is active in the northern areas which can cause strong wind. Fig. 1 presents the monthly mean days (a) and yearly mean series (b) of dust weather for 46 years (1954-1999) in Beijing. From Fig. 1(a) we can see that all these types of dust weather (dust haze, blowing dust and dust storm) mainly happened in springtime. It is in Fig. 1. Monthly mean days (a) and yearly mean series (b) of dust weather for 46 years (1954-1999) in Beijing. 1, Dust weather; 2, dust storm; 3, blowing dust; 4, dust haze. In Fig. 1(b), a decreasing trend can be clearly seen from the curve. The dust frequencies drop quickly especially after the mid-1970s, and reach quite a low value in the 1990s. The mean dust weather frequency over North China has the same trend [18]. Power spectra analysis (Fig.2) shows there are three main peaks above the 95% significance line: one year, ten years and twenty years or so, which verify the decadal scale variation of dust weather. Furthermore, wavelet analysis reveals the two decades that pass the 95% significant test: 1954-early 1970s and mid- 1980s-1999. Based the above analysis, 1956-1979 and 1985-1999 are chosen as the high-frequency and low-frequency dust storm decades respectively. Elaborate and systematic works are designed and performed to compare the spring and wintertime anomalous atmospheric circulation between these two decades. Here Winter is defined to the average of January, February of the

[ 2262 Science in China Ser. D Earth Sciences present year and December (DJF) of the prior year; Spring is the average of March, April and May (MAM) of the present year. Fig. 2. Power spectra of the dust frequency of Beijing. The dashed line indicates the 95% significance level (red noise as background spectra). 2 Changes of polar vortex, westerly, and East Asian major trough Figure 3 compares mean wintertime 500 hpa geopotential height anomalies of these two decades in North hemisphere. In the former decade, there is a weaker polar vertex and negative anomaly in the midlatitude areas around the Arctic; in the latter there is reverse situation, but both show a zonal symmetric pattern. From the distribution of wintertime height difference (Fig. 5(a)), we can see there are a strengthened polar vortex, enhanced westerly near 50 N, and a weak East Asian major trough in the latter decade. The distributions of mean wintertime see-level pressure anomalies of the two decades (Figs. 4, 5(b)) Fig. 3. DJF mean geopotential heights anomalies (in gpm) at 500 hpa during 1956-1979 (a) and 1985-1999 (b) in the Northern Hemisphere. Fig. 4. DJF mean sea-level pressure anomalies (in Pa) during 1956-1979 (a) and 1985-1999 (b) in the Northern Hemisphere.

Analysis on the decadal scale variation of the dust storm in North China 2263 Fig. 5. The differences of DJF atmospheric circulation anomalies between 1985-1999 and 1956-1979. (a) 500 hpa geopotential height/gpm; (b) sea-level pressure/pa. Shaded areas indicate significant changes at 95% level, estimated by a local student t-test. are also significantly different. It can be seen that the north and center parts of the Siberian high and the Aleutian low become weak. So the northwest wind from the west of the Aleutian low becomes weak. All these factors are not in favor of the occurrence and development of dust weather in North China. It can also be noticed from Fig. 5 that the anomalous atmospheric circulation between these two decades in the Southern hemisphere is even more obvious than in Northern hemisphere. We can see clear zonal symmetric pattern in the Southern hemisphere. These indicate that the decadal scale variation of the Dust Storm in North China corresponds to the global scale anomalous atmospheric circulation. Figure 6 also provides the yearly series of the dust frequency, zonal index, and Arctic Oscillation (AO) Index. For the zonal index, we follow the definition of Gong Daoyi [20] et al.: H 40 N -H 65 N. According to Thompson et al., AO index is defined as the time-index corresponding to the leading EOF of wintertime sea-level pressure north of 20 N [21]. There are significant

2264 Science in China Ser. D Earth Sciences out-of-phase relationships between dust frequency and wintertime westerly intensity, and also between dust frequency and Artic Oscillation, where the correlation coefficients are 0.36 and 0.43 respectively. Fig. 6. Yearly series of standardized zonal index (1), dust weather frequency of Beijing (2), and AO index (3). 3 Changes of wind and East Asian monsoon Iwasaka et al. [22] pointed out that the trajectory of dust moving is mainly along the wind direction of 850 hpa. Fig. 7 shows the difference of mean springtime wind between the two decades. The southerly and easterly wind anomalies appear over the north and east parts of China, which implies the weakening of East Asian winter monsoon. An anomalous anticyclone over northeast Asia can be seen clearly at 850, 500, 200 hpa levels in Fig. 7, which indicates that it is a barotropic affecting system (A). The southern and southeastern wind anomalies on its west are in favor of the weakening of western and northwestern wind in East China. Also we can notice there is an anomalous cyclone-anticyclone series along the eastern Asia, i.e. the anomalous anticyclone over northeast Asia the anomalous cyclone to the east of Philippines the anomalous anticyclone over the northern Australia. This anomalous series plays an important role in connecting the anomalous circulation in the Southern hemisphere with the extreme event in the Northern hemisphere, and is in close relationship with the dust storm. The shade part in Fig. 7(a) indicates the East Asian monsoon area (110-125 E, 20-40 N). Wang Huijun [23] defined an EAM index as the average of the wind velocity anomalies over this area. We discuss the relationship between dust frequency and East Asian monsoon by using this index. Fig. 8 is the interannual curve of them, from which we can see the long-term trend and interannual variability are rather remarkable and consistent with each other. The correlation coefficients between dust frequency and winter and springtime East Asian monsoon intensity are 0.40 and 0.47 respectively. From the above analysis, it can be seen clearly that the East Asian monsoon index and AO index are good indicators for dust storm activity. Fig. 7. The differences of MAM wind (m/s) between 1985-19991 and 1956-1979. (a) 850 hpa; (b) 500 hpa; (c) 200 hpa. A: anticyclone; C: cyclone. Shaded areas indicate East Asian monsoon area (110-125 E, 20-40 N).

Analysis on the decadal scale variation of the dust storm in North China 2265 Fig. 8. Yearly series of standardized East Asian Monsoon Index (1) and dust weather frequency of Beijing (2). The EAMI is the area-mean velocity over the area indicated in Fig. 7. 4 Changes of temperature and precipitation Some studies showed that there is an out-of-phase relationship between dust frequency and prior winter temperature [24,25], which can be clearly seen from Fig. 9(a). Except the small part of northeast and west, there are all significantly positive anomalies over China. This implies that China experienced warmer winter in the low dust frequency decade. Precipitation is another important factor that influences dust weather. From the difference distribution of the two decades (Fig. 9(b)), we can see the springtime precipitation plays a more important role. In the low dust frequency decade, most areas in North China have more precipitation in the spring, especially the dust source areas. The humid climate can prevent the dust from blowing into the sky, so is unfavorable to the occurrence of dust storms. 5 Summary There is significant decadal variation of dust weather in North China, which is accompanied by the decadal variation of atmospheric circulation. Analysis results clearly show that the spring and wintertime anomalous atmospheric circulation between high and low dust frequency decades are significantly different. Compared to the former decade, the latter one has the following circulation characteristics: Fig. 9. The differences of DJF temperature ( ) (a) and MAM precipitation/mm (b) between 1985-1999 and 1956-1979. Shaded areas in (a) indicate significant changes at 95% level, estimated by a local student t-test. (1) There are a strengthened polar vortex, enhanced westerly near 50 N, and a weak East Asian major trough in the prior winter, the north and center parts of the Siberian high and the Aleutian low become weak, the northwest wind from the west of the Aleutian low becomes weak. (2) The southerly and easterly wind anomalies appear over the north and east parts of China, which implies the weakening of East Asian winter monsoon, the anomalous anticyclone over northeast Asia, which plays an important role in connecting the anomalous circulation in the Southern Hemisphere with the extreme event in the Northern Hemisphere, is in close relationship with the dust storm. (3) The anomalous atmospheric circulation between these two decades is of global scale.

2266 Science in China Ser. D Earth Sciences (4) North China experiences warmer winters and wetter springs, which are in favor of the weakening of dust storm intensity in North China. We do some research to explore the interactions between the dust storm frequency and several important circulation systems. We find there are significant out-of-phase relationships between dust frequency and wintertime westerly intensity, and also between dust frequency and Artic Oscillation. It is also found that the frequency of dust weather is strongly related to winter and springtime East Asian monsoon intensity. Our work in this paper provides an approach to forecast the trend of dust storm intensity in North China. If the anomalous circulation situation in prior winter is predicted, we can get a basic preview of the trend of dust storm in the following spring. 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