Relationship between vegetation coverage and spring dust storms over northern China

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi:10.1029/2003jd003913, 2004 Relationship between vegetation coverage and spring dust storms over northern China Xukai K. Zou and Panmao M. Zhai Laboratory for Climate Studies, National Climate Center, China Meteorological Administration, Beijing, China Received 25 June 2003; revised 11 December 2003; accepted 16 December 2003; published 6 February 2004. [1] On the basis of normalized difference vegetation index (NDVI) data from 1982 to 2001 and dust storm observations in China the relationship between vegetation and spring dust storms over northern China is discussed. The results show that poor vegetation coverage in northern China is one important factor for the frequent occurrence of spring dust storms. In addition, vegetation cover plays an important role in interannual variations of dust storms. In general, a negative correlation is noted between vegetation coverage and occurrence of dust storms in northern China for spring during the period 1982 2001. The correlation coefficient between vegetation coverage and areas affected by dust storms is 0.59, which is statistically meaningful at 99% confidence level. The sharp decrease of spring vegetation coverage in recent years is one of the major contributors to frequent spring dust storms over northern China specifically during 2000 and 2001. A negative correlation is especially significant in the eastern part of northern China, mainly in central and eastern Inner Mongolia. When vegetation decreases (increases), the occurrence of dust storms increases (decreases). Furthermore, statistics show that abundant vegetation in previous seasons could help reduce dust storms in the coming spring. The effect of prior summer vegetation on the variation of spring dust storms is particularly evident in the central and eastern part of northern China. Because of the presence of little to no vegetation in the desert areas of northwest China the variation in occurrence of spring dust storms seems unrelated to the vegetation. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 1610 Global Change: Atmosphere (0315, 0325); 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); KEYWORDS: dust storm, NDVI, China Citation: Zou, X. K., and P. M. Zhai (2004), Relationship between vegetation coverage and spring dust storms over northern China, J. Geophys. Res., 109,, doi:10.1029/2003jd003913. 1. Introduction [2] Dust storms blow away vast amounts of topsoil, destroy crops, grasslands, and roads, result in rapid desertification over arid and semiarid regions, and have a negative effect on human health through air pollution. China is located in the east Asian monsoon region. Arid and semiarid climate dominates the northern parts of its territory [Domrös and Peng, 1988]. The strength of the monsoon circulation can cause not only drought/flood and cold/warm events, but also windy conditions in northern China [Qian et al., 2002]. Observations of the recent 50 years show that the most frequent dust storms occurred in northern China in spring [Zhai and Li, 2003]. It is known that mainly three conditions are needed to cause dust storms: strong wind, instability in the lower troposphere, and a dry land surface to provide a dust source [Fang et al., 1997]. In previous studies [Sun et al., 2001; Qian et al., 2002], the temporalspatial changes of China s dust storm frequencies and dust transport routes have been widely studied. On the basis of Copyright 2004 by the American Geophysical Union. 0148-0227/04/2003JD003913 station-averaged means of the annual time series of dust storm days from 1954 to 1998, in which 83 stations of dust weather in northern China were used, Qian et al. [2002] pointed out a decreasing trend during the study period. Using the relationship between dust weather, winter air temperature, and spring cyclone frequency, a dust-index was also developed to simulate the trend of dust events. The climate controls of dust storms were also researched. Zhai and Li [2003] studied the temporal and spatial characteristics of China s dust storms by using more than 700 station observations from the past 50 years, and analyzed many meteorological factors that were related to dust storms. The study revealed the importance of strong winds, precipitation, and humidity on the seasonal cycle and interannual variability of dust storm frequency. Using the data of NDVI and the number of dust storm days in central and western Inner Mongolia, a negative correlation between vegetation coverage and dust storm days was recently revealed by Gu et al. [2002]. Although a decreasing trend of dust storms is observed from mid-1950s to mid-1990s in observations, an obvious increasing trend is seen from 1997 to 2002 [Lu et al., 2003]. The recent increase of dust storms has attracted particular concern from both the Chinese government and 1of9

general public. Is the recent increase of dust storms related to the global warming, human activity, or is it just purely a part of natural variability? Although some achievements have been made in understanding the spatial distribution, climate controls, and history of dust storms, the causes and mechanisms of dust storm variability, and the role of land surface conditions which are strongly related to the occurrence of dust storms, have not been carefully investigated. The purpose of this research is to study the relationship of vegetation coverage with dust storms in northern China, and its impact on variability of the dust storm frequency. [3] Data of satellite observed vegetation indices are often used to measure the density and distribution of green vegetation over wide regions. It is widely recognized that the variations of vegetation conditions are responding to the climate change. Studies show a good correlation between NDVI and temperature during the growing season over mid and high latitudes in the north hemisphere [Tucker et al., 2001; Zhou et al., 2001; Gong et al., 2002]. A warming climate, associated with a reduction in annual snow cover and an earlier disappearance of snow in spring [Groisman et al., 1994; Konstantin et al., 1999], and the increases of CO 2 could generally promote biological activity, result in a longer growing season and therefore favor a higher vegetation index [Zhou et al., 2001; Gong and Ho, 2003]. In some areas, however, higher temperatures could induce drought if increased temperatures are not accompanied by increased precipitation [Barber et al., 2000]. The relationship between vegetation and climate is therefore complicated. During the past 50 years, the climate has become warmer and wetter in northwest China [Wang et al., 2002], which is the major source region for dust storms [Sun et al., 2001]. In north China, a warming and drying trend is observed during the last two decades [Qian and Zhu, 2001]. The variation in temperature and precipitation may be related to changes in vegetation, while variation in vegetation conditions may be linked to dust storm occurrence. In this study, we attempt to reveal links between variations of China s dust storms and vegetation conditions. 2. Data Source and Methods [4] Vegetation indices are generally calculated from the visible and near-infrared light reflected by vegetation. NDVI is widely used as an indicator of the vegetation activity [Tucker and Sellers, 1986]. Previous studies suggest that NDVI is highly correlated with vegetation parameters such as green-leaf biomass and green-leaf area [Justice et al., 1985]. The NDVI data used in this study are monthly grid data, with a horizontal resolution of 1 longitude by 1 latitude, produced by the NOAA/NASA Earth Observing System Pathfinder AVHRR Land Program and are available from the NASA Goddard Space Flight Center Distributed Active Archive Center via the Internet at http://eosdata. gsfc.nasa.gov. The period of the NDVI data used in this study is from 1982 to 2001. The mean NDVI for spring is averaged from monthly data of March, April and May. Dust storm, precipitation, temperature and wind speed data are derived from surface meteorological observations by the National Meteorological Center of China. The original data set includes data from about 700 stations, covering vast regions over mainland China. Dust storm in China is Figure 1. Distribution of the number of spring dust storm days in northern China (averaged from 1982 to 2001, shaded areas indicate average dust storm days of more than 1 day) and locations of 283 stations where dust storms have been observed during the period 1982 2001. defined as a severe weather phenomenon with strong turbulent wind systems blowing dust particles into the air, causing the visibility to be reduced to 1km or less [China Meteorological Administration (CMA), 1979]. The number of dust storm days is derived from surface meteorological records according to a daily interval of 24 hours from 20:00 (Beijing local time, BLT) [CMA, 1979]. The number of dust storm days in spring (MAM) is the total number days with dust storms observed during the three months. In this study, dust storm data are from the period of 1982 2001. During this period, dust storms were observed in 283 stations, most of which are located in northern China (Figure 1). There are two dust storm centers: one located in western China, and the other in western central Inner Mongolia [e.g., Sun et al., 2001]. The positions of the two centers are within the largest deserts of China and their surrounding regions (Figure 2). [5] To compare the interannual variations of NDVI and dust storms in northern China, the areas of vegetation coverage and regions affected by dust storms in spring during 1982 2001 are estimated. During the calculation, 604 grid pins in northern China are used. Since small NDVI values can produce large errors in regions where the mean NDVI values are near zero [Gong and Ho, 2003], only areas with NDVI values more than 0.1 are used in the calculation for each spring. For each one-degree box, the area covered by vegetation is S i ¼ rlong rlati cos f i where rlong indicates the length in the longitudinal direction in km, and rlati denotes the length in latitudinal direction in 1 box at the equator, rlong rlati is approximation of area with 1 longitude by 1 latitude grid in this region. cos f i is a correction factor to latitude. The total area with vegetation cover in the spring of year j is S j ¼ Xn S ij i¼1 The amount of the area of 1 box affected by dust storms is calculated based on the percentage amount of observation 2of9

Figure 2. Distribution of the main deserts, provinces, and five selected stations in northern China. Deserts are as follows: A, Taklimakan Desert; B, Gurbantunggut Desert; C, Badain Jaran Desert; D, Tengger Desert; E, Horqin Desert; F, Mu Us Desert; G, Hunshandake Desert. Provinces are as follows: XJ, Xinjiang; QH, Qinghai; GS, Gansu; IM, Inner Mongolia; NX, Ningxia; SAX, Shaanxi; SX, Shanxi; HB, Hebei; HN, Henan; SD, Shandong; LN, Liaoning; JL, Jilin; HLJ, Heilongjiang. The five stations are as follows: 1, Qitai; 2, Wuwei; 3, Mandula; 4 Xilinhaote; 5, Baicheng. The geography regions are as follows: northwest China consists of Xinjiang, Qinghai, Gansu, Ningxia, Shannxi and western Inner Mongolia; north China consists of central Inner Mongolia, Shanxi, Hebei, Henan, and Shandong; northeast China consists of Heilongjiang, Jilin, Liaoning, and eastern Inner Mongolia. stations in that box. Missing records are common in dust storms data, as is a lack of stations in deserts or remote areas. Percentage areas are finally calculated excluding missing data as well as NDVI data in boxes without dust storm observations. If the total area of available data of year j is SN j, the total area with NDVI more than 0.1 or a dust storm frequency more than 1 day is S j, then the percentage area is, SS j ¼ S j SN j 100 It is widely recognized that the interannual variations of dust storms are affected by many climate and geographic factors [Qian et al., 2002; Sun et al., 2001; Zhai and Li, 2003]. A comparison of the relationships of dust storm variability with climate parameters and vegetation in different regions and different seasons is presented in Table 1, which lists the correlation coefficients for five selected stations that are scattered from west to east in northern China (Figure 2). Although several stations are located near the desert (e.g., Qitai and Xilinhaote), they all are vegetated areas, in which dust storms have been observed over 7 years during the whole 20 years period. 3. Vegetation Control on Spring Dust Storms [6] In China, the main source regions of dust storms are the deserts and their surrounding regions, including the Taklimakan Desert, the Badain Jaran Desert, the Tengger Desert, and the Mu Us Desert etc [Sun et al., 2001] (Figure 2). The NDVI values in these areas are usually less than 0.05. Natural vegetation distributions are quite different from west to east in northern China [Domrös and Peng, 1988]. Barren soils spread over the west part of northern China, where desert, semi-desert or grassland is distributed. In comparison, vegetation grows better in the east territory, where the natural vegetation is mostly grass- Table 1. Correlation Coefficients Among Spring Dust Storm and Various Parameters (Precipitation, NDVI, Temperature, and Wind Speed) at Five Selected Stations a Factors Qitai Wuwei Mandula Xilinhaote Baicheng JJA( 1) precipitation 0.26 0.41 0.65 0.63 0.29 SON( 1) precipitation 0.07 0.19 0.04 0.21 0.33 DJF( 1) precipitation 0.14 0.20 0.19 0.29 0.13 MAM(0) precipitation 0.01 0.06 0.35 0.06 0.19 JJA( 1) NDVI 0.34 0.45 0.71 0.62 0.69 SON( 1) NDVI 0.34 0.32 0.60 0.47 0.43 DJF( 1) NDVI 0.02 0.44 0.65 0.34 0.12 MAM(0) NDVI 0.22 0.49 0.72 0.56 0.32 MAM(0) temperature 0.16 0.18 0.08 0.01 0.09 MAM(0) wind speed 0.04 0.35 0.68 0.57 0.65 a Here a 1 means prior seasons and 0 represents the same spring season. The bold values are statistically significant at 95% confidence level. 3of9

Figure 3. Distribution of ratios (%) of the number of spring dust storm days to those in a whole year over northern China (averaged from 1971 to 2000). land, forest-steppe, and even forest. Dust storms often occur more frequently in the west than east regions (Figure 1) in northern China. [7] Figure 3 displays the spatial distribution of percentage ratios of number of dust storm days in spring (MAM) to those in whole year. In northern China, spring is the season with highest frequency of dust storms, winter (DJF) is the next, while summer (JJA) and autumn (SON) are the least frequent seasons (figure not shown). [8] The continental anticyclone dominates northern China in winter [Domrös and Peng, 1988]. The climate is cold and dry in this season, and the wind is weaker than that in spring [Zhai and Li, 2003]. In most areas of northern China, the lowest NDVI is observed in winter, when vegetation cover is at its lowest level during whole year. Although winter has the least vegetation cover, because the soil is heavily frozen due to the low temperature or snow cover in most parts of northern China, the frequency of dust storms is actually less than that in spring since there is no dust source. [9] In spring, the rising temperature results in the melting of frozen soil and snow cover. In this season the synoptic systems over east Asia are highly unstable which cause high-frequency strong winds [Sun et al., 2001; Qian et al., 2002]. Also some areas experience the driest climate of the year in spring due to higher temperature, less precipitation and increasing evaporation [Zhai and Li, 2003; Sun et al., 2001]. The NDVI values increase slightly in this season, but the values are still low (Figure 4a), reflecting that vegetation is still sparsely distributed, especially in vast areas of northwest China. The naked and loose land surface provides a rich dust source for dust storms. From the difference of NDVI values between winter and spring (Figure 4b), it is found that the NDVI values increase more than 0.1 from winter to spring only in a small domain including some eastern regions of northern China, and northern Xinjiang. In western parts of northern China and most of Inner Mongolia, changes of NDVI values are very limited, mostly less than 0.1. Vegetation conditions haven t been improved obviously from winter to spring in these regions. Compared with Figure 1, the regions with little changes of NDVI between winter and spring are very consistent with the areas where dust storms usually occur in spring. It can be concluded that sparse vegetation coverage, associated with favorable climate parameters result in frequent dust storms in spring in northern China. Figure 5 gives the distributions of average monthly dust storm days, NDVI, and wind speed for five selected stations. Frequent dust storms seem to occur when vegetation condition is rather poor. Poor vegetation coverage in northern China is one important factor for the frequent occurrence of dust storms in spring. In the areas and months which NDVI is small, the frequency of dust storms is mainly dominated by wind speed. [10] From Figures 1 and 3 it can be seen that although there are less dust storms in the eastern part of northern China than the western part (except northern Xinjiang) in spring, the number of spring dust storm days accounts for 70 80% of the whole year in the eastern part, while it accounts for 40 50% in the western part. It has been revealed by Zhai and Li [2003] that in the western regions of northern China, where more frequent dust storms are observed, dust storms may occur in any season of the year and are prevalent from spring to summer. In the eastern part of northern China, however, dust storms occur mostly in spring and decrease significantly into early summer. It is known that the seasonal variations of dust storms are closely related to many climate and geographic parameters [Zhai and Li, 2003]. Vegetation also plays an important role. East of northern China has more vegetation than west. In the east, the wind speed must exceed a certain speed to blow the dust and sand upward, while in the west, the dust sources are so abundant that even weak winds can bring dust weather. [11] In summer and autumn, climate is relatively more warm and humid. Accompanying the cessation of the winter monsoon, wind velocities become smaller in northern China during summer [Domrös and Peng, 1988]. It is widely recognized that climate affects plant growth directly through temperature and precipitation. In general, warm temperatures and high rainfall increase vegetation activity because of increased plant photosynthesis [Zhou et al., 2003]. Summer is the warmest and most rainy season in northern China. Precipitation in summer accounts for 50 to 80% of the annual precipitation in most regions of Figure 4. (a) Distribution of the mean spring NDVI in northern China (averaged from 1982 to 2001, shaded areas represent NDVI values less than 0.2). (b) Same as Figure 4a except for the NDVI difference between spring and winter (shaded areas represent NDVI difference less than 0.1). 4of9

Figure 5. Distribution of averaged dust storm days (histogram), NDVI (solid line with diamond) and wind speed (dashed line with triangle) from January to December for five selected stations (averaged from 1982 to 2001, left axis for dust storm days and wind speed, right axis for NDVI). northern China (Figure 6), while spring precipitation only accounts for 10 to 20% (figure not shown). Favorable climate supports the highest NDVI values in summer. Dense and green vegetation cover reduces the dust source. All these factors limit the occurrence of dust storms. Dust storms decrease evidently in summer, especially in the eastern parts of northern China. It is known that NDVI in any given season may be determined by current and earlier climate conditions [Zhou et al., 2003]. In autumn, climate is relatively warm and humid and quite high NDVI values are also observed in this season. In addition, windy days are less frequent than that in spring. Climate factors and good vegetation are the main reasons for fewer dust storms in summer and autumn. 4. Relationship Between Variation of Vegetation and Spring Dust Storms [12] Changes of climate result in variation of vegetation activity that may have important impacts on interannual changes of dust storms. The following sections discuss the changes of NDVI, and the occurrence of dust storms in northern China in recent 20 years, and the relationship between them. 4.1. Simultaneous Relationship Between Vegetation Coverage and Dust Storms [13] Previous studies show that NDVI over northern mid and high latitude had risen steadily during the growing season since the early 1980s [Zhou et al., 2001; Tucker et Figure 6. Ratios (%) of summer precipitation to annual totals over northern China (averaged from 1971 to 2000). 5of9

Figure 7. Time series of percentage areas with mean spring NDVI value greater or more than 0.1 (solid line) and areas affected by dust storms (dash line) in spring in northern China from 1982 to 2001. al., 2001]. However, variations in vegetation are not uniform in terms of geographical distribution. Decreases in NDVI can also be observed in some regions [Zhou et al., 2001; Gong and Ho, 2003]. Figure 7 delineates variations of percentage area of dust storms and vegetation in spring in northern China from 1982 to 2001. It is very clear that there is an obvious opposite correlation between areas of vegetation coverage and dust storms in northern China. The correlation coefficient is 0.59, which is statistically significant at 99% confidence level. From the variation of spring vegetation, it can be seen that vegetation coverage in northern China increased gradually during period early 1980s to mid 1990s, the maximum coverage having occurred in 1994. Since the mid 1990s, NDVI has decreased, and reaching a minimum in 2001. The year 2000 and 2001 experienced the sparsest vegetation cover during the past two decades. The decrease of vegetation cover may be due to prolonged periods of drought in late 1990s and early 2000s. In 1997, serious drought hit northern China resulting in huge economic damage, and some sections of the Yellow River had dried up for 226 days. Successive severe droughts devastated these regions again in 1999, 2000 and 2001 [Li et al., 2002]. The area hit by dust storms has decreased during the period from the early 1980s to the mid 1990s. In spring 1997, the smallest domain was affected by dust storms, but an obvious increase can be noticed in 2000 and 2001. Lu et al. [2003] pointed out that 14 large-scale dust storms occurred during March to mid- May in 2000, and 18 dust storms in the same period of 2001 in northern China. The rapid decrease of vegetation cover may be one of the main factors to China s frequent dust events since 1997. [14] The anomalies of NDVI and the number of dust storm days in spring for every five years from 1982 to 2001 are shown in Figure 8. For the period of 1982 1986, NDVI has negative departures over eastern parts of northern China, where positive anomalies of dust storm days are found during the same time span. However, it is worth noting that most parts of northwest China except northern Xinjiang have not shown a clear contrasting relationship between NDVI and dust storms. For the 1987 1991 and 1992 1996 periods, vegetation cover improved in most areas of northern China, where negative anomalies of dust storm days occur in almost the same territories except for a few regions. For the 1997 2001 period, most of the eastern parts of northern China are covered by NDVI and dust storms anomalies of opposite sign, but this relationship does not exist in northwest China. From the above analysis, it can be concluded that spring vegetation and dust storms all reflect obvious multiyear timescale variability. In addition, the vegetation and dust storm relationship seems to be negatively related in the eastern parts of northern China and some regions in northern Xinjiang, which is located in northwest China. It is well known that vast deserts are distributed in northwest China such as the Taklimakan Desert, the Badain Jaran Desert, the Tengger Desert (Figure 2), where no vegetation or very sparse vegetation exists. The vegetation condition is comparatively better in the eastern parts of northern China. The negative correlations between NDVI and dust storms mean that when vegetation decreases in those vegetated regions, dust storms become more frequent, and vice versa. [15] Figure 7 reveals that 1997 had the fewest dust storms, while 1983, 1984 and 2001 rank as having the most widespread dust storms in the recent two decades. Figure 9 presents the anomalies of spring NDVI in northern China in 1997, 1983, and 2001. In the spring of 1997, a wide region of positive NDVI anomalies occurred in northern China, while a large area of negative departures of NDVI appeared in the eastern parts of northern China in 1983, and negative departures existed in most areas of northern China in 2001. These results show that the anomalies of vegetation are closely related to the occurrence of dust storms from year to year. [16] To identify the regional differences of the relationship between NDVI and dust storms in spring, the spatial distribution of correlation coefficients is presented in Figure 10. Obviously, negative correlation dominates many areas of northern China. Significant negative correlations are found in central and eastern Inner Mongolia, northern Xinjiang, etc. The vegetation types in the regions with significant correlations are mainly grassland. The result of meaningful negative correlations suggests that in the regions with some extent of vegetation cover, an increase of vegetation cover in these regions can suppress the occurrence of dust storms, while a decrease would induce more frequent dust storms. [17] It is known that the occurrence of dust storms is affected by many factors. The relationship between NDVI and dust storms only reflects the influence of the land surface state on dust storms. Correlation coefficients of dust storm with other parameters, such as precipitation, temper- 6of9

Figure 8. (a) Anomalies of spring mean NDVI for every 5 years from 1982 to 2001 in northern China (shaded areas and solid lines stand for positive anomalies, dotted lines represent negative anomalies). (b) Same as Figure 8a except for dust storm days (shaded areas and solid lines stand for positive anomalies, dotted lines represent negative anomalies). ature and wind speed are presented in Table 1 for five selected stations. Those results indicate that vegetation (the land surface condition) and wind speed (the dynamical factor) in the spring contribute highly to the frequency of dust storms. Frequent dust storms mostly occurred in the years when vegetation was very sparse and stronger winds were present. 4.2. Impact of Early Vegetation on Spring Dust Storms [18] In previous studies, efforts have been made to detect early factors influencing variations of spring dust storms. Qian et al. [2002] proved that the cold air temperatures in the previous winter season were strongly related to spring dust storms. If the soil is heavily frozen by the lower winter temperature, desertification happens more readily after melting in spring. Can the vegetation condition in previous seasons affect the occurrences of spring dust storm? Can NDVI be used for dust storm predictions? To answer these questions, the relationship between spring dust storm and NDVI in the seasons leading up to spring are investigated. Because of the lower values of NDVI in winter ( possibly caused by sparse vegetation and snow coverage), the relationship of the previous winter NDVI and spring dust storms is not very evident (figure not shown). However, opposite correlations are still found in some areas of Inner Mongolia. The NDVI in the prior summer and autumn (figure not shown) has similar negative correlations with spring dust storms in the east part of northern China as spring NDVI. Noticeably, the negative correlations between the prior summer NDVI with spring dust storms are more remarkable. A recent study [Zhai and Li, 2003] showed that the frequency of dust storms in spring in northern China had a good relationship with prior summer precipitation. This is possibly due to the large proportion of summer precipitation to annual totals (Figure 6). And precipitation may be one of the main factors controlling the occurrences of dust storms 7of9

Figure 11. Same as Figure 10 except for correlation between summer NDVI and number of dust storm days of the next spring (minus and plus signs represent negative and positive correlations, respectively, minus with circle signs represent significant negative correlations with values less than 0.44, and the calculation period is 1982 to 2001). Figure 9. Distribution of anomalies of spring mean NDVI in 1997, 1983, and 2001 over northern China (dotted lines represent negative anomalies and solid represent positive anomalies). [Sun et al., 2001; Zhai and Li, 2003]. Zhai and Li [2003] pointed out that the significant negative correlation was especially obvious in the frequent dust storm regions. The reasons are not known. This relationship between previous summer precipitation and spring dust storms may be linked by vegetation. Abundant rainfalls, associated with favorable temperature support lush vegetation in summer when NDVI values generally are at their highest in China. Table 1 gives the correlation coefficients of previous precipitation and NDVI with spring dust storms for five selected stations. The results indicate that prior summer precipitation and NDVI Figure 10. Correlation coefficients between NDVI and number of dust storm days in the same spring over northern China (minus and plus signs represent negative and positive correlations, respectively, minus with circle signs represent significant negative correlations with values less than 0.44, and the calculation period is 1982 to 2001). have a similar relationship with spring dust storms in some regions. The correlations of prior summer NDVI with spring dust storms are even closer than those of prior summer precipitation in several stations such as Mandula and Baicheng. The opposite correlation between the previous summer NDVI and spring dust storm are statistically significant at 95% confidence level for Wuwei, Mandula, Xilinhaote and Baicheng stations. This suggests that good vegetation in prior seasons may act to reduce the next spring dust storms. Figure 11 exhibits the spatial distribution of correlation coefficients between summer NDVI and number of dust storm days of the next spring. Obviously, negative correlations cover most parts of northern China, and the significant correlated regions include western parts of northeast China, central and eastern Inner Mongolia, and northern Xinjiang etc. Obviously, better growing vegetation in summer will help reduce occurrence of dust storms in the coming spring in areas with some certain vegetation in northern China. 5. Conclusions [19] 1. Poor vegetation coverage in northern China is one of the important factors for the frequent occurrence of dust storms in spring. In the western part of northern China, where vegetation coverage shows no important improvement from winter to spring, the sparse vegetation, associated with rising temperature, strong winds, and dry climate result in the frequent dust storms in spring in northern China. [20] 2. In general, a negative correlation between vegetation coverage and occurrence areas of dust storms exists in northern China in spring during the period 1982 2001. The correlation coefficient between vegetation coverage and areas affected by dust storms is 0.59, which is statistically meaningful at 99% confidence level. From the late 1990s, especially in 2000 and 2001, vegetation cover was reduced sharply, which seems to be one of the important causes for frequent dust storms in northern China during the same period. [21] 3. There is significant correlation between the number of dust storm days and NDVI in spring (e.g., central and east Inner Mongolia) in their interannual variations. An increase of vegetation cover can suppress the occurrence of dust storms, while vegetation deterioration will induce 8of9

frequent dust storms. However, in desert areas without vegetation or with very sparse vegetation, the variation of dust storms seems not related to the vegetation. [22] 4. A negative correlation between NDVI and the occurrence of dust storms not only occurs during the same spring, the previous summer vegetation condition can also affect the occurrence of dust storms in the coming spring. Regionally, significant negative correlations of prior summer NDVI and spring dust storm are found in central and eastern Inner Mongolia and northern Xinjiang, etc. [23] Acknowledgments. This study was supported by projects G1999043405 and 2001BA611B-01. We thank the anonymous reviewers for their constructive comments and Susan Bates for her help in modifying English. References Barber, V. A., G. P. Juday, and B. P. Finney (2000), Reduced growth of Alaska white spruce in the twentieth century from temperature-induced drought stress, Nature, 405, 668 672. China Meteorological Administration (1979), Practices for Surface Meteorological Observations (in Chinese), 186 pp., China Meteorol. Press, Beijing. Domrös, M., and G. Peng (1988), The Climate of China, pp. 360, Springer- Verlag, New York. Fang, Z. Y., et al. (1997), Study of Dust-Storms in China (in Chinese), 158 pp., China Meteorol. Press, Beijing. Gong, D. Y., P. J. Shi, and X. Z. He (2002), Spatial features of the coupling between spring NDVI and temperature over the Northern Hemisphere (in Chinese), Acta Geogr. Sin., 57(5), 505 514. Gong, D. Y., and C. H. Ho (2003), Detection of large-scale climate signals in spring vegetation index (NDVI) over the Northern Hemisphere, J. Geophys. Res., 108(D16), 4498, doi:10.1029/2002jd002300. Groisman, P. Y., T. R. Karl, and R. W. Knight (1994), Observed impact of snow cover on the heat balance and the rise of continent spring temperature, Science, 263, 198 200. Gu, W., X. P. Cai, F. Xie, Z. J. Li, and X. H. Wu (2002), Study on relationship between vegetation cover and distribution of days of sandstorm Taking central and western Inner Mongolia for example (in Chinese), Adv. Earth Sci., 17(2), 273 277. Justice, C. O., J. R. G. Townshend, B. N. Holben, and C. J. Tucker (1985), Analysis of the phenology of global vegetation using meteorological satellite data, Int. J. Remote Sens., 6, 1271 1318. Konstantin, Y. V., A. Robock, R. J. Stouffer, J. E. Walsh, D. A. Robinson, D. Garrett, and V. Zakharov (1999), Detection of global warming using observed Northern Hemisphere snow cover and sea ice areas, paper presented at The Tenth Symposium on Global Change Studies, Am. Meteorol. Soc., Dallas, Tex. Li, Q. X., X. N. Liu, and X. Q. Li (2002), Drought trend in north China in recent half century, J. Nat. Disasters, 11(3), 50 56. Lu, J. T., X. K. Zou, J. G. Wang, and G. Y. Ren (2003), Analyses of the causes for frequent dust weather that occurred in China during the last three years (in Chinese), Clim. Environ. Res., 8(1), 107 113. Qian, W. H., and Y. F. Zhu (2001), Climate change in China from 1880 to 1998 and its impact on the environmental condition, Clim. Change, 50, 419 444. Qian, W. H., L. S. Quan, and S. Y. Shi (2002), Variations of dust storm in China and its climatic control, J. Clim., 15, 1216 1229. Sun, J. M., M. Y. Zhang, and T. S. Liu (2001), Spatial and temporal characteristics of dust storms in China and its surrounding regions, 1960 1999: Relations to source area and climate, J. Geophys. Res., 106(D10), 10,325 10,333. Tucker, C. J., and P. J. Sellers (1986), Satellite remote sensing of primary production, Int. J. Remote Sens., 7, 1395 1416. Tucker, C. J., D. A. Slayback, J. E. Pinzon, S. O. Los, R. B. Myneni, and M. G. Taylor (2001), Higher northern latitude NDVI and growing season trends from 1982 1999, Int. J. Biometeorol., 45, 184 190. Wang, S. W., D. Y. Gong, and P. M. Zhai (2002), Climate change in western China, in Environmental Evolvement in Western China (in Chinese), edited by D. H. Qin, pp. 31 80, China Sci. Press, Beijing. Zhai, P. M., and X. Y. Li (2003), On climate background of duststorms over northern China (in Chinese), Acta Geogr. Sin., 58, suppl., 125 131. Zhou, L. M., et al. (2001), Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981 to 1999, J. Geophys. Res., 106(D17), 20,069 20,083. Zhou, L. M., R. K. Kaufmann, Y. Tian, R. B. Myneni, and C. J. Tucker (2003), Relation between interannual variations in satellite measures of northern forest greenness and climate between 1982 and 1999, J. Geophys. Res., 108(D1), 4004, doi:10.1029/2002jd002510. P. M. Zhai and X. K. Zou, Laboratory for Climate Studies, National Climate Center, CMA, 100081 Beijing, China. (zxk@cma.gov.cn) 9of9