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Why does precipitation in northwest China show a significant increasing trend from 1960 to 2010?
Atmospheric Research 167 (2016) 275–284
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Why does precipitation in northwest China show a significant increasing trend from 1960 to 2010? Baofu Li a,b,⁎, Yaning Chen b, Zhongsheng Chen c, Heigang Xiong d, Lishu Lian a a
College of Geography and Tourism, Qufu Normal University, Rizhao 276826, China State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China Key Laboratory of Geographic Information Science, Ministry of Education of the People’s Republic of China, East China Normal University, Shanghai 200241, China d College of Applied Arts and Science of Beijing Union University, Beijing 100083, China b c
a r t i c l e
i n f o
Article history: Received 29 October 2014 Received in revised form 4 August 2015 Accepted 25 August 2015 Available online 2 September 2015 Editor: J.L. Sanchez Keywords: Precipitation Atmospheric circulations Northwestern China
a b s t r a c t Based on monthly precipitation data from 74 weather stations in the arid region of northwest China, we employed statistical methods to analyse the characteristics of precipitation and investigated the relationships between precipitation and 11 atmospheric circulations. The results showed that the precipitation in northwest China had a significantly increasing trend (P b 0.01), at a rate of 0.61 mm/year, which is higher than the average rate of China (−0.16 mm/year) for the same period. Annual precipitation increased markedly after 1987, but the increase in precipitation gradually declined from north to south and from west to east. We found that the precipitation variation in spring, summer, autumn, and winter plays an important role in the yearly change, accounting for 21.6%, 42.4%, 18.4%, and 17.6%, respectively. The correlation analysis indicated that the annual precipitation revealed strong and significant associations with the West Pacific Subtropical High (WPSH, R = 0.60, P b 0.001) and the North America Subtropical High (NASH, R = 0.57, P b 0.001) from 1960 to 2010. We therefore suggest that the strengthening of the WPSH and NASH after the mid-1980s is probably the main cause for the significant increasing trend of precipitation in northwest China. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Global climate change has become a generally accepted fact in recent years, as many scientists and government institutions attempt to address this arising environmental challenge (Meehl et al., 2005; Tian et al., 2011; Pingale et al., 2014). However, some limitations exist for analyses and simulations of climate change at a global scale (Pierce et al., 2009). Firstly, the use of different simulation methods may lead to different research results in the same study areas, which is especially true in the case of precipitation (Mariotti et al., 2011; Sun et al., 2010; Tebaldi et al., 2005). Secondly, global-scale simulation results cannot be fully applied to regional research, nor do they facilitate accurate and comprehensive formulation of regional counter measure strategies. Recently, greater attention has been given to research on regional climate change (Sanchez et al., 2011; Pingale et al., 2014). As a particular case, precipitation in the arid region of northwest China (Fig. 1) has shown a significant increasing trend in the last 50 years, which is considerably different from that of other areas that exhibit slightly rising trends or even downward trends in China for the same period (Sun
⁎ Corresponding author at: College of Geography and Tourism, Qufu Normal University, No. 80 Yantai Road, Rizhao 276826, China. E-mail address: [email protected] (B. Li).
http://dx.doi.org/10.1016/j.atmosres.2015.08.017 0169-8095/© 2015 Elsevier B.V. All rights reserved.
and Yin, 2010). In this study, we intended to identify the possible cause for this regional precipitation change. Chen et al. (2005, 2009) showed that all parts of Xinjiang had experienced an increase in precipitation since the mid-1980s. Li et al. (2011) found that precipitation in Xinjiang has increased by 26 mm, at a rate of 4.12 mm/decade for the last 50 years. Li et al. (2013) noted that precipitation has increased at a rate of 10.15 mm/10 a, 6.29 mm/10 a and 0.87 mm/10 a in the mountain, oasis, and desert areas of northwest China, respectively. Wang et al. (2013) and Deng et al. (2014) revealed that substantial spatial variations in temperature and precipitation extremes were observed in different regions of northwest China and that precipitation exhibited a significant increasing trend that is especially marked in northern Xinjiang. Many scholars have analysed precipitation change trends in the northwest China; however, this topic requires further investigation to elucidate reasons for the significant rising trend in precipitation from 1960 to 2010 in northwest China. Zhang et al. (2002) demonstrated the substantial influence of the East-Asian monsoon on drought formation in northwest China. Li et al. (2012) noted that the weakening of the Siberian High during the 1980s to the 1990s was an important reason for the higher rate of temperature increase in northwest China. Chen et al. (2014) analysed the relationships between climate extremes in northwest China and 8 atmospheric circulations selected at random and showed that Index B of the Tibetan Plateau (TPI_B) was probably an important factor in the
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Fig. 1. Map of the location and distribution of the meteorological stations in the study area.
abrupt change in annual temperature and precipitation extremes. However, the study does have some limitations. The influences of other atmospheric circulations on precipitation over the northwest region cannot be considered based on the region’s climatology. Additionally, they only explore the impact of atmospheric circulation on climate change on an interannual timescale, but seasonal climate change and its influence were ignored. Considering the complexity of the climate system, we also infer that the dynamic change in mean precipitation in northwest China might be related to the regional atmospheric circulation. Yang (2005) noted that water vapour in summer has markedly increased in the western region of northwest China over the past 20 years. Dai et al. (2006) found that water vapour in Xinjiang is mainly derived from the lakes or oceans in the western region of Xinjiang, and in July, it was mainly from the north Atlantic and Arctic Ocean. Zeng (1993) illustrated that very little water vapour in Xinjiang was derived from the Arctic Ocean and water vapour from eastern Asia can reach the Hexi Corridor as well as southern Xinjiang. Huang et al. (2015) studied the physical mechanisms of precipitation variations during summer in the Tarim Basin of northwest China and revealed that summer precipitation variations in southern Xinjiang are affected by the water vapour fluxes from the south and east, albeit the long-term mean water vapour was mostly from the west. Feng et al. (2004) concluded that the interannual changes of summer water vapour in northwest China mainly resulted from westerly wind field convergence or divergence. These studies have helped us understand where water vapour originates from in this arid region. Furthermore, we need to analyse the influence of water vapour sources on precipitation from the perspective of atmospheric dynamics. Water vapour in the arid region of northwest China may originate from the western Atlantic, northern Eurasia, the Arctic Ocean, the eastern Pacific Ocean, and the southern Indian Ocean. Therefore, considering water vapour sources, we attempt to explore the relationships between precipitation and atmospheric circulation in different directions, including the Western Pacific Subtropical High (WPSH) and El Niño-Southern Oscillation (ENSO) in the east; the North America Subtropical High (NASH), the Atlantic Subtropical High (ASH) and the Pacific North America Pattern (PNAP) in the west; the South China Sea
Subtropical High(SCSSH), the India Subtropical High Area (ISHA), and TPI_B in the south; and the Siberian High Index (SHI), Eurasian Meridional Circulation (EMC), and Eurasian Zonal Circulation (EZC) in the north. The intensity index of the Western Pacific Subtropical High (WPSH) is defined as (Zhao, 1999) the encoding sum of the mean height value from 588 dagpm (the encoding of 588 dagpm, 589 dagpm, and 590 dagpm represents 1, 2, and 3, respectively, and so on) in the Western Pacific on monthly mean circulation maps at a height of 500 hPa along 10°N and 110°E–180°E. Similarly, according to the above definition, we computed these intensity indexes as follows: NASH (110°W– 60°W), ASH (55°W–25°W), SCSSH (100°E–120°E), and ISHA (65°E– 95°E) (Zhao, 1999). ENSO is featured by large-scale sea surface temperature (SST) anomalies in the eastern equatorial Pacific Ocean. The most direct effect of ENSO is an interaction in surface pressure, related to a modulation of trade winds and a shift of tropical Pacific precipitation (Bellenger et al., 2014). The Pacific-North American (PNA) pattern is the leading mode of atmospheric circulation over North America, which strongly affects interannual climatic variations (Barnston and Livezey, 1987). Index B of the Tibetan Plateau (TPI_B) is defined as (Zhao, 1999) the accumulative value of the 500 hPa height value minus 500 dagpm, ranging from 30°N to 40°N and 75°E to 105°E. It roughly reflects the activities of low vortex and high pressure at 500 hPa over the Tibetan Plateau (Wang and Wu, 1997). As the source of both cool and warm air, the Tibetan Plateau exerts some influence on its surrounding and global climate in general. The Siberian High is a cold or very cold dry air mass formed in the Mongolian–Siberian region. It has a huge impact on the weather patterns in most of the Northern Hemisphere (Gong and Ho, 2002). Eurasian meridional (zonal) circulation is defined by mean meridional (zonal) air transportation in Eurasia per unit time at the 500 hPa height from 45°–65°N to 0°–150°E (Zhao, 1999). In this study, we detected the mean precipitation change in northwest China over the past 50 years (1960–2010) and investigated the statistical relationships between the precipitation changes and eleven modes of atmospheric circulations. This study is conducted to suggest the possible direct factors for the mean precipitation change in the arid region of northwest China.
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2. Data and methods
and uk as the normalized variable statistic is given in the following formula:
2.1. Data
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uk ¼ t k −t k = vanðt k Þ
In this study, the monthly and annual precipitations from 74 meteorological stations (Fig. 1) were utilized to investigate the precipitation variation for the period of 1960–2010 in northwest China. All of the meteorological stations selected for this study were maintained following the standards of the National Meteorological Administration of China (NMAC). The standard requires strict quality control processes including extreme inspection, consistency checks, and others before releasing these data. Meanwhile, we performed data quality control and a homogeneity assessment using the RClimDex software package (available at the ETCCDI website, http://etccdi.pacificclimate.org/software.shtml) (You et al., 2011). The monthly circulation indices data (WPSH, NASH, TPI_B, ASH, ISHA, SCSSH, AMC, EZC and EMC) were taken from China’s National Climate Centre (http://cmdp.ncc.cma.gov.cn/cn/download. htm). The SHI data were obtained from Li et al. (2012). The ENSO data were obtained from http://research.jisao.washington.edu/data_sets/ globalsstenso/. 2.2. Methods The Mann–Kendall non-parametric statistical test (Mann, 1945; Kendall, 1975) was applied to detect trends in the precipitation time series. For a time series X = {x1, x2, … , xn}, when n N 10, the standard normal statistic Z is estimated as follows: 8 pffiffiffiffiffiffiffiffiffiffiffiffiffiffi < ðS−1Þ= varðSÞ S N 0 Z¼ 0pffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0 : ðS þ 1Þ= varðSÞ S N 0
n −1 X
n X
We used the mean value from the 74 meteorological stations as the representative value of northwest China. Meanwhile, the annual and monthly mean precipitation was calculated in the study area for the period of 1960–2010. The monthly values were used to further compute the seasonal mean precipitation and atmospheric circulation values. In this study, we defined spring as March–May, summer as June–August, autumn as September–November, and winter as December–February. We employed Pearson’s correlation coefficients to detect the relationship between the mean precipitation and the atmospheric circulation indices. To determine if a certain season plays a stronger role in terms of the yearly change, we calculated the importance of the precipitation change for each individual season in the yearly change. Moreover, we used the mean precipitation for the period of 1961–1990 as the benchmark to measure the change. Previous studies (Chen et al., 2009; Li et al., 2013) and this research showed that the step change point in mean precipitation occurred in 1987; after this date, precipitation increased markedly. For this reason, we calculated the precipitation change during 1987–2010 based on the benchmark value. The formula can be represented as follows: P s;1987−2010 −P s;1961−1990 Is ¼ X 100% P s;1987−2010 −P s;1961−1990
ð9Þ
s∈ðspring;summer;autumn;w inter Þ
ð1Þ
where S¼
ð8Þ
sgn x j −xi
ð2Þ
where Is is the importance of each season’s precipitation variation on the overall precipitation variation for the same period; Ps,1961–1990 and Ps,1987–2010 are the mean precipitation of the each season of year during 1961–1990 and 1987–2010, respectively. 3. Results
i¼1 j¼iþ1
8 < þ1; sgnðθÞ ¼ 0; : −1;
3.1. Precipitation characteristics θN0 θ¼0 θb0
ð3Þ
i. h varðSÞ ¼ nðn−1Þð2n þ 5Þ− Σ t ðt−1Þð2t−5Þ 18 t
ð4Þ
where t is the extent of any given time, and Σ denotes the summation of t
all ties. Furthermore, the non-parametric Mann–Kendall–Sneyers test (Mann, 1945; Kendall, 1975; Sneyers, 1975) was employed in this study for determining the occurrence of step change points of precipitation. x1,…, xn represent the data points. The number mi of elements preceding xj (j b i) such that xj b xi is computed for each element xi. Under the null hypothesis (no step change point), the normally distributed statistic tk can be described as follows:
tk ¼
k X
mi ð2 ≤ k ≤ nÞ
ð5Þ
i¼1
where tk as the mean and variance of the normally distributed statistic can be calculated as follows: t k ¼ Eðt k Þ ¼ kðk−1Þ=4
ð6Þ
varðt k Þ ¼ kðk−1Þð2k þ 5Þ=72
ð7Þ
3.1.1. Annual precipitation Mean annual precipitation is 153 mm in northwest China for the period of 1960–2010 (Fig. 2), which belongs to the typical temperate continental climate. Mean annual precipitation is less than 100 mm at 41.9% of weather stations, approximately between 100 and 200 at 31.1% of weather stations, and more than 200 mm at only 27% of weather stations, which are mainly distributed in the Tianshan, Altai and Qilian Mountains. Spatially, the mean precipitation in northern Xinjiang (the north of Tianshan Mountains) during the period of 1960–2010 was the highest with 219.7 mm, followed by Hexi Corridor (the north of Qilian Mountains) with 171.1 mm, and the lowest was southern Xinjiang (the south of Tianshan Mountains) with 90.9 mm. The northwest arid areas are characterized by landforms of the mountain-basin interphase, and most of the mountains run from east to west, which affects water meridional vapour transport. For example, as a consequence of the Tianshan Mountains blocking water vapour in northern Xinjiang, reaching southern Xinjiang is difficult. Similarly, because of the Himalayas, water vapour from the Indian Ocean is has difficulty reaching southern and northern Xinjiang. However, water vapour transports relatively smoothly in the east–west direction. For example, west-wind drift has a huge influence on precipitation in northern Xinjiang and water vapour in the eastern summer monsoon region can also reach the Hexi Corridor. Northern Xinjiang is located in the westerlies of the Northern Hemisphere. Strong western winds blow water vapour in the Atlantic to the inland and exerts a significant influence on precipitation in the Tianshan region due to few tall mountains in Western, Central and
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Fig. 2. Spatial distribution of mean annual precipitation for a 51-year period in the arid region of northwest China.
Eastern Europe, and Central Asia. The Tianshan Mountains result in orographic rain forming to the north of the mountains, causing more precipitation here compared to southern Xinjiang. Thus, orographic rain in northern Xinjiang and Hexi Corridor is prevalent in summer. 3.1.2. Seasonal precipitation In spring, the mean precipitation is 32.8 mm in northwest China for the period of 1960–2010 (Fig. 3). Mean precipitation in spring is less than 15 mm at 33.7% of weather stations, between 15 and 40 mm at 37.8% weather stations, and more than 40 mm at only 28.4% weather stations, mainly distributed in the Tianshan and Qilian Mountains. Spatially, mean precipitation in northern Xinjiang is the highest with 45.4 mm, followed by Hexi Corridor with 30.2 mm, and the lowest is southern Xinjiang with 20.8 mm. In summer, mean precipitation can reach 73.9 mm in northwest China for the period 1960–2010. Mean summer precipitation of less than 30 mm was observed at 21.6% of weather stations, between 30 and 80 mm was calculated at 50% of weather stations, and only 28.4% of weather stations, mainly distributed in the Tianshan and Qilian Mountains, had mean precipitation levels greater than 80 mm. Spatially, mean precipitation in Hexi Corridor is the highest with 103.1 mm, followed by northern Xinjiang with 69.3 mm and southern Xinjiang with 51.1 mm. In autumn, mean precipitation can reach 29.5 mm in northwest China for the period of 1960–2010 (Fig. 3). Mean precipitation in autumn is less than 15 mm at 40.5% of weather stations, between 15 and 50 mm at 41.9% weather stations, and more than 50 mm at only 17.6% of weather stations, which are mainly located in the Tianshan and Qilian Mountains. Mean precipitation in northern Xinjiang is the highest with 39.4 mm, followed by Hexi Corridor with 34.1 mm, and the lowest is in southern Xinjiang with 14.0 mm. In winter, mean precipitation in northwest China is only 10.3 mm for the period 1960–2010. Mean precipitation in winter is less than 4 mm at 33.8% of weather stations, between 4 and 15 mm at 47.3% of weather stations, and more than 15 mm at only 18.9% of weather stations, mainly in the Tianshan and Altai Mountains. Mean precipitation in northern Xinjiang is the highest with 20.2 mm, followed by southern Xinjiang with 5.0 mm, and Hexi Corridor is the lowest with 3.7 mm (Fig. 3). The above results show that only summer precipitation in Hexi Corridor is the highest, associated with the water vapour transport of the eastern Asian monsoon. Meanwhile, in the other seasons, northern Xinjiang precipitation is the highest as a result of water vapour transport of western wind circulation.
3.2. Precipitation changes 3.2.1. Annual precipitation changes The Mann–Kendall statistical test revealed a significant (P b 0.01) increasing trend in annual precipitation in northwest China during 1960– 2010, with a rate of 0.61 mm/year (Fig. 4), which is higher than the average rate of China (−0.16 mm/year) for the same period (Sun and Yin, 2010). These values are consistent with previous studies (Li et al., 2013). The Mann–Kendall–Sneyers test showed that step change points in mean precipitation in northwest China occurred in 1987 (P b 0.01), which indicates that mean precipitation for the period 1960–1986 was significantly different than that of 1987–2010. The results are consistent with the findings of previous studies (Chen and Xu, 2005; Li et al., 2013; Deng et al., 2014). Fig. 4 also shows that precipitation has experienced significant variability but maintained relatively high mean values during the period 1987–2010. In addition, precipitation showed slightly decreasing trends in the last10 years (1998–2010), which should arouse our attention. In this study, we divided the 51-year period into two periods, namely, 1961–1990 and 1987–2010, according to the results of the step change analysis. Fig. 5 shows the spatial distribution of mean annual precipitation changes during 1987–2010 in comparison with that of 1961–1990. After 1987, precipitation increased by 10–30 mm at 47% of meteorological stations, less than 10 mm at 29% of stations and more than 30 mm at 24% of stations. The increasing amount of mean annual precipitation is 19.7 mm in northwest China. Regionally, the mean annual precipitation in northern Xinjiang for the period of 1987–2010 increased by 91 mm. Compared with that of 1961–1990, however, precipitation levels in southern Xinjiang and Hexi Corridor are only 20 mm and 8 mm, respectively. 3.2.2. Seasonal precipitation changes In spring, mean precipitation increased by 4.3 mm during 1987– 2010 in the arid region of northwest China compared with that of 1961–1990. Precipitation increased by 0–5 mm at 47% of meteorological stations and 5–10 mm at 32% of meteorological stations; however, at 6 meteorological stations, precipitation decreased by 0–5.9 mm (Fig. 6). In summer, an increase in mean precipitation by 8.4 mm occurred after 1987 in northwest China. Precipitation decreased by 0–13.6 mm at 21% of stations, most of which are located in the Hexi Corridor. Nevertheless, precipitation increased by 0–10 mm at 40% of stations and by 10–30 mm at 32% of stations. In autumn, precipitation increased by 3.6 mm in recent years. Precipitation increased by 0–5 mm at 54% of stations and by 5–10 mm at
B. Li et al. / Atmospheric Research 167 (2016) 275–284
Fig. 3. Spatial distribution of seasonal mean precipitation for a 51-year period in the arid region of northwest China.
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Fig. 4. Variation of annual precipitation totals in northwest China for the period 1960– 2010, expressed as deviations from the average value for the period 1961–1990.
20% of stations. In addition, 17% of stations showed decreased precipitation of 0–6.3 mm (Fig. 6). In winter, an increase in mean precipitation by 3.5 mm occurred after 1986 in northwest China. Most of the meteorological stations (54%) showed increased precipitation of 0–3 mm, and 17% of stations showed an increase of 3–6 mm. Based on seasonal precipitation change analysis, we calculated the seasonal importance to the yearly change for each year between 1987 and 2010. Over this period, spring, summer, autumn, and winter accounted for 21.6%, 42.4%, 18.4%, and 17.6% of the annual change, respectively. These results indicate that the summer precipitation change is the most important factor in the annual precipitation change compared to the other seasons in northwest China. 3.3. Potential causes of precipitation change The Pearson correlation coefficient values (Table 1) show that the annual precipitation in northwest China has a strong and significant correlation with the annual WPSH (R = 0.60, P b 0.001) and NASH (R = 0.57, P b 0.001) but exhibits a weaker relationship with the other tested atmospheric circulations. Fig. 7 shows that the WPSH and NASH and the annual precipitation in the region had an almost perfect “mirroring” relationship during 1960–2010, suggesting the direct impact of the WPSH and NASH on annual precipitation. In particular, the annual precipitation increased following the marked increase in the WPSH and NASH. Another feature worth mentioning is that the correlations between precipitation and atmospheric circulation are different in various
seasons (Table 1). In spring, the R values for the precipitation with NASH (R = 0.54, P b 0.001), ISHA (R = 0.57, P b 0.001), and ENSO (R = 0.54, P b 0.001) are higher than those with other factors. In summer, the values for precipitation with NASH (R = 0.37, P b 0.01), WPSH (R = 0.36, P b 0.01), and SCSSH (R = 0.35, P b 0.05) are also much stronger and more significant than those with other circulations. In autumn, only the correlation between precipitation and EMC (R = − 0.36, P b 0.01) is significant (Fig. 7). In winter, precipitation has a significant correlation with the WPSH (R = 0.36, P b 0.01), NASH (R = 0.35, P b 0.05), and TPI_B (R = 0.42, P b 0.01). As a consequence, the great influences of the WPSH and NASH are observed in spring, summer, winter and the annual precipitation. To better understand how the other indices might be related to precipitation in northwest China, we constructed a scatter plot of annual precipitation and the various circulations from 1960 to 2010, with linear regressions shown in Fig. 8. Based on these figures, the correlation coefficients between the annual precipitation and ENSO, TPI_B, SCSSH, and EMC are weaker than those between the precipitation and WPSH and NASH. 4. Discussion In recent years, many scholars have paid attention to climate variations and their influencing factors, especially the effects of human activities on climate change (Li et al., 2014). However, the impact of atmospheric circulation and other natural factors on climate change should also be noted (Li et al., 2012). In this study, we detected the spatial and temporal change in precipitation in northwest China and conducted a detailed analysis of the relationships between the precipitation change and atmospheric circulations. Zhang et al. (2002) found that the influence of the East Asian monsoon on the drought formation of Northwest China was significant. This indirectly demonstrates our result that the WPSH has an important influence on the precipitation change in the arid region of northwest China. The latest research by Chen et al. (2014) showed that TPI_B was probably an important factor in the abrupt change in precipitation extremes in northwest China because the correlation coefficient between the precipitation extremes in northwest China and TPI_B was 0.44 (P b 0.01), which is obviously lower than that between the WPSH (0.60, P b 0.001), NASH (0.57, P b 0.001) and annual precipitation in the same area, suggesting that the WPSH and NASH had a stronger influence on the precipitation in northwest China than the TPI_B. Li et al. (2012) found that the weakening of the Siberian High during the 1980s to the 1990s is an important reason for the higher rate of
Fig. 5. Spatial distribution of annual mean precipitation change during 1987–2010 compared with that of 1961–1990 in the arid region of northwest China (black spots mean the precipitation change trend is significant at the P b 0.1 level).
B. Li et al. / Atmospheric Research 167 (2016) 275–284 Fig. 6. Spatial distribution of the mean seasonal precipitation change during 1987–2010 compared with that of 1961–1990 in the arid region of northwest China (black spots mean the precipitation change trend is significant at the P b 0.05 level).
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Table 1 Pearson’s correlation coefficients between mean precipitation and certain factors that may affect the precipitation during 1960–2010. Factors
Spring
Summer
Autumn
Winter
Annual
WPSH NASH TPI_B ASH SCSSH ISHA SHI PNAP ENSO EZC EMC
0.39** 0.54*** 0.31* 0.31* 0.35* 0.57*** 0.19 0.16 0.54*** 0.09 −0.48***
0.36** 0.37** 0.06 0.09 0.35* – 0.25 0.19 0.26 −0.11 0.17
0.23 0.26 0.12 0.17 0.14 −0.1 0.01 0.19 −0.11 −0.01 −0.36**
0.36** 0.35* 0.42** −0.09 0.29 0.28 0.01 0.08 0.13 0.12 −0.21
0.60*** 0.57*** 0.45*** 0.18 0.51*** 0.47*** 0.24 0.51*** 0.39* 0.07 −0.34*
*Significant at P b 0.05; ** Significant at P b 0.01; *** Significant at P b 0.001.
temperature rise in northwest China because the winter temperature in this region has a strong association with the Siberian High (correlation coefficient: R = − 0.715, P b 0.001). However, this study shows that the correlation coefficient between precipitation in northwest China and the Siberian High was not obvious, indicating that the influence of temperature and precipitation changes can be different in the same area. The subtropical high is one of the main atmospheric circulation entities affecting climate change in China, which reflects the atmospheric circulation changes in the middle–low latitudes to a large extent. In general, the stronger NASH is, the weaker the Atlantic mid-ocean ridge is, which is conducive to the prevailing westerly winds. For this reason, water vapour over the Atlantic will be smoothly transported to Eurasia. Because central Asia, including the arid areas of northwest China, is affected by westward flowing wind throughout the year, rainfall increases significantly. Thus, the significant relationship between NASH and precipitation in this study is reasonable. Additionally, the increase of WPSH is conducive to the westward movement of the East Asian summer monsoon and more water vapour transport to the arid areas of northwest China. Precipitation in the arid region is affected by the East Asian summer monsoon, especially in the eastern region. Therefore, the WPSH shows a significant relationship with precipitation.
Furthermore, when the subtropical high in North America and the western Pacific subtropical high were significantly enhanced, it created a large Indo-Burmese groove. Thus, south of the equator, western airflow from the Indian Ocean and from the western Pacific subtropical high formed southwest and southeast flow, respectively. The two airflows together provided a warm, moist airflow for the northwest arid areas, leading to more precipitation (Shi, 2003). In addition, summer is the most important season for the regional precipitation change. Our results indicate that the correlation between summer precipitation and SCSSH is also substantial because the strengthening of the SCSSH enables more water vapour transport from the Tibetan plateau to the Tarim Basin. Moreover, the latest research (Huang et al., 2015) states that the variability of summer precipitation in southern Xinjiang is strongly influenced by the water vapour from the south and east, which is consistent with the results in the study. Based on the above analysis, we hypothesize that the water vapour in the northwest arid region of China mainly originates from the Atlantic, followed by the western Pacific, with a small amount from the Indian Ocean. The indices used are not independent (e.g., ENSO has a dominant control over regions such as where the WPSH is calculated, etc.). Therefore, correlations between precipitation and many indices may be on different timescales. Thus, determining the most significant correlation between precipitation and circulation is an important task. However, the influence mechanism of these indices on northwest precipitation is complex (Shi, 2003) and thus requires further study. 5. Conclusions In this study, we state that the precipitation change in northwest China over the past 50 years might be associated with regional atmospheric circulation, and thus, we investigate the statistical correlation between the precipitation change and 11 modes of atmospheric circulations. The correlation analysis shows that precipitation had a strong and significant association with the WPSH and NASH on different timescales. In particular, annual precipitation exhibited stronger and more significant associations with the WPSH (R = 0.60, P b 0.001) and NASH (R = 0.57, P b 0.001) from 1960 to 2010. Based on this finding,
Fig. 7. The precipitation in northwest China and certain factors from 1960 to 2010.
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Fig. 8. Scatter plot of the annual precipitation and circulations (ENSO, TPI_B, SCSSH, EMC) from 1960 to 2010 and their linear regressions.
we suggest that WPSH and NASH are probably important factors influencing the precipitation change in the arid region of northwest China over the past 50 years. Accordingly, a model aiming to predict the precipitation of the arid region of northwest China should take into account the WPSH and NASH, which should be a major item in future research agenda. Acknowledgment This research is supported by the National Natural Science Foundation of China (41501211), Startup Fund Scientific Research Project of Qufu Normal University (BSQD20130102), Open Found of State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences (2015). We are grateful to Dr. Jose Luis Sánchez, and anonymous reviewers for their helpful comments on improving the manuscript. References Barnston, A.G., Livezey, R.E., 1987. Classification, seasonality and persistence of lowfrequency atmospheric circulation patterns. Mon. Weather Rev. 115 (6), 1083–1126. Bellenger, H., Guilyardi, É., Leloup, J., Lengaigne, M., Vialard, J., 2014. ENSO representation in climate models: from CMIP3 to CMIP5. Clim. Dyn. 42 (7-8), 1999–2018. Chen, Y.N., Xu, Z.X., 2005. Plausible impact of global climate change on water resources in the Tarim River Basin. Sci. China Ser. D Earth Sci. 48, 65–73. Chen, Y.N., Deng, H.J., Li, B.F., Li, Z., Xu, C.C., 2005. Abrupt change of temperature and precipitation extremes in the arid region of Northwest China. Quat. Int. 336, 35–43. Chen, Y.N., Xu, C.C., Yang, Y.H., Hao, X.M., Shen, Y.P., 2009. Hydrology and water resources variation and its responses to regional climate change in Xinjiang. Acta Geograph. Sin. 64 (11), 1331–1341. Chen, Y., Deng, H., Li, B., Li, Z., Xu, C., 2014. Abrupt change of temperature and precipitation extremes in the arid region of Northwest China. Quat. Int. 336, 35–43. Dai, X., Li, W., Ma, Z., 2006. In recent years water vapor source variation characteristics in Xinjiang (in Chinese with English abstract). Prog. Nat. Sci. 16 (12), 1651–1656. Deng, H.J., Chen, Y.N., Shi, X., Li, W.H., 2014. Dynamics of temperature and precipitation extremes and their spatial variation in the arid region of northwest China. Atmos. Res. 138, 346–355. Feng, W., Wang, K., Jiang, H., 2004. Influences of westerly wind interannual change on water vapor transport over Northwest China Summer (in Chinese with English abstract). Plateau Meteorol. 23 (2), 271–275. Gong, D.Y., Ho, C.H., 2002. The Siberian High and climate change over middle to high latitude Asia. Theor. Appl. Climatol. 72 (1-2), 1–9.
Huang, W., Feng, S., Chen, J., Chen, F., 2015. Physical mechanisms of the summer precipitation variations in the Tarim Basin, Northwest China. J. Clim. http://dx.doi.org/10. 1175/JCLI-D-14-00395.1. Kendall, M.G., 1975. Rank Correlation Methods. Griffin, London. Li, Q.H., Chen, Y.N., Shen, Y.J., Li, X.G., Xu, J.H., 2011. Spatial and temporal trends of climate change in Xinjiang, China. J. Geogr. Sci. 21 (6), 1007–1018. Li, B.F., Chen, Y.N., Shi, X., 2012. Why does the temperature rise faster in the arid region of northwest China. J. Geophys. Res. 117, D16115. http://dx.doi.org/10.1029/ 2012JD017953. Li, B.F., Chen, Y.N., Shi, X., Chen, Z.S., Li, W.H., 2013. Temperature and precipitation changes in the diverse environments in the arid region of northwest China. Theor. Appl. Climatol. 112, 589–596. Li, B., Su, H.B., Chen, F., Li, H.B., Zhang, R.H., Tian, J., Chen, S.H., Yang, Y.M., Rong, Y., 2014. Separation of the impact of climate change and human activity on streamflow in the upper and middle reaches of the Taoer River, northeastern China. Theor. Appl. Climatol. 118, 271–283. Mann, H.B., 1945. Nonparametric tests against trend. Econometrica 13, 124–259. Mariotti, L., Coppola, E., Sylla, M.B., Giorgi, F., Piani, C., 2011. Regional climate model simulation of projected 21st century climate change over an all-Africa domain: comparison analysis of nested and driving model results. J. Geophys. Res.-Atmos. 116. http:// dx.doi.org/10.1029/2010JD015068. Meehl, G.A., Washington, W.M., Collins, W.D., Arblaster, J.M., Hu, A.X., Buja, L.E., Strand, W.G., Teng, H.Y., 2005. How much more global warming and sea level rise? Science 307 (5716), 1769–1772. Pierce, D.W., Barnett, T.P., Santer, B.D., Gleckler, P.J., 2009. Selecting global climate models for regional climate change studies. Proc. Natl. Acad. Sci. U. S. A. 106 (21), 8441–8446. Pingale, S.M., Khare, D., Jat, M.K., Adamowski, J., 2014. Spatial and temporal trends of mean and extreme rainfall and temperature for the 33 urban centers of the arid and semi-arid state of Rajasthan, India. Atmos. Res. 138, 73–90. Sanchez, E., Dominguez, M., Romera, R., de la Franca, N.L., Gaertner, M.A., Gallardo, C., Castro, M., 2011. Regional modeling of dry spells over the Iberian Peninsula for present climate and climate change conditions. Clim. Chang. 107 (3-4), 625–634. Shi, Y.F., 2003. An assessment of the issues of climate shift from warm-dry to warm-wet in northwest China. China Meteorological Press, Beijing, pp. 17–25. Sneyers, R., 1975. Sur l'analyse statistique des séries d'observations. O.M.M., Note Technique No 143. Suisse, Gencve. Sun, L., Yin, Y., 2010. Change of precipitation distribution and trends in China from 19602009. The 27th Annual Meeting of China Meteorological Society Proceedings of Atmospheric Physics and Atmospheric Conditions at the Venue, Beijing. Sun, F., Roderick, M.L., Farquhar, G.D., Lim, W.H., 2010. Partitioning the variance between space and time. Geophys. Res. Lett. 37, L12704. http://dx.doi.org/10.1029/ 2010GL043323. Tebaldi, C., Smith, R.L., Nychka, D., Mearns, L.O., 2005. Quantifying uncertainty in projections of regional climate change: A Bayesian approach (vol 18, pg 1524, 2005). J. Clim. 18 (16), 3405. Tian, H., Xu, X., Lu, C., Liu, M., Ren, W., Chen, G., Melillo, J., Liu, J., 2011. Net exchanges of CO2, CH4, and N2O between China's terrestrial ecosystems and the atmosphere and
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Atmospheric Research journal homepage: www.elsevier.com/locate/atmos
Why does precipitation in northwest China show a significant increasing trend from 1960 to 2010? Baofu Li a,b,⁎, Yaning Chen b, Zhongsheng Chen c, Heigang Xiong d, Lishu Lian a a
College of Geography and Tourism, Qufu Normal University, Rizhao 276826, China State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China Key Laboratory of Geographic Information Science, Ministry of Education of the People’s Republic of China, East China Normal University, Shanghai 200241, China d College of Applied Arts and Science of Beijing Union University, Beijing 100083, China b c
a r t i c l e
i n f o
Article history: Received 29 October 2014 Received in revised form 4 August 2015 Accepted 25 August 2015 Available online 2 September 2015 Editor: J.L. Sanchez Keywords: Precipitation Atmospheric circulations Northwestern China
a b s t r a c t Based on monthly precipitation data from 74 weather stations in the arid region of northwest China, we employed statistical methods to analyse the characteristics of precipitation and investigated the relationships between precipitation and 11 atmospheric circulations. The results showed that the precipitation in northwest China had a significantly increasing trend (P b 0.01), at a rate of 0.61 mm/year, which is higher than the average rate of China (−0.16 mm/year) for the same period. Annual precipitation increased markedly after 1987, but the increase in precipitation gradually declined from north to south and from west to east. We found that the precipitation variation in spring, summer, autumn, and winter plays an important role in the yearly change, accounting for 21.6%, 42.4%, 18.4%, and 17.6%, respectively. The correlation analysis indicated that the annual precipitation revealed strong and significant associations with the West Pacific Subtropical High (WPSH, R = 0.60, P b 0.001) and the North America Subtropical High (NASH, R = 0.57, P b 0.001) from 1960 to 2010. We therefore suggest that the strengthening of the WPSH and NASH after the mid-1980s is probably the main cause for the significant increasing trend of precipitation in northwest China. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Global climate change has become a generally accepted fact in recent years, as many scientists and government institutions attempt to address this arising environmental challenge (Meehl et al., 2005; Tian et al., 2011; Pingale et al., 2014). However, some limitations exist for analyses and simulations of climate change at a global scale (Pierce et al., 2009). Firstly, the use of different simulation methods may lead to different research results in the same study areas, which is especially true in the case of precipitation (Mariotti et al., 2011; Sun et al., 2010; Tebaldi et al., 2005). Secondly, global-scale simulation results cannot be fully applied to regional research, nor do they facilitate accurate and comprehensive formulation of regional counter measure strategies. Recently, greater attention has been given to research on regional climate change (Sanchez et al., 2011; Pingale et al., 2014). As a particular case, precipitation in the arid region of northwest China (Fig. 1) has shown a significant increasing trend in the last 50 years, which is considerably different from that of other areas that exhibit slightly rising trends or even downward trends in China for the same period (Sun
⁎ Corresponding author at: College of Geography and Tourism, Qufu Normal University, No. 80 Yantai Road, Rizhao 276826, China. E-mail address: [email protected] (B. Li).
http://dx.doi.org/10.1016/j.atmosres.2015.08.017 0169-8095/© 2015 Elsevier B.V. All rights reserved.
and Yin, 2010). In this study, we intended to identify the possible cause for this regional precipitation change. Chen et al. (2005, 2009) showed that all parts of Xinjiang had experienced an increase in precipitation since the mid-1980s. Li et al. (2011) found that precipitation in Xinjiang has increased by 26 mm, at a rate of 4.12 mm/decade for the last 50 years. Li et al. (2013) noted that precipitation has increased at a rate of 10.15 mm/10 a, 6.29 mm/10 a and 0.87 mm/10 a in the mountain, oasis, and desert areas of northwest China, respectively. Wang et al. (2013) and Deng et al. (2014) revealed that substantial spatial variations in temperature and precipitation extremes were observed in different regions of northwest China and that precipitation exhibited a significant increasing trend that is especially marked in northern Xinjiang. Many scholars have analysed precipitation change trends in the northwest China; however, this topic requires further investigation to elucidate reasons for the significant rising trend in precipitation from 1960 to 2010 in northwest China. Zhang et al. (2002) demonstrated the substantial influence of the East-Asian monsoon on drought formation in northwest China. Li et al. (2012) noted that the weakening of the Siberian High during the 1980s to the 1990s was an important reason for the higher rate of temperature increase in northwest China. Chen et al. (2014) analysed the relationships between climate extremes in northwest China and 8 atmospheric circulations selected at random and showed that Index B of the Tibetan Plateau (TPI_B) was probably an important factor in the
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Fig. 1. Map of the location and distribution of the meteorological stations in the study area.
abrupt change in annual temperature and precipitation extremes. However, the study does have some limitations. The influences of other atmospheric circulations on precipitation over the northwest region cannot be considered based on the region’s climatology. Additionally, they only explore the impact of atmospheric circulation on climate change on an interannual timescale, but seasonal climate change and its influence were ignored. Considering the complexity of the climate system, we also infer that the dynamic change in mean precipitation in northwest China might be related to the regional atmospheric circulation. Yang (2005) noted that water vapour in summer has markedly increased in the western region of northwest China over the past 20 years. Dai et al. (2006) found that water vapour in Xinjiang is mainly derived from the lakes or oceans in the western region of Xinjiang, and in July, it was mainly from the north Atlantic and Arctic Ocean. Zeng (1993) illustrated that very little water vapour in Xinjiang was derived from the Arctic Ocean and water vapour from eastern Asia can reach the Hexi Corridor as well as southern Xinjiang. Huang et al. (2015) studied the physical mechanisms of precipitation variations during summer in the Tarim Basin of northwest China and revealed that summer precipitation variations in southern Xinjiang are affected by the water vapour fluxes from the south and east, albeit the long-term mean water vapour was mostly from the west. Feng et al. (2004) concluded that the interannual changes of summer water vapour in northwest China mainly resulted from westerly wind field convergence or divergence. These studies have helped us understand where water vapour originates from in this arid region. Furthermore, we need to analyse the influence of water vapour sources on precipitation from the perspective of atmospheric dynamics. Water vapour in the arid region of northwest China may originate from the western Atlantic, northern Eurasia, the Arctic Ocean, the eastern Pacific Ocean, and the southern Indian Ocean. Therefore, considering water vapour sources, we attempt to explore the relationships between precipitation and atmospheric circulation in different directions, including the Western Pacific Subtropical High (WPSH) and El Niño-Southern Oscillation (ENSO) in the east; the North America Subtropical High (NASH), the Atlantic Subtropical High (ASH) and the Pacific North America Pattern (PNAP) in the west; the South China Sea
Subtropical High(SCSSH), the India Subtropical High Area (ISHA), and TPI_B in the south; and the Siberian High Index (SHI), Eurasian Meridional Circulation (EMC), and Eurasian Zonal Circulation (EZC) in the north. The intensity index of the Western Pacific Subtropical High (WPSH) is defined as (Zhao, 1999) the encoding sum of the mean height value from 588 dagpm (the encoding of 588 dagpm, 589 dagpm, and 590 dagpm represents 1, 2, and 3, respectively, and so on) in the Western Pacific on monthly mean circulation maps at a height of 500 hPa along 10°N and 110°E–180°E. Similarly, according to the above definition, we computed these intensity indexes as follows: NASH (110°W– 60°W), ASH (55°W–25°W), SCSSH (100°E–120°E), and ISHA (65°E– 95°E) (Zhao, 1999). ENSO is featured by large-scale sea surface temperature (SST) anomalies in the eastern equatorial Pacific Ocean. The most direct effect of ENSO is an interaction in surface pressure, related to a modulation of trade winds and a shift of tropical Pacific precipitation (Bellenger et al., 2014). The Pacific-North American (PNA) pattern is the leading mode of atmospheric circulation over North America, which strongly affects interannual climatic variations (Barnston and Livezey, 1987). Index B of the Tibetan Plateau (TPI_B) is defined as (Zhao, 1999) the accumulative value of the 500 hPa height value minus 500 dagpm, ranging from 30°N to 40°N and 75°E to 105°E. It roughly reflects the activities of low vortex and high pressure at 500 hPa over the Tibetan Plateau (Wang and Wu, 1997). As the source of both cool and warm air, the Tibetan Plateau exerts some influence on its surrounding and global climate in general. The Siberian High is a cold or very cold dry air mass formed in the Mongolian–Siberian region. It has a huge impact on the weather patterns in most of the Northern Hemisphere (Gong and Ho, 2002). Eurasian meridional (zonal) circulation is defined by mean meridional (zonal) air transportation in Eurasia per unit time at the 500 hPa height from 45°–65°N to 0°–150°E (Zhao, 1999). In this study, we detected the mean precipitation change in northwest China over the past 50 years (1960–2010) and investigated the statistical relationships between the precipitation changes and eleven modes of atmospheric circulations. This study is conducted to suggest the possible direct factors for the mean precipitation change in the arid region of northwest China.
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2. Data and methods
and uk as the normalized variable statistic is given in the following formula:
2.1. Data
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uk ¼ t k −t k = vanðt k Þ
In this study, the monthly and annual precipitations from 74 meteorological stations (Fig. 1) were utilized to investigate the precipitation variation for the period of 1960–2010 in northwest China. All of the meteorological stations selected for this study were maintained following the standards of the National Meteorological Administration of China (NMAC). The standard requires strict quality control processes including extreme inspection, consistency checks, and others before releasing these data. Meanwhile, we performed data quality control and a homogeneity assessment using the RClimDex software package (available at the ETCCDI website, http://etccdi.pacificclimate.org/software.shtml) (You et al., 2011). The monthly circulation indices data (WPSH, NASH, TPI_B, ASH, ISHA, SCSSH, AMC, EZC and EMC) were taken from China’s National Climate Centre (http://cmdp.ncc.cma.gov.cn/cn/download. htm). The SHI data were obtained from Li et al. (2012). The ENSO data were obtained from http://research.jisao.washington.edu/data_sets/ globalsstenso/. 2.2. Methods The Mann–Kendall non-parametric statistical test (Mann, 1945; Kendall, 1975) was applied to detect trends in the precipitation time series. For a time series X = {x1, x2, … , xn}, when n N 10, the standard normal statistic Z is estimated as follows: 8 pffiffiffiffiffiffiffiffiffiffiffiffiffiffi < ðS−1Þ= varðSÞ S N 0 Z¼ 0pffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0 : ðS þ 1Þ= varðSÞ S N 0
n −1 X
n X
We used the mean value from the 74 meteorological stations as the representative value of northwest China. Meanwhile, the annual and monthly mean precipitation was calculated in the study area for the period of 1960–2010. The monthly values were used to further compute the seasonal mean precipitation and atmospheric circulation values. In this study, we defined spring as March–May, summer as June–August, autumn as September–November, and winter as December–February. We employed Pearson’s correlation coefficients to detect the relationship between the mean precipitation and the atmospheric circulation indices. To determine if a certain season plays a stronger role in terms of the yearly change, we calculated the importance of the precipitation change for each individual season in the yearly change. Moreover, we used the mean precipitation for the period of 1961–1990 as the benchmark to measure the change. Previous studies (Chen et al., 2009; Li et al., 2013) and this research showed that the step change point in mean precipitation occurred in 1987; after this date, precipitation increased markedly. For this reason, we calculated the precipitation change during 1987–2010 based on the benchmark value. The formula can be represented as follows: P s;1987−2010 −P s;1961−1990 Is ¼ X 100% P s;1987−2010 −P s;1961−1990
ð9Þ
s∈ðspring;summer;autumn;w inter Þ
ð1Þ
where S¼
ð8Þ
sgn x j −xi
ð2Þ
where Is is the importance of each season’s precipitation variation on the overall precipitation variation for the same period; Ps,1961–1990 and Ps,1987–2010 are the mean precipitation of the each season of year during 1961–1990 and 1987–2010, respectively. 3. Results
i¼1 j¼iþ1
8 < þ1; sgnðθÞ ¼ 0; : −1;
3.1. Precipitation characteristics θN0 θ¼0 θb0
ð3Þ
i. h varðSÞ ¼ nðn−1Þð2n þ 5Þ− Σ t ðt−1Þð2t−5Þ 18 t
ð4Þ
where t is the extent of any given time, and Σ denotes the summation of t
all ties. Furthermore, the non-parametric Mann–Kendall–Sneyers test (Mann, 1945; Kendall, 1975; Sneyers, 1975) was employed in this study for determining the occurrence of step change points of precipitation. x1,…, xn represent the data points. The number mi of elements preceding xj (j b i) such that xj b xi is computed for each element xi. Under the null hypothesis (no step change point), the normally distributed statistic tk can be described as follows:
tk ¼
k X
mi ð2 ≤ k ≤ nÞ
ð5Þ
i¼1
where tk as the mean and variance of the normally distributed statistic can be calculated as follows: t k ¼ Eðt k Þ ¼ kðk−1Þ=4
ð6Þ
varðt k Þ ¼ kðk−1Þð2k þ 5Þ=72
ð7Þ
3.1.1. Annual precipitation Mean annual precipitation is 153 mm in northwest China for the period of 1960–2010 (Fig. 2), which belongs to the typical temperate continental climate. Mean annual precipitation is less than 100 mm at 41.9% of weather stations, approximately between 100 and 200 at 31.1% of weather stations, and more than 200 mm at only 27% of weather stations, which are mainly distributed in the Tianshan, Altai and Qilian Mountains. Spatially, the mean precipitation in northern Xinjiang (the north of Tianshan Mountains) during the period of 1960–2010 was the highest with 219.7 mm, followed by Hexi Corridor (the north of Qilian Mountains) with 171.1 mm, and the lowest was southern Xinjiang (the south of Tianshan Mountains) with 90.9 mm. The northwest arid areas are characterized by landforms of the mountain-basin interphase, and most of the mountains run from east to west, which affects water meridional vapour transport. For example, as a consequence of the Tianshan Mountains blocking water vapour in northern Xinjiang, reaching southern Xinjiang is difficult. Similarly, because of the Himalayas, water vapour from the Indian Ocean is has difficulty reaching southern and northern Xinjiang. However, water vapour transports relatively smoothly in the east–west direction. For example, west-wind drift has a huge influence on precipitation in northern Xinjiang and water vapour in the eastern summer monsoon region can also reach the Hexi Corridor. Northern Xinjiang is located in the westerlies of the Northern Hemisphere. Strong western winds blow water vapour in the Atlantic to the inland and exerts a significant influence on precipitation in the Tianshan region due to few tall mountains in Western, Central and
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Fig. 2. Spatial distribution of mean annual precipitation for a 51-year period in the arid region of northwest China.
Eastern Europe, and Central Asia. The Tianshan Mountains result in orographic rain forming to the north of the mountains, causing more precipitation here compared to southern Xinjiang. Thus, orographic rain in northern Xinjiang and Hexi Corridor is prevalent in summer. 3.1.2. Seasonal precipitation In spring, the mean precipitation is 32.8 mm in northwest China for the period of 1960–2010 (Fig. 3). Mean precipitation in spring is less than 15 mm at 33.7% of weather stations, between 15 and 40 mm at 37.8% weather stations, and more than 40 mm at only 28.4% weather stations, mainly distributed in the Tianshan and Qilian Mountains. Spatially, mean precipitation in northern Xinjiang is the highest with 45.4 mm, followed by Hexi Corridor with 30.2 mm, and the lowest is southern Xinjiang with 20.8 mm. In summer, mean precipitation can reach 73.9 mm in northwest China for the period 1960–2010. Mean summer precipitation of less than 30 mm was observed at 21.6% of weather stations, between 30 and 80 mm was calculated at 50% of weather stations, and only 28.4% of weather stations, mainly distributed in the Tianshan and Qilian Mountains, had mean precipitation levels greater than 80 mm. Spatially, mean precipitation in Hexi Corridor is the highest with 103.1 mm, followed by northern Xinjiang with 69.3 mm and southern Xinjiang with 51.1 mm. In autumn, mean precipitation can reach 29.5 mm in northwest China for the period of 1960–2010 (Fig. 3). Mean precipitation in autumn is less than 15 mm at 40.5% of weather stations, between 15 and 50 mm at 41.9% weather stations, and more than 50 mm at only 17.6% of weather stations, which are mainly located in the Tianshan and Qilian Mountains. Mean precipitation in northern Xinjiang is the highest with 39.4 mm, followed by Hexi Corridor with 34.1 mm, and the lowest is in southern Xinjiang with 14.0 mm. In winter, mean precipitation in northwest China is only 10.3 mm for the period 1960–2010. Mean precipitation in winter is less than 4 mm at 33.8% of weather stations, between 4 and 15 mm at 47.3% of weather stations, and more than 15 mm at only 18.9% of weather stations, mainly in the Tianshan and Altai Mountains. Mean precipitation in northern Xinjiang is the highest with 20.2 mm, followed by southern Xinjiang with 5.0 mm, and Hexi Corridor is the lowest with 3.7 mm (Fig. 3). The above results show that only summer precipitation in Hexi Corridor is the highest, associated with the water vapour transport of the eastern Asian monsoon. Meanwhile, in the other seasons, northern Xinjiang precipitation is the highest as a result of water vapour transport of western wind circulation.
3.2. Precipitation changes 3.2.1. Annual precipitation changes The Mann–Kendall statistical test revealed a significant (P b 0.01) increasing trend in annual precipitation in northwest China during 1960– 2010, with a rate of 0.61 mm/year (Fig. 4), which is higher than the average rate of China (−0.16 mm/year) for the same period (Sun and Yin, 2010). These values are consistent with previous studies (Li et al., 2013). The Mann–Kendall–Sneyers test showed that step change points in mean precipitation in northwest China occurred in 1987 (P b 0.01), which indicates that mean precipitation for the period 1960–1986 was significantly different than that of 1987–2010. The results are consistent with the findings of previous studies (Chen and Xu, 2005; Li et al., 2013; Deng et al., 2014). Fig. 4 also shows that precipitation has experienced significant variability but maintained relatively high mean values during the period 1987–2010. In addition, precipitation showed slightly decreasing trends in the last10 years (1998–2010), which should arouse our attention. In this study, we divided the 51-year period into two periods, namely, 1961–1990 and 1987–2010, according to the results of the step change analysis. Fig. 5 shows the spatial distribution of mean annual precipitation changes during 1987–2010 in comparison with that of 1961–1990. After 1987, precipitation increased by 10–30 mm at 47% of meteorological stations, less than 10 mm at 29% of stations and more than 30 mm at 24% of stations. The increasing amount of mean annual precipitation is 19.7 mm in northwest China. Regionally, the mean annual precipitation in northern Xinjiang for the period of 1987–2010 increased by 91 mm. Compared with that of 1961–1990, however, precipitation levels in southern Xinjiang and Hexi Corridor are only 20 mm and 8 mm, respectively. 3.2.2. Seasonal precipitation changes In spring, mean precipitation increased by 4.3 mm during 1987– 2010 in the arid region of northwest China compared with that of 1961–1990. Precipitation increased by 0–5 mm at 47% of meteorological stations and 5–10 mm at 32% of meteorological stations; however, at 6 meteorological stations, precipitation decreased by 0–5.9 mm (Fig. 6). In summer, an increase in mean precipitation by 8.4 mm occurred after 1987 in northwest China. Precipitation decreased by 0–13.6 mm at 21% of stations, most of which are located in the Hexi Corridor. Nevertheless, precipitation increased by 0–10 mm at 40% of stations and by 10–30 mm at 32% of stations. In autumn, precipitation increased by 3.6 mm in recent years. Precipitation increased by 0–5 mm at 54% of stations and by 5–10 mm at
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Fig. 3. Spatial distribution of seasonal mean precipitation for a 51-year period in the arid region of northwest China.
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Fig. 4. Variation of annual precipitation totals in northwest China for the period 1960– 2010, expressed as deviations from the average value for the period 1961–1990.
20% of stations. In addition, 17% of stations showed decreased precipitation of 0–6.3 mm (Fig. 6). In winter, an increase in mean precipitation by 3.5 mm occurred after 1986 in northwest China. Most of the meteorological stations (54%) showed increased precipitation of 0–3 mm, and 17% of stations showed an increase of 3–6 mm. Based on seasonal precipitation change analysis, we calculated the seasonal importance to the yearly change for each year between 1987 and 2010. Over this period, spring, summer, autumn, and winter accounted for 21.6%, 42.4%, 18.4%, and 17.6% of the annual change, respectively. These results indicate that the summer precipitation change is the most important factor in the annual precipitation change compared to the other seasons in northwest China. 3.3. Potential causes of precipitation change The Pearson correlation coefficient values (Table 1) show that the annual precipitation in northwest China has a strong and significant correlation with the annual WPSH (R = 0.60, P b 0.001) and NASH (R = 0.57, P b 0.001) but exhibits a weaker relationship with the other tested atmospheric circulations. Fig. 7 shows that the WPSH and NASH and the annual precipitation in the region had an almost perfect “mirroring” relationship during 1960–2010, suggesting the direct impact of the WPSH and NASH on annual precipitation. In particular, the annual precipitation increased following the marked increase in the WPSH and NASH. Another feature worth mentioning is that the correlations between precipitation and atmospheric circulation are different in various
seasons (Table 1). In spring, the R values for the precipitation with NASH (R = 0.54, P b 0.001), ISHA (R = 0.57, P b 0.001), and ENSO (R = 0.54, P b 0.001) are higher than those with other factors. In summer, the values for precipitation with NASH (R = 0.37, P b 0.01), WPSH (R = 0.36, P b 0.01), and SCSSH (R = 0.35, P b 0.05) are also much stronger and more significant than those with other circulations. In autumn, only the correlation between precipitation and EMC (R = − 0.36, P b 0.01) is significant (Fig. 7). In winter, precipitation has a significant correlation with the WPSH (R = 0.36, P b 0.01), NASH (R = 0.35, P b 0.05), and TPI_B (R = 0.42, P b 0.01). As a consequence, the great influences of the WPSH and NASH are observed in spring, summer, winter and the annual precipitation. To better understand how the other indices might be related to precipitation in northwest China, we constructed a scatter plot of annual precipitation and the various circulations from 1960 to 2010, with linear regressions shown in Fig. 8. Based on these figures, the correlation coefficients between the annual precipitation and ENSO, TPI_B, SCSSH, and EMC are weaker than those between the precipitation and WPSH and NASH. 4. Discussion In recent years, many scholars have paid attention to climate variations and their influencing factors, especially the effects of human activities on climate change (Li et al., 2014). However, the impact of atmospheric circulation and other natural factors on climate change should also be noted (Li et al., 2012). In this study, we detected the spatial and temporal change in precipitation in northwest China and conducted a detailed analysis of the relationships between the precipitation change and atmospheric circulations. Zhang et al. (2002) found that the influence of the East Asian monsoon on the drought formation of Northwest China was significant. This indirectly demonstrates our result that the WPSH has an important influence on the precipitation change in the arid region of northwest China. The latest research by Chen et al. (2014) showed that TPI_B was probably an important factor in the abrupt change in precipitation extremes in northwest China because the correlation coefficient between the precipitation extremes in northwest China and TPI_B was 0.44 (P b 0.01), which is obviously lower than that between the WPSH (0.60, P b 0.001), NASH (0.57, P b 0.001) and annual precipitation in the same area, suggesting that the WPSH and NASH had a stronger influence on the precipitation in northwest China than the TPI_B. Li et al. (2012) found that the weakening of the Siberian High during the 1980s to the 1990s is an important reason for the higher rate of
Fig. 5. Spatial distribution of annual mean precipitation change during 1987–2010 compared with that of 1961–1990 in the arid region of northwest China (black spots mean the precipitation change trend is significant at the P b 0.1 level).
B. Li et al. / Atmospheric Research 167 (2016) 275–284 Fig. 6. Spatial distribution of the mean seasonal precipitation change during 1987–2010 compared with that of 1961–1990 in the arid region of northwest China (black spots mean the precipitation change trend is significant at the P b 0.05 level).
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Table 1 Pearson’s correlation coefficients between mean precipitation and certain factors that may affect the precipitation during 1960–2010. Factors
Spring
Summer
Autumn
Winter
Annual
WPSH NASH TPI_B ASH SCSSH ISHA SHI PNAP ENSO EZC EMC
0.39** 0.54*** 0.31* 0.31* 0.35* 0.57*** 0.19 0.16 0.54*** 0.09 −0.48***
0.36** 0.37** 0.06 0.09 0.35* – 0.25 0.19 0.26 −0.11 0.17
0.23 0.26 0.12 0.17 0.14 −0.1 0.01 0.19 −0.11 −0.01 −0.36**
0.36** 0.35* 0.42** −0.09 0.29 0.28 0.01 0.08 0.13 0.12 −0.21
0.60*** 0.57*** 0.45*** 0.18 0.51*** 0.47*** 0.24 0.51*** 0.39* 0.07 −0.34*
*Significant at P b 0.05; ** Significant at P b 0.01; *** Significant at P b 0.001.
temperature rise in northwest China because the winter temperature in this region has a strong association with the Siberian High (correlation coefficient: R = − 0.715, P b 0.001). However, this study shows that the correlation coefficient between precipitation in northwest China and the Siberian High was not obvious, indicating that the influence of temperature and precipitation changes can be different in the same area. The subtropical high is one of the main atmospheric circulation entities affecting climate change in China, which reflects the atmospheric circulation changes in the middle–low latitudes to a large extent. In general, the stronger NASH is, the weaker the Atlantic mid-ocean ridge is, which is conducive to the prevailing westerly winds. For this reason, water vapour over the Atlantic will be smoothly transported to Eurasia. Because central Asia, including the arid areas of northwest China, is affected by westward flowing wind throughout the year, rainfall increases significantly. Thus, the significant relationship between NASH and precipitation in this study is reasonable. Additionally, the increase of WPSH is conducive to the westward movement of the East Asian summer monsoon and more water vapour transport to the arid areas of northwest China. Precipitation in the arid region is affected by the East Asian summer monsoon, especially in the eastern region. Therefore, the WPSH shows a significant relationship with precipitation.
Furthermore, when the subtropical high in North America and the western Pacific subtropical high were significantly enhanced, it created a large Indo-Burmese groove. Thus, south of the equator, western airflow from the Indian Ocean and from the western Pacific subtropical high formed southwest and southeast flow, respectively. The two airflows together provided a warm, moist airflow for the northwest arid areas, leading to more precipitation (Shi, 2003). In addition, summer is the most important season for the regional precipitation change. Our results indicate that the correlation between summer precipitation and SCSSH is also substantial because the strengthening of the SCSSH enables more water vapour transport from the Tibetan plateau to the Tarim Basin. Moreover, the latest research (Huang et al., 2015) states that the variability of summer precipitation in southern Xinjiang is strongly influenced by the water vapour from the south and east, which is consistent with the results in the study. Based on the above analysis, we hypothesize that the water vapour in the northwest arid region of China mainly originates from the Atlantic, followed by the western Pacific, with a small amount from the Indian Ocean. The indices used are not independent (e.g., ENSO has a dominant control over regions such as where the WPSH is calculated, etc.). Therefore, correlations between precipitation and many indices may be on different timescales. Thus, determining the most significant correlation between precipitation and circulation is an important task. However, the influence mechanism of these indices on northwest precipitation is complex (Shi, 2003) and thus requires further study. 5. Conclusions In this study, we state that the precipitation change in northwest China over the past 50 years might be associated with regional atmospheric circulation, and thus, we investigate the statistical correlation between the precipitation change and 11 modes of atmospheric circulations. The correlation analysis shows that precipitation had a strong and significant association with the WPSH and NASH on different timescales. In particular, annual precipitation exhibited stronger and more significant associations with the WPSH (R = 0.60, P b 0.001) and NASH (R = 0.57, P b 0.001) from 1960 to 2010. Based on this finding,
Fig. 7. The precipitation in northwest China and certain factors from 1960 to 2010.
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Fig. 8. Scatter plot of the annual precipitation and circulations (ENSO, TPI_B, SCSSH, EMC) from 1960 to 2010 and their linear regressions.
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