Climatology and Trends of High Temperature Extremes across China in Summer

Transcription

1 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2009, VOL. 2, NO. 3, Climatology and Trends of High Temperature Extremes across China in Summer WEI Ke and CHEN Wen Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing , China Received 6 March 2009: revised 7 May 2009; accepted 7 May 2009; published 16 May 2009 Abstract Based on the daily maximum air temperature data from 300 stations in China from 1958 to 2008, the climatological distribution of the number of days with high temperature extremes (HTEs, maximum temperatures higher than 35ºC) are studied with a focus on the long-term trends. Although the number of HTE days display well-defined sandwich spatial structures with significant decreasing trends in central China and increasing trends in northern China and southern China, the authors show that the decrease of HTE days in central China occurs mainly in the early period before the 1980s, and a significant increase of HTE days dominates most of the stations after the 1980s. The authors also reveal that there is a jump-like acceleration in the number of HTE days at most stations across China since the mid 1990s, especially in South China, East China, North China, and northwest China. Keywords: high temperature extremes, hot days, longterm trend, climate regime Citation: Wei, K., and W. Chen, 2009: Climatology and trends of high temperature extremes across China in summer, Atmos. Oceanic Sci. Lett., 2, Introduction High temperature extremes (HTEs) can cause massive deaths from hyperthermia, large scale catastrophic crop failures and shortages of water resources and power supplies (Haines et al., 2006; McMichael et al., 2006). Officials and the public are beginning to recognize the need for enhanced systematic mitigation actions to reduce the increasing risk to national economics, the environment and society from this kind of extreme heat. Therefore, it is necessary to make an overall evaluation to understand the climatology and long-term trend in variations of such extreme events in China, especially in densely populated eastern China. The frequency of HTEs has strong interannual and decadal variability as well as a long-term trend. Using Chinese station datasets from , Zhai and Pan (2003a) detected a slightly decreasing trend in the number of hot days (T max over 35ºC) in China. This was confirmed by Su et al. (2006) in the Yangtze River basin, despite its minor amplitude. However, there are also studies claiming an upward trend in the frequency and intensity of the high-temperature events in the Yellow River basin (Zhang et al., 2008). Meanwhile, some studies (Ma et al., 2003; Corresponding author: WEI Ke, weike@mail.iap.ac.cn Zhai and Pan, 2003b) detected a remarkable, increasing trend in high maximum temperature days after 1990 over arid and semi-arid northern China. Studies have also revealed that the linear trends in the frequency of low and high temperature extremes are probably different in different regions of China (Gong et al., 2004; Qian and Qin, 2006). Therefore, the trend and variation in HTE days in China is still not very clear. Thus, the main motivation for this study is to characterize the climatology and variability of HTE days in China and to further explore the long-term trends and distribution of HTE days in China. 2 Data and methodology The primary datasets employed in this study are the daily maximum surface temperatures of 730 stations over China derived from the China Meteorological Administration. Although the earliest observation of this dataset at some stations is from 1 January 1951, the number of stations throughout China is low until the mid 1950s. We adopted the beginning date of 1 January 1958 to achieve enough station coverage and also have a time period long enough to conduct trend analysis. The missing values were screened and stations with too many missing values were dropped. Finally, datasets from a total of 300 stations with coverage from were utilized in this study (see the black dots in Fig. 1). As the extreme temperature events that can cause large-scale turmoil occur mainly in summer season of June, July, and August, we conducted our study for only the June August period. There are usually two criteria to define HTE days. One uses temperature percentiles, such as above 90% or 95%, and the other uses a temperature threshold such as 35ºC or 38ºC (e.g., DeGaetano and Allen, 2002; Yan et al., 2002; Zhai and Pan, 2003a). Considering the serious effects on human health and public concerns over when the maximum temperature (T max ) exceeds 35ºC, we employ the criterion of 35ºC in this paper. In addition, using percentile criteria actually obtains similar trends and variation (not shown). For a single station, an HTE day is defined as when the T max is over 35ºC. We calculate the number of HTE days at each station as the cumulative number of HTE days for that station during June August of each year. The number of HTE day was calculated annually for all of the selected stations in order to investigate trends. A linear regression was fitted to each time series of the number of HTE days by using a least square regression for each station. Two-sided Student s t-tests were used to check the statistical significance levels for the regression coefficients.

2 154 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 2 Figure 1 (a) Climatology (average from 1958 to 2008) of the number of days with HTEs (with the maximum temperature greater than 35 C) across China. (b) The distribution of the standard deviation of HTE days over the period of 1958 to The diameter of a mark is proportional to the number of HTE days in (a) and the standard deviation of the number of HTE days in (b). The scales are shown in the bottom left in each panel. 3 Climatology Being a country with complex topography, western China is mainly mountainous Tibetan Plateau with an average elevation of more than 4000 meters. The temperature there rarely reaches as high as 35ºC in the summer; therefore, the HTE defined in this study mainly occurs over eastern China. Figure 1a shows the climatology of the number of HTE days in China for the period of 1958 to The diameters of the dots are proportional to the number of HTE days. Light and dark shadings outline the regions with elevations of more than 1000 and 3000 meters, respectively. Topography has an important role in the distribution of HTE events across China. The stations with the most HTE days are confined to east of the mountainous regions, below the height of 1000 meters. Another region with a lot of HTE days is northwestern China, primarily in the Xinjiang Province. The station with the most HTE days in China is the northwest China Turpan station (latitude: 42º56 N, longitude: E, Xinjiang Province), with an average value of 79.6 HTE days in summer. For the whole year around in Turpan, there are 98.5 HTE days on average, and there is an average of 35.3 broiling days with temperatures above 40ºC. The neighboring stations at Tikanlik (40 38 N, E) and Ruoqiang (39 02 N, E), north and south of the Taklimakan Desert, also have 36.1 and 45.8 HTE days in summer, respectively. The main regions with a large number of HTE days are the mid and lower reaches of the Yangtze River, especially the region to the south of Yangtze River. The average number of HTE days amount to an extreme of 38.3 in Lishui (28 27 N, E, Zhejiang Province), and 32.8 in Nanping (26 39 N, E, Fujian Province). The traditional Stove cities along the Yangtze River, Chongqing (29 31 N, E), Wuhan (30 37 N, E) and Nanjing (32 00 N, E) have 27.2, 18.9, and 12.9 HTE days, respectively. Due to their high humidity and dense populations, these cities are more vulnerable to heat waves. The megalopolis cities, Shanghai and Beijing, have 7.7 and 5.2 HTE days, respectively. The statistics of HTE days in the main provincial capital cities are listed in Table 1. Another region worthy of attention is southwestern China, with Yuanjiang station (23 36 N, E, Yunnan Province) registering an average of 35.5 HTE days and Baise (23 54 N, E, Guangxi Province) 27.5 HTE days in summer. The cities along the Yellow River, Xi'an (34 18 N, E), Yuncheng (35 02 N, E), and Jinan (36 41 N, E) have on average 20.1, 22.7, and 12.1 HTE days, respectively. Although less than those in southern China, these frequencies are still higher than the values observed for cities in North America at the same latitudes (DeGaetano and Allen, 2002). The variability in the number of HTE days in China is illustrated in Fig. 1b, which shows the standard deviation of HTE days across China for the period of The standard deviations are the largest over the lower reaches of Yangtze River. Keeping in mind that this region also has the most HTE days, heat wave disasters can, therefore, be especially severe in extreme years. For example, Lishui station has the largest standard deviation of 12.5 days. In the extreme cases, Lishui station saw maximums of 70 HTE days in 1971, 62 in 1961, and 56 in 2005, while only 11 HTE days occurred in 1999 and 15 in The Stove cities along the Yangtze River, Chongqing, Wuhan, and Nanjing, have standard deviations of 11.1, 10.7, and 8.6 days. All of these are larger than the 4.2 days standard deviation at the northwest Turpan station, in spite of the fact that it has the most HTE days in China. In the most extreme case, Turpan station had 89 HTE days in 1977, while the fewest number of HTE days there was 69, in 1979 and The small standard deviation in northwestern China reflects the stabilization of HTE days in this arid region. At northern China stations near Beijing, and along the Yellow River, although the HTE days are small, their variations are comparable to their counterpart stations in the Yangtze River basin and southern China. For example, Beijing saw zero HTE days in the summer of 1976 and 1977, but 21 and 26 HTE days were recorded in the summer of 1999 and 2000, respectively. 4 Trends analysis Using linear regression analysis, the trends in HTE

3 NO. 3 WEI AND CHEN: HIGHT TEMPERATURE EXTREMES ACROSS CHINA 155 Table 1 The statistics of HTE days in main China capital cities. Station Location HTE days Max HTE days Min HTE days T max record lat lon mean std number year number year ( C) date Harbin 45º45 126º * Changchun 43º54 125º Shenyang 41º44 123º /72/2000/ Urumqi 43º47 87º Xining 36º43 101º Hohhot 40º49 111º Yinchuan 38º29 106º Taiyuan 37º47 112º Beijing 39º48 116º /77/ Tianjin 39º05 117º /80/ Jinan 36º36 117º Guiyang 26º35 106º / Xi'an 34º18 108º Shijiazhuang 38º02 114º /77/82/ Zhengzhou 34º43 113º / Wuhan 30º37 114º Chongqing (Shapingba) 29º35 106º / Nanchang 28º36 115º Nanjing 32º00 118º Hefei 31º47 117º / Hangzhou 30º14 120º Fuzhou 26º05 119º Guangzhou 23º10 113º / Nanning 22º38 108º / Haikou 20º02 110º Lhasa 29º40 91º Chengdu 30º40 104º Kunming 25º01 102º Shanghai (Longhua) *2 31º10 121º Lanzhou 36º03 103º Changsha *3 28º13 112º *1 There are more than 4 years. *2 The station of Shanghai (Longhua) is continuous to the end of After that, we use Shanghai station. *3 The Changsha station moved at the end of 1986 (from station number to 57687). The climatology and std are based on the period from 1958 to days across China were computed. Figure 2a illustrates the linear trend in HTE days over the period of A decreasing trend is apparent in central China. In the region between the lower reaches of the Yangtze River and Yellow River, the largest, significant decreasing trend occurred at a rate of about 3 days per decade. This confirms the results of Zhai and Pan (2003a) that a decreasing trend can be observed in northern China during the second half of 20th century. A strong increase occurred in the eastern part of northwestern China in the Xinjiang Province, with a rate of increase of about 4.5 days per decade. A large increasing trend also occurs along the south coastal region of the Zhejiang, Fujian, and Guangdong provinces. Weak, but significantly increasing, trends also occurred across Northeast China, Southwest China, and the northernmost part of North China, verifying the results of Qian and Qin (2006). However, if we divide the data period into two halves, different features are found. During the first half of the period of study, from 1958 to 1983 (Fig. 2b), there was a

4 156 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 2 strong decrease in the number of HTE days in eastern China, especially in central China, northern China and the mid and lower reaches of the Yangtze River. An extreme decrease was recorded in central China in Gushi (32 10 N, E, Henan Province), with a rate of decrease of 10.1 days per decade, and in Nanyang (33 02 N, E, Henan Province, 8.9 days per decade), Laohekou (32 23 N, E, Hubei Province, 8.7 days per decade) and Xinyang (32 08 N, E, Henan Province, 8.0 days per decade). This strong decreasing trend helped to reduce the variation in the temperature range Figure 2 Linear trends in the number of HTE days (with the maximum temperatures greater than 35 C): (a) for the years from 1958 to 2008, (b) for the years from 1958 to 1983, and (c) for the years from 1984 to The marks are proportional to the values of the trend with plus signs for positive values and dots for negative values. Only the stations with a significant trend level above 95% are displayed. Units: day per decade. The gray boxes indicate the regions analyzed in Fig. 3. and led to the documented decadal cooling in the 1980s in central China (Lin et al., 1995; Tang and Ren, 2005). In the latter period of 1984 to 2008 (Fig. 2c), an overall warming was recorded across most Chinese stations, except for sporadic cooling in the west of Guangxi Province (e.g., Baise station with a decreasing trend of 6.6 days per decade). Of the whole 300 stations, only 17 stations show a tendency for cooling. The strongest increasing trend occurred in northwestern China, where the number of HTE days increased 16.2 days per decade in Qijiaojing (43 13 N, E), 9.1 days per decade in Tikanlik and 4.9 days per decade in Ruoqiang. This is consistent with the findings of previous studies that northwestern China is experiencing a warmer and wetter period (Yang et al., 2006; Zhou and Huang, 2003). A strong increasing trend in HTE days also occurred in southwestern China in the Sichuan Basin (an increasing rate of about 5 days per decade), in the mid and lower reaches of Yangtze River (about 6 days per decade), in South China (about 4.5 days per decade) and in North China (about 2.5 days per decade). The contrast between Figs. 2a, 2b, and 2c illustrates that the long-term trend studies are sensitive to the chosen time period; a trend can only be recognized for a specific time period. We further show the evolution of the number of HTE days in Fig. 3, which shows the time series of the regionally averaged numbers of HTE days and their 9-year running mean. The regions we analyzed are southern China bounded by N, E; eastern China N, E; central China N, E; northern China N, E; and northwestern China N, E (as indicated with gray boxes in Figs. 2a). The interannual variation, as well as decadal and long-term trends, can be observed in the time series. For southern China (mainly Guangdong and Fujian provinces), the 9-year running mean shows a decreasing trend from late 1950s to mid 1970s, followed by an increasing trend after the mid 1970s. After 2000, a persistent and strong increase is evident, and extremely hot summers occurred in 2003 and 2007 in this region. Decadal variation is also apparent, with more HTE days in the early 1960s, early 1990s and 2000s, and relatively less HTE days in the 1970s and mid 1990s. In eastern China (mainly Jiangsu, Shanghai, Zhejiang, and part of Anhui and Jiangxi provinces), consistently less HTE days occurred during the 1970s and 1980s, and more HTE days were recorded during the 1960s and 2000s, which results in a decreasing trend before the 1980s and an increasing trend after early 1980s. The most HTE days were recorded in 1971, followed by Six consecutive years with more HTE days occurred from 2003 to 2008, which caused a burden on the water resource and energy supply in this economically booming region. Central China (mainly Henan Province and parts of Shanxi, Shaanxi, Hebei, and Hubei provinces) had the most HTE days in years in the 1950s and 1960s. In 1959 and 1966, the number of HTE days reached 31.4 and 25.4, which are 2.6 and 2.1 times the mean value in this region.

5 NO. 3 WEI AND CHEN: HIGHT TEMPERATURE EXTREMES ACROSS CHINA 157 sharp increase in the number of HTE days since the mid 1990s, from about 2 days to about 6 days. The number reaches as high as 9.4 days and 9.2 days in 1999 and 2000, respectively, both of which are 2.8 times its climatological value. In contrast, in the early 40 years, from the 1950s to 1990s, the number of HTE days had a steadily decreasing trend. Most of years in the late 1970s, 1980s, and early 1990s had less than two HTE days. The jump of HTE days from less than 2 days to more than 8 days may suggest a climate shift in the mid 1990s in this region. The shift of the climate regime of the HTE days has also occurred in northwestern China. Before this climate shift, the number of HTE days averaged about 17 days for the time from the 1950s to mid 1990s, dropping to even only 10 days in After the shift, the number of HTE days has climbed up to a level of about 23 days, with an extreme number of more than 26 days in both 2002 and Figure 3 Time series of regional mean numbers of HTE days in summer (JJA) from 1958 to The curves indicate the 9-year running mean. The regions averaged are (a) southern China (stations in the region bounded by N, E), (b) eastern China (27 32 N, E), (c) central China (30 37 N, E), (d) northern China (37 42 N, E), and (e) northwestern China (38 45 N, E). Dark and light bars indicate values above and below the long-term mean values. The number of HTE days decreased and remained low from mid the 1970s onwards and dropped to a minimum value of only 2.8 days in Since then, it has started to increase, but the rate of increase rate is slow and not comparable to that in other regions during this period. The new century has seen an above-average number of HTE days from 2001 to 2006, while the value dropped to below average again in 2007 and The northern China region (Beijing, Tianjin, part of Hebei, Shanxi, and Shandong provinces) has recorded a 5 Conclusion and discussion Considering global warming, there are more concerns over extreme climate and weather events. We made a statistical study of the climate distribution of HTE days across China. The number of HTE days to the south of lower reach of the Yangtze River is at a level of days, and it is about days in central China and northern China. The stations with most HTE days are in northwestern China in the Xinjiang Province, probably due to the lower elevation and adjacency to arid desert regions. The variability in the number of HTE days is large in eastern China, especially in the mid and lower reaches of the Yangtze River, suggesting that this region is vulnerable to HTE disasters in extreme years. The trend analysis shows that there has been an overall increase in the number of HTE days across China, while this increase happened mainly in the latter half of our 50-year study period. In the first half of the study period, from late 1950s to early 1980s, a decrease in the number of HTE days was recorded in most of central China, northern China, and the lower Yangtze River Valley. The latter period experienced an increase that had a jump-like feature over southern China, northern China, and northwestern China, with the shift occurring in the mid 1990s. Global warming plays a key role in the variation in HTE days across China. The most recent evaluation by the Intergovernmental Panel on Climate Change (IPCC, 2007) reported an acceleration of warming, which is mainly caused by the effect of greenhouse gases. In East Asia, the black carbon produced by incomplete combustion (Ramanathan et al., 2007; Ramanathan and Carmichael, 2008), the reduction of clouds and the associated increase in incoming radiation (Ding et al., 2004; Wild et al., 2005) all help to warm up the region and cause the increase. It is worthwhile to note that the long period of low HTE activity from 1970s to early 1990s was probably related to the summer cooling across central China (Hu et al., 2003; Yu et al., 2004; Yu and Zhou, 2007), which suppressed the warming trend. As global warming con-

6 158 ATMOSPHERIC AND OCEANIC SCIENCE LETTERS VOL. 2 tinued to develop, the competition between regional cooling and global warming became biased toward the latter one, which then leads to the abrupt change in climate extremes in China, as the nonlinearity effect of extreme events indicates that a small increase in the mean can result in a large change in the frequency of high extremes (Mearns et al., 1984; Meehl et al., 2000). The exclusion of western China s high topography region and northeastern China from the analysis of HTE events does not mean that these regions are excluded from the increase in high temperature extremes. Large-scale warming will spur on the increase in extreme warm/hot events across the whole of China, which will have profound influences on economic activities and social life, especially when projected into the future under conditions of continuous global warming. Acknowledgements. The authors thank the editor and two anonymous reviewers for their constructive comments that helped improve this manuscript. This work was supported by the National Basic Research Program of China under Grant No. 2009CB421405, the National Natural Science Foundation of China under Grant No , and the Innovation Project of Chinese Academy of Sciences under Grant No M301. References DeGaetano, A. T., and R. J. Allen, 2002: Trends in twentieth-century temperature extremes across the United States, J. Climate, 15(22), Ding, S., G. Shi, and C. Zhao, 2004: Analyzing global trends of different cloud types and their potential impacts on climate by using the ISCCP D2 dataset, Chinese Sci. Bull., 49(12), Gong, D. Y., Y. Z. Pan, and J. A. Wang, 2004: Changes in extreme daily mean temperatures in summer in eastern China during , Theor. Appl. Climatol., 77(1 2), Haines, A., R. S. Kovats, D. 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