Spatial-temporal characteristics of temperature variation in China

Transcription

1 MAP-0/758 Meteorol Atmos Phys 000, 1 16 (2005) DOI /s Monsoon and Environment Research Group, School of Physics, Peking University, Beijing, China Spatial-temporal characteristics of temperature variation in China W. Qian and A. Qin With 12 Figures Received June 6, 2005; revised July 14, 2005; accepted August 19, 2005 Published online: * *, 2005 # Springer-Verlag 2005 Summary Spatial-temporal characteristics of temperature variations were analyzed from China daily temperature based on 486 stations during the period The method of hierarchical cluster analysis was used to divide the territory into sub-regional areas with a coherent evolution, both annually and seasonally. Areas numbering 7 9 are chosen to describe the regional features of air temperature in mainland China. All regions in mainland China experienced increasing trends of annual mean temperature. The trend of increasing temperature was about C=10 yr in northern China and less than 0.1 C=10 yr in southern China. In the winter season, the increasing trend of temperature was about C=10 yr in northern China and about C=10 yr in southern China. The increasing trend of autumn temperature was mainly located in northwestern China and southwestern China including the Tibetan Plateau. In spring, the rising trend of temperature was concentrated in Northeast China and North China while there was a declining temperature trend of 0.13 C=10 yr in the upper Yangtze River. In summer, the declining trend of temperature was only concentrated in the mid-low valley of the Yangtze and Yellow Rivers while surrounding this valley there were increasing trends in South China, Southwest China, Northwest China, and Northeast China. Rapid changes in temperature in various regions were detected by the multiple timescale t-test method. The year 1969 was a rapid change point from a high temperature to a low temperature along the Yangtze River and South China. Present address: Linfen Meteorological Office, Linfen , Shanxi Province, China. In the years , temperature significantly increased from a lower level to a higher level in many places except for regions in North China and the Yangtze River. Another rapid increasing temperature trend was observed in In the years , a positive rapid change of summer temperature occurred in northwestern China and southwestern China while a decreasing temperature was found between the Yellow River and the Yangtze River. A rapid increase of winter temperature was found for and in many places. There were increasing events of extreme temperature in broad areas except in the north part of Northeast China and the north part of the Xinjiang region. In winter, increasing temperature of the climate state and weakening temperature extremes are observed in northern China. In summer, both increasing temperature of the climate state and enhancing temperature extremes were commonly exhibited in northern China. 1. Introduction Climate conditions or climate changes can have important impacts on human society (Zhang D et al, 2005), the economy and the environment (Boo et al, 2004). The global average surface temperature has increased over the 20th century by about 0.6 C and there is evidence that the warming of the latter half of the twentieth century is due primarily to human activities (IPCC, 2001), but the mean annual temperatures over northwestern North America have risen 1 2 C in the same period (Hansen et al, 1999;

2 2 W. Qian and A. Qin Cayan et al, 2001). This is a piece of evidence that the climate changes in various regions are different. For decision makers in a country or a region, it is insufficient to merely know the global average climate change. In China, there are substantial long-term temperature rises with magnitudes comparable to those observed in the global average temperature (Wang and Gong, 2000; Wang et al, 2001) and the regional features of temperature variations from place to place (Zhang D and Gaston, 2004; Zhang Q et al, 2005; Lu et al, 2004; Liu et al, 2004). In a recent study, regional trends in recent temperature indices in China have been analyzed from the daily surface air temperature maximum and minimum during (Qian and Lin, 2005). The results show significant changes in important temperature indices over the past 40 years, especially in northern China, the Yangtze River valleys and Xinjiang. Decreasing trends of diurnal temperature range are found in mainland China as a whole during with stronger decreases in Northeast China, central South China and the Xinjiang region. Trends of cool days display a significant decrease in the middle latitudes near 40 N along the Yellow River valley. There are increasing trends of warm days in the upper-middle Yellow River valley and other regions such as along the coast of South China. Consecutive warm days have significantly increased in northern China, while consecutive cold days have decreased largely in the regions of Xinjiang, the Yellow River valley, the southern part of Northeast China, and along the southeast coast of the country. These basic characteristics of extreme temperature distribution are also revealed by Gong et al (2004a, b, c) with different regional data resolutions focused in winter or summer. China is strongly influenced by the East Asian monsoon (Ding, 2004). During the winter half year, the climate is mostly cold and dries (Lau and Lau, 1984; Chan and Li, 2004). Cold days and strong winds accompanied by dust storms are the major climate disasters particularly observed in northern China (Boyle and Chen, 1987; Qian et al, 2002b). During the summer period, the rain belt moves gradually from south to north with the hot and humid climate in eastern China (Tao and Chen, 1987; Ding, 1992; Qian et al, 2002a). The regional characteristics of climate variations are outstanding in China. Regionally-averaged temperature variations, climate-state temperature changes, frequency change of temperature extremes as well as their linkage are largely unknowns in China. In the present study, we analyzed the spatialtemporal characteristics of temperature variation seasonally and annually in China by using daily data. The daily temperature data from 486 stations (Fig. 1a) used in this study was obtained from the National Climate Center of China. Although daily temperature data are available since 1951, there are many unreliable and missing data before 1960 in most of China. Thus, this paper focused on the 41-year period for which more reliable observation data are available. The quality control (QC) of this dataset was based on the daily values of temperature Fig. 1a. 486 stations in mainland China, and (b) interannual rainfall division. In (b): dashed line, dot-dashed line, and solid line indicate the within-group correlation, between-group correlation and their difference, respectively

3 Spatial-temporal characteristics of temperature variation in China 3 and precipitation from individual stations with established extreme values. A detailed description of the QC method can be found in the recent work of Feng et al (2004), and Qian and Lin (2004). In this paper, the method of division and numerical analysis are described in Sect. 2. Interannual temperature variations are given in Sect. 3. Regional temperature variations for the four seasons are shown in Sect. 4. Changes in the climate state of temperature from seasonal to annual means for two 30-year periods are illustrated in Sect. 5. Conclusions and discussions are given in Sect Method of division and numerical analysis Climate changes are different from place to place and from period to period. In China many methods such as empirical orthogonal function (EOF) or rotated EOF or rotated complex EOF were used in the division of climate components (Li et al, 2002). EOF (or REOF) analysis provides a convenient mathematical method for extracting major information and studying the spatial and temporal variability of a variable. But for the division the irregular boundary of sub-regional areas is hard to be determined from the EOF or REOF analysis. To reduce this limitation, the method of hierarchical cluster analysis was used in this paper. This method requires that the correlation coefficient between two individual series within a group needs to be large enough while the correlation coefficient between group-averaged series needs to be small enough. The Pearson correlation is formulated as r ðx;yþ ¼ Xn i¼1 ðz xi z yi =ðn 1ÞÞ; ð1þ where, z xi and z yi are the sample series of standardization. There are many steps to get the final division. First, a new group series is calculated by their original series with a determined criterion of significant correlation. After many steps, the last group series is calculated based on the series of current groups with a significant correlation. For the interannual temperature division, the goal is to find the situation where the anomalous temperature (such as above normal or below normal temperature) should be very similar from station to nearby station. The data is annual mean time series from In Fig. 1b, 100 groups are finally obtained. The withingroup correlation averaged from all division groups is gradually increased as we move from 2 to 100 groups while the between-group correlation averaged from all division groups is rapidly decreased as we move from 2 to 10 groups. The difference between the two correlations, namely the within-group correlation minus the between-group correlation, is also rapidly increased from a value of 0.25 for 2 groups to about 0.5 for 20 groups. Moving from the 20 to 100 groups, three lines in Fig. 1b reach their stable values so that 20 areas (groups) can well describe regional coherent evolution in mainland China. Moving from 2 to 8 groups the withingroup correlation rapidly increases from 0.6 to 0.8 while the between-group correlation sharply decreases from 0.9 to 0.5. To describe clearly, 7 9 areas in this paper are chosen for temperature division in both annually and seasonally. If the goal is for the downscaling forecast, the division should at least reach 20 areas in mainland China. A trend analysis is used in each series of groups. A linear function of time t can be written as Y ¼ at þ b; ð2þ where a and b are two experience constants based on the group s series. In order to find whether there are long-term fluctuations in the group s series, a cubic function of time t can be written as Y ¼ b 0 þ b 1 t þ b 2 t 2 þ b 3 t 3 ; ð3þ where b 0, b 1, b 2 and b 3 are three experience constants based on the group s series. 3. Interannual temperature variation The correlation consistency of yearly temperature in different areas can be found in the final panel of Fig. 2. Nine areas are given, with areas Northeast China (I), north Xinjiang (II), south Xinjiang (III), Northwest China (IV), the lower Yangtze River (V), the upper Yangtze River (VI), south coast China (VII), Southwest China (VIII), and the Tibetan Plateau (IX).

4 4 W. Qian and A. Qin Fig. 2. Area-averaged temperature series. The five dashed lines from top to bottom in each panel indicate the standard deviations of þ1.5, þ1.0, 0, 1.0, and 1.5, respectively. The trend of each regional series is indicated by the solid line. The curved lines indicate the cubic function fit. The last panel shows nine areas divided from annual temperature The area-averaged annual temperature series are displayed in Fig. 2. In the area of Northeast China (I), positive anomalies of annual temperature of more than þ1.5 found in the years of 1990, 1994, 1997, 1998 and 1999 while negative anomalies of less than 1.5 are only observed in The increasing trend of annual temperature has a value 0.3 C=10 yr. A trend of rapid increase can be noted after In the north Xinjiang region (II), the highest temperature larger than þ1.5 is found in 1997 while two years of the lowest temperature occurred in 1969 and The increasing trend of annual temperature reaches a value 0.3 C=10 yr the highest temperature happened in north Xinjiang (II) in 1997 is different from other places or in China as a whole with the highest temperature occurred in 1998 (Qian and Zhu, 2001; Wang and Gong,

5 Spatial-temporal characteristics of temperature variation in China ; Wang et al, 2004). In the south Xinjiang region (III), the increasing trend is 0.2 C=10 yr with the two lowest years in 1967 and 1974 while the highest temperature is found in 1997, 1998, 1999, and In Northwest China (IV), the increasing trend still has a value of 0.3 C=10 yr with the lowest temperature in 1967 and the highest temperature in In other areas, the increasing trend of annual temperature is equal to or lowers than 0.1 C=10 yr with the highest temperature in From the cubic function fitted lines, it can be observed that there was a cooler period in the 1970s 1980s while a significant increasing trend appears after the 1990s, particularly in southern China. The result shows that warming trend is larger in northern China (I, II, III, and IV) with a rate C=10 yr while it is smaller in southern China with a rate less than 0.1 C=10 yr including central eastern China (V and VI) and south coast China (VII). Warming trends of annual mean temperature were also investigated by Tang and Zhai (2005), who indicated a rate of 0.26 C=10 yr in eastern China and 0.18 C=10 yr in western China. Usually, the running t-test method can be used to find the transition or rapid-change point of a long-term series. Let the temperature series be fy i g, i ¼ 1; 2;...; nðn ¼ 41Þ and the two subregional series be fy i1 g, fy i2 g, i 1 ¼ 1; 2;...; m 1, i 2 ¼ 1; 2;...; m 2, m 1 þ m 2 n. We have the t-test formula y 2 y 1 t ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiq ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; ð4þ ðm 1 1Þs 2 1 þðm 2 1Þs 2 2 m 1 þ m m 1 þ 1 m 2 to estimate if the two series of fy i1 g and fy i2 g have a significant difference. Here, y 1, s 1 and y 2, s 2 are the mean value and standard deviation, respectively, of the two series. The degree of freedom of t is ¼ m 1 þ m 2 2. In this paper, a variable timescale was used in the moving t-test for all temperature series. The timescales are from 2 to 20 years based on the total 41-year series. For example, if the timescale is 10 years, the first step is to take the first 10 temperature series from 1960 to 1969 to form a set of series and the next 10 temperature series from 1970 to 1979 to form the other set of series. Fig. 3. The results of the t-test calculation with different timescales from 2 to 20 years in various areas from I to IX. The value 2.30 (3.35) indicates that the t-test reaches the 0.05 (0.01) level of significance at a determined time point in the different timescales. Positive values denote the series transition from a below-normal to above-normal temperature and negative values denote the reversed situation

6 6 W. Qian and A. Qin The two sets of series are used to calculate the t-value and estimate whether a rapid transition happened between 1969 and The second step is to take the 10 temperature series from 1961 to 1970 to form a set of series and the 10 temperature series from 1971 to 1980 form the other set of series. The t-test is then used with the second step s two sets of series. After many steps, it is to take the 10 temperature series from 1981 to 1990 to form a set of series and the last 10 series from 1991 to 2000 to form the other set of series. The t-test is then used on the last two sets of series. For the timescale of 20 years, only two steps are calculated with two sets of series: ( and ) and ( and ). The results of the t-test calculation with different timescales from 2 to 20 years are shown in Fig. 3. The longer timescale signals are important for finding the climate transition. In Northeast China (I), a significant temperature increase starts from 1977 and lasts to 1991, particularly in 1988 for all timescales. In the north Xinjiang region (II), a significant temperature increase also starts from 1977 at the 6 16-year timescale and lasts for about 7 years. In the south Xinjiang region (III), there is no trend of significant change from any timescale. In Northwest China (IV), temperature variations in both timescales and time points are similar to those in Northeast China (I). In the lower Yangtze River (V), the temperature decrease is found in 1969 at the 6 8-year timescales and the temperature increase appears after A significant temperature decrease in 1969 is commonly observed along the Yangtze River (V and VI) and in South China (VII). In South China (VII), there is a short timescale increase of temperature in 1977 and a long timescale increase in In the Tibetan Plateau (IX), there is no significant temperature increase. The most significant temperature increase is found in Southwest China (VIII) starting from 1977 at all timescales. 4. Seasonal temperature variation 4.1 Spring temperature variation The seasonal temperature division in mainland China is different from annual one. The last panel in Fig. 4 shows seven regions of division for the spring temperature. These areas are named as follow: Northeast China (I), the Xinjiang region (II), Northwest China (III), North China (IV), central-western China (V), South China (VI), Southwest China and the Tibetan Plateau (VII). Two large-valued increasing trends for the spring temperature are located in Northeast China (I: 0.36 C=10 yr) and North China (IV: 0.21 C= 10 yr) while a region with a decreasing trend is found in central-western China (V: 0.13 C= 10 yr). The year 1998 has an extreme high spring temperature over the regions of Northeast China (I), the Xinjiang region (II), Northwest China (III), South China (VI), and Southwest China (VII). In North China (IV), the highest temperature appears in 2000 while the highest temperature in 1969 occurs in central-western China (V). Long-term oscillations are obviously exhibited in the regions of China except for the Xinjiang region (II), the Tibetan Plateau (VII) and Northeast China (I). After the t-test analysis it was found that there are no significant changes of spring temperature in many regions expect for the increasing transition in Northeast China (I), Northwest China (III) and North China (IV) as well as the decreasing transition in central-western China (V). Transition time points can be observed in 1981 and 1989 in Northeast China (I), in 1991 in Northwest China (III), in 1993 in North China (IV), and in 1976 in central-western China (V). 4.2 Summer temperature variation The summer temperature division is shown in the last panel of Fig. 5. Eight regions are distinguished, including Northeast China (I), north Xinjiang (II), south Xinjiang (III), Northwest China (IV), the Yellow River (V), the Yangtze River (VI), South China (VII), Southwest China (VIII). The increasing trends appear in Northeast China (I: 0.22 C=10 yr), the north Xinjiang region (II: 0.20 C=10 yr), Northwest China (IV: 0.14 C=10 yr), South China (VII: 0.11 C=10 yr) and Southwest China (VIII: 0.24 C=10 yr). The decreasing trends are observed in the Yellow River (V: 0.08 C=10 yr) and the Yangtze River (VI: 0.05 C=10 yr). In summer, the highest temperature of 1998 only appears in north Xinjiang (II), South China (VII) and Southwest

7 Spatial-temporal characteristics of temperature variation in China 7 Fig. 4. As in Fig. 2 and Fig. 3, except for the spring (MAM) temperature in 7 areas China (VIII). Long-term temperature variations are complex in the summer in China. In Northeast China (I), there is no significant temperature trend before A trend of temperature increase only appears after 1992 in Northeast China (I). Large fluctuations of summer temperature are obvious in north Xinjiang (II) with the lowest values in 1960, 1972, and 1993, and the highest values in 1962, 1973, and A period of low temperature in Northwest China (IV) and the Yellow River (V) is experienced in the late 1970s and in the 1980s. In summer, the declining trend of temperature is only concentrated in the mid-low valleys of the Yangtze and Yellow Rivers (V and VI), while surrounding these valleys, there are increasing trends of summer temperature in South China (VII), Southwest China (VIII), Northwest China (IV), and Northeast China (I). From the t-test in summer, temperature in five areas experienced significant changes. In north Xinjiang (II), a significant temperature increase is found in 1973 while a significant temperature decrease is observed in In south Xinjiang (III) and Northwest China (IV), significant temperature increases are separately found in 1977 and 1988 near the decadal timescale. Rapid decreases of temperature in the years 1976 and 1979 are centered in the Yellow River (V). In south coast China and Southwest China (VII and VIII), a rapid increase of temperature is situated in 1977 for all timescales.

8 8 W. Qian and A. Qin Fig. 5. As in Fig. 4, except for the summer (JJA) temperature in 8 areas 4.3 Autumn temperature variation The autumn temperature division and regional temperature evolutions are shown in Fig. 6. Seven regions are distinguished, including Northeast China (I: 0.17 C=10 yr), the north Xinjiang region (II: 0.31 C=10 yr), northern China (III: 0.22 C=10 yr), the upper Yangtze River (IV: 0.08 C=10 yr), Southeast China (V: 0.11 C= 10 yr), Southwest China (VI: 0.21 C=10 yr), and the Tibetan Plateau (VII: 0.24 C=10 yr) with sincreasing trends. Consistent increases of autumn temperature are obvious in all regions,

9 Spatial-temporal characteristics of temperature variation in China 9 Fig. 6. As in Fig. 4, except for the autumn (SON) temperature in 7 areas except that there are long-term oscillations with low temperatures from the 1970s to the early 1980s in Northeast China (I) and Southeast China (V). The highest temperature in 1998 is observed in northern China (III), the upper Yangtze River (IV), Southeast China (V), and Southwest China (VI). The increasing trend of autumn temperature is mainly located in northern and western China, i.e., regions II, III and VII. The t-test figure shows that there were four areas with rapid increase temperature in Northeast China (I: 1982 and 1988), north Xinjiang (II: 1977), Southwest China (VI: 1980 and 1987), and Tibetan Plateau (VII: 1987). 4.4 Winter temperature variation The winter temperature division and regional temperature evolutions are shown in Fig. 7. Eight regions are distinguished including Northeast China (I) with the largest increasing trend 0.73 C=10 yr, the north Xinjiang region (II) with the second largest increasing trend 0.68 C=10 yr. Other increasing trends exist in the south Xinjiang region (III: 0.60 C=10 yr), northern China (IV: 0.53 C=10 yr), the mid-upper Yangtze River (V: 0.19 C=10 yr), the southeast coast (VI: 0.31 C=10 yr), Southwest China (VII: 0.31 C= 10 yr), and the Tibetan Plateau (VIII: 0.21 C= 10 yr). The large increasing trend of winter

10 10 W. Qian and A. Qin Fig. 7. As in Fig. 4, except for the winter (DJF) temperature in 8 areas temperature mainly appears in the northern part of China including Northeast China (I), the Xinjiang region (II and III) and northern China (IV). A relatively weak increasing trend of winter temperature is found in the central part of China, i.e., the mid-upper Yangtze River (V). In the

11 Spatial-temporal characteristics of temperature variation in China 11 northern part of China, a low winter temperature is found in the late 1960s and in the 1970s. The t- test figure shows that the most significant temperature increase in winter are found in Northeast China (I: 1977 and 1986), south Xinjiang (III: 1977 and 1985), Northwest China (IV: 1986), and Southwest China (VII: 1984). 5. Changes of climate state 5.1 Difference between two climate states The climate state is usually determined by the 30-year average of observational components. Obviously, the climate state changes with different 30-year periods, such as to the reference periods or or In many literatures, climate component anomalies yearly for a long series, for example 100 years, are related to a determined mean value of climate state. In this section, the annual and seasonal temperatures in China will be used to detect the changes of the climate state regionally. Figure 8 displays the difference between two 30-year periods of the average temperature of minus Figure 8a shows the climate-state change of annual temperature. It is found that an increasing annual temperature covers China as a whole except for 3 spots in the southwestern part of China. Increasing temperature with a value of more than 0.2 Cis mainly located in northern China, north of 35 N, and in Southwest China (the Tibetan Plateau). There are 4 centers of temperature increase with values of more than 0.4 C 0.6 C in Northeast China, the northern part of North China and Southwest China. Actually, the climate state change showed in Fig. 8 only is the two-decadal difference of temperature for minus However, this result only tell us that Fig. 8. Temperature difference (unit: C) between the two 30-year periods of minus

12 12 W. Qian and A. Qin references are really changed from two 30 years. Long-term temperature anomaly series in Northeast China will have larger negative departures if the reference for is used. In spring, a rising temperature of the climate state is located over the Tibetan Plateau (>0.2 C), East China (>0.2 C), and from North to Northeast China (0 0.2 C). A declining temperature of the climate state is situated from Northwest China to the Xinjiang region (0 0.2 C) and Southwest China. In summer, a declining temperature of the climate state is found in the mid-low Yangtze River and south of the lower Yellow River. The maximum decreasing temperature with a value of 0.2 C is located in the central part of China. A summer declining temperature of the climate state can also be observed in two spots of the Xinjiang region. Otherwise, there are many regions with increasing temperature of the climate state, with values of more than 0.2 C. In autumn, a declining temperature of the climate state is found only at several small spots, while there are broader regions with increasing temperature over northern China and the Tibetan Plateau. Increasing temperatures in winter have a huge contribution to the climate state change. In the north of 35 N, the increase in temperature reaches C between the two 30-year periods. A declining temperature is found only in one spot of the Tibetan Plateau. 5.2 Difference of two standard deviations The standard deviation of temperature can well reveal the extreme variation in a given period. Figure 9 shows the difference of two standard deviations of annual temperature and seasonal temperature between the two 30-year periods. Positive (negative) values in the figure indicate that there are increasing (decreasing) deviations Fig. 9. Temperature differences of standard deviations (unit: C) between the two 30-year periods of and

13 Spatial-temporal characteristics of temperature variation in China 13 or extreme temperature variations during the period of compared to For annual temperature, there are increasing events of extreme temperature in broader areas except in the north part of Northeast China and the north part of the Xinjiang region. In spring, the coverage of increasing and decreasing events of extreme temperature anomalies is half to half in China. The increasing temperature extremes in spring are mainly located from the middle Yellow River to the midlow Yangtze River and the east part of Xinjiang. Decreasing temperature extremes appears from the upper Yellow River to the upper Yangtze River, North China and the northern part of Northeast China. In summer, increasing temperature extremes cover northern China, i.e., north of 30 N, while decreasing extremes observe along the Yangtze River. In autumn, there are mainly increasing temperature extremes over China particularly in the Xinjiang region. The situation of winter is different from that of summer and autumn. The decreasing temperature extremes cover in many areas particularly in the northern parts of the Xinjiang region, Northwest China, North China and Northeast China. This result of winter temperature extreme is similar to that the winter daily temperatures are becoming less variable in East Asia (Gong and Ho, 2004a). It is interesting to note that the increasing temperature of the climate state and the weakening temperature extremes in winter are observed in northern China. In summer, however, both increasing temperature of the climate state and enhancing temperature extremes are commonly exhibited in northern China. 6. Conclusions and discussions Spatial-temporal characteristics of temperature variations were analyzed from daily data based on 486 stations during 1960 to The major conclusions, with discussion, are given as follows: (1) The method of hierarchical cluster analysis was used in the divisions of temperature over mainland China, both annually and seasonally. Areas numbering 7 9 are chosen to describe the regional features of air temperature in mainland China. Actually, an ideal division of air temperature in mainland China should reach 20 areas. For the downscaling forecast, the division should at least reach 20 areas in mainland China. (2) Trends of temperature variations for the annual and seasonal series in various subregional areas are summarized in Table 1. For the annual series, all areas in mainland China experienced a trend of increasing temperature. In northern China, the trend of increasing temperature was about C=10 yr, while in southern China it was less than 0.1 C=10 yr over the last 4 decades. In winter season, the increasing trend of temperature was C=10 yr in northern China while it was about C=10 yr in southern China. The increasing trend of autumn temperature was mainly located in northwestern China and southwestern China including the Tibetan Plateau. Table 1. Trends of regional temperature variation (unit: C=10 yr) Region Annual Spring Summer Autumn Winter Northeast China North Xinjiang South Xinjiang Northwest China North China Lower Yangtze River Upper Yangtze River South China Southwest China Tibetan Plateau

14 14 W. Qian and A. Qin Table 2. Temperature rapid changes in various regions. Sample þ1977(14 16) indicates the temperature rapid increase in 1977 at the timescales of years while sample 1983 (4 7) indicates the temperature rapid decrease in 1983 at the timescales of 4 7 years. All denotes all timescales Region Annual Spring Summer Autumn Winter Northeast China þ1977 (14 16) þ1981 (16 20) þ1982 (12 18) þ1977 (13 16) þ1988 (all) þ1989 (4 12) þ1988 (all) þ1986 (all) North Xinjiang þ1977 (6 16) þ1973 (4 11) þ1977 (13 15) þ1977 (11 16) 1984 (4 7) South Xinjiang þ1977 (8 12) þ1977 (10 16) þ1985 (9 15) Northwest China þ1977 (12 16), þ1987 (all) North China þ1992 (9 10) 1976 (10 15) 1979 (11 18) Lower Yangtze 1969 (6 9) River Upper Yangtze 1969 (6 9) 1976 (13 15) River South China 1969 (4 9), þ1977 (4 8), þ1985 (12 16) þ1991 (8 10) þ1988 (12 13) þ1977 (11 16) þ1986 (all) þ1986 (12 15) Southwest China þ1977 (all), þ1979 (all) þ1977 (all) þ1980 (18 20) þ1979 (17 20) þ1987 (10 13) þ1984 (11 16) Tibetan Plateau þ1977 (all) þ1987 (9 10) In spring, the rising trend of temperature was concentrated in Northeast China and North China, while there was a declining temperature of 0.13 C=10 yr in the upper Yangtze River. In summer, the declining trend of temperature was only concentrated in the midlow valley of the Yangtze and Yellow Rivers, while surrounding this valley there was an increasing trend of summer temperature in South China, Southwest China, Northwest China, and Northeast China. (3) Rapid changes in temperature in various areas were detected by the multiple timescale t-test method. Table 2 shows the rapid change points of temperature in the last 4 decades. For the annual series, the year 1969 was a rapid change point from a high temperature to a low temperature along the Yangtze River and South China. In , the temperature significantly increased from a lower level to a higher level in many places except in the areas of North China and the Yangtze River. Another rapid increase in temperature was observed in The rapid increase of spring temperature appeared in mainly in northern China, while a rapid temperature decrease was found in 1976 over the upper Yangtze River. In , a positive rapid change of temperature occurred in northwestern China and southwestern China while a decreasing temperature was found in North China. The rapid increase of autumn temperature mainly appeared in Northeast China from 1982 to The rapid increase of winter temperature was found for and in many places. (4) Changes in the climate state in seasonal and annual temperature were analyzed. For annual temperature, there were increasing events of extreme temperature in broad areas except in the north part of Northeast China and the north part of the Xinjiang region. Changes in the increasing temperature of the climate state were mainly found in winter, but the extreme events in winter were greatly weakened. In summer, both the increasing temperature of the climate state and the enhancing temperature extreme were commonly exhibited in northern China. These results are consistent with the difference between two 20-year periods of the average temperature of minus

15 Spatial-temporal characteristics of temperature variation in China 15 In summery, the increasing rate of temperature for the last 41 years is lower in the upper Yangtze River than its surrounding regions for the annual and autumn winter series while the negative trends are located in the upper Yangtze River in spring and over the mid-low reaches of Yangtze- Yellow river valley in summer. There are two questions. One is why the enhancing trend of temperature is concentrated in winter in northern China while the declining trend is situated in summer along the Yangtze River. Other is why temperature rapid change is found in the late of 1970s. In winter, temperature variations in China particularly in northern China are strongly sensitive to the Siberian high activity (Gong and Ho, 2004a). The strength and position of the Siberian high experienced an abrupt change since the 1980s, which coincided with the rising of winter temperature particularly in northern China (Qian et al, 2001). The reduction of cool nights in winter and the increase of warm days in winter are observed over northern China (Qian and Lin, 2004). These should have a positive contribution to the rising trend of air temperature. In summer, an abnormal summer climate pattern of north drought in North China and south flooding along the Yangtze River started in the late 1970s-early 1980s was widely revealed (Xu, 2001; Zhang, 1999; Huang et al, 2003). The abrupt change of regional temperature around 1977 may be caused by other climate factors, such as more summer rainfall in the Yangtze River basin and dry climate along the Yellow River valley (Qian and Lin, 2004), and non-climate factors, such as urbanization and industrial aerosols (Xu, 2001). Actually, the monsoon circulation in East Asia (Huang et al, 2003), including the winter summer monsoon weakening (Qian et al, 2003) and the subtropical high strengthening (Gong and Ho, 2002) may be attributed to the evolution of regional temperature in China. Acknowledgments The authors wish to thank two reviewers whose comments and suggestions are helpful for improving the quality of this paper. This research was supported by the National Basic Research Program of China under contracts 2006CB and NNFC References Boo KO, Kwon WT, Kim JK (2004) Vegetation change I the regional surface climate over East Asia due to global warming using BIOME4. Geophys Space Phys 27(4): Boyle JS, Chen TJ (1987) Synoptic aspects of the wintertime. In: East Asian monsoon (Chang CP, Krishnamurti TN, eds), Monsoon meteorology. Oxford University Press, pp Cayan D, Kammerdiener S, Dettinger M, Caprio J, Peterson D (2001) Changes in the onset of spring in the western United States. Bull Amer Meteorol Soc 82: Chan J, Li CY (2004) The East Asia winter monsoon. In: East Asian monsoon (Chang CP, ed), World Scientific Publishing, pp Ding YH (1992) Summer monsoon rainfall in China. J Meteor Soc Japan 70: Ding YH (2004) Seasonal march of the East-Asian summer monsoon. In: East Asian monsoon (Chang CP, ed), World Scientific Publishing, pp 3 53 Feng S, Hu SQ, Qian WH (2004) Quality control of daily meteorological data in China, : a new dataset. Int J Climatology 24: Gong DY, Ho CH (2002) Shift in the summer rainfall over the Yangtze River valley in the late 1970s. Geophys Res Lett 29(10): 1436 (May 15) Gong DY, Ho CH (2004a) Intra-seasonal variability of wintertime temperature over East Asia. Int J Climatology 24: Gong DY, Pan YZ, Wang JA (2004b) Changes in extreme daily mean temperatures in summer in eastern China during Theor Appl Climatol 77: Gong DY, Wang SW, Zhu JH (2004c) Arctic oscillation influence on daily temperature variation in winter over China. Chinese Sci Bull 49(6): Hansen JE, Ruedy R, Glascoe J, Sato M (1999) GISS analysis of surface temperature change. J Geophys Res 104: Huang RH, Zhou LT, Chen W (2003) The progresses of recent studies on the variabilities of the East Asian monsoon and their causes. Adv Atmos Sci 20: IPCC (2001) The science of climate change. In: Contribution of Working Group I to the 3rd Assessment Report of the Intergovernmental Panel on Climate Change (Houghton JT et al, eds), UK and New York: Cambridge University Press, Cambridge Lau NC, Lau KM (1984) The structure and energetics of midlatitude disturbances accompanying cold-air outbreaks over East Asia. Mon Wea Rev 112: Li XD, Zhu YF, Qian WH (2002) Spatio-temporal variations of summer rainfall over eastern China during Adv Atmos Sci 19: Liu BH, Xu M, Henderson M, et al (2004) Taking China s temperature: daily range, warming trends, and regional variation, J Climate 17(22): Lu AG, He YQ, Zhang ZL, et al (2004) Regional structure of global warming across China during the twentieth century. Climate Res 27(3):

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