The Decadal Shift of the Summer Climate in the Late 1980s over Eastern China and Its Possible Causes

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1 NO.4 ZHANG Renhe, WU Bingyi, ZHAO Ping et al. 435 The Decadal Shift of the Summer Climate in the Late 1980s over Eastern China and Its Possible Causes ZHANG Renhe ( ), WU Bingyi ( ), ZHAO Ping ( ), and HAN Jinping ( ) State Key Laboratory of Severe Weather (LaSW), Chinese Academy of Meteorological Sciences, Beijing (Received September 28, 2008) ABSTRACT In this paper, it is pointed out that a notable decadal shift of the summer climate in eastern China occurred in the late 1980s. In association with this decadal climate shift, after the late 1980s more precipitation appeared in the southern region of eastern China (namely South China), the western Pacific subtropical high stretched farther westward with a larger south-north extent, and a strengthened anticyclone at 850 hpa appeared in the northwestern Pacific. The decadal climate shift of the summer precipitation in South China was accompanied with decadal changes of the Eurasian snow cover in boreal spring and sea surface temperature (SST) in western North Pacific in boreal summer in the late 1980s. After the late 1980s, the spring Eurasian snow cover apparently became less and the summer SST in western North Pacific increased obviously, which were well correlated with the increase of the South China precipitation. The physical processes are also investigated on how the summer precipitation in China was affected by the spring Eurasian snow cover and summer SST in western North Pacific. The change of the spring Eurasian snow cover could excite a wave-train in higher latitudes, which lasted from spring to summer. Because of the wave-train, an abnormal high appeared over North China and a weak depression over South China, leading to more precipitation in South China. The increase of the summer SST in the western North Pacific reduced the land-sea thermal contrast and thus weakened the East Asian summer monsoon, also leading to more precipitation in South China. Key words: summer climate in eastern China, decadal climate variability, decadal climate shift 1. Introduction The summer climate in eastern China is under the influence of the East Asian summer monsoon and is characterized by multi-time scale variations (Wang et al., 2005; Ding, 2007), among which the decadal variation is one of the distinguished features (Li et al., 2004; Zhao and Nan, 2006). Wang et al. (2000) showed that during the period of , eastern China registered a noticeable decadal variation featured with dry and wet cycles, and there were no long term trends for both annual and seasonal rainfalls. The investigation made by Shi et al. (1995) indicated that in the past century, eastern China witnessed five decadal climate shifts between wet and dry episodes. For example, 1902/03 was featured with a shift from abnormal dry to abnormal wet, 1918/19 from abnormal wet to abnormal dry, 1930/31 from dry to wet and dry cyclings, 1944/45 into abnormal wet, and 1964/65 into abnormal dry. The decadal rainfall variations in eastern China were featured with a sophisticated spatial distribution (Qin, 2005). In the decadal times cale, there was a significant difference in the rainfall among the areas of North China, the Yangtze River Valley, and South China. For example, the precipitation change in North China would frequently go opposite to that in the middle and lower reaches of the Yangtze River. Sometimes the precipitation variation in South China would go along with that in North China, though occasionally acted oppositely. The limited observational data in the remote past have confined people studying the decadal rainfall variation to using the data available after the 1950s. Previous studies show that eastern China experienced a marked decadal shift of the summer climate in the late 1970s. Based on an analysis of the summer rainfall data in China during the period of , Huang et al. (1999) have detected a large change of the summer rainfall between Supported by the National Key Program for Developing Basic Sciences under Grant No. 2004CB and National Natural Science Foundation of China under Grant No Corresponding author: renhe@cams.cma.gov.cn.

2 436 ACTA METEOROLOGICA SINICA VOL.22 the 1970s and 1980s. Both the Yangtze River Valley and the Huaihe River Valley have evidently increased flood disaster events caused by excessive rainfall since the late 1970s. Meanwhile, both the southern and northern parts of China witnessed noticeably reduced rainfall in the 1980s, compared with the 1970s. Examining the rainfall data from 1958 to 1999, Zhang and Wu (2001) detected abnormally less rainfall, or droughts in the climatic term, across the Yangtze River Valley before the late 1970s. The same region has shifted to a wet period from the 1980s. Having analyzed the decadal climate variation of the summer rainfall over North China, Chen (1999) pointed out that during the period of , two abrupt drying processes occurred in North China. One appeared in the mid-1960s, and the other in the late 1970s, with the latter having a scale and scope that was larger than the former. The studies made by Chen et al. (1998) and Shi and Xu (2007) also indicated that both Northeast China and the lower and middle reaches of the Yangtze River had abnormally less rainfall during the periods of 1960s and 1970s, with a decadal climate shift appeared in the late 1970s. The two regions have registered more rainfall in the 1980s. On the contrary, both South and North China had shifted from wet to dry since the late 1970s. Having examined the physical attributes of the decadal climate shift of the summer rainfall in eastern China in the late 1970s from different angles, many investigators believed that the decadal climate shift of the summer rainfall in eastern China in the late 1970s was associated with the decadal variation of a range of other natural elements in the climate system, including the decadal ENSO cycle (Huang et al., 1999), the thermal forcing of the Tibetan Plateau (Zhao and Chen, 2001), the Pacific decadal oscillation (PDO) (Li and Xian, 2003; Yang et al., 2004), the decadal cooling of upper troposphere in summer across the east part of China (Yu et al., 2004), the decadal variation of sea surface temperature (SST) in the northern Atlantic Ocean (Lu et al., 2006), the decadal change of the difference between the surface temperature over Northwest Pacific and East Asian continent in spring (Xu et al., 2007), and the decadal variation of the difference between the disturbed summer temperature in the middle and upper troposphere over the Asian continent and that over the northern Pacific Ocean (Zhao et al., 2007). In addition, some other studies pointed out that the increased aerosol concentration derived from human activities in eastern China could also be a possible cause of the decadal climate shift of the summer rainfall across eastern China in the late 1970s (Xu, 2001; Menon et al., 2002). All the cited studies shared the view that the decadal variation of these physical elements was associated with the weakening of the East Asian summer monsoon, which facilitated the southbound movement of the summer rain belt in the late 1970s, allowing abundant rainfall in the Yangtze and Huaihe River Valleys, but reduced rainfall across both South and North China. Apparently, we have gained an improved understanding of the decadal climate shift of the summer rainfall in eastern China in the late 1970s and associated possible attributions. However, what about the decadal climate variation of the summer rainfall over eastern China since the end 1970s? Has it also a decadal shift? These are the questions this paper to discuss. In Section 2 it is proposed that decadal climate shifts of the summer rainfall and circulation occurred too over East China in the late 1980s. Based on the authors latest findings, in Sections 3 and 4 the attributes of this decadal climate shift are analyzed. The conclusion and discussion are given in Section Decadal climate shift of the East Asian summer monsoon in the late 1980s Because of the sophisticated nature of the East Asian monsoon, many scientists have defined East Asian summer monsoon indices from different angles (Zhao and Zhou, 2005). Wang et al. (2001) defined the western North Pacific-East Asian summer monsoon (WNP-EASM) index in the context of the horizontal shear of the zonal wind at 850 hpa in south of 30 N. Wu et al. (2008a) obtained two modes through the Hermit matrix decomposition of 850-hPa wind fields. Each mode has two sub-modes. The four modes could

3 NO.4 ZHANG Renhe, WU Bingyi, ZHAO Ping et al. 437 be employed to depict well the variability of the East Asian summer monsoon. To illustrate the decadal variability of the East Asian summer monsoon, Fig.1 presents the 7-yr running mean of the WNP-EASM index defined by Wang et al. (2001) (Fig.1a), and the 7-yr running mean of the principle component of the first East Asian summer monsoon mode obtained by Wu et al. (2008a) (Fig.1b). One can see from Fig.1 both indeices exhibited significant signals of the decadal variability, with a fine consistency between the two indices. In the period ranging from the mid-1970s to the end 1980s, the summer monsoon indices showed high values. In the 1990s, the summer monsoon indices had lower values. Both indices have indicated a shift from high to low values in the late 1980s, suggesting that the East Asian summer monsoon has experienced a decadal climate shift in the late 1980s. Figure 2 shows the difference between the summer (JJA) mean rainfall averaged from and that averaged from , based on the precipitation data observed at 618 stations in China, in an attempt to demonstrate the summer rainfall variation across China in association with the decadal climate shift of the East Asian summer monsoon in the late 1980s. It is apparent that the rainfall difference between the two periods has a relatively large positive value in eastern China to the south of 30 N, except a small area near the eastern part of Yunnan around 104 E. In other words, the southern areas in eastern China (namely South China) experienced a remarkably increased rainfall in the period of , compared with the period of In addition, there appeared an abnormally wet area, though small in size, in the western part of the Shandong Peninsula. Undoubtedly, summer climate in eastern China featured with unique variation, though the globe, including China, has witnessed a sustained ascending surface temperature since the 1980s (Qin, 2005). The East Asian summer monsoon indices in Fig.1 shows that the East Asian monsoon does not go along with a consistent changing trend, but with a distinct decadal variability and a pronounced decadal climate shift. The East Asian summer monsoon changed from strong to weak in the late 1980s. In China, a rainfall increase was seen in eastern China to the south of 30 N from the period of compared to that Fig.1. The 7-yr running means of (a) the WNP-EASM Index from Wang et al. (2001) (unit: m s 1 ) and (b) the principal component of the leading mode of East Asian monsoon from Wu et al. (2008a).

4 438 ACTA METEOROLOGICA SINICA VOL.22 Fig.2. Difference of summer (JJA) rainfall in China between and (unit: mm; interval: 10 mm). Solid and dashed lines represent positive and negative differences, respectively. Thick lines denote zero. of In order to investigate the circulation change associated with the decadal climate shift of the summer rainfall as shown in Fig.2, based on the ERA40 reanalysis data from ECMWF, Fig.3 shows the 5880-gpm contour at 500 hpa in summer averaged in and that averaged in , to illustrate the extent of the western Pacific subtropical high (WPSH). The difference between summer averaged wind at 850 hpa in and that averaged in is shown in Fig.4. From Fig.3, we can see that the WPSH in the period of became stronger, stretching farther westward with a larger south-north extent, compared to that in the period of , which is favorable for the development of the southerlies over South China. Corresponding to the strengthened WPSH, in Fig.4, we can see that a salient anticyclone at 850 hpa appeared in the northwestern Pacific. The air flows along the western part of the anticyclone led to the southerlies developing in the south of the Yangtze River. The strengthened southerlies were beneficial to the strengthening of the water vapor transportation and therefore more precipitation in the south of the Yangtze River. Here we can see that, corresponding to the decadal climate shift of the East Asian summer monsoon in the late 1980s, both the summer circulation over East Asia and the summer rainfall in China changed significantly. In the next Fig.3. Summer (JJA) mean 5880-gpm contour at 500 hpa averaged in (dashed line) and (solid line), respectively.

5 NO.4 ZHANG Renhe, WU Bingyi, ZHAO Ping et al. 439 Fig.4. Difference of summer (JJA) mean wind fields between and (unit: m s 1 ). two sections we will discuss the physical mechanisms which are possibly responsible for the decadal climate shift. 3. Association with Eurasian spring snow cover Utilizing the singular value decomposition (SVD) approach, Wu et al. (2008b) analyzed the relationship between the spring snow water equivalent (SWE) across the Eurasian continent and summer rainfall in China. Figure 5 shows distributions of the SVD first mode for the Eurasian spring SWE and China s summer rainfall. It is not difficult to find that the major part of the Eurasian continent has shown a basically consistent variation of spring snow cover (Fig.5a), except the south and east border areas. Corresponding to the consistent snow cover distribution, in eastern China the rainfall in the south is opposite to that in the north (Fig.5b). The summer rainfall in the south agrees well with the rainfall difference before and after the decadal climate shift occurred in the late 1980s, as indicated in Fig.2. A comparison between Figs.2 and 5b implies that in addition to an almost identical distribution in South China, the two even have a consistent distribution in the west of the Shandong Peninsula, and in some smaller areas near the Hetao Plain. The consistency can also be spotted in the eastern part of Yunnan near 104 E. Undoubtedly, the decadal summer rainfall shift in eastern China in the late 1980s was closely associated with the changing of the Eurasian spring snow cover. Figure 6 presents the time series of the SVD first mode for the Eurasian spring SWE and the summer rainfall in China, showing a quite consistent variation between the two, with the correlation coefficient reaching as high as Like the East Asian summer monsoon, both snow cover and rainfall have witnessed a striking change in the late 1980s. The values after the late 1980s have turned mainly positive, though being negative before that, indicating that the reduced Eurasian spring snow cover went along with the decreased summer rainfall in North China, and with the increased summer rainfall in South China. A comparison with Fig.5 also reveals that before the late 1980s, the abnormally abundant Eurasian spring snow corresponded to the abnormally less rainfall in the southern part of China. After the late 1980s, the Eurasian spring snow cover had become abnormally low, with abnormally abundant rainfall in South China, indicating that the Eurasian spring snow cover was closely associated with the increased rainfall in South China, which was associated with the decadal summer climate shift. To illustrate the impact of the Eurasian spring snow cover on the summer rainfall in China and the associated physical process, Wu et al. (2008b) calculated the linear regressed geopotential height fields at 500 hpa in spring and summer, respectively, based on the time series of the SVD first mode for the Eurasian spring snow cover. The results showed that the regressed 500-hPa geopotential height fields in spring presented a teleconnection wave train triggered by the snow cover over the Eurasian continent in the north of 40 N. In association with the teleconnection wave train, in the regressed geopotential height fields a high appeared to the west of the Baikal Lake. This high controlled the area in the northern part of China. In summer, the regressed 500-hPa geopotential height fields were similar to those in spring, indicating that the teleconnection wave train triggered by the Eurasian spring snow cover had sustained from the spring till the summer, placing North China under the high pressure, which was not desirable for rainfall. The southern part of China was dominated by a weak

6 440 ACTA METEOROLOGICA SINICA VOL.22 Fig.5. Spatial distributions of the left (a; SWE) and right (b; summer rainfall in China) fields of the leading SVD mode (from Wu et al., 2008b). Fig.6. Time series of the SVD leading mode for SWE (solid line) and summer rainfall in China (dotted line)(from Wu et al., 2008b).

7 NO.4 ZHANG Renhe, WU Bingyi, ZHAO Ping et al. 441 depression, facilitating the southbound movement of summer rain belt for more rainfall in the south. Here we can see that the increased rainfall in South China from the late 1980s is closely associated with the high latitude teleconnection wave train triggered by the Eurasian spring snow cover. The fact that the wave train is able to sustain till the summer provides the physical process by which the decadal change of the Eurasian spring snow cover affects the decadal summer climate shift over the east part of China. 4. Association with the Northwest Pacific sea surface temperature Wu and Zhang (2007) decomposed the sea surface temperature (SST) in Northwest Pacific by using empirical orthogonal function (EOF) analysis. Figures 7 and 8 present first two principal components and evolutions of the corresponding time series, respectively. The first (EOF1) and second (EOF2) principal components have a variance contribution at 30.5% and 15.2%, respectively. In Fig.7, EOF1 shows a wholly consistent SST distribution (Fig.7a), and EOF2 a tripole distribution pattern as + + in the northsouth direction (Fig.7b). Figure 8 presents evolutions of the time series, indicating a sustained decadal variability in the first principal component (Fig.8a). The decadal climate shift occurred in the late 1980s. The values before the late 1980s were basically negative, then became positive after the late 1980s. From Fig.7a, we can know that in the late 1980s, the uniformly consistent SST in Northwest Pacific had a significant decadal climate shift, namely the SST was abnormally cooler before the late 1980s, with a negative anomaly, and abnormally warmer after that, with a noted positive anomaly. The time series of the second principal component (Fig.8b) does not show a marked decadal variability, but rather an interannual variation. A 5-yr running mean is performed on both the summer rainfall and EOF1 time series, to understand the effect of the Northwest Pacific SST on the decadal climate variation of the summer rainfall in China, and on the climate shift of the East Asian summer monsoon in the late 1980s. The correlation coefficients between the 5-yr running mean of the EOF1 time Fig.7. (a) EOF1 and (b) EOF2 of the SST in the western North Pacific (from Wu and Zhang, 2007).

8 442 ACTA METEOROLOGICA SINICA VOL.22 Fig.8. Time series of (a) EOF1 and (b) EOF2 of the SST in the western North Pacific (from Wu and Zhang, 2007). series and that of the China s summer rainfall are calculated and the results are shown in Fig.9. We can see in Fig.9 that there is a clear positive correlation in South China, namely the abnormally low Northwest Pacific SST is associated with the abnormal dry condition in South China, and the abnormally high Northwest Pacific SST is associated with the abnormal wet condition in South China. Figure 8a indicates that the Northwest Pacific Ocean had an abnormally low SST before the late 1980s, and an abnormally high SST afterwards, indicating that China s decadal summer climate shift in the late 1980s is closely associated with the decadal change of the Northwest Pacific SST. A comparison of Figs.9 and 2 shows that the distribution patterns resemble each other nicely. The consistent distribution can even be spotted in smaller areas, such as the west of the Shandong Peninsula, the area near the Hetao Plain, and the east of Yunnan around 104 E. The consistent patterns of these smaller areas further indicate that the ascending of the Northwest Pacific SST from the late 1980s is closely associated with the increasing of the rainfall in South China. 5. Conclusions and discussion Based on the evolutions of two indices of the East Asian summer monsoon, it is reported that both monsoon indices consistently exhibit significant signals of the decadal variability. The summer monsoon indices show high values from the mid-1970s to the late 1980s. In the 1990s low index values appeared. Both summer monsoon indices changed from high to low values in the late 1980s, indicating decadal climate shift in the late 1980s. Corresponding to the decadal climate shift of the East Asian summer monsoon in the late 1980s, both the summer rainfall in China and the circulation over East Asia changed remarkably. The difference of the summer rainfall averaged in the period from 1990 to 2001 and that from 1975 to 1989 is positive in South China, showing a noticeable rainfall increase there after the late 1980s. The WPSH in the period of became stronger, stretching farther westward with a larger south-north extent, compared to that in the period of , which was favorable

9 NO.4 ZHANG Renhe, WU Bingyi, ZHAO Ping et al. 443 Fig.9. Correlation coefficients between 5-yr running means of the summer precipitation in China and the EOF1 time series of the western North Pacific SST. for the development of the southerlies over South China. The difference between sumer averaged wind at 850 hpa in and that averaged in shows a salient anticyclone at 850 hpa appearing in the northwestern Pacific. The air flows along the western part of the anticyclone led to the southerlies developing in the south of the Yangtze River. The strengthened southerlies were beneficial to the strengthening of the water vapor transportation and therefore more precipitation in the south of the Yangtze River. Furthermore, an SVD method is employed to analyze the relationship between the Eurasian spring SWE and China s summer rainfall. The results indicate that, like the East Asian summer monsoon, both snow cover and rainfall had a noticeable change in the late 1980s. The abnormally abundant Eurasian snow cover before the late 1980s corresponded to the abnormally less rainfall in the southern part of eastern China. After the late 1980s, the Eurasian spring snow cover had become abnormally less, with abnormally abundant rainfall in South China, indicating that the Eurasian spring snow cover was closely related to the increased summer rainfall in South China. To illustrate the impact of Eurasian spring snow cover on China s summer rainfall and the associated physical process, the time series of the SVD first mode for the Eurasian spring snow cover is used to calculate the linear regressed geopotential height fields at 500 hpa in spring and in summer, respectively. It is found that the Eurasian spring snow cover had triggered a teleconnection wave train lasting from the spring till the summer over the Eurasian continent in the north of 40 N, allowing the northern part of eastern China under the dominance of a high pressure, which was not desirable for rainfall. The southern part of eastern China was dominated by a weak depression, facilitating the southbound movement of the summer rain belt for more rainfall in the south, indicating that the high latitude system can play an important role in the decadal climate change in East Asia. As a matter of fact, Wu et al. (2008c) found that, like the East Asian summer monsoon, the summer atmospheric circulation over Arctic also saw a distinguished decadal climate shift in the late 1980s. The EOF analysis has been applied to the Northwest Pacific SST. The EOF1 presents a wholly consistent SST distribution, with the time series showing a

10 444 ACTA METEOROLOGICA SINICA VOL.22 significant decadal change. The decadal climate shift occurred in the late 1980s. Before the late 1980s the Northwest Pacific was basically dominated by negative SSTs, and afterwards by positive SSTs, indicating that the uniformly consistent Northwest Pacific SST had a pronounced decadal climate shift in the late 1980s, namely the SST was abnormally cooler before the late 1980s, mainly with a negative anomaly, and abnormally warmer afterwards, with a marked positive anomaly. The distribution of the correlation coefficients between China s summer rainfall and the time series of EOF1 for the Northwest Pacific SST shows that there is a distinct positive correlation in the southern part of eastern China, namely the abnormally low Northwest Pacific SST is associated with the abnormal dry condition in the southern part of eastern China, and the abnormally high Northwest Pacific SST is associated with the abnormal wet condition there. The Northwest Pacific had abnormally low SSTs before the late 1980s and abnormally high SSTs after that, indicating that China s summer climate shift in the late 1980s is closely associated with the changing of the Northwest Pacific SST. It is believed that the increased SSTs have narrowed down the temperature difference between the ocean and the land, weakened the East Asian summer monsoon, and caused more rainfall to occur in the southern part of eastern China. Analyses in this paper show that the decadal changes of both the Eurasian spring snow cover and the northwestern Pacific summer SST are important factors contributing to the decadal summer climate shift in the east part of China in the late 1980s. As seen in Figs.5 and 9, it is apparent that both the Eurasian spring snow cover and the Northwest Pacific summer SST are closely associated with the summer rainfall in the northwestern part of China. In fact, a previous study by Shi et al. (2008) has indicated that Xinjiang registered an abrupt change of rainfall in the late 1980s, with more rainfall after that. Therefore, impacts of the Eurasian spring snow cover and the Northwest Pacific summer SST on the rainfall in the northwest part of China in the decadal scale is a topic deserving further investigation. Additionally, it is also necessary to supplement the present diagnostic analyses with model simulations to further verify the relationship between the Eurasian spring snow cover and the summer rainfall in China, and that between the latter and the Northwest Pacific summer SST. Furthermore, future studies should be conducted to probe the decadal changes of the Eurasian spring snow cover and the Northwest Pacific summer SST themselves, as it remains unclear why such decadal changes take place. Chen Lieting, 1999: REFERENCES Regional features of interannual and interdecadal variations in summer precipitation anomalies over North China. Plateau Meteor., 18(4), (in Chinese) Chen Longxun, Zhu Wenqin, Wang Wen, et al., 1998: Study on climate change in China in recent 45 years. Acta Meteor. Sinica, 56(3), (in Chinese) Ding Y. H., 2007: The variability of the Asian summer monsoon. J. Meteor. Soc. Japan, 85B, Huang Ronghui, Xu Yuhong, and Zhou Liantong, 1999: The interdecadal variation of summer precipitations in China and the drought trend in North China. Plateau Meteor., 18(4), (in Chinese) Li C. Y., and P. Xian, 2003: Atmospheric anomalies related to interdecadal variability of SST in the North Pacific. Adv. Atmos. Sci., 20, Li C. Y., J. H. He, and J. Zhu, 2004: A review of decadal/interdecadal climate variation studies in China. Adv. Atmos. Sci., 21, Lu R. Y., B. W. Dong, and H. Ding, 2006: Impact of the Atlantic multidecadal oscillation on the Asian summer monsoon. Geophys. Res. Lett., 33, L24701, doi: /2006gl Menon, S., J. Hasen, L. Nazarenko, et al., 2002: Climate effects of black carbon aerosols in China and India. Science, 29, Qin Dahe, 2005: Climate and Environment Changes in China. Science Press, Beijing, 562 pp. Shi Neng, Chen Jiaqi, and Tu Qipu, 1995: 4-phase climate change features in the last 100 years over China. Acta Meteor. Sinica, 53(4), (in Chinese) Shi X. H., and X. D. Xu, 2007: Regional characteristics of the interdecadal turning of winter/summer climate

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