Abstract

In this study, the authors found that the summer precipitation over China experienced different decadal variation features from north to south after the late 1990s. In northeastern and North China and the lower–middle reaches of the Yangtze River, precipitation decreased after 1999, while precipitation experienced a significant reduction over South and southwestern China and a significant increase over the southern parts of Hetao region and Huaihe River valley after 2003. The authors next analyzed the associated decadal variation of the atmospheric circulation and attempted to identify the mechanisms causing the two decadal variations of precipitation. The wind anomalies for the former exhibit a barotropic meridional dipole pattern, with anticyclonic anomalies over Mongolia to northern China and cyclonic anomalies over the southeastern Chinese coast to the northwestern Pacific. For the latter, there is a southeast–northwest-oriented dipole pattern in the middle and lower troposphere, with cyclonic anomalies over the northern parts of the Tibetan Plateau and anticyclonic anomalies over the lower–middle reaches of the Yangtze River to southern Japan. An anomalous anticyclone dominates the upper troposphere over China south of 40°N. The authors further found that the summer sea surface temperature (SST) warming over the tropical Atlantic played an important role in the decadal variation around 2003 via inducing teleconnections over Eurasia. In contrast, the decadal variation around 1999 may be caused by the phase shift of the Pacific decadal oscillation (PDO), as has previously been indicated.

1. Introduction

China, with its large zonal and meridional range, is located in eastern Eurasia. Climate change and its causes over China are very complicated. The main atmospheric circulation systems impacting summer climate over China are the western North Pacific subtropical high (WNPSH), the South Asian high (SAH), the East Asian subtropical westerly jet, stationary Rossby waves and blocking highs in the middle–high latitudes over Eurasia, and so on (Tao and Chen 1987). These atmospheric circulation systems also act as a bridge between summer climate over China and other climate systems around the globe. For example, Arctic ice (Wu et al. 2009a,c), the North Atlantic Oscillation (NAO) (Liu and Yin 2001; Sung et al. 2006; Sun et al. 2008; Wu et al. 2009; Sun and Wang 2012), and thermal conditions over the North Atlantic (Sun et al. 2009a; Wu et al. 2009; Gao et al. 2013) in the upstream area of Eurasia can impact summer climate over China by influencing stationary Rossby wave activities and blocking highs in the middle–high latitudes over Eurasia. The variation of thermal conditions over the tropical Indian Ocean, northwestern Pacific, and Indo-Pacific warm pool can lead to anomalies of summer atmospheric circulation and climate over China by changing the local atmospheric circulation or by exciting the stationary Rossby waves over Eurasia or the coast of East Asia (Huang and Li 1987; Li and Mu 2001; Yang et al. 2007; Li et al. 2008; Yuan et al. 2008; Xie et al. 2009; Chen and Huang 2012). By changing the thermal conditions over these three regions, remote climate systems, such as the Atlantic multidecadal oscillation (AMO) (Lu et al. 2006), the Antarctic Oscillation (AAO) (Fan and Wang 2004; Wang and Fan 2005; Sun et al. 2009b), and the eastern tropical Pacific SST anomalies (Wang et al. 2000; Yang et al. 2007; Zhou and Chan 2007; Xie et al. 2009; Wang et al. 2014), can make a contribution to summer climate change over China. The thermal forcing of the Tibetan Plateau also has a significant influence on summer climate over China (Ye and Gao 1979; Yanai et al. 1992; G. Wu et al. 2012). Besides, atmospheric aerosols (Menon et al. 2002) and land surface processes, such as previous-season snowmelt over Eurasia (Yang and Xu 1994; Zhang et al. 2004; Wu and Kirtman 2007; Ding et al. 2009; Wu et al. 2009b; Wu et al. 2010), have important impacts on summer climate over China through their modulation of the thermal contrast over local regions or between Eurasia and the oceans. In short, the summer climate of China exhibits interannual and decadal variability with spatiotemporal structure, under the combined influence of the climate systems over the tropics and middle–high latitudes, as well as its own natural variability and feedback to atmospheric circulation anomalies (Lu and Lin 2009).

The interannual and decadal variability of the summer precipitation are obvious over China. The summer precipitation over eastern China, which is located in the East Asian summer monsoon (EASM) area, is closely associated with the variation of the EASM circulation on interannual and decadal time scales. The summer precipitation over southwestern China is also influenced by the summer monsoon circulation over there. As northwestern China is far away from the ocean and cannot receive the vapor transported by summer monsoon circulation, it has far less summer precipitation but also with significant interannual and decadal variability. Because of the sparse observation networks, relatively less attention has been given to the variability of summer precipitation over western China. Since the middle of the twentieth century, the summer precipitation over eastern China has experienced three decadal variations, in the mid and late 1970s, early 1990s, and late 1990s (Ding et al. 2008; Si et al. 2009; Zhou et al. 2009; Wu et al. 2010; Huang et al. 2011; Liu et al. 2011; Zhu et al. 2011; Huang et al. 2012). The summer precipitation pattern over eastern China shifted abruptly in the mid- and late 1970s, as a result of the weakening of the EASM (Wang 2001). The anomalies exhibited a triple meridional pattern, with more precipitation over the lower–middle reaches of the Yangtze River but less precipitation over South and North China. The warming over the tropical Indian Ocean to the warm pool and the warming over the central and eastern tropical Pacific Ocean corresponding to the phase shift of the Pacific decadal oscillation (PDO) (Hu 1997; Huang et al. 1999; Weng et al. 1999; Gong and Ho 2002; Yang and Lau 2004; Li et al. 2010; Qian and Zhou 2014; Yu et al. 2015), the increase of snow depth over the Tibetan Plateau in winter and spring (Zhang et al. 2004; Ding et al. 2009), anthropogenic activity (Menon et al. 2002; Wang et al. 2013), and natural decadal variability (Jiang and Wang 2005) are the possible causes for the shift of the precipitation regime. In the early 1990s, the summer precipitation over South China increased remarkably (Kwon et al. 2007; Ding et al. 2008; Yao et al. 2008; Zhang et al. 2008; Wu et al. 2010). The cause may be the combined contribution of the warming of the Indian Ocean SST in summer and the increase of snow over the Tibetan Plateau in winter and spring and the decrease in northern Eurasia in spring (Zhang et al. 2008; Wu et al. 2010). Since the late 1990s, the summer mei-yu belt over eastern China has moved northward, which has led to a summer precipitation decrease over the lower–middle reaches of the Yangtze River but an increase over the Huaihe River valley and the precipitation over North and northeastern China also decreased (Si et al. 2009; Huang et al. 2011; Liu et al. 2011; Zhu et al. 2011; Huang et al. 2012). Zhu et al. (2011) showed that the shift of the PDO to a negative phase was probably responsible for the decadal variation of summer precipitation over eastern China around the late 1990s.

There are pronounced interannual and decadal variability in summer precipitation over western China (Huang et al. 1999; Wei et al. 2003; Yang and Zhang 2007; Zhou and Huang 2008, 2009; Chen et al. 2012). The summer precipitation over northwestern China has increased obviously since the late 1970s, which may be related to the local surface warming (Zhou and Huang 2008, 2009).

The present work first addresses the spatial patterns of the decadal variation of summer precipitation over China after the late 1990s and then compares atmospheric circulation anomalies associated with the decadal variations. Finally, we attempt to identify the mechanisms of the decadal variations. Section 2 describes the data used and the design of sensitivity experiments. Section 3 describes the basic features of summer precipitation over China. The spatial patterns of decadal variation in summer precipitation over China after the late 1990s are described in section 4. Section 5 compares atmospheric circulation anomalies for the decadal variations around 1999 and 2003. Section 6 studies the mechanisms that the summer warming over the tropical Atlantic contributes to the decadal variation around 2003. Section 7 provides conclusions and a discussion.

2. Data and methods

A daily precipitation dataset from 756 stations in China for the period 1961–2012, supplied by the National Meteorological Information Center (NMIC) of the China Meteorological Administration (CMA), was used in this study. After removing stations with continuous missing values based on the data description file, 542 of the 756 stations were selected (see Fig. 2b). The daily precipitation data were first transformed to monthly data, and then the seasonal mean was calculated. The Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) with a horizontal resolution of 2.5° × 2.5° (Xie and Arkin 1996) was used. The summer season in this study was taken to include the months of June, July, August, and September. The monthly reanalysis data used, with a horizontal resolution of 2.5° × 2.5°, were provided by the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) (Kalnay et al. 1996). In addition, we used SST from the Extended Reconstructed SST, version 3b (ERSST.v3b) dataset, with a horizontal resolution of 2.0° × 2.0° (Smith et al. 2008). We applied three methods, including the moving t test (MTT), Mann–Kendall (MK) test (Fu and Wang 1992; Wei 1999), and Lepage test (Lepage 1971; Liu et al. 2011), to detect the abrupt changepoints of the decadal variation of summer precipitation over China. If the abrupt changepoint of decadal variation detected by the three methods is N, this implies that a decadal variation occurred in the period (N − 1)/N. For example, if N is equal to 1999, the decadal variation occurred in 1998/99. The abrupt changepoint of a time series, as detected by the MTT and Lepage test with a 9-yr moving window, is the year with the most statistical significance and is significant at the 0.05 significance level. The MK test was applied to the time series of summer precipitation for the period 1961–2012.

To better detect the decadal variation of summer precipitation over China after the late 1990s, several subregions over China were selected based on the rotated empirical orthogonal function (REOF) analysis of the standardized summer precipitation over China during 1961–2012. The first 12 EOFs, which account for 55.6% of the total variance, are rotated orthogonally. Based on the anomalous center of each REOF (Fig. 1), eight subregions were selected: northeastern China (NEC; 43.5°–52.0°N, 115.0°–135.0°E; Fig. 1b), the lower–middle reaches of the Yangtze River (YZR; 28.0°–32.5°N, 110.0°–122.0°E; Fig. 1c), North China (NC; 36.5°–43.5°N, 115.0°–130.0°E; Fig. 1d), South China (SC; 20.0°–28.0°N, 107.0°–122.0°E; Fig. 1e), southwestern China (SWC; 20.0°–30.0°N, 97.0°–107.0°E; Fig. 1f), the Huaihe River valley (HHR; 32.5°–36.5°N, 110.0°–122.0°E; Fig. 1g), the southern parts of the Hetao region (SHT; 31.0°–36.5°N, 97.0°–110.0°E; Fig. 1i), and the northern parts of Xinjiang Province (NXJ; 40.0°–50.0°N, 80.0°–100.0°E; Fig. 1j). The area average was calculated by the arithmetic mean among stations. The correlation coefficients between area-averaged summer precipitation and the time coefficients of corresponding REOF were above 0.90 for NEC, NC, HHR, and SC; 0.78 for SWC and SHT; and 0.63 for NXJ. The relatively weak correlations for SWC, SHT, and NXJ partly result from relatively sparse observation networks over western China. If the boundaries of each subregion are altered simultaneously by one degree, the results change slightly. Therefore, the selection of eight subregions is reasonable. We performed harmonic analysis for time series to reveal the decadal component with a period greater than 9 yr.

Fig. 1.

The spatial modes of the 12 REOFs of the standardized summer precipitation over China during 1961–2012. The contribution of each mode to the total variance is displayed in the top center of each panel. Black dashed rectangles indicate the area used to calculate the time series of summer precipitation of the eight subregions.

Fig. 1.

The spatial modes of the 12 REOFs of the standardized summer precipitation over China during 1961–2012. The contribution of each mode to the total variance is displayed in the top center of each panel. Black dashed rectangles indicate the area used to calculate the time series of summer precipitation of the eight subregions.

Sensitivity experiments were performed to study the contribution of summer SST anomalies to the decadal variation of summer precipitation. The ECHAM5 atmospheric general circulation model (AGCM) developed by the Max Planck Institute for Meteorology was employed. ECHAM5 is a global spectral model that provides several horizontal and vertical resolution options. We selected a horizontal resolution with T63 spectral truncation and a vertical resolution with 19 levels. The details of the model are described in Roeckner et al. (2003). The experiments were performed as follows: First, a control experiment (EXP_Ctrl) was run, in which AMIP II climatological midmonth SST (Taylor et al. 2000) was prescribed, the simulation was integrated for 50 years, and the last 30 years of results were used as the samples. Then, the SST anomaly sensitivity experiments were run, for which the climatological SST in June, July, August, and September plus observational summer mean SST anomalies in the region were prescribed, and the simulations were integrated from 31 December to 30 September of the following year with the initial conditions taken from the control experiment. Each sensitivity experiment included 30 samples. How the observational summer SST anomalies were calculated is given in section 6. The anomalies of sensitivity experiments are the difference between sensitivity experiments and the control experiment.

3. Basic features of summer precipitation over China

a. Climatology and linear trend

According to Fig. 2a, the climatology of summer precipitation over China decreases from southeast to northwest. The summer precipitation is generally greater than 5 mm day−1 over southern China (south of 32°N) and lies in the range 2–5 mm day−1 for SHT, HHR, NC, and NEC but is less than 1 mm day−1 for most parts of northwestern China. The spatial variation described above is closely related to the difference of the summer water vapor transport between eastern and western China (Huang and Chen 2010). In eastern China and southwestern China, the summer monsoon circulation prevails and brings (especially via the low-level flow) sufficient water vapor to those regions. Geographically, northwestern China is located in the interior of Eurasia. As revealed by Huang and Chen (2010), because of the topographic barrier of the Tibetan Plateau, the water vapor transported by summer monsoon circulation cannot reach there. The water vapor transported by the midlatitude westerlies is the main source of summer precipitation over northwestern China. The water vapor transported by the midlatitude westerlies is far less than that transported by the summer monsoon circulation, which is an important cause for the summer precipitation over northwestern China being much less than that over eastern China and SWC.

Fig. 2.

The (a) climatology (mm day−1) and (b) linear trend (mm day−1 decade−1) of summer precipitation over China during 1961–2012. Black dashed rectangles in (a) indicate the eight subregions: NEC, NC, HHR, YZR, SC, SWC, SHT, and NXJ. Dots in (b) indicate the stations for which the linear trend is statistically significant at the 0.05 level.

Fig. 2.

The (a) climatology (mm day−1) and (b) linear trend (mm day−1 decade−1) of summer precipitation over China during 1961–2012. Black dashed rectangles in (a) indicate the eight subregions: NEC, NC, HHR, YZR, SC, SWC, SHT, and NXJ. Dots in (b) indicate the stations for which the linear trend is statistically significant at the 0.05 level.

Figure 2b shows a linear trend of summer precipitation over China during the period 1961–2012. The linear trend is quite different and even opposite over different regions of China. In eastern China, the linear trend exhibits a “south wet–north dry” meridional pattern. The summer precipitation increases over SC, YZR, and HHR but decreases over NC and NEC. Over western China, the linear trend is negative over most stations of SWC and SHT, and positive over most stations of Qinghai Province, Xinjiang Province, and the northeastern parts of Gansu Province. The linear trend of summer precipitation over China is generally consistent with the results of previous studies (Liu 2005).

b. Decadal variability

Figure 3 exhibits the decadal component of the summer precipitation of the eight subregions. It can be seen that the summer precipitation over all of the subregions exhibits pronounced decadal variability. In the late 1970s, the summer precipitation over YZR and SHT experienced a decadal increase (Figs. 3d,g), but a decadal decrease was observed over NC and SC (Figs. 3b,e). In the early and mid-1980s, the summer precipitation over NEC increased (Fig. 3a), but a decrease was observed over HHR and SHT (Figs. 3c,g). In the late 1980s, the summer precipitation over NXJ shifted to a positive phase (Fig. 3h). In the early 1990s, the summer precipitation increased over SC on the decadal time scale (Fig. 3e). After the late 1990s, the summer precipitation over all the subregions but NXJ, SC, and SHT underwent a decadal variation (Fig. 3), and the precipitation decreased significantly over SC (Fig. 3e) and increased significantly over SHT (Fig. 3g). For the decadal variation after the late 1990s, previous studies have mainly focused on the summer precipitation over eastern China and less so on that over western China. In the next section, we analyze the spatial patterns and corresponding abrupt changepoints of the decadal variations in summer precipitation over China after the late 1990s using several statistical methods.

Fig. 3.

Time series of summer precipitation (gray bars; mm day−1) and 9-yr low-pass-filtered components (black curve) for the eight subregions during 1961–2012.

Fig. 3.

Time series of summer precipitation (gray bars; mm day−1) and 9-yr low-pass-filtered components (black curve) for the eight subregions during 1961–2012.

4. Spatial patterns of decadal variation in summer precipitation over China after the late 1990s

As described in the previous section, after the late 1990s, the summer precipitation experienced a pronounced decadal variation not only over eastern China but also over the SWC subregion of western China. Figure 4 shows the stations for which summer precipitation underwent a decadal variation after the late 1990s and the corresponding abrupt changepoints detected by the MTT and Lepage test. The summer precipitation over NEC and the northern parts of NC was the first to experience a decadal variation, with most stations over the two subregions shifting to a negative phase during the period 1997–99 (Fig. 4a). Following NEC and the northern parts of NC, the summer precipitation at some stations over YZR and the northern parts of HHR underwent a shift of phase during 2000–01. Finally, a decadal variation of the summer precipitation of stations over SC, SWC, SHT and the southern parts of HHR occurred during 2002–04. Additionally, there are some stations over NXJ at which the summer precipitation experienced a decadal variation. However, the abrupt changepoints are quite different among these stations. The results detected by the MTT (Fig. 4b) and Lepage test (Fig. 4a) are basically identical, albeit with some differences in the number of stations showing decadal variation and the abrupt changepoints of decadal variation.

Fig. 4.

The abrupt changepoints of the decadal variation of summer precipitation over China after the late 1990s detected by the (a) MTT and (b) Lepage test.

Fig. 4.

The abrupt changepoints of the decadal variation of summer precipitation over China after the late 1990s detected by the (a) MTT and (b) Lepage test.

We also detected abrupt changepoints of decadal variation for the area-averaged summer precipitation in the eight subregions by the MTT, Lepage test, and MK test. The results are summarized in Fig. 5. It can be seen from Fig. 5 that decadal variation of summer precipitation over NEC and NC occurred in 1999 and 2000 for YZR and 2003 for HHR, SC, SWC, and SHT. NXJ did not experience any decadal variation after the late 1990s. The area-averaged precipitation over SC and SHT did not exhibit a phase shift relative to the mean of 1961–2012 (Figs. 2e,g), but they are regarded as having experienced a decadal variation by the MMT or and Lepage test because the precipitation over the two regions decreased/increased remarkably during the past two decades. As discussed in sections 5 and 6, the marked decrease precipitation over SC and increase over SHT both correspond to the decadal variation that occurred around 2003. Therefore, we regard the precipitation over SC and SHT as having experienced a decadal variation around 2003. The abrupt changepoints of the decadal variation detected by the three methods differ to some extent. This difference may result partly from the intrinsic differences among the three methods and the artificial choice for the boundaries of each subregion. The results obtained by the statistical methods generally support what was revealed in the previous section. After the late 1990s, the summer precipitation over both eastern and western China experienced a decadal variation. The decadal variation of area-averaged summer precipitation over NEC and NC occurred first in 1999; then over YZR in 2000; and finally over HHR, SC, SWC, and SHT in 2003. The decadal variation over eastern China, especially over NEC, occurred about 2–3 yr earlier than that over western China.

Fig. 5.

The abrupt changepoints of the decadal variation of summer precipitation over the eight subregions detected by the MTT, Lepage test, and MK test after the late 1990s. An upward arrow denotes decadal increase for precipitation; a downward arrow denotes decadal decrease; “Non” denotes no decadal variation is detected by the three methods; and an asterisk denotes the abrupt changepoint is only statistically significant at the 0.10 level.

Fig. 5.

The abrupt changepoints of the decadal variation of summer precipitation over the eight subregions detected by the MTT, Lepage test, and MK test after the late 1990s. An upward arrow denotes decadal increase for precipitation; a downward arrow denotes decadal decrease; “Non” denotes no decadal variation is detected by the three methods; and an asterisk denotes the abrupt changepoint is only statistically significant at the 0.10 level.

As described above, the summer precipitation over China exhibited two decadal variations after the late 1990s. One occurred mainly over eastern China in around 1999, especially over NEC, the northern parts of NC, YZR, and the northern parts of HHR. The other occurred over China south of 40°N in around 2003; it was especially pronounced from SHT to the southern parts of HHR and from SWC to SC. Therefore, we selected 1999 and 2003 as representative years for the two decadal variations over China to study and compare their basic features. To better distinguish the features of the two decadal variations, the anomalies of variables for the decadal variation around 1999 were computed using the mean of the period 1999–2007 minus that of the period 1990–98; for the decadal variation around 2003, the mean of the period 2003–11 minus that of the period 1994–2002 was used. We also used 7-yr mean and 11-yr mean for each period instead of 9-yr mean, to calculate the anomalies of variables for the two decadal variations, and the results change slightly. Figure 6 shows summer precipitation anomalies over China for the two decadal variations. It can be seen that the precipitation anomalies are mainly located in eastern China for the decadal variation around 1999, with less precipitation over NEC, NC, YZR, and some parts of SWC and more precipitation over the northern parts of HHR. The anomalies over NEC, NC, and YZR are generally greater than 0.6 mm day−1. The area-averaged precipitation decreased 0.80, 0.74, and 0.51 mm day−1 over NEC, NC, and YZR, about 24.9%, 18.4%, and 9.3% of the mean for 1990–98, respectively. For the decadal variation around 2003, the precipitation anomalies were mainly located south of 40°N, with increased precipitation over SHT to the southern parts of HHR and decreased precipitation over SWC–SC. The anomalies for majority of SC, SWC, and the southern parts of HHR are greater than 0.6 mm day−1. In the northern parts of SC, the southern parts of HHR, and some parts of SHT, the anomalies are greater than 1.2 mm day−1. The area-averaged precipitation decreases 1.08 and 0.87 mm day−1 over SC and SWC (i.e., about 14.3% and 13.5% of the mean for 1994–2002) and increases 0.47 and 0.90 mm day−1 over SHT and HHR (i.e., about 15.3% and 23.5% of the mean for 1994–2002), respectively.

Fig. 6.

The summer precipitation anomalies (mm day−1) corresponding to the decadal variation of summer precipitation over China around (a) 1999 and (b) 2003. Dots indicate the stations for which summer precipitation anomalies are statistically significant at the 0.05 level.

Fig. 6.

The summer precipitation anomalies (mm day−1) corresponding to the decadal variation of summer precipitation over China around (a) 1999 and (b) 2003. Dots indicate the stations for which summer precipitation anomalies are statistically significant at the 0.05 level.

The two decadal variations of summer precipitation over China after the late 1990s are also depicted by the first two leading modes of the EOF analysis of the standardized summer precipitation over China. Figure 7 displays the first two leading modes and the corresponding time coefficients of the EOF analysis of the standardized summer precipitation over China east of 95°E. Considering the first EOF (EOF1; Fig. 7a) and the corresponding time coefficients (Fig. 7c), the EOF1 indicates the decadal variation of summer precipitation over China around 2003, revealing a meridional dipole pattern over China south of 40°N (Fig. 7b). Analogously, the EOF2 reveals the decadal variation over eastern China around 1999 with decreased precipitation over NEC, NC, and YZR (Fig. 7d). The corresponding time coefficients of the EOF1 and EOF2 exhibit a decadal phase shift around 2002/03 and 1998/99 (Figs. 7c,d), respectively, indicating that it is reasonable to select 1999 and 2003 as the representative years for the two decadal variations of summer precipitation over China.

Fig. 7.

(a),(b) The first two leading modes and (c),(d) the corresponding standardized time coefficients of the EOF analysis of the standardized summer precipitation over China east of 95°E during 1990–2012. The contributions of the first two leading modes to the total variance account for 12.6% and 11.8%, respectively.

Fig. 7.

(a),(b) The first two leading modes and (c),(d) the corresponding standardized time coefficients of the EOF analysis of the standardized summer precipitation over China east of 95°E during 1990–2012. The contributions of the first two leading modes to the total variance account for 12.6% and 11.8%, respectively.

5. Summer atmospheric circulation anomalies corresponding to the decadal variation around 1999 and 2003

The decadal variation of summer precipitation over China is accompanied by atmospheric circulation variation. Therefore, we study and compare the summer atmospheric circulation anomalies associated with the two decadal variations.

a. Local atmospheric circulation anomalies

Figure 8 shows the summer local wind anomalies for the decadal variation around 1999 and 2003. As shown, they are different from one another; for the former, the wind anomalies exhibit a barotropic meridional dipole pattern, with anticyclonic anomalies over Mongolia to northern China and cyclonic anomalies over the southeastern Chinese coast to the northwestern Pacific (Figs. 8a,c,e). Because of the influence of the dipole wind anomalies, there is anomalous divergence over NEC, NC, and YZR in the lower troposphere. Therefore, the summer precipitation over those regions decreases. This is consistent with the results of previous studies (Si et al. 2009; Zhu et al. 2011; Huang et al. 2012). However, no pronounced anomalies can be found in the middle and lower troposphere over SWC, which is consistent with the fact that summer precipitation over SWC did not experience a pronounced decadal variation around 1999.

Fig. 8.

The summer wind anomalies (m s−1) corresponding to the decadal variation of summer precipitation over China around (a),(c),(e) 1999 and (b),(d),(f) 2003 at the 700-, 500-, and 200-hPa levels. Light and dark shading indicates the anomalies are statistically significant at the 0.10 and 0.05 level, respectively.

Fig. 8.

The summer wind anomalies (m s−1) corresponding to the decadal variation of summer precipitation over China around (a),(c),(e) 1999 and (b),(d),(f) 2003 at the 700-, 500-, and 200-hPa levels. Light and dark shading indicates the anomalies are statistically significant at the 0.10 and 0.05 level, respectively.

For the decadal variation around 2003, there are cyclonic anomalies over the northern parts of the Tibetan Plateau and anticyclonic anomalies over YZR to southern Japan in the middle and lower troposphere, which is a southeast–northwest-oriented dipole pattern (Figs. 8b,d). This indicates that the WNPSH enhances and moves northwestward and the heat low over the Tibetan Plateau enhances in summer, leading to more water vapor being transported to the regions from SHT to HHR, resulting in increased precipitation over those regions. The enhancement and northwestward movement of the WNPSH lead to weakened vertical motion over SC and SWC. In addition, the northward water vapor transport at the northern boundary of SC and SWC increases because of the southerly anomalies related to the enhanced WNPSH and heat low over the Tibetan Plateau. Therefore, the summer precipitation over SC and SWC decreases. An anomalous anticyclone is dominant over China south of 40°N at the 200-hPa level (Fig. 8f), which is related to the enhanced SAH. Therefore, the East Asian subtropical westerly jet moves northward. This also results in weakened vertical motion and less precipitation over the northern parts of SC and SWC but enhanced vertical motion and more precipitation over SHT and HHR. The wind anomalies are weak and not statistically significant in the middle and low troposphere over NC and NEC. Therefore, no significant decadal variation of summer precipitation occurs over NC and NEC around 2003.

As described above, both in this subsection and section 4, although the decadal variations of summer precipitation around 1999 and 2003 are close in terms of time, the spatial pattern of summer precipitation anomalies and the associated local atmospheric circulation anomalies are quite different.

b. Atmospheric circulation anomalies over Eurasia

According to the analysis in section 5a, the local atmospheric circulation anomalies for the decadal variations around 1999 and 2003 are different. What about the anomalies of remote atmospheric circulation in summer? Huang et al. (2012) revealed that the teleconnection patterns over Eurasia and the coast of East Asia changed during the decadal variation around 1999. It can be seen from Fig. 9 that the summer atmospheric circulation anomalies over Eurasia are also distinct between the two decadal variations. For the decadal variation around 1999, the summer 200-hPa geopotential height anomalies over Eurasia exhibit a zonal positive–negative–positive (“+ − +”) wavelike structure (Fig. 9a). An anomalous high dominates over northern Europe to the Arabian Peninsula and over Mongolia to northern China, but an anomalous low dominates over central Asia. The geopotential height anomalies at the 700- and 500-hPa levels (figure not shown) are similar to that at the 200-hPa level. For the decadal variation around 2003, the summer 200-hPa geopotential height anomalies over Eurasia also exhibit a wavelike structure (Fig. 9b), with an anomalous high over the Ural Mountains and over China south of 40°N and an anomalous low over Lake Baikal to Lake Balkhash. The geopotential height anomalies over Eurasia also display a barotropic structure, excluding the baroclinic structure over the Tibetan Plateau. For the decadal variation around 1999, an anomalous high dominates over northern Europe (Fig. 9a); however, an anomalous high is located in the Ural Mountains for the decadal variations around 2003 (Fig. 9b), about 30° of longitude farther east than that of the former (Fig. 9a). There was a strong anomalous high over northern China to Mongolia (Fig. 9a) for the former but a weak anomalous low for the latter (Fig. 9b). Therefore, for the two decadal variations, the summer atmospheric circulation anomalies over Eurasia are also different.

Fig. 9.

Anomalies in the summer geopotential height field (contours; gpm) at the 200-hPa level, corresponding to the decadal variation of summer precipitation over China around (a) 1999 and (b) 2003. Light and dark shading indicate that anomalies are statistically significant at the 0.10 and 0.05 level, respectively.

Fig. 9.

Anomalies in the summer geopotential height field (contours; gpm) at the 200-hPa level, corresponding to the decadal variation of summer precipitation over China around (a) 1999 and (b) 2003. Light and dark shading indicate that anomalies are statistically significant at the 0.10 and 0.05 level, respectively.

We applied the EOF analysis to the summer 300-hPa meridional wind velocity anomalies to reveal the teleconnections over Eurasia. The first two modes account for 31.1% and 15.0% of the total variance, respectively, and can be isolated from other modes based on the criteria by North et al. (1982). As seen in Fig. 10a, the EOF1 displays a zonal pattern. Analysis on the related wave activity flux reveals that there are two main Rossby waves propagating over Eurasia (Fig. 11a). The southern one propagates along the Asian jet stream, resembling the Silk Road teleconnection (Lu et al. 2002; Enomoto et al. 2003; Kosaka et al. 2012). The northern one, which propagates through an arc path, is relatively weak. The three anomalous centers (Fig. 11a) are similar to those of the summer mean [June–August (JJA)] Silk Road teleconnection revealed by Kosaka et al. (2012). We regard this teleconnection as a Silk Road pattern–like teleconnection. The anomalous centers over northern Europe, central Asia and Mongolia to northern China are similar to those of the decadal variation around 1999 (cf. Figs. 9a and 11a), and the time coefficients of EOF1 have been predominantly in the positive phase since 1999 (Fig. 10c). Therefore, the Silk Road pattern–like teleconnection provides important contributions to atmospheric circulation anomalies over Eurasia for the decadal variation around 1999.

Fig. 10.

(a),(b) The first two leading modes and (c),(d) the corresponding standardized time coefficients of the EOF analysis of meridional wind velocity anomalies at the 300-hPa level over the region of (30°–60°N, 30°–130°E) during 1990–2012. The contributions of the first two leading modes to the total variance account for 31.1% and 15.0%, respectively.

Fig. 10.

(a),(b) The first two leading modes and (c),(d) the corresponding standardized time coefficients of the EOF analysis of meridional wind velocity anomalies at the 300-hPa level over the region of (30°–60°N, 30°–130°E) during 1990–2012. The contributions of the first two leading modes to the total variance account for 31.1% and 15.0%, respectively.

Fig. 11.

(a) Regressions of summer 200-hPa geopotential height (contour; gpm) and wave activity flux (vector; m2 s−2) and (c) precipitation (mm day−1) against the time coefficients corresponding to EOF1 of summer 300-hPa meridional wind velocity anomalies over the region of (30°–60°N, 30°–130°E) during 1990–2012. (b),(d) As in (a),(c), but for the time coefficients corresponding to EOF2. The wave activity flux is calculated according to Takaya and Nakamura (2001). Light and dark shading in (a),(b) indicate that anomalies are statistically significant at the 0.10 and 0.05 level, respectively. Dots in (c),(d) indicate the stations for which the regressions are statistically significant at the 0.10 level.

Fig. 11.

(a) Regressions of summer 200-hPa geopotential height (contour; gpm) and wave activity flux (vector; m2 s−2) and (c) precipitation (mm day−1) against the time coefficients corresponding to EOF1 of summer 300-hPa meridional wind velocity anomalies over the region of (30°–60°N, 30°–130°E) during 1990–2012. (b),(d) As in (a),(c), but for the time coefficients corresponding to EOF2. The wave activity flux is calculated according to Takaya and Nakamura (2001). Light and dark shading in (a),(b) indicate that anomalies are statistically significant at the 0.10 and 0.05 level, respectively. Dots in (c),(d) indicate the stations for which the regressions are statistically significant at the 0.10 level.

The EOF2 also displays a wavelike structure (Fig. 10b). The related wave activity flux (Fig. 11b) shows that it propagates from Scandinavia to the Tibetan Plateau through an arc path over Eurasia. We regard this teleconnection as a Eurasian (EU) pattern–like teleconnection. The anomalous centers over the Ural Mountains, southwest of Lake Balkhash, and over the Tibetan Plateau are similar to those of the decadal variation around 2003 (cf. Figs. 9b and 11b) and the time coefficients of EOF2 have been predominantly in a positive phase since 2003 (Fig. 10d). Besides, the positive phase of EU pattern–like teleconnection is accompanied by anticyclonic circulation anomalies over East Asia and the northwestern Pacific in the lower level, with anomalous centers located around 20°N (figure not shown). This means that the WNPSH enhances, which is important to the decadal variation around 2003 revealed in section 5a. Therefore, the EU pattern–like teleconnection provides important contributions to atmospheric circulation anomalies over Eurasia for the decadal variation around 2003. To study the linkage between atmospheric circulation over Eurasia and the two decadal variations, Figs. 11c,d show the correlations between Silk Road pattern–like teleconnection and EU pattern–like teleconnection and summer precipitation during 1990–2012. As seen Fig. 11c, there are significantly negative correlations over NEC and the northern part of NC, where the summer precipitation experiences a decadal variation around 1999. As for Fig. 11d, there are significantly negative correlations over some parts of SWC and SC and positive correlations over throughout most of China. Therefore, the atmospheric circulation anomalies over Eurasia are also closely related to the two decadal variations.

6. Role of the tropical Atlantic SST warming in the decadal variation around 2003

For the decadal variation around 1999, the summer geopotential height anomalies exhibit a barotropic Silk Road pattern–like wave over Eurasia. Using an AGCM, Zhu et al. (2011) verified that the summer 500-hPa geopotential height anomalies over Eurasia are related to the phase shift of the PDO (see their Fig. 8) and noted that the shift of the PDO to a negative phase was probably responsible for the decadal variation of summer precipitation over eastern China around 1999/2000. The summer SST anomalies associated with Silk Road pattern–like teleconnection during 1990–2012 resemble the negative phase of the PDO over the North Pacific (figure not shown). Therefore, the atmospheric circulation anomalies over Eurasia and precipitation anomalies for the decadal variation around 1999 may be closely related to the phase shift of the PDO. As previous studies have researched the decadal variation around 1999, we mainly focus on the decadal variation around 2003.

By comparing the SST anomalies in summer for the two decadal variations, the primary difference is located in the North Pacific, the tropical Pacific, and the tropical Atlantic. For the decadal variation around 1999, the SST anomalies are in a negative phase of the PDO over the North Pacific and the SST over the tropical Atlantic exhibits a weak warming (Fig. 12a). Compared with the SST anomalies for the decadal variation around 1999, the negative phase of the PDO over the North Pacific and the SST anomalies in the tropical Pacific weaken significantly, and the tropical Atlantic experiences a pronounced warming after 2003 (Fig. 12b). On the interannual time scales, the summer atmospheric circulation anomalies in the upper troposphere over the North Atlantic and Europe can excite stationary Rossby waves over Eurasia by modulating the meridional position of the North Atlantic storm track and thereby influence precipitation over the Tibetan Plateau (Bothe et al. 2010). Sun et al. (2009a) demonstrated that, on decadal time scales, the warming of the tropical Atlantic SST in summer could influence the summer NAO, a major mode of atmospheric variability over the North Atlantic and Europe. Chen and Huang (2012) indicated that the Europe–China pattern of stationary Rossby waves in July over Eurasia could be excited by the heating anomalies over the tropical Atlantic. Therefore, the EU pattern–like wave over Eurasia for the decadal variation around 2003 may be closely related to the decadal warming of the tropical Atlantic SST in summer. In fact, the SST anomalies associated with the EU pattern–like teleconnection during 1990–2012 exhibit warming over the tropical Atlantic (figure not shown).

Fig. 12.

The summer SST anomalies (°C) corresponding to the decadal variation of summer precipitation over China around (a) 1999 and (b) 2003. Dot-filled areas indicate that anomalies are statistically significant at the 0.05 level.

Fig. 12.

The summer SST anomalies (°C) corresponding to the decadal variation of summer precipitation over China around (a) 1999 and (b) 2003. Dot-filled areas indicate that anomalies are statistically significant at the 0.05 level.

In addition to exciting stationary Rossby waves over Eurasia by changing the atmospheric circulation over the North Atlantic and Europe, the diabatic heating anomalies related to SST anomalies in summer over the tropical Atlantic can produce a Gill–Matsuno-type pattern in atmospheric circulation to influence the WNPSH and thereby precipitation over China (Z. Wu et al. 2012). Hong et al. (2014) indicated that the influence of July–August tropical Atlantic SST anomalies on the WNPSH strengthened on interannual time scales after the early 1980s and on decadal time scales after the early 1990s. As revealed by Hong et al. (2014), on interannual time scales, the warming over the tropical Atlantic in July–August induces a zonally overturning circulation anomaly over the equatorial central Pacific to the tropical Atlantic, leading to descending over the equatorial central Pacific. The anomalous descent likely induces a low-level anticyclonic anomaly to the west and enhances the WNPSH. Therefore, the warming of the tropical Atlantic SST may contribute to the decadal variation around 2003 via inducing a zonally overturning circulation anomaly over the equatorial central Pacific to the tropical Atlantic.

To verify the underlying mechanisms discussed above for the decadal variation around 2003, several SST anomaly experiments were performed. The summer SST anomalies are the difference between the means of 2003–11 and 1994–2002 (Fig. 12b). The summer SST anomalous centers over the western tropical Atlantic (0°–20°N, 70°–40°W) and the eastern tropical Atlantic (0°–25°N, 30°W–0°) were selected to force the ECHAM5; they were called EXP_WTA and EXP_ETA, respectively.

First, we analyzed the summer precipitation anomalies for EXP_WTA and EXP_ETA. As seen in Fig. 13, the summer SST warming over the western and eastern tropical Atlantic can lead to less precipitation over about China south of 35°N and more precipitation along the Yellow River region (Figs. 13b,c). Although existing obvious difference over the Yangtze River valley compared with the observations (Fig. 13a), it is true that the summer warming over the western and eastern tropical Atlantic contributed to the decadal variation around 2003 over China south of 40°N, especially for SWC and SC. As for SC, the area-averaged precipitation anomalies are −0.49 mm day−1 in EXP_WTA and −0.25 mm day−1 in EXP_ETA, which are about 38.0% and 19.4% of the observations. The linear trend in the observations from 1979 to 2012 was removed before calculating the anomalies (Figs. 13a, 15a, 16a, and 17a). This is because, as described in the following, we found the atmospheric circulation anomalies over Europe associated with long-term trends were “noise” when analyzing the contribution of the western tropical Atlantic warming to atmospheric circulation anomalies over Eurasia. Corresponding to summer precipitation anomalies, there are an anomalous anticyclone over SWC to the northwestern Pacific at the 700-hPa level (Figs. 14a,b) and a southeast–northwest-oriented dipole over East Asia at the 500-hPa level (Figs. 14c,d); an anomalous high dominates at the 200-hPa level over southern China (Figs. 14e,f). The spatial pattern of summer circulation anomalies over East Asia for EXP_WTA (Figs. 14a,c,e) and EXP_ETA (Figs. 14b,d,f) have similarity with the observations (Figs. 8b,d,f), in spite of being relatively weak and exhibiting differences in the location of the anomalous centers.

Fig. 13.

Summer precipitation anomalies (mm day−1): (a) observations (2003–11 minus 1994–2002; linear trend from 1979 to 2012 is removed first), (b) EXP_WTA, and (c) EXP_ETA. Dots in (a) indicate the stations for which summer precipitation anomalies are statistically significant at the 0.05 level. Dot-filled areas in (b) and (c) indicate the anomalies are statistically significant at the 0.05 level.

Fig. 13.

Summer precipitation anomalies (mm day−1): (a) observations (2003–11 minus 1994–2002; linear trend from 1979 to 2012 is removed first), (b) EXP_WTA, and (c) EXP_ETA. Dots in (a) indicate the stations for which summer precipitation anomalies are statistically significant at the 0.05 level. Dot-filled areas in (b) and (c) indicate the anomalies are statistically significant at the 0.05 level.

Fig. 14.

Summer wind anomalies (m s−1): (a),(c),(e) EXP_WTA and (b),(d),(f) EXP_ETA at the 700-, 500-, and 200-hPa levels. Light and dark shading indicate that anomalies are statistically significant at the 0.10 and 0.05 level, respectively.

Fig. 14.

Summer wind anomalies (m s−1): (a),(c),(e) EXP_WTA and (b),(d),(f) EXP_ETA at the 700-, 500-, and 200-hPa levels. Light and dark shading indicate that anomalies are statistically significant at the 0.10 and 0.05 level, respectively.

As discussed above, the tropic Atlantic warming may induce teleconnections over Eurasia to influence the summer precipitation over China. Because the linear Rossby waves propagate further away from the source region in the upper level, we mainly focus on the atmospheric circulation anomalies at the 200-hPa level. According to Figs. 15b,c, the warming of the western and eastern tropical Atlantic can induce teleconnections over Eurasia. As seen in Fig. 15b, the teleconnection induced by the western tropical Atlantic warming propagates along the Asian jet and turns southeast over East Asia, which also can be indicated by the related wave activity flux (figure not shown). This teleconnection is similar to the July Silk Road teleconnection revealed by Chen et al. (2012). It propagates southeastward over East Asia, which may be associated with heating anomalies over the tropical western Pacific, as indicated by Hsu and Lin (2007). Upon comparing the observations (Fig. 15a) and EXP_WTA (Fig. 15b), the anomalies of EXP_WTA over areas north of the Black Sea, south of the Caspian Sea to the Aral Sea, in the vicinity of the western Tibetan Plateau, and in China south of 40°N, which exhibit a wave structure, are similar to those of the observations. The anomalies of EXP_WTA over the western tropical Atlantic and the extratropical eastern Atlantic to western Europe are also similar to those observed (cf. Figs. 15a,b). This means that the warming over the tropical western Atlantic is important to the anomalies over those regions. Therefore, the western tropic Atlantic warming may contribute to the decadal variation around 2003 by inducing a teleconnection, similar to July Silk Road teleconnection, that enhances the summer WNPSH (Figs. 14a,c). We show the observations with the linear trend of 1979–2012 removed first, instead of the raw observations. This is because the anomalous cyclonic circulation over north of the Black Sea of EXP_WTA (Fig. 15b) is consistent with the detrended observations but not with the raw observations (Fig. 9b) and the anticyclonic circulation anomalies of EXP_TWA (Fig. 15b) over the south of the Caspian Sea to the Aral Sea is more similar to the detrended observations (Fig. 15a) than the raw observations (Fig. 9b). According to the Fig. 15c, the warming over the eastern tropical Atlantic can excite a wave over Eurasia propagating through an arc path indicated by the related wave activity flux (figure not shown). The anomalies of EXP_ETA (Fig. 15c) over the Ural Mountains, the vicinity of the western Tibetan Plateau, and China south of 40°N are similar to those observed (Fig. 15a). Therefore, the warming over the eastern tropical Atlantic may contribute to the decadal variation around 2003 by inducing a teleconnection over Eurasia, which leads to the enhanced WNPSH (Figs. 14b,d). The spatial pattern of anomalies in EXP_ETA (Fig. 15c), anomalous centers over Eurasia (west of 120°E), and the four anomalous centers over the extratropical North Atlantic share some similarity with those of the EU pattern–like teleconnection (Fig. 15d), in spite of existing differences regarding the location of the anomalous centers. The wave activity flux is also similar to each other (figure not shown). Therefore, the teleconnection over Eurasia inducing by summer warming over the eastern tropical Atlantic are partly related to the EU pattern–like teleconnection.

Fig. 15.

Summer 200-hPa wind anomalies (m s−1): (a) observations (2003–11 minus 1994–2002; linear trend from 1979 to 2012 is removed first), (b) EXP_WTA, (c) EXP_ETA, and (d) regressions of 200-hPa wind against the time coefficients corresponding to EOF2 of summer 300-hPa meridional wind velocity anomalies over the region of (30°–60°N, 30°–130°E) during 1990–2012. Light and dark shading indicate that anomalies are statistically significant at the 0.10 and 0.05 level, respectively.

Fig. 15.

Summer 200-hPa wind anomalies (m s−1): (a) observations (2003–11 minus 1994–2002; linear trend from 1979 to 2012 is removed first), (b) EXP_WTA, (c) EXP_ETA, and (d) regressions of 200-hPa wind against the time coefficients corresponding to EOF2 of summer 300-hPa meridional wind velocity anomalies over the region of (30°–60°N, 30°–130°E) during 1990–2012. Light and dark shading indicate that anomalies are statistically significant at the 0.10 and 0.05 level, respectively.

As noted by Hong et al. (2014), the July–August WNPSH is closely associated with the tropical Atlantic SST (mainly in the west) on interannual time scales after the early 1980s. We display the results of EXP_WTA. We also assessed the results of EXP_ETA and found that the summer precipitation anomalies were weak over the equatorial central Pacific (figure not shown), which means that the zonally overturning circulation anomaly over the equatorial central Pacific to the tropical Atlantic is weak.

As seen in Fig. 16b, warming over the western tropical Atlantic can lead to more precipitation over the western tropical Atlantic and west of Mexico and less precipitation over the equatorial central Pacific and northwestern Pacific to southern China. Corresponding to precipitation anomalies, there are anomalous cyclonic circulation over the tropical Atlantic to west of Mexico and anomalous anticyclonic circulation over the northwestern Pacific (Fig. 17b), which is similar to that revealed by Hong et al. (2014). This indicates that the zonally overturning circulation anomaly over the equatorial central Pacific to the tropical Atlantic also plays an important role for the enhancement of the WNPSH and precipitation anomalies over southern China in EXP_WTA. The more precipitation over the western tropical Atlantic to the west of Mexico (Fig. 16b) is generally similar to the observations (Fig. 16a) and 850-hPa streamfunction anomalies over the Caribbean Sea to the Gulf of Mexico and over the northwestern Pacific to southern China also share some resemblance between observations (Fig. 17a) and EXP_WTA (Fig. 17b). However, the precipitation anomalies over the equatorial central Pacific and northwestern Pacific to southern China are different in spite of existing resemblances. In the observations (Fig. 17a), the pronounced precipitation anomalies over the equatorial western Pacific are basically not reproduced in EXP_WTA (Fig. 17b). This indicates that other mechanisms made an important contribution to the circulation anomalies over the tropical Pacific, corresponding to the decadal variation around 2003. Therefore, the zonally overturning circulation anomaly over the equatorial central Pacific to the tropical Atlantic, which influences the WNSPH via the precipitation anomalies over the equatorial central Pacific, may not be an important mechanism for the decadal variation around 2003.

Fig. 16.

Summer precipitation anomalies (mm day−1): (a) CMAP precipitation (2003–11 minus 1994–2002; linear trend from 1979 to 2012 is removed first) and (b) EXP_WTA. Dot-filled areas indicate that anomalies are statistically significant at the 0.05 level.

Fig. 16.

Summer precipitation anomalies (mm day−1): (a) CMAP precipitation (2003–11 minus 1994–2002; linear trend from 1979 to 2012 is removed first) and (b) EXP_WTA. Dot-filled areas indicate that anomalies are statistically significant at the 0.05 level.

Fig. 17.

Summer 850-hPa streamfunction anomalies (m s−1): (a) observations (2003–11 minus 1994–2002; linear trend of 1979–2012 is removed first) and (b) EXP_WTA. Light and dark shading indicate that anomalies are statistically significant at the 0.10 and 0.05 level, respectively.

Fig. 17.

Summer 850-hPa streamfunction anomalies (m s−1): (a) observations (2003–11 minus 1994–2002; linear trend of 1979–2012 is removed first) and (b) EXP_WTA. Light and dark shading indicate that anomalies are statistically significant at the 0.10 and 0.05 level, respectively.

7. Conclusions and discussion

Based on station data, we revealed a new decadal variation around 2003 and pointed out that the tropical Atlantic warming may play an important role in this decadal variation via numerical simulations. The summer precipitation over China exhibited two different decadal variations after the late 1990s. One occurred mainly over eastern China around 1999. The summer precipitation over NEC, NC, and YZR decreased, but it increased over the northern parts of HHR. The other occurred over China south of 40°N around 2003. The summer precipitation over SHT to the southern parts of HHR increased, but it decreased over SWC–SC. Detected by various statistical methods, the decadal variation of area-averaged summer precipitation over NEC and NC occurred first in 1999; then over YZR in 2000; and finally over HHR, SC, SWC, and SHT in 2003. The decadal variation over eastern China, especially over NEC, occurred about 2–3 yr earlier than that over western China.

The summer local atmospheric circulation anomalies corresponding to the two decadal variations are quite different. The wind anomalies for the decadal variation around 1999 show a barotropic meridional dipole pattern, with anticyclonic anomalies over Mongolia to northern China and cyclonic anomalies over the southeastern Chinese coast to the northwestern Pacific. This leads to anomalous divergence in the lower troposphere over NEC, NC, and YZR and then decreased precipitation over those regions. For the decadal variation around 2003, there is a southeast–northwest-oriented dipole pattern in the middle and lower troposphere, with cyclonic anomalies over the northern parts of the Tibetan Plateau and anticyclonic wind anomalies over YZR to southern Japan, indicating the enhanced WNPSH and moving northwestward and the enhanced heat low over the Tibetan Plateau. An anomalous anticyclone dominates over China south of 40°N at the 200-hPa level, which is related to the enhanced SAH and northward movement of the subtropical westerly jet. Influenced by such atmospheric circulation anomalies, anomalous water vapor divergence and weakened vertical motion dominate over SC and SWC, which lead to less precipitation. In contrast, the summer precipitation increases over SHT and HHR because of anomalous water vapor convergence and enhanced vertical motion.

The atmospheric circulation anomalies over Eurasia show distinct differences between the decadal variation around 1999 and 2003. There is a wavelike structure over Eurasia in the 200-hPa geopotential height anomalies for the decadal variation around 1999, with the anomalous centers located in northern Europe, central Asia, and Mongolia to northern China, the anomalies for the decadal variation around 2003 also exhibit a wavelike structure but with the anomalous centers located in the Ural Mountains, from Lake Baikal to Lake Balkhash, and southern China (south of 40°N). The atmospheric circulation anomalies over Eurasia for the two decadal variations are closely associated with the variation of the teleconnections over there. The shift of the PDO to a negative phase was probably responsible for the decadal variation around 1999, as has been previously indicated. The warming over the tropical Atlantic may contribute to the decadal variation around 2003 via wave activity anomalies over Eurasia, which are basically reproduced by the numerical simulations.

According to the analysis in section 5a, the atmospheric circulation anomalies are barotropic and baroclinic over the Tibetan Plateau for the decadal variation of summer precipitation over China around 1999 and 2003, respectively. Previous studies (e.g., Liu et al. 2012) have indicated that the enhanced thermal forcing of the Tibetan Plateau leads to an anomalous cyclone in the lower troposphere and an anomalous anticyclone in the upper troposphere over the Tibetan Plateau in summer. Therefore, the thermal forcing anomalies over the Tibetan Plateau may not be the dominant factor leading to the decadal variation around 1999. In contrast, the baroclinic atmospheric circulation anomalies over the Tibetan Plateau for the decadal variation around 2003 is evidence that the thermal forcing over the Tibetan Plateau is a factor that contributes to this decadal variation. In fact, recent studies (Liu et al. 2012; Si and Ding 2013) have revealed that the surface sensible heat flux over the eastern Tibetan Plateau in spring and summer has enhanced since around 2003. Xu et al. (2013) revealed that the apparent heating in spring over the Tibetan Plateau has increased since 2003. In other words, the East Asian summer monsoon may have intensified since then, which was revealed in section 5a. Therefore, the enhanced thermal forcing over the Tibetan Plateau in spring and summer since around 2003 is an important factor affecting the decadal variation of summer precipitation over China around 2003 but not around 1999. As discussed in section 6, the sensitivity experiments revealed the warming over the western and eastern tropical Atlantic can cause surface (500 hPa) anomalous westerlies over the Tibetan Plateau (Figs. 14c,d). The variation of the eastern Tibetan Plateau summer surface sensible heat flux closely follows that of surface wind speed on decadal time scales (Liu et al. 2012). Therefore, the tropical Atlantic warming in summer is likely an important factor leading to the enhancement of summer surface sensible heat flux over the eastern Tibetan Plateau around 2003.

Previous studies revealed that the Silk Road teleconnection may be closely related to the latent heating released by precipitation over India (e.g., Krishnan and Sugi 2001; Guan and Yamagata 2003; Ding and Wang 2005; Sato and Takahashi 2006). The precipitation anomalies over northwestern India decreased significantly, corresponding to the decadal variation around 1999 (figure not shown). The area-averaged (23.75°–31.25°N, 71.25°–78.75°E) precipitation of CMAP dataset over northwestern India turned into negative phase around 1999, but it quickly shifted to positive phase around 2006 (figure not shown). This is different from the Silk Load pattern–like teleconnection (Fig. 10c) and precipitation over NEC and NC (Figs. 3a,b). Therefore, the contribution of summer precipitation anomalies over India to the decadal variation around 1999 may be probably small.

Chen and Zhai (2014) reported the changing of precipitation structure and its linkage to drought over southwestern China during 1961–2012 and pointed out that the decrease of summer precipitation during the past decade mainly resulted from the decrease of long-lasting precipitation. They proposed several mechanisms for the change in annual or seasonal precipitation structure and associated long-term drying tendency. We also checked the contribution of some of those mechanisms to the decadal variation around 2003. As revealed in section 6, the summer SST anomalies over the tropical Pacific, which are weak and insignificant, have on contribution to the decadal variation around 2003. Ju et al. (2005) indicated winter Arctic Oscillation (AO) has influence on the weakening of EASM since the late 1970s. The winter AO exhibited a decrease trend during the past two decades and did not experience a decadal variation around 2003 (figure not shown). Besides, the spring Arctic sea ice and snow over Eurasia also can exert impact on the summer precipitation on decadal time scales (Wu et al. 2009b,c; Li and Leung 2013). Therefore, the contribution of the previous-season decadal signals to the decadal variation around 2003 need to be further studied in the future.

Acknowledgments

The authors are grateful to the three anonymous reviewers for their insightful comments, which led to a significant improvement of the manuscript. This research was jointly supported by the National Natural Science Foundation for Distinguished Young Scientists of China (Grant 41325018), the strategic technological program of the Chinese Academy of Sciences (Grant XDA05090426), the National Natural Science Foundation of Innovation (Grant 41421004), and the National Natural Science Foundation of China (Grant 41175071).

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