Persistent severe drought in southern China during winter–spring 2011: Large-scale circulation patterns and possible impacting factors

Authors


Abstract

[1] Severe drought persisted in southern China from January to May in 2011. In this study, a statistical analysis is carried out to discuss the multiple possible impacting factors including La Niña, the North Atlantic Oscillation (NAO), and the thermal condition of the Tibetan Plateau (TP). The La Niña event in 2010–11 excited a lower-tropospheric anomalous cyclone over the northwestern Pacific, weakening the northwestern Pacific subtropical high and caused an eastward shift of the high. As a result, transportation of wet and warm moisture from tropical oceans to southern China decreased. The La Niña event also strengthened the upper-tropospheric East Asian jet stream and deepened the East Asian trough, favoring a southward intrusion of dry northerly flow from the Siberia. The La Niña condition in the previous two seasons also seemed to provide precursory signals for the drought. Moreover, in January–May 2011, the NAO was in a positive phase and it tended to excite stationary Rossby waves that were distributed along the sub-polar and subtropical waveguides, respectively. The sub-polar one induced an anomalous anticyclone over the Siberia, favoring a southward intrusion of high-latitude northerly flow to southern China. The subtropical one, associated possibly with the enhanced convection over the broad region from the Mediterranean to Sahara, was favorable for an influence of upper-tropospheric flow on southern China. The TP might also exert an influence on the drought by weakening the westerly flow to the southern flank of TP and reducing water vapor transport from the Bay of Bengal to southern China.

1. Introduction

[2] From the peak winter to the late spring in 2011, severe drought occurred persistently in southern China, especially in the middle-lower reaches of the Yangtze River basin. It was reported that more than 98.9 million hectares of crops were destroyed and more than 4.9 million people and 3.4 million livestock were short of drinking water. Merely in Hubei Province, the economic loss exceeded 1.09 billion dollars (http://jzs.mca.gov.cn/article/zqkb/201105/20110500156549.shtml). Since southern China is densely populated and is an important economic region of the country, this drought exerted a significant influence on the economic growth of China and the daily lives of millions of people and raised tremendous public and scientific interests. Thus, understanding the features of this persistent and severe drought and the associated large-scale climate anomalies, as well as the possible forcing, is important for both scientific research and disaster mitigation service.

[3] The cold-season climate in China is significantly influenced by the East Asian winter monsoon (EAWM) [e.g., Huang et al., 2007]. When the EAWM is strong, cold surges are active [Chen et al., 2000; Zhu, 2008; Park et al., 2011] and precipitation often decreases [Shi, 1996]. Among all the influencing factors of EAWM, the variability of tropical sea surface temperature (SST) especially that associated with El Niño-Southern Oscillation (ENSO) is perhaps one of the most important causes. It has been shown that drought in Asia can be caused by La Niña, the western Pacific SST, and the tropical Indian Ocean SST [Barlow et al., 2002; Chen et al., 2003; Hoerling and Kumar, 2003; Wu et al., 2003, 2012; Chen and Qian, 2005; Zhou and Wu, 2010; Feng and Li, 2011]. During La Niña (El Niño) winter, a strong (weak) EAWM is often observed [Li, 1990; Zhang et al., 1999; Chen and Wu, 2000; Chen, 2002; Yang et al., 2002; Chen et al., 2005]. When EAWM is strong, temperature decreases over eastern China and precipitation decreases over southern China, and vice versa [Zhang et al., 1996, 1999; Zhang and Sumi, 2002]. The above relationship may be explained by the enhanced (reduced) Hadley and Ferrel cells and the persistent anomalous anti-cyclonic (cyclonic) circulation over the western Pacific during El Niño (La Niña) winters [Wu and Cubasch, 1987; Li, 1990; Wang et al., 2000], as well as the remote response of the Asian jet stream to ENSO-related tropical convective forcing [Sakai and Kawamura, 2009]. In the second half of 2010, a La Niña event occurred and persisted into the spring of 2011 (http://www.wmo.int/pages/prog/wcp/wcasp/enso_updates.html). Thus, our analysis is first conducted to understand whether the persistent drought in China can be explained by the influence of La Niña, among others.

[4] Previous studies have also shown that the climate anomalies of the North Atlantic may play an important role in the variability of East Asian climate. In winter, the North Atlantic Oscillation (NAO) creates either a favorable or an unfavorable background condition, depending on its phase, for the development of extratropical atmospheric circulation systems over East Asia such as the Siberian high and the East Asian jet stream, which are important phenomena that influence the variability of EAWM [e.g., Ding and Krishnamurti, 1987; Ding, 1990; Wu and Wang, 2002; Jeong and Ho, 2005; Takaya and Nakamura, 2005a, 2005b; Wang and Ding, 2006; Mao et al., 2007]. It has also been shown that NAO can influence the East Asian climate in spring. Gong and Ho [2003] showed that a positive NAO in late spring could even lead to a northward shift of the summertime East Asian jet stream. Changes in the position of jet stream manipulate the distribution of rainfall over the Yangtze River valley, southern Japan, and southern China. Li et al. [2005, 2008] proposed that springtime NAO extended its influence downstream to cool southwestern China in March via an atmospheric teleconnection pattern. During January–May 2011, significantly positive NAO was detected, with appearances of negative geopotential height anomalies near Greenland and positive height anomalies over the eastern Atlantic Ocean and West Europe (see Figure 4a). However, was the NAO possibly linked to the persistent drought from winter to spring in southern China? If yes, how could the signals propagate from the North Atlantic to East Asia and intensify the drought?

[5] Furthermore, the Tibetan Plateau (TP) is believed to be important for the variability of climate in China through its thermal and dynamical effects [Ye and Gao, 1979; Murakami, 1988; Chen et al., 2001; Wu and Qian, 2003; Yu et al., 2004; Yanai and Wu, 2006; Mailler and Lott, 2009]. Wu and Qian [2003] found that different winter snow anomalies over TP exerted different influences on the East Asian summer monsoon. If heavy snow occurs over eastern or southwestern TP, the subsequent summer monsoon tends to be weak, with less rainfall over southern and southeastern Asia. Wu and Kirtman [2007] also revealed a relationship between the winter snow over Eurasia and the spring rainfall over southern China. Gao and Yang [2009] showed that the above normal tropospheric temperatures over TP led to the severe drought in northern China in winter 2008/09. Liu and Wang [2011] found that the thermal condition over TP might influence the variation of rainfall over southeast China in late spring and early summer. Since temperature was above normal over the vicinity of TP in the winter and the spring of 2010/11 (Beijing Climate Center, http://cmdp.ncc.cma.gov.cn/Monitoring/bulletin.php), we will also investigate whether the drought event can be linked to the TP thermal condition.

[6] The data sets applied in the study are described in section 2. In section 3, we depict the features of precipitation and large-scale circulation patterns that are related to the drought. In section 4, we investigate the possible influences of La Niña and NAO on the drought analyzed. We further discuss the possible ENSO-related precursory signals for the drought and the possible effect of TP on the drought in section 5. A summary of the study is presented in section 6.

2. Data

[7] The data sets analyzed in this study include the CPC unified global daily precipitation analysis [Xie et al., 2007; Chen et al., 2008], the National Centers for Environmental Prediction - National Center for Atmospheric Research (NCEP-NCAR) reanalysis [Kalnay et al., 1996], and the NOAA Extended Reconstructed Sea Surface Temperature data set [Smith et al., 2008]. The analyzed variables from the NCEP-NCAR reanalysis include winds, geopotential height, air temperature, and specific humidity at various pressure levels. Analyzed also includes the NAO index from the NOAA Climate Prediction Center (http://www.cpc.ncep.noaa.gov/products/precip/CWlink). Seasonal means are calculated from daily or monthly values, and anomalies are computed relative to the means of the entire analysis period (from 1979 to 2011).

3. Major Precipitation and Circulation Features Associated With the 2011 Severe Drought

3.1. Main Characteristics of the Drought

[8] Figure 1 shows the distributions of monthly precipitation anomalies from December 2010 to May 2011. In December 2010, precipitation was at least 50 mm above average over most of southern China, i.e., to south of 36°N (the Huai River) in eastern China. In January 2011, however, light drought appeared in majority of southern China. After that, the drought persisted and intensified gradually, especially in the middle-lower reaches of the Yangtze River basin, where precipitation was at least 80–100 mm below average. In June 2011, positive precipitation anomalies over 200 mm were observed over the middle-lower reaches of the Yangtze River basin, ending the persistent drought (figure not shown). Therefore, in this study, we specifically focus on the drought period from January to May 2011.

Figure 1.

Monthly precipitation anomalies (unit: mm) from December 2010 to May 2011.

[9] Climatologically, the January–May (JFMAM) total precipitation in southern China ranges from 500 mm to 800 mm (Figure 2a), making up as much as 35–45% of the annual total precipitation (figure not shown). However, the JFMAM precipitation of southern China was at least 200 mm below average in 2011, especially over the middle-lower reaches of the Yangtze River basin where precipitation was 400 mm below average (Figure 2b). Figure 2c presents the normalized JFMAM precipitation anomalies over the core drought region (hereafter CD index; for the box shown in Figure 2b) during 1979–2011. It reveals that the CD index value in 2011 is below normal by more than two standard deviations and measures a most severe drought event since 1979.

Figure 2.

(a) January–May precipitation climatology and (b) anomalies of 2011 (unit: mm). (c) Standardized January–May precipitation anomalies in the core drought region (outlined by the box in Figure 2b) from 1979 to 2011.

3.2. Large-Scale Circulation Features

[10] Figure 3 presents the climatology of JFMAM 850-hPa winds and specific humidity as well as their anomalies in 2011. Climatologically, the moisture over southern China is transported from the northwestern Pacific by the subtropical high and the Indian Ocean along the southern flank of TP (Figure 3a). Dry northerly flow intrudes southward from the Siberian region to merge with the moist southerly wind over the middle-lower reaches of the Yangtze River basin. For JFMAM 2011, the most notable feature is that the moisture transportation from the south was significantly weakened (Figure 3b). The anomalous dry northerly flow expanded deeply into southern China from the middle latitudes, associated with the effect of an anti-cyclonic circulation over the Siberia. Indeed, the northerly flow was 2–3 m/s stronger than normal and humidity was at least 0.9 g/kg less than normal over southern China. Associated with the weakened southerly moisture transportation, an anomalous cyclonic circulation was evident over the northwestern Pacific, as noted by Wang et al. [2000]. That is, the western Pacific subtropical high was weaker than normal and was located farther eastward than normal, as depicted by the 587 contour in Figure 4a. The other notable features in Figure 3b include the dramatically weaker westerly moisture transportation from the Indian Ocean. This feature may be associated with the anomalous anticyclone over western TP, which will be discussed in section 5 of this paper.

Figure 3.

(a) January–May climatology and (b) anomalies of 2011 for 850-hPa winds (ms−1) and specific humidity (g kg−1).

Figure 4.

(a) Z500 anomalies (in m; shaded) and the position of subtropical high (contours) in January–May 2011 (solid line; dash line for climatology). (b) 200-hPa zonal wind anomalies (shaded) in January–May 2011. The solid and dashed curves denote the 40 ms−1 contours of 200-hPa wind for January–May 2011 and for climatology (1979–2011), respectively.

[11] Figure 4a presents the anomalous circulation pattern at 500-hPa level in JFMAM 2011. Over the high latitudes, there appeared an anomalous wave train, which emanated eastward from the eastern North Atlantic to East Asia along the sub-polar waveguide. In particular, negative height anomalies were observed over the coastal region of East Asia, and positive anomalies were seen over the Siberian region. This pattern usually reflects a deepened East Asian trough and a developed Siberian high and facilitates an intrusion of cold air from the high latitudes into East Asia [Ding, 1994; Takaya and Nakamura, 2005a, 2005b]. It is also noteworthy that the negative anomalies over the vicinity of Greenland and the positive anomalies over the eastern North Atlantic and western Europe resembled the feature of a positive phase of NAO [Hurrell, 1995]. In the pattern associated with NAO, there was another anomalous wave pattern with negative height anomalies over northern Africa and East Asia and positive height anomalies in between, in the vicinity of TP. As usually expected, the geopotential height anomalies in the subtropics were weaker than those in the high latitudes. However, the distribution of standardized geopotential height anomalies reveals that both of the two wave train patterns are noteworthy. A similar wave-like structure was also seen at the 850-hPa and 200-hPa levels, reflecting an equivalent barotropic vertical structure of the wave train (figure not shown). The coexistence of aforementioned twin anomalous wave patterns in the middle and higher latitudes and the positive NAO-like pattern suggest a possible effect of NAO on the East Asian monsoon, which will be discussed in section 4.

[12] Previous studies have reported that the variability of upper-troposphere Asian jet stream has an appreciable influence on EAWM [e.g., Yang et al., 2002, 2004; Sakai and Kawamura, 2009; Li and Yang, 2010]. As seen from Figure 4b, in JFMAM 2011 the 200-hPa westerly jet core was along about 30°N. The jet streams over both East Asia and the Middle East were stronger than normal. The western edge of the East Asian jet core extended westward to merge with the eastward expanded Middle East jet stream. As a result, the entrance of the East Asian jet stream shifted westward from its normal location, and the associated upper-tropospheric divergence to the south of the jet core also migrated westward, consistent with the positive rainfall anomalies over southwestern China and the negative anomalies over southern China (Figure 2b). The enhanced Asian jet could also trap disturbances near the jet entrance and the induced anomalies could be communicated to the east by the propagation of quasi-stationary Rossby-wave energy along the jet waveguide [e.g., Hsu and Lin, 1992; Hoskins and Ambrizzi, 1993; Branstator, 2002; Enomoto et al., 2003; Watanabe, 2004], which will be disused in the next sections.

4. Possible Impacting Factors of the Severe Drought

4.1. La Niña Event

[13] The distribution of SSTA (SST anomaly) in JFMAM 2011 is shown in Figure 5a. La Niña condition was evident, with negative SSTA below −0.5°C in the center-eastern Pacific and the Indian Ocean and positive SSTA in the western Pacific. The JFMAM correlation between the negative CD index and grid point SSTs for 1979–2011 (Figure 5b) indicates a possible link of southern China drought to La Niña. The most significant correlation appears over the tropical center-eastern Pacific, where the values exceed the 95% confidence level (Student t-test). Other significant correlation includes the positive values over the western Pacific and the negative values over the tropic Indian Ocean. Thus, the spatial pattern of correlation exhibits several features similar to those shown in Figure 5a. Figure 6, which shows the correlation between negative Niño3.4 index and grid point JFMAM precipitation, also indicates that droughts may occur in southern China during La Niña years. Indeed, values of significant negative correlation cover a large portion of southeastern and northern China. A comparison of Figure 6 with Figures 3d and 3e of Wu et al. [2003] indicates that the significant correlation over southern China can appear in both winter and spring, and the correlations over northern and southwestern China may be weighed by spring features.

Figure 5.

(a) SST anomalies (°C) in January–May 2011. (b) January–May correlation between the negative precipitation index (in core drought area) and grid point SST for 1979–2011. Values significantly exceeding the 95% confidence level (t-test) are shaded.

Figure 6.

January–May correlation between negative Niño3.4 SST and grid point precipitation for 1979–2011. Values significantly exceeding the 95% confidence level (t-test) are shaded.

[14] To understand why there is a potential relationship between La Niña and southern China drought, we analyze the possible influence of La Niña on drought-associated moisture transportation, whose variability is important for southern China in various seasons including winter and spring [Li et al., 2010]. Figure 7a displays the moisture transport pattern associated with southern China drought as shown in the JFMAM correlation between the negative CD index and grid point 850-hPa moisture fluxes. Three main features can be seen in association with drought: (1) the significant anomalous southward moisture transportation over much of southern China, (2) the anomalous cyclone over the northwestern Pacific, with its western flank extended to the Asian continent, and (3) the significant anomalous westward moisture transportation along the southern flank of TP, which will be further discussed in section 5. Figure 7b displays the possible influence of La Niña on the variability of 850-hPa moisture fluxes. It shows that La Niña tends to cause an anomalous cyclone over the northwestern Pacific, as emphasized by Wang et al. [2000], which reflects a weaker-than-normal northwestern Pacific subtropical high with an eastward-withdrawn position [Wen et al., 2009]. Corresponding to the weakened subtropical high, the moisture transport along its western flank to southern China is reduced simultaneously.

Figure 7.

(a) January–May correlation between the negative precipitation index (in core drought area) and grid point 850-hPa moisture fluxes. (b) Same as in Figure 7a but for negative Niño3.4 index. Values significantly exceeding the 95% confidence level (t-test) are shaded.

[15] Previous studies have shown that a strong EAWM is usually observed during La Niña winters [Li, 1990; Zhang et al., 1999; Chen and Wu, 2000; Chen, 2002; Yang et al., 2002; Chen et al., 2005]. The JFMAM correlation between a meridional wind index (hereafter MW index; 850-hPa meridional wind averaged over 110°E–130°E, 20°N–40°N) and grid point SSTA also shows that significant correlation appears over the central and eastern Pacific (Figure 8a). To understand how the southward intrusion of dry northerly flow is linked to La Niña, we further examine the features of correlation between negative Niño3.4 index and 200-hPa zonal wind (see Figure 8b). Significant positive correlation is found in East Asia, indicating an enhancement of the East Asian jet stream (EAJS), which is consistent with the result of previous studies for winter [e.g., Yang et al., 2002]. When the EAJS is stronger than normal, the East Asian trough tends to deepen (Figures 9a and 9b), providing a favorable mid-tropospheric circulation background for the intrusion of cold air from the high latitudes to East Asia [e.g., Ding, 1994], as seen in Figure 3b.

Figure 8.

(a) January–May correlation between negative MW index (V850 averaged over 110°E–130°E, 20°N–40°N) and SST during 1979–2011. (b) January–May correlation between negative Niño3.4 index and 200-hPa zonal wind. Values significantly exceeding the 95% confidence level (t-test) are shaded.

Figure 9.

(a) Standardized time series of the January–May EAJS index (U200 averaged over 30°N–35°N, 130°E–160°E). (b) January–May correlation between EAJS index and Z500. Values significantly exceeding the 95% confidence level (t-test) are shaded.

[16] Hence, the possible influence of La Niña on southern China drought during January to May can be explained as follows. At the lower troposphere, La Niña tends to cause an anomalous cyclonic circulation over the northwestern Pacific, which weakens the northward moisture transportation from the western Pacific to southern China. Further, La Niña may enhance the 200-hPa EAJS and induce a deeper-than-normal mid-tropospheric East Asian trough, which provides a favorable circulation background for dry northerly flow to intrude from the high latitudes to southern China, blocking the northward transportation of moisture from tropical oceans.

4.2. The North Atlantic Oscillation

[17] In the above section we have discussed the southward intrusion of northerly flow from the mid-high latitudes to southern China during La Niña. However, a more direct origin of the northerly flow over southern China may be associated with NAO and the positive geopotential height anomalies over the vicinity of Siberia, as shown in the JFMAM correlations between NAO and the 850-hPa meridional wind, and between the MW index and grid point Z500 (figures not shown). That is, the anomalous northerly wind may be associated with the positive phase of NAO and the positive height anomalies over the Siberia, as noted in Figure 4a. Therefore, in this section, we examine whether the anomalous northerly flow over the southern China drought region is related to the positive phase of NAO.

[18] We first assume that NAO influences downstream circulation through a wave train. To depict the possible link to this wave train (see Figure 4a), we analyze the anomalies of wave activity flux and stream function at 200-hPa level in JFMAM 2011 (Figure 10). Wave activity flux is a useful tool for diagnosing stationary disturbances that propagate through a zonally varying basic flow [Takaya and Nakamura, 2001]. The sub-polar anomalous stationary wave energy clearly emanated from the positive geopotential height anomalies over the vicinity of eastern North Atlantic and western Europe, the southern part of positive NAO spatial pattern (Figure 4a). Since previous studies have shown strong correlation between the European and North Atlantic blocking systems and NAO in winter [e.g., Shabbar et al., 2001; Luo, 2005; Scherrer et al., 2006; Sung et al., 2011], it is interesting to know whether the NAO in JFMAM also influences the blocking anomalies over or near the North Atlantic and further leads to positive height anomalies over the Siberia through its downstream effect. The JFMAM correlation between the NAO index and 500-hPa geopotential height during 1979–2011 shows that, when NAO is positive, northern Eurasian continent is featured by a wave train pattern with significant positive height anomalies over the eastern North Atlantic, western Europe, and the Siberia (Figure 11b). The above result indicates that the positive NAO phase of 2011 might excite an anomalous anti-cyclonic circulation over the North Atlantic and western Europe regions and wave energy that emanated from the western Europe blocking systems and propagated eastward to induce the positive height anomalous over the Siberia.

Figure 10.

200-hPa horizontal Rossby wave activity flux (vectors; m2s−2) and stream function (contours; 1.0 × 106 m2s−2) anomalies in January–May 2011. The definition of wave activity flux is based on the computation of Takaya and Nakamura [2001].

Figure 11.

(a) Standardized January–May NAO index from 1979 to 2011 and January–May correlation of January–May NAO (b) with Z500 and (c) with 200-hPa zonal wind. Values significantly exceeding the 95% confidence level (t-test) are shaded.

[19] We further demonstrate the JFMAM correlation between NAO and 200-hPa zonal wind during 1979–2011 (Figure 11c). It is seen that the Middle East and Asian jet streams tend to become stronger when NAO is in a positive phase, as emphasized by Watanabe [2004] and Wen et al. [2009]. Thus, when the Rossby waves propagate southward from the eastern North Atlantic to the jet entrance, they will be trapped by the strong Asian jet waveguide to form a wave train along the jet stream (see Figure 10). These features are similar to the result about the downstream influence of NAO [e.g., Watanabe, 2004; Hong et al., 2008]. To demonstrate the possible impact of this Asian wave train on the northerly flow over southern China, we also analyze the longitude-height cross section of meridional wind anomalies along the Asian jet core (20°N–30°N) and the JFMAM correlation between the MW index and 200-hPa meridional wind. It is revealed by Figure 12a and the correlation analysis (figure not shown) that the low-level anomalous northerly flow over southern China is clearly associated with an upper-level barotropic wave train emanating from the Mediterranean-Sahara region to southern China along the subtropical waveguide [also see Hsu and Lin, 1992; Hoskins and Ambrizzi, 1993; Branstator, 2002]. Since previous studies have proposed that the enhanced convection over the vicinity of Mediterranean-Sahara region may act as a Rossby wave source to excite a wave train along the subtropical jet stream [Sardeshmukh and Hoskins, 1988; Watanabe, 2004; Hong et al., 2008], we also compute the 200-hPa velocity potential to show the upper-level convergence condition over the region. Indeed, enhanced convection can be found over the Mediterranean-Sahara region, to the southern part of the overall NAO spatial pattern (Figure 12b). A similar phenomenon was also found by previous studies and it was considered as a result from the influence of NAO [e.g., Wang, 2003; Hong et al., 2008].

Figure 12.

(a) Vertical cross-section of meridional wind anomalies (ms−1) averaged over 20°N–30°N for January–May 2011. (b) 200-hPa velocity potential anomalies (1.0 × 106 m2s−2) for January–May 2011.

[20] The more direct link between NAO and southern China drought is shown in Figure 13, which presents the JFMAM correlation between the negative MW index and grid point precipitation during 1979–2011. Significant negative correlation is found over a broad region from northern China to southern China, which is consistent with the precipitation anomaly distribution shown in Figure 2b. Therefore, the overall link of NAO to precipitation anomalies is dynamically consistent with the general relationship between NAO and the key circulation parameters discussed above.

Figure 13.

January–May correlation between negative MW index and grid point precipitation for 1979–2011. Values significantly exceeding the 95% confidence level (t-test) are shaded.

5. Further Discussion

5.1. Precursory Signals Related to ENSO

[21] Since the anomalies of equatorial central and eastern Pacific SST are relatively more persistent compared to atmospheric signals, we further analyze the possible precursory SST signals for the JFMAM-2011 persistent drought over southern China in SST anomalies. Figure 14 shows the correlations between the negative JFMAM CD index and grid point SSTs in the previous two seasons. The pattern in the central-eastern Pacific is generally similar to the simultaneous correlation pattern shown in Figure 5b, meaning that the SSTs in the tropical central and eastern Pacific may provide significant signals for southern China drought. The correlations between grid point JFMAM precipitation and the negative Niño3.4 index in previous two seasons also show the link of drought to previous La Niña conditions (Figure 15). The most notable feature is the significant negative correlation over southern China and North China, which is consistent with the distribution of precipitation anomalies in JFMAM 2011 (Figure 2b) and the simultaneous correlation between JFMAM precipitation and negative Niño3.4 index (Figure 6). Thus, La Niña may have a predictive potential for southern China droughts during winter and spring seasons.

Figure 14.

Correlation of negative January–May precipitation index (in core drought area) with grid point SSTs in (a) previous July–September and (b) previous October–December. Values significantly exceeding the 95% confidence level (t-test) are shaded.

Figure 15.

(a) Correlation of negative Nino3.4 SST in previous July–September with January–May precipitation and (b) correlation of negative Nino3.4 SST in previous October–December with January–May precipitation. Values significantly exceeding the 95% confidence level (t-test) are shaded.

5.2. Possible Relationship With the Tibetan Plateau Thermal Condition

[22] The report of snow monitoring issued by the CMA National Climate Center indicates that the number of snow cover days in late 2010 and early 2011 was below normal over most of the TP region (http://ncc.cma.gov.cn/Monitoring/snow_ice.php). Given the usually strong snow-temperature relationship, it is speculated that the below-normal snow condition is linked to an increase in surface and possibly tropospheric temperatures and thus an anomalous anticyclone over the region [Gao and Yang, 2009]. Figure 16 shows the 700-200-hPa mean temperature, 500-hPa geopotential height, and 700-hPa wind anomalies in October–December 2010 and JFMAM 2011. From northern India to TP, the anomalies of tropospheric temperatures were 0.8–1.6°C (above normal) in October–December 2010 and 0–0.8°C in the following JFMAM. Associated with the positive tropospheric temperature anomalies, an anomalous anti-cyclonic circulation (or ridge) pattern appeared over the vicinity of TP, which weakened the westerly flow to the southern flank of the plateau. It is recalled that the southern branch westerly flow is important for the winter climate in China, via its control on water vapor supply [He et al., 2006; Li et al., 2007; Wang et al., 2011].

Figure 16.

Anomalies of 700-200-hPa mean temperature (shading, °C), 700-hPa winds (vectors, ms−1), and Z500 (contours) in (a) October–December 2010 and (b) January–May 2011.

[23] To further reveal the possible link between southern China drought and the lower-tropospheric anomalous anticyclone over TP, we analyze the time series of TP westerly wind index (hereafter, TP index) during 1979–2011 (Figure 17a), which is defined as the JFMAM 850-hPa zonal wind averaged over 18°N–25°N/80°E–100°E, to the southern flank of TP (i.e., location of the India-Burma trough). The southern branch westerlies cover the India-Burma trough, which is an important phenomenon for precipitation over China [Wang et al., 2011]. The JFMAM correlations of the negative TP index with Z500 and with 700-200-hPa mean temperature reveals that the variability of the southern branch westerly flow is statistically more closely associated with the anomalous mid-troposphere ridge over the vicinity of TP (Figure 17b) than with the whole troposphere temperatures (figure not shown). On the other hand, the lag correlation between the negative JFMAM TP index and the Z500 and tropospheric temperatures of previous October–December indicates that the positive height anomalies and the warm temperature anomalies over TP may both provide precursory signals for the variability of the southern branch westerly flow in the following JFMAM (Figure 17c). We finally analyze the possible effect of TP on southern China drought by examining the correlations of the negative JFMAM TP index with 850-hPa moisture transport and precipitation (Figure 18). It is seen that the weakened southern branch westerly flow over the southern flank of TP tends to reduce the moisture transportation from the Bay of Bengal to southern China. The reduction in water vapor supply decreases precipitation and intensify southern China drought.

Figure 17.

(a) Standardized January–May 850-hPa zonal wind index over the southern flank of the Tibetan Plateau (TP index; U850 averaged for 18°N–25°N, 80°E–100°E) from 1979 to 2011. Shown also are correlations of negative January–May TP index (b) with January–May Z500 and (c) with previous October–December Z500. Values significantly exceeding the 95% confidence level (t-test) are shaded.

Figure 18.

Correlation of the negative TP index with grid point 850-hPa moisture (a) fluxes and (b) precipitation. Values significantly exceeding the 95% confidence level (t-test) are shaded.

6. Summary

[24] In this study, we have applied the CPC unified global daily precipitation analysis and the NCEP-NCAR reanalysis to depict the extraordinarily persistent southern China drought from peak winter to spring 2011 and its dynamical relationship with multiple possible impacting factors including La Niña, NAO, and the Tibetan Plateau thermal condition. La Niña seemed to be one of the main influencing factors of southern China droughts. In JFMAM, the SSTs in central and eastern tropical Pacific are significantly correlated with the drought in southern China. The La Niña event in 2010–2011 seemed to cause an anomalous cyclonic circulation at the lower troposphere over the northwestern Pacific, weakening the northwestern Pacific subtropical high and reducing the moisture transportation from tropical oceans. The La Niña event also tended to reinforce the East Asian jet stream and deepen the middle-tropospheric East Asian trough, inducing a southward intrusion of high-latitude dry northerly flow to southern China and further blocking the northward transportation of moisture from tropical oceans. In addition, the equatorial central-eastern Pacific SSTs in the previous two seasons might provide precursory signals of the JFMAM drought over southern China.

[25] In JFMAM 2011, the NAO was in a positive phase and it might excite a wave train propagating eastward from the vicinity of eastern North Atlantic and western Europe to East Asian along the sub-polar waveguide. The downstream influence of this wave train further induced positive height anomalies over the Siberian region, which is usually viewed as a favorable condition for intrusion of cold surges into East Asia. Furthermore, the NAO-associated stationary Rossby wave energy emanated southward to the broad region from Mediterranean to Sahara, enhancing local convective activity. The enhanced convection served as a Rossby wave source [Sardeshmukh and Hoskins, 1988; Watanabe, 2004; Hong et al., 2008] to excite upper-tropospheric Rossby waves along the East Asian jet stream and bring dry northerly flow from the upper levels.

[26] In JFMAM 2011, above-normal tropospheric temperatures occurred over the vicinity of TP, associated with the less snow cover over the plateau. Correspondingly, an anomalous anticyclone occurred over the plateau. The associated change in westerlies over the southern flank of TP, especially over the India-Burma trough region, might provide another favorable condition for the southern China drought. The westerlies were weaker and the trough was less active in JFMAM 2011, reducing moisture transportation from the Bay of Bengal to eastern China.

[27] Several other southern China drought events in the past decades have also been analyzed. When the CD index is less than 0.5 standard deviations, twelve drought years can be selected for the period of 1979–2010: 1982, 1985, 1986, 1993, 1994, 1995, 1996, 1997, 1999, 2007, 2008, and 2009. The corresponding distributions of SST anomalies in the tropical Pacific, the phase of NAO, the thermal condition of TP, and the intensity of the associated southern branch of westerlies over TP are also validated. Two of the cases (1986 and 2009) occurred when negative SST anomalies below −0.5°C appeared in the equatorial center-eastern Pacific, the NAO was in a positive phase, above-normal tropospheric temperatures or anomalous ridge occurred over the vicinity of TP, and the associated zonal wind in the India-Burma trough region was weaker than normal (figure not shown). Thus, La Niña, positive phase of NAO, and the TP thermal condition may be among the important influencing factors for droughts in southern China.

[28] It should be pointed out that although the persistent and severe southern China drought analyzed in this study may be attributed to the effects of La Niña, NAO, and the Tibetan Plateau, other factors should also be investigated. Since previous studies have shown that upper tropospheric temperatures, aerosol, and others may also be important for the variability of Asian climate [Cheng et al., 2005; Xin et al., 2006], more analyses are needed to understand whether the particular southern China drought in JFMAM 2011 was also related to the influences of these possible factors. Nevertheless, in this study we have only obtained the results using a linear correlation analysis, which has limitations in distinguishing cause and effect directly. When cautions should be taken when applying the conclusions of this study about the possible impacting factors of drought, experiments with state-of-the-art dynamic models may be necessary in future investigations on the causes of the persistent severe drought.

Acknowledgments

[29] The authors thank Peiqun Zhang of the CMA National Climate center, the two anonymous reviewers, and the Editor whose constructive comments are helpful for improving the overall quality of the paper. This research was supported by grants from the National Basic Research Program of China (973 Program) (2012CB955901, 2010CB950501, and 2010CB950404) and the National Science and Technology Support Program of China (2009BAC51B05 and 2007BAC29B04).