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  • View in gallery
    Fig. 1.

    Shown are (a) the locations of 93 stations in the MLRYR (dots) along with the distribution of total precipitation (shaded; 50 mm contour interval) during the 2011 JFMAM, (b) the percentages of precipitation anomalies compared to its mean climatology (1961–2011), and (c) the time series of precipitation anomalies (mm) averaged over MLRYR. The thick black curves in (a) and (b) indicate for the Yangtze River.

  • View in gallery
    Fig. 2.

    (a) Anomalous zonal–vertical circulation averaged over 25°–35°N, with the gray shades indicating the zonal component of divergent wind anomalies (m s−1); (b) anomalous meridional–vertical circulation averaged over 110°–122.5°E, with gray shades indicating the meridional component of divergent wind anomalies (m s−1); and (c) anomalies of water vapor flux integrated from the earth surface up to 300 hPa (vectors; kg m−1 s−1) along with the velocity potential of the vapor flux (shaded; 106 m2 s−1). The black filled areas in (a) and (b) are for topography while the thick black curve in (c) indicates the Yangtze River. Dashed contours are for negative values. (d)–(f) As in (a)–(c), but for quantities after having ENSO signal removed.

  • View in gallery
    Fig. 3.

    (a) Sea level pressure anomalies (shaded; hPa) and wave activity flux at 850 hPa (vectors; m2 s−2). (b) Geopotential height anomalies at 500 hPa (shaded; dgpm) and wave activity flux at 500 hPa (vectors; m2 s−2); (c) As in (b), but for 200 hPa. (d) Zonal–vertical cross section for the wave activity fluxes (m2 s−2) of anomalous circulation averaged over 50°–65°N with shades for anomalous geopotential height (dgpm). (e)–(h) As in (a)–(d), but for quantities after having NAO signal removed.

  • View in gallery
    Fig. 4.

    Longitude–time diagram for 15-day running mean of the meridional wind along the thick red dotted line shown in Fig. 3b.

  • View in gallery
    Fig. 5.

    (a) Sea surface temperature anomalies (shaded; °C) and wind field anomaly at 850 hPa (vectors; m s−1) during JFMAM of 2011. (b) Anomalies of dynamic heating (K day−1) obtained by integrating the horizontal advection of potential temperature vertically from the earth surface up to 100 hPa. (c) As in (b), but for convection of potential temperature (K day−1). (d) Anomalies of vertically integrated diabatic heating (K day−1). Dashed contours are for negative values.

  • View in gallery
    Fig. 6.

    As in Fig. 5, but for quantities after having ENSO signal removed.

  • View in gallery
    Fig. 7.

    Anomalous velocity potential (shaded; 106 m2 s−1) and related divergent component of winds (vectors; m s−1) at (a) 850, (b) 500, and (c) 200 hPa. (d) The anomalous zonal circulation averaged over 5°S–5°N with the shades indicating the zonal component of divergent wind anomalies (m s−1). Dashed contours are for negative values. (e)–(h) As in (a)–(d), but for quantities after having ENSO signal removed.

  • View in gallery
    Fig. 8.

    (left) Anomalous velocity potential (contours; 106 m2 s−1) and the divergent winds at 850 hPa. (top)–(bottom) The quantities averaged over the periods 1–10 Jan, 11–31 Jan, February, March, April, 1–20 May, and 21–25 May 2011, respectively. (center) As in (left), but for 200 hPa. (right) The SSTAs (shaded; °C) along with the anomalous winds (vectors; m s−1) at 850 hPa.

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The Extreme Drought Event during Winter–Spring of 2011 in East China: Combined Influences of Teleconnection in Midhigh Latitudes and Thermal Forcing in Maritime Continent Region

Dachao JinKey Laboratory of Meteorological Disaster, Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China

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Zhaoyong GuanKey Laboratory of Meteorological Disaster, Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China

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Weiya TangKey Laboratory of Meteorological Disaster, Ministry of Education, Nanjing University of Information Science and Technology, Nanjing, China

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Abstract

The middle and lower reaches of the Yangtze River (MLRYR) in China experienced an extremely severe and persistent drought event from January to May of 2011. Using both the observational data and NCEP–NCAR reanalysis, features of the drought event and the related circulation anomalies were investigated. It is found that the precipitation during the investigated period of 2011 was deficient mostly along the Yangtze River. The water vapor diverged from MLRYR southward into the Bay of Bengal, South China Sea, and the Philippines. There were two factors facilitating the drought event. One was the quasi-stationary Rossby wave–related teleconnection, which propagated eastward at midhigh latitudes from the North Atlantic to East Asia, reinforcing the Siberian high and the East Asian trough, henceforth resulting in the divergence anomalies in MLRYR in the lower troposphere. This quasi-stationary wave train, though originating from the North Atlantic region, was not essentially related to the North Atlantic Oscillation. Another factor for the drought event was the persistent anomalous thermal forcing over the Maritime Continent, which induced the anomalous divergence in the upper troposphere in this region, building up an anomalous Hadley circulation with its ascent branch over the Maritime Continent and descent branch over MLRYR. This thermal forcing was possibly, but not necessarily, related to the La Niña event. The persistence of the drought event over MLRYR was due to the maintenance of the quasi-stationary waves at midhigh latitudes and the persistent anomalous thermal forcing in the Maritime Continent.

Corresponding author address: Zhaoyong Guan, Key Laboratory of Meteorological Disaster, Ministry of Education, Nanjing University of Information Science and Technology, 219 Ningliu Rd., Nanjing 210044, Jiangsu, China. E-mail: guanzy@nuist.edu.cn

Abstract

The middle and lower reaches of the Yangtze River (MLRYR) in China experienced an extremely severe and persistent drought event from January to May of 2011. Using both the observational data and NCEP–NCAR reanalysis, features of the drought event and the related circulation anomalies were investigated. It is found that the precipitation during the investigated period of 2011 was deficient mostly along the Yangtze River. The water vapor diverged from MLRYR southward into the Bay of Bengal, South China Sea, and the Philippines. There were two factors facilitating the drought event. One was the quasi-stationary Rossby wave–related teleconnection, which propagated eastward at midhigh latitudes from the North Atlantic to East Asia, reinforcing the Siberian high and the East Asian trough, henceforth resulting in the divergence anomalies in MLRYR in the lower troposphere. This quasi-stationary wave train, though originating from the North Atlantic region, was not essentially related to the North Atlantic Oscillation. Another factor for the drought event was the persistent anomalous thermal forcing over the Maritime Continent, which induced the anomalous divergence in the upper troposphere in this region, building up an anomalous Hadley circulation with its ascent branch over the Maritime Continent and descent branch over MLRYR. This thermal forcing was possibly, but not necessarily, related to the La Niña event. The persistence of the drought event over MLRYR was due to the maintenance of the quasi-stationary waves at midhigh latitudes and the persistent anomalous thermal forcing in the Maritime Continent.

Corresponding author address: Zhaoyong Guan, Key Laboratory of Meteorological Disaster, Ministry of Education, Nanjing University of Information Science and Technology, 219 Ningliu Rd., Nanjing 210044, Jiangsu, China. E-mail: guanzy@nuist.edu.cn

1. Introduction

An extremely severe and persistent drought occurred in the middle and lower reaches of the Yangtze River (MLRYR) in China from late winter in 2010-11 to the spring in 2011 (from 11 January to 20 May 2011; Li et al. 2012). This was a record-breaking event for this region over the past five decades. Deficient rainfall prevailed in the study area (Fig. 1), which resulted in very low water stages and the drying of many local lakes, hence exerting serious impacts on agricultural production, fisheries, river transportation, hydroelectricity supply, and the daily life of local people and bringing about huge economic losses (Sun and Yang 2012).

Fig. 1.
Fig. 1.

Shown are (a) the locations of 93 stations in the MLRYR (dots) along with the distribution of total precipitation (shaded; 50 mm contour interval) during the 2011 JFMAM, (b) the percentages of precipitation anomalies compared to its mean climatology (1961–2011), and (c) the time series of precipitation anomalies (mm) averaged over MLRYR. The thick black curves in (a) and (b) indicate for the Yangtze River.

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00652.1

The MLRYR is located in eastern China (25°–35°N, 110°–122.5°E) in the East Asian monsoon region near the extratropical west Pacific (Fig. 1a). Some studies have reported that winter and spring precipitation in MLRYR has strong interannual variations, while exhibiting a decreasing trend over the past 60 yr (e.g., Hu et al. 2003; Yang and Lau 2004; Li et al. 2005; Zhai et al. 2005; Xin et al. 2006). The precipitation in MLRYR is directly affected by the East Asian winter monsoon (EAWM) significantly (e.g., Ding 1994; Wang 2006; Zhou and Wu 2010). However, the EAWM and the related winter precipitation in MLRYR can be influenced by many factors, including circulation disturbances and the diabatic and dynamic forcings from both the higher latitudes and tropical regions.

Yang et al. (2002) found that the Asian–Pacific–American winter climate anomalies are related to the variations of the East Asian jet stream (EAJS). Anomalous changes in the westerly jet over north Asia and the polar vortex are found to have influences on spring precipitation in MLRYR (Wang et al. 2002) and the mid- and upper-tropospheric temperature anomalies (Xin et al. 2006). Some teleconnections, such as the Eurasian teleconnection pattern (EU), the Arctic Oscillation (AO; Thompson and Wallace 1998), and North Atlantic Oscillation (NAO; Hurrell 1995), are found to have impacts on the winter–spring precipitation and temperature changes in southern China (Zhang et al. 2009; Li et al. 2005). During spring, the precipitation can also be affected by an Asia–Pacific teleconnection, as reported in a study by Zhou and Zhao (2010).

Winter and spring rainfall in MLRYR is sometimes related to the Pacific sea surface temperature anomalies (SSTAs). Wang et al. (2000) revealed that the Asian climate anomalies are affected by ENSO via a Pacific–East Asia teleconnection that is characterized by an anomalous anticyclonic circulation east of Philippines. This ENSO–EAWM relation has also been examined in many other literatures (e.g., Wang et al. 2000; Wang et al. 2002; Zhang and Sumi 2002; Yang and Lau 2004; Li et al. 2005; Zhang et al. 2009; Zhou and Wu 2010).

Recently, Sun and Yang (2012) have examined the persistent severe drought event in January–May (JFMAM) of 2011, concluding that the La Niña event and the NAO are the possible factors contributing to this extreme drought event in MLRYR.

However, there are still some other related changes in our climate system to be clarified. First, a weak correlation with a coefficient of about 0.3 exists between JFMAM rainfall and the La Niña signal (Sun and Yang 2012), suggesting that there were some other possible anomalous changes in tropics besides the La Niña event in 2011. If we remove the ENSO signal from the circulation anomalies, what scenarios will there be for the drought event in 2011? Second, the NAO can influence the precipitation in MLRYR as mentioned above. Sun and Yang (2012) emphasized that the NAO-induced Rossby wave propagated eastward to East Asia along the Asian jet stream. However, the NAO-induced disturbances in the westerly may also propagate in the midlatitudes north of the wintertime Asian jet, with the path of wave energy propagation curving southward around Lake Baikal. In 2011, was this the possible scenario observed? Further, by removing the NAO signal from the time series of physical quantities, was there anything to change in the circulations in midlatitudes responsible for the drought event? Third, during JFMAM, the rainfall amount was too small to end the persistent drought in spite of some rainfall events. Is this persistence due to the La Niña event or because of the persistent thermal forcing from tropical oceans? And if it is, which part of the tropics is the most important? These questions need to be answered.

In this study, we examine the drought event in section 3 after a short description of data sources in section 2. We investigate the local circulation anomalies that induce the extreme drought event during JFMAM of 2011. After section 3, the Rossby wave dispersions related to NAO is described in section 4 in connection with the atmospheric teleconnection north of the wintertime Asian jet. In section 5, we document the influences of SSTAs in the tropical Pacific, with emphasis on the thermal forcings from the Maritime Continent region, on the drought event in MLRYR in JFMAM of 2011. We examine the persistence of drought in JFMAM of 2011 in section 6. Section 7 covers the concluding remarks and discussion.

2. Data and methodology

Daily precipitation data in MLRYR during JFMAM during the years from 1961 to 2009 at 93 stations (Fig. 1a) are obtained from the National Climate Center of China Meteorological Administration, and the daily precipitation data of 2010–11 are retrieved from the Meteorological Information Combined Analysis and Process System 2.0 (MICAPS 2.0). The daily data of winds, air temperature, and geopotential heights are from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis (Kalnay et al. 1996) for the JFMAM during the period of 1979–2011 with 17 levels in vertical and a grid mesh of 2.5° × 2.5° in the horizontal. The National Oceanic and Atmospheric Administration (NOAA) Extended Reconstructed Sea Surface Temperature (ERSSTv3) dataset (Smith et al. 2008) with a grid mesh of 2° × 2° are employed in the present work.

A regression is used to partly remove the ENSO signal from a time series. Suppose that () is a normalized time series for the ENSO signal (NAO index). Let and be a time series for a physical quantity and the remaining part of after having the ENSO signal removed, respectively. Then can be expressed as
e1
where is the covariance of and (). In fact, the regression method employed in the present work is similar to the partial correlations in a statistical sense. More details can be found in some other climate studies (e.g., Ashok et al. 2003; Guan et al. 2003).
The thermal dynamical equation is employed, which is expressed as
e2
In Eq. (2), the first, second, and third terms on the right-hand side stand for dynamic heating from the horizontal advection of potential temperature, dynamic heating from convection, and diabatic heating, respectively. The symbols in Eq. (2) represent the conventional meteorological variables. The terms in Eq. (2) were calculated for the discussion in section 5 of the paper.

3. The extreme drought event in MLRYR in 2011 and local circulation features

The precipitation is spatially distributed nonuniformly in MLRYR during JFMAM of 2011 (Fig. 1a). Maximum precipitation (>400 mm) occurred in the southern MLRYR while the minimum (<100 mm) occurred in the northern part of the region. The precipitation was characterized spatially by its larger amount in the south and relatively smaller amount in the north.

Compared with the mean precipitation amount in the corresponding period over the past five decades, a deficient rainfall zone (Fig. 1b) in 2011 was evident along the MLRYR, with an extreme precipitation anomaly percentage lower than −60%. The time series of average precipitation anomalies over MLRYR (Fig. 1c) indicate that precipitation in this region changes substantially on interannual and interdecadal time scales. In particular, the precipitation anomaly in 2011 amounted to −245 mm, which was the lowest in the past 51 years.

Some rainfall events occurred in MLRYR during the period from 11 January to 20 May 2011. In Table 1, all rainy days with areal-averaged rainfall above 5 mm day−1 are listed. As seen from Table 1, such rainy days were only 19 out of 151 days of JFMAM. Without any doubt, both the rainfall amount and the number of rainy days were not large enough to end the drought episode in MLRYR in JFMAM of 2011.

Table 1.

Dates of regional mean precipitation above 5 mm day−1 during the period 11 Jan–31 May 2011 over MLRYR.

Table 1.

Descending air in MLRYR is responsible for the occurrence of the drought event. The zonal–vertical circulation is represented by the divergent component of the zonal wind and vertical velocity averaged over 25°–35°N (Fig. 2a) and is displayed by the strong downdrafts in the region 110°–135°E. The upward motion of the atmosphere, as seen in Fig. 2b, is observed in the Pacific warm pool, whereas the downward motion branch of the meridional circulation is in region from 20° to 30°N. This meridional circulation strengthened the Hadley circulation over East Asia, leading to an anomalously stronger descent of air in MLRYR, hence resulting in the extreme drought event of 2011.

Fig. 2.
Fig. 2.

(a) Anomalous zonal–vertical circulation averaged over 25°–35°N, with the gray shades indicating the zonal component of divergent wind anomalies (m s−1); (b) anomalous meridional–vertical circulation averaged over 110°–122.5°E, with gray shades indicating the meridional component of divergent wind anomalies (m s−1); and (c) anomalies of water vapor flux integrated from the earth surface up to 300 hPa (vectors; kg m−1 s−1) along with the velocity potential of the vapor flux (shaded; 106 m2 s−1). The black filled areas in (a) and (b) are for topography while the thick black curve in (c) indicates the Yangtze River. Dashed contours are for negative values. (d)–(f) As in (a)–(c), but for quantities after having ENSO signal removed.

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00652.1

Water vapor diverges from MLRYR and south Japan as well as the Kuroshio region into both the Maritime Continent and Bay of Bengal (Fig. 2c). This divergence was not favorable for producing rainfall in MLRYR. It is seen in the tropical region that a strong convergence center of water vapor fluxes is located in the Maritime Continent with two vapor divergence centers to its left and right hand sides. This distribution of water vapor transports implies that a matured La Niña event occurred in 2011. Usually, when a La Niña event occurs, a vapor convergence center tends to appear in the Maritime Continent, inducing floods in Indonesia. The divergence in MLRYR is therefore related to the La Niña event via the water vapor convergence in the Maritime Continent. However, as for the drought event in MLRYR from the winter to the spring of 2011, the Maritime Continent seemed to be a key region where the convergence of water vapor is linked to the divergence of vapor in MLRYR, even though the La Niña signal has been removed (Figs. 2d–f).

4. Impacts of teleconnections in midhigh latitudes

The local factors that facilitated the strong negative anomalous precipitation in JFMAM of 2011 were the strengthened descent of air and water vapor divergence over the MLRYR. However, the reason why the drought event occurred in 2011 deserves further investigation into the circulation anomalies related to the upstream perturbations of the westerlies. The term anomaly hereafter refers to the deviation of the quantities in JFMAM of 2011 from the multiyear mean climatology of the quantities over JFMAM. The wave activity fluxes are calculated according to Takaya and Nakamura (2001) to look for insight into Rossby wave propagations (Figs. 3a–d).

Fig. 3.
Fig. 3.

(a) Sea level pressure anomalies (shaded; hPa) and wave activity flux at 850 hPa (vectors; m2 s−2). (b) Geopotential height anomalies at 500 hPa (shaded; dgpm) and wave activity flux at 500 hPa (vectors; m2 s−2); (c) As in (b), but for 200 hPa. (d) Zonal–vertical cross section for the wave activity fluxes (m2 s−2) of anomalous circulation averaged over 50°–65°N with shades for anomalous geopotential height (dgpm). (e)–(h) As in (a)–(d), but for quantities after having NAO signal removed.

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00652.1

The East Asian winter monsoon anomalies are usually associated with the East Asian trough anomalies (Wang et al. 2009). The negative anomalies of geopotential height at 500 hPa during JFMAM of 2011 are found over the Korean Peninsula and Japanese archipelago (Fig. 3b), indicating that the East Asian trough was intensified though its location apparently did not shift in JFMAM of 2011. This intensified trough facilitates the cold air to migrate from the Siberian region into mainland China. The positive sea level pressure anomalies (SLPAs) as seen in Fig. 3a are just a result of the intensification of the East Asian trough.

The strengthening of the East Asia trough can be related to the NAO through Rossby wave energy propagation. A negative SLPA was centered over Greenland and a positive SLPA was centered over the North Atlantic (Fig. 3a), assuming an NAO structure. Negative SST anomalies were observed over the central North Atlantic (30°–45°N, 45°–60°W), while positive anomalies were to the south and north (Fig. 5a, see below) of the region. This SSTA distribution was consistent with the SSTA distribution pattern related to NAO (Peng et al. 2002). Moreover, a Rossby wave train in association with NAO propagated eastward along the mid- and high latitudes that can be seen from wave activity flux (Figs. 3a–d).

The SLP anomalies (Fig. 3a) show us a negative–positive–negative–positive–negative wave train structure in the midlatitudes from southern Greenland eastward to the Norwegian Sea, the northern Ural Mountains, the Mongolian Plateau, and the Japanese archipelago. Correspondingly, circulation anomalies of cyclone–anticyclone–cyclone–anticyclone–cyclone were observed at 850 hPa from North Atlantic to Okhotsk (Fig. 5a, see below). This wave train structure can also be seen in geopotential height anomalies at 500 and 200 hPa (Figs. 3b,c). Particularly, the geopotential height anomalies averaged over 50°–65°N as displayed with the zonal–vertical cross section in Fig. 3d, which clearly shows a quasi-stationary wave train with barotropic structure west of the far east of Asia.

To verify the teleconnection pattern propagating along the mid- and high latitudes from the central North Atlantic to Japan, the wave activity fluxes were calculated according to Takaya and Nakamura (2001; Figs. 3a–d). The wave activity fluxes clearly displayed that the wave energy propagated eastward from the central North Atlantic to East Asia along the mid- and high latitudes (Figs. 3a–c). The Rossby wave train was excited at 60°W and then propagated eastward in 2011. The wave activity fluxes averaged over 50°–65°N (Fig. 3d) clearly show this scenario. Part of the wave energy propagated downward and accumulated over Siberia (80°–100°E), inducing the positive anomalies of geopotential height there. Fluxes of wave activity converge near northeast China and Japan (120°–150°E), causing the East Asian trough to be intensified. As a result, the high pressure cold air prevailed in eastern China, inducing an anomalously strong descent of air in JFMAM of 2011. In more detail, a 15-day running mean is performed on the daily wind data for JFMAM of 2011 along the thick red dotted line shown in Fig. 3b, yielding a longitude–time diagram for the meridional wind as shown in Fig. 4. The quasi-stationary waves were clearly seen between the high-latitude Atlantic and Japan regions with increased intensification of the anomalous zonal wind in JFMAM of 2011 and eastward propagation of the stronger winds (Fig. 4).

Fig. 4.
Fig. 4.

Longitude–time diagram for 15-day running mean of the meridional wind along the thick red dotted line shown in Fig. 3b.

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00652.1

Note that the teleconnection discussed by Sun and Yang (2012) describes a possible way in which the wave energy of NAO-related disturbances propagated along the Asian jet stream. However, the aforementioned wave train from the Ural Mountains to Japan matches the Eurasian–Pacific (EUP) teleconnection pattern discovered by Wallace and Gutzler (1981). A highly negative correlation between the EUP and the winter precipitation in MLRYR was reported in Zhang et al. (2009). This EUP pattern looks like what is shown in Fig. 3. Li et al. (2008) suggested that NAO-related disturbances propagated along a high-latitude route from the North Atlantic to northwest Europe, the Ural Mountains, and northern China and thereby affect the climate of East Asia. They named it the North Atlantic–Ural–East Asian (NAULEA) teleconnection pattern. The wave train during JFMAM of 2011 was similar to some extent to the NAULEA teleconnection pattern, but it did not propagate directly to East Asia after passing through the Ural Mountains; instead it continued to propagate eastward to the Far East through Siberia.

The NAO and the AO are highly correlated with each other and are usually considered as one phenomenon with two paradigms (Thompson and Wallace 1998; Deser 2000; Wallace 2000). Gong et al. (2001) and Wu and Wang (2002) argued that the winter AO was able to impact the Siberian high and thereby affect the winter monsoon and winter climate of East Asia. Here, close attention was paid to the NAO as the wave train was connected to the disturbances of the North Atlantic and East Asia. Therefore, the drought in MLRYR in 2011 JFMAM might be related to the NAO.

The NAO was partly responsible for the quasi-stationary waves that determined the positive geopotential height anomalies over the Asian continent. However, if the NAO signal is removed from the time series of physical quantities by means of a regression method, the scenarios displayed in Figs. 3a–d do not change much. Figures 3e–h show the wave activity fluxes along with geopotential height anomalies. It is seen that the NAO structure in the northern Atlantic disappears, but the wave train is still clearly observed in midhigh latitudes from Europe to East Asia, which corresponds to the wave propagation as indicated by the activity fluxes. These suggest that the NAO was not a necessary factor for the drought event occurrence in China in 2011.

5. Impacts of thermal forcing over the Maritime Continent

The SSTAs (Fig. 5a) were positive in the western equatorial Pacific during JFMAM of 2011 while they were negative in the central and eastern equatorial Pacific, indicating a La Niña event in the tropical Pacific. This La Niña might facilitate a less than normal winter rainfall in the MLRYR (Zhang et al. 2009; Sun and Yang 2012). Note that negative SST anomalies were observed in the central equatorial Indian Ocean (IO) in contrast to positive SSTAs in the tropical western Pacific, assuming a dipole pattern of SSTAs (Fig. 5a). As a response to the anomalous SSTAs, latent heat is released anomalously in the atmosphere (Fig. 5c), inducing cooling over the tropical Indian Ocean and warming over the Maritime Continent as seen in the vertically integrated diabatic heating (Fig. 5d).

Fig. 5.
Fig. 5.

(a) Sea surface temperature anomalies (shaded; °C) and wind field anomaly at 850 hPa (vectors; m s−1) during JFMAM of 2011. (b) Anomalies of dynamic heating (K day−1) obtained by integrating the horizontal advection of potential temperature vertically from the earth surface up to 100 hPa. (c) As in (b), but for convection of potential temperature (K day−1). (d) Anomalies of vertically integrated diabatic heating (K day−1). Dashed contours are for negative values.

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00652.1

The anomalous heating induced circulation anomalies in the Asian winter monsoon region during JFMAM of 2011. An anomalous Walker circulation was well defined in the tropical Indo-Pacific, linking to a dipole-like pattern of velocity potential with the divergence center in the equatorial Indian Ocean and a convergence center over the Indonesian archipelagos in the lower troposphere (Fig. 7a, see below). The anomalous winds converged into the Maritime Continent from east China in the lower troposphere (Fig. 7a, see below) while they emanated in the upper troposphere from the Maritime Continent region northward to MLRYR where the convergence was observed, resulting in an anomalous meridional circulation around 120°E and hence contributing to the drought event in MLRYR.

To examine whether ENSO is a necessary factor in influencing drought in MLRYR, a regression is performed to partly remove the ENSO signal (Fig. 6) from the time series of SSTAs and other quantities. It is clearly seen from Figs. 2d–f and 7e–h that the anomalous circulation pattern does not change much as compared with those in Figs. 2a–c and 7a–d. In the lower troposphere, the divergence anomalies are found in MLRYR, while the convergence anomalies are found in the upper troposphere there. On the other hand, a strong divergence center is found over the Maritime Continent region in the upper troposphere while a convergence center is in the lower troposphere. These convergence–divergence centers are responsible for the formation of the anomalous meridional circulation in East Asia. Obviously, the ENSO signal is not explicitly shown in Fig. 6a. This implies that the thermal forcing in the Maritime Continent, rather than that in eastern equatorial Pacific, is a more important factor for inducing the drought event in MLRYR in 2011.

Fig. 6.
Fig. 6.

As in Fig. 5, but for quantities after having ENSO signal removed.

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00652.1

Fig. 7.
Fig. 7.

Anomalous velocity potential (shaded; 106 m2 s−1) and related divergent component of winds (vectors; m s−1) at (a) 850, (b) 500, and (c) 200 hPa. (d) The anomalous zonal circulation averaged over 5°S–5°N with the shades indicating the zonal component of divergent wind anomalies (m s−1). Dashed contours are for negative values. (e)–(h) As in (a)–(d), but for quantities after having ENSO signal removed.

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00652.1

Note that the SSTA signal in the central tropical IO is essentially related to ENSO in boreal winter due to the lagging response in the tropical IO to the remote forcing of SSTAs in the central–eastern equatorial Pacific (e.g., Ashok et al. 2003; Saji et al. 2006; Tozuka et al. 2008; Taschetto et al. 2011). The dipole-like pattern in divergence in the Maritime Continent and central IO, as seen in Fig. 7 (left column), might be attributed to both the La Niña in the equatorial Pacific and the La Niña–induced IO SSTAs in 2011. However, after having the ENSO signal removed using the regression method, the dipole structure is still seen in Figs. 2, 5, and 7. This kind of anomalous circulation pattern is therefore obviously independent of La Niña in the eastern equatorial Pacific though it is associated with the weak IO SSTAs (Fig. 6a). In other words, after removing the ENSO signal the weak SSTAs in the tropical IO, which might be a result of ENSO forcing, may have different impacts on the anomalous divergence in the Maritime Continent. While the divergence anomalies in the Maritime Continent region induced by the thermal contrasts between the Indonesian archipelagoes and their surrounding regions merge as vorticity sources, a Matsuno–Gill-type response in the troposphere (Matsuno 1966; Gill 1980) may occur (e.g., Guan et al. 2003; Ashok et al. 2013) and hence a Pacific–Japan teleconnection pattern (Nitta 1987) might be excited in East Asia, inducing the anomalous climate conditions.

Therefore, La Niña events tend to influence the reduction of the winter rainfall in MLRYR (Zhang et al. 2009; Sun and Yang 2012). However, thermal heating anomalies in the Maritime Continent region, rather than the SSTAs in the central–eastern equatorial Pacific, may have also played a crucial role in inducing the anomalous meridional circulation in East Asia. The La Niña might not be the only remote forcing on the 2011 drought event in MLRYR.

6. Further examining the persistence of the drought event

A severe persistent drought event can be ended by a strong daily rainfall event. However, during the period from 11 January to 20 May 2011, the continuous rainy days were very few and the rainfall amount was very small, although several daily rainfall events occurred as listed in Table 1. This suggested that the long-term mean circulation anomalies that represented the quasi-stationary disturbances in the troposphere were not favorable for the occurrence of rainy weather systems over MLRYR.

By averaging the anomalous circulations and SSTAs over periods of several days or a month, we can easily find the scenarios of anomalous circulations from 1 January to 25 May 2011 (Fig. 8). During the period from 11 January to 20 May, the scenarios look similar to those in the JFMAM mean anomalies. Before the drought event started, the divergence center was observed over Japan rather than in MLRYR in early January of 2011. Instead, the weak convergence anomalies at 850 hPa were observed in MLRYR, while weak divergence anomalies were observed at 200 hPa (Fig. 8a). After 20 May 2011, a relatively stronger rainfall event occurred that lasted at least 4 days during the period from 21 to 24 May 2011 in MLRYR (Table 1). Out of the 4 days, there were 2 days when the daily rainfall averaged over MLRYR was above 10 mm day−1; it was a moderate rain event on a regional scale. The divergence was clearly seen over MLRYR at 200 hPa after the drought event (Fig. 8t).

Fig. 8.
Fig. 8.

(left) Anomalous velocity potential (contours; 106 m2 s−1) and the divergent winds at 850 hPa. (top)–(bottom) The quantities averaged over the periods 1–10 Jan, 11–31 Jan, February, March, April, 1–20 May, and 21–25 May 2011, respectively. (center) As in (left), but for 200 hPa. (right) The SSTAs (shaded; °C) along with the anomalous winds (vectors; m s−1) at 850 hPa.

Citation: Journal of Climate 26, 20; 10.1175/JCLI-D-12-00652.1

Although some location shifts of convergence and divergence centers can be found in Fig. 8 during the period from 11 January to 20 May 2011, the scenarios do not change much and are characterized by a convergence center at 850 hPa in the Maritime Continent and a divergence center at 200 hPa in the same region. The East Asian trough around the region east of Japan did not change its location much, as shown by the zonal shears of meridional wind (around 130°E) in Fig. 4. Therefore, both the thermal forcing in the Maritime Continent and the teleconnections in the midhigh latitudes facilitate the quasi-stationary disturbances of circulations in MLRYR. These quasi-stationary forcings and disturbances resulted in the persistence of the extreme drought event in the region in JFMAM of 2011.

7. Conclusions and discussion

Using the daily precipitation data from MICAPS 2.0 and from the China Meteorological Administration and NCEP–NCAR reanalysis, the persistent extreme drought event in MLRYR during JFMAM of 2011 was investigated in the present paper. The conclusions are as follows:

The entire MLRYR experienced extremely severe drought in JFMAM of 2011, which was a record-breaking event over the past 50 years. Despite the uneven geographical distribution of rainfall amounts, the worst drought-stricken area was along the Yangtze River.

During the drought event, a divergence center was observed in the lower troposphere of MLRYR. The water vapor was transported from the Kuroshio region and the MLRYR southward to the Bay of Bengal, South China Sea, and Philippine region. There were two important factors responsible for the formation of the circulation anomalies over MLRYR.

One such factor was the teleconnections in midhigh latitudes, which was related to a Rossby wave train excited over the North Atlantic. The NAO might play a part in exciting this Rossby wave train. However, after the NAO signal had been removed from the anomalies of circulations, the teleconnection patterns were still found from the North Atlantic eastward to East Asia. Both the quasi-stationary waves indicated by the geopotential height anomalies and the wave activity fluxes demonstrated the existence of this Rossby wave train. The Rossby wave energy could accumulate over Siberia and the region around Japan, resulting in positive geopotential height anomalies over Eurasia and intensifying the East Asian trough. The reinforced Siberian high and the East Asian trough led to the anomalous divergence in MLRYR in the lower troposphere.

The other factor was the anomalous thermal forcing in the Maritime Continent. During JFMAM of 2011, the central and eastern equatorial Pacific had negative SSTAs in a La Niña episode. The La Niña event had a weak link with the deficit wintertime rainfall over MLRYR (Sun and Yang 2012). However, if the ENSO signal had been removed simultaneously from circulation anomalies, the anomalous circulation patterns did not change much in JFMAM of 2011. This suggested that the thermal forcing in the tropical region close to MLRYR was more important for the drought event there. In fact, a dipole pattern of anomalous heating was observed in the tropical region, which was generated from the positive SSTAs of the western equatorial Pacific and the negative SSTAs of the central and eastern equatorial Indian Ocean. The eastern pole over the Maritime Continent played an important role in the low MLRYR precipitation and its persistence for a meridional circulation with its ascent branch over the Maritime Continent and the descent branch over MLRYR. These were conducive to the maintenance of the descent over the MLRYR, thus leading to the extreme drought event.

The record-breaking drought event in MLRYR lasted for almost 5 months. The persistence of this event was due to both the maintenance of the quasi-stationary waves in midhigh latitudes and the persistent thermal forcing in the Maritime Continent. In the background circulation anomalies, the weather system that brought about stronger rainfall over MLRYR could not stay long enough to eliminate the long-lasting drought episode.

Note that the NAO-related perturbations that propagated along the Asian jet and the La Niña event might be two possible factors that facilitated the drought event in MLRYR in 2011, as discussed in Sun and Yang (2012). But the quasi-stationary waves in midhigh latitudes and the persistent anomalous thermal forcing in the Maritime Continent might be more important, though neither NAO nor La Niña was directly involved. However, how and why both the quasi-stationary waves in midhigh latitudes and the constant anomalous thermal forcing in the Maritime Continent were maintained are still not clear. Moreover, during JFMAM of 2011, other regions such as Europe and the Korean Peninsula also suffered severe drought (e.g., http://www.wmo.int/pages/mediacentre/news/drought_en.html; http://ncc.cma.gov.cn/upload/upload2/disaster/201104_GL_20110518115356.png), which seemed to be associated with the wave train propagating eastward from the central North Atlantic. In addition, we also analyzed the ground surface air pressure anomalies in 2011 and found that the drought in MLRYR might also be associated with the interhemispheric oscillation (IHO; Guan and Yamagata 2001). These possibilities deserve further investigation.

Acknowledgments

The authors thank the anonymous reviewers of this paper for their helpful comments. This work is jointly supported by the National Technology Support Project of China (2007BAC29B02), the National Basic Research Program of China (2013CB430202), NSFC project with Grant 41175262, and the PAPD project of Jiangsu Province along with the Jiangsu Qinglan project for research teams. D.C. Jin is also supported by the Research Innovation Program for college graduates of Jiangsu Province.

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