Clim Dyn (2011) 36:1463–1473 DOI 10.1007/s00382-010-0852-9
Recent changes in the summer precipitation pattern in East China and the background circulation Yali Zhu • Huijun Wang • Wen Zhou Jiehua Ma
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Received: 31 August 2009 / Accepted: 15 May 2010 / Published online: 2 June 2010 Ó Springer-Verlag 2010
Abstract This study documents the decadal changes of the summer precipitation in East China, with increased rainfall in the Huang-Huai River region (HR) and decreased in the Yangtze River region (YR) during 2000–2008 in comparison to 1979–1999. The main features of the atmospheric circulation related to the increased precipitation in the HR are the strengthened ascending motion and slightly increased air humidity, which is partly due to the weakened moisture transport out of the HR to the western tropical Pacific (associated with the weakened westerly over East Asia and the warming center over the Lake Baikal). The rainfall decrease in the YR is related to the weakened ascending motion and reduced water vapor content, which is mainly related to the weakened southwesterly moisture flux into the YR (associated with the eastward recession of the Western Pacific Subtropical High). The global sea surface temperature (SST) also shows significant changes during 2000–2008 relative to 1979–1999. The shift of the Pacific decadal oscillation (PDO) to a negative phase probably induces the
Y. Zhu H. Wang J. Ma Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China Y. Zhu (&) H. Wang Climate Change Research Center, Chinese Academy of Sciences, Beijing 100029, China e-mail:
[email protected] W. Zhou School of Energy and Environment, City University of Hong Kong, Hong Kong, China J. Ma Graduate School of Chinese Academy of Sciences, Beijing 100029, China
warming over the Lake Baikal and the weakened westerly jet through the air-sea interaction in the Pacific, and thus changes the summer precipitation pattern in East China. Numerical experiments using an atmospheric general circulation model, with prescribed all-Pacific SST anomalies of 2000–2008 relative to 1979–1999, also lend support to the PDO’s contribution to the warming over the Lake Baikal and the weakened westerlies over East China. Keywords East China Precipitation Interdecadal variability Climate change
1 Introduction East China is one of the world’s most populated agricultural regions, dominated by the well-known East Asian monsoon. It’s society and economy are quite vulnerable to the variability in the summer precipitation, which is closely associated with the East Asian summer monsoon. One of the most prominent features of the precipitation pattern in East China is the belt-like distribution of the climatological and anomalous rainfall. There are three typical precipitation patterns (which have major centers over the Yangtze River valley, the Huang River, and the Huai River) that have been identified as keys to improve the seasonal rainfall forecast (Zhao 1999). These major rain-belts can also be distinguished with the empirical orthogonal function analysis method (Zhou and Yu 2005). The interdecadal variability is a main contributor to the variation in the summer rainfall in East China; identifying different interdecadal periods is important for seasonal forecasting. A prominent interdecadal shift in the East China rainfall pattern happened in the late 1970s (Wang 2001; Wu and Wang 2002; Han and Wang 2007; Ding et al. 2009), with
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more precipitation in the Yangtze River valley and less in North China since then. This shift appeared accompanying the global scale interdecadal shift. Wang (2001) attributed this rainfall change to the weakening of the Asian summer monsoon. Variations in the sea surface temperature (SST) can explain a large fraction of this weakening (Chang et al. 2000; Fu et al. 2009). The Pacific decadal oscillation (PDO), which is the leading mode of the interdecadal variability in the North Pacific SST, resembles the El Nino–Southern oscillation (ENSO) and is considered to be a major contributor to the 1970s shift (Zhang et al. 1997; Chang et al. 2000; Mantua and Hare 2002; Wang and An 2002; Yang and Lau 2004). The Indian and Atlantic Ocean SST are also suggested as the possible sources of this interdecadal change (Li et al. 2001; Yang and Lau 2004; Dong et al. 2006; Li et al. 2008; Wang et al. 2009; Sun et al. 2009a; Sun and Yuan 2009). Ding et al. (2009) attributes the shift to the cooperation between the abrupt increase in the winter and spring snow over the Tibetan Plateau and the major rapid warming events of the SST in the central and eastern tropical Pacific. North China (the area north of 35°N in East China) has suffered from severe drought since the late 1970s, while South China (the area south of 28°N in East China) has received increased rainfall since the early 1990s to 2000; these features form the so-called ‘‘southern flood and northern drought’’ pattern in East China (Fig. 1). The drought in North China over the last decade has been the severest during past 58 years. However, obvious changes in the precipitation pattern in East China occurred after about 1999, with increased rainfall over the Huang-Huai River region (HR) and somewhat decreased over the Yangtze River region (YR). Investigating this change is important for understanding and forecasting the East China climate, especially given the recent debate on the possibility of the precipitation pattern changes, namely, from ‘‘southern flood and northern drought’’ to ‘‘northern flood and southern drought’’. The topic becomes even more important under the debate on the recent global climate trend (Easterling and Wehner 2009).
Fig. 1 The latitude-time cross-section of the 7 year smoothed summer-mean (June–July–August) rainfall anomalies averaged over 110°E–121°E, shown as the percentage of the rainfall departure from
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Recent studies have identified new interdecadal shifts in the global climate. Cummins et al. (2005) found that a new shift occurred around 1998 using their upper ocean climate index; Bratcher and Giese (2002) suggested a weakening global warming trend from about 2002 based on the tropical Pacific decadal variability; Peterson and Schwing (2003) found a rapid and striking transition in the North Pacific’s climate in late 1998; and Swanson and Tsonis (2009) revealed a shifting point in 2001/2002 based on a network of observed climate indices (Tsonis et al. 2007). Besides, Kwon et al. (2007) used the Climate Research Unit’s monthly precipitation data to identify a climate shift in the summertime circulation over East Asia during the mid-1990s. This paper investigates the characteristics of the recent changes in the summer precipitation pattern in East China and explores the related background circulation. The employed datasets are listed in Sect. 2. Section 3 covers the specific features of the precipitation change, the background circulation, sea surface temperature (SST) changes, and results of numerical experiments. Brief discussions and conclusions are presented in Sect. 4.
2 Datasets and model The datasets used in this study include: (1) the 160-station precipitation dataset from the China Meteorological Administration, with most of the 160 stations located in East China; (2) the soil moisture, horizontal and vertical wind, specific humidity, geopotential height and air temperature from the NCEP/NCAR reanalysis; (3) the Hadley Centre sea surface temperature (SST); and (4) the PDO index from the website http://jisao.washington.edu/pdo/. We also carry out several sets of experiments with a global atmospheric general circulation model (AGCM). The model is developed in the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences, with horizontal resolution of 4° latitude by 5° longitude, and nine levels in the vertical direction (IAP 9L-AGCM for short).
the 1951–2008 seasonal mean. Light and deep shadings cover the positive and negative values, respectively. The thick vertical lines approximate the changing point
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The model’s capability in simulating the realistic climate has been examined and verified by previous studies (Bi 1993; Wang and Bi 1996).
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3 Analysis and results 3.1 Features of the summer precipitation pattern changes
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Figure 1 shows the percentage of the 7 year smoothed rainfall anomalies averaged over 110°E–121°E relative to the long-term seasonal mean of 1951–2008. North China has suffered from severe droughts since the late 1990s, while during the late 1950s to late 1970s, the rainfall amount was normal or slightly above average. The precipitation increased after the late 1970s over the YangtzeHuai River (28°N–34°N), while the rainfall in South China experienced obvious changes during the late 1970s and early 1990s (Ding et al. 2008). On closer examination Fig. 1 shows prominent changes over the last decade: the precipitation clearly increases in the HR region (32°N– 36°N, 110°E–121°E), decreases in the YR (28°N–31°N, 110°E–121°E), and increases over South China (SC for short, 22°N–26°N, 110°E–121°E). We thus define three regional rainfall indices as the mean rainfall anomalies of the three areas relative to the mean summer precipitation during 1951–2008. Both the interannual and interdecadal variability are clearly seen in the three indices. Figure 2a shows the three original indices. The amplitudes of the interannual variation in them are comparable. The 7 year smoothed indices reveal the interdecadal variation over the period (Fig. 2b). The HR suffered from persistent drought from the late 1960s to about 1999, when the rainfall increased abruptly thereafter. The YR received sufficient rainfall since the 1970s (especially during the 1990s), while the amount of rainfall decreased sharply after the late 1990s’ peak. In addition, the rainfall over SC increased quickly at the beginning of the 1990s and continuously increased. This obvious rainfall increase over SC can be partly explained by the increased typhoon precipitation (Kwon et al. 2007). Thus, we first calculate the difference in the mean summer precipitation between 2000–2008 and 1977–1999 (Fig. 3a). Significant positive anomalies appear over the HR (the top rectangle), slightly positive over the SC (the bottom rectangle) and negative over the YR (the middle rectangle). The difference pattern in the surface soil moisture (Fig. 3b) is in good agreement with that of the rainfall. The pattern partly resembles the first mode of the empirical orthogonal function analysis on the summermean precipitation anomalies in Zhou and Yu (2005) and the second one in Ding et al. (2008).
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Fig. 2 The normalized a original and b 7 year smoothed indices derived from the station averaged precipitation over South China (SC, 22°N–26°N, 110°E–121°E), the Yangtze River region (YR, 28°N– 31°N, 110°E–121°E), the Huang-Huai River region (HR, 32°N–36°N, 110°E–121°E). The thick vertical line approximates the changing point
3.2 Regional circulation Convective activity and water vapor content are two major factors that directly affect the summer rainfall amount in East China. The differences in the vertical velocity and vertically integrated specific humidity from 300 hPa to 1,000 hPa between 2000–2008 and 1979–1999 are shown in Fig. 4a, b, respectively. The ascending anomalies and increased atmospheric humidity increased the rainfall over the HR during 2000–2008, while the descending anomalies and decreased water vapor content depressed the rainfall over the YR. The difference field in the water vapor content in Fig. 4b can be explained by the moisture transport changes in Fig. 4c. During 2000– 2008, the westerlies (see climatological moisture transport, Fig. 4d) that transport moisture away from HR across the eastern boundary became weaker, which increased the water vapor content over HR, while the significant southward anomalies near 100°E caused less moisture transport into the SC and YR, which decreased the air humidity there. The moisture divergence differences between the two periods in the HR, YR and SC are -0.78, 0.29, 0.43 (units: 10-5), respectively, showing increased (decreased) moisture convergence in the HR (YR and SC). This result is in consistent with the study of Sun et al. (personal communication). The vertical wind and moisture flux changes together can provide local explanations for the increased (decreased) rainfall in the HR (YR). However, to determine whether the anomalies are signals that can persist into a decadal change
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or just stochastic noises, examining the background circulation may provide a stronger explanation. 3.3 Background circulation The intensity and position of the Western Pacific Subtropical High (WPSH) have significant influence on the strength of the East Asian summer monsoon and the
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Fig. 4 The difference between 2000–2008 and 1979–1999 in the a 850 hPa omega. The negative values represent upward motion, unit: 10-3 hPa s-1; b vertically integrated water vapor content between surface and 300 hPa, unit: g kg-1; c vertically integrated (1,000–300 hPa) moisture flux; d climatology of vertically integrated (1,000–300 hPa) moisture flux, unit: kg m-1 s-1. Shadings as in Fig. 3
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precipitation in the YR (Hu 1997; Huang and Sun 1994; Chang et al. 2000). We therefore examine the changes in the 500 hPa geopotential height. Figure 5a shows strong positive height anomalies that are centered over the Lake Baikal, which is in agreement with the anomalous southeasterly moisture transport to the HR in Fig. 4c. The warming center over the Lake Baikal may be related to the negative PDO phase through the air–sea interaction. This
Y. Zhu et al.: Recent changes in the summer precipitation pattern in East China
researches have shown the close linkage between the PDO and East Asian climate. Gong and Ho (2002) showed that the SST anomalies in the eastern tropical Pacific and tropical Indian Ocean are responsible for the shift of the summer rainfall over the YR via the changes in the WPSH. Yang and Lau (2004) showed that the downward trend of summertime precipitation over northern China is due to the warming trend of the ENSO-like mode, namely, the positive-phase PDO. Ma (2007) suggested that the decadal variation of the SST in the North Pacific Ocean is one possible cause of the decadal dry/wet trend and the shift of the central part of North China. Based on these studies, we speculate that the negative-phase PDO may be linked to the recent positive (negative) summer rainfall anomalies in the HR (YR). The top panel of Fig. 6 illustrates the difference in global SST between 2000–2008 and 1979–1999. The SST anomalies in the Pacific Ocean corresponds to a negative PDO pattern, with negative SST anomalies in the eastern tropical Pacific and positive in the North and South Pacific. In addition, significant changes occur over most of the North Atlantic with warming above 0.5°C. The PDO indices in January to December during 1900–2008 (Fig. 6, bottom panel) show several interdecadal changes. Both the changes during the 1940s and 1970s are consistent with significant changes in the global-scale climate (Zhang et al. 1997; Yang and Lau 2004). The most recent interdecadal shift from positive to negative phase occurred during the late 1990s and at the beginning of the 21st century, which implies a new shift in the global climate and may also be linked with the summer rainfall pattern changes in East China mentioned in Sect. 3.1. In order to explore the connection between the atmospheric circulation changes, which are related with the
point is discussed in the next section. The WPSH recedes somewhat eastward during 2000–2008 relative to 1979– 1999 (Fig. 5b). The eastward recession of the WPSH weakens the southerly moisture transport into the YR. The subtropical jet stream can give information on the strength of the interaction between climate systems in high and low latitudes. When the jet is stronger, the cooler and warmer air is separated to the north and south of the jet, and the atmospheric interaction between high and low latitudes is relatively weak; for weaker jets, the blocking effect is decreased, and the mixing of the cooler and warmer air increases. Figure 5c also shows the weakened jet stream that is centered over the northern part of East China at 200 hPa level, which indicates a stronger cool– warm air interaction near the jet. How does the jet stream become weaker in the later period? The warming center over the Lake Baikal is related with the decreased air temperature (averaged over 110°E–120°E) gradient between 32°N and 42°N at 200 hPa level (Fig. 5d). According to the thermal wind relation, the decreased meridional temperature gradient is responsible for the weakened jet stream. 3.4 Sea surface temperature (SST) The SST changes in the 1970s can significantly influence the interdecadal shift in the East Asian summer monsoon and have been highlighted by many studies (e.g. Hu 1997; Li et al. 2001; Wu and Wang 2002; Gong and Ho 2002; Ma 2007; Fu et al. 2009). As the first leading mode of the interdecadal variability of the Pacific SST, the PDO strongly contributes to the interdecadal shift in the East Asian summer monsoon. Many
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Fig. 5 a Same as Fig. 3a, but for the 500 hPa geopotential height, unit: gpm. b The Western Pacific Subtropical High (5,860 gpm contour). c 200 hPa jet stream (24 m s-1 contour). d The meridional gradient of the mean air temperature between 110°E– 120°E at 200 hPa level. Solid and dashed lines in (b) (c) and (d) denote the mean state of the period during 2000–2008 and 1979–1999, respectively
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rainfall pattern changes in East China at about 1999, and the PDO shift, we draw the correlation pattern between the summer-mean PDO index and 500 hPa geopotential height during 1979–2008 (Fig. 7). A belt with significant negative values appears from the Lake Baikal to the central North Pacific. The results suggest that the warming center over the Lake Baikal in Fig. 5a is significantly linked with the negative PDO phase during 2000–2008. The PDO can probably impact the summer climate in East China through modulating the atmospheric circulation over the Lake Baikal. 3.5 Model experiments Many researchers have studied the mid-low latitude air–sea interaction by various general circulation models (e.g. Peng and Whitaker 1999). In this part, we try to explore the
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possible responses of the atmosphere to the Pacific SST anomalies with a low resolution AGCM. With the IAP 9L-AGCM model, we carried out two sets of experiments: (1) the control run, the monthly multi-year mean SST during 1979–2008 is used as the boundary forcing; (2) the sensitive run, the boundary conditions are set as the difference in the monthly SST between 2000– 2008 and 1979–1999 in the Pacific (shown as the SST anomalies in the area surrounded by the thick lines in the top panel of Fig. 6) adding to the monthly mean SST in the control run. Each run is integrated for 17 years, and the results in the last 15 years are analyzed. Figure 8 shows the difference field in the 500 hPa geopotential height between 2000–2008 and 1979–1999 in the reanalysis (top panel), and the difference between the sensitive and control run (bottom panel). In the reanalysis, significant positive height anomalies appear over the Lake
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Baikal (which is in close linkage with the westerly jet weakening, based on the thermal wind relation, see Vallis 2006), and western Bering Sea. The IAP 9L-AGCM can approximately capture the two major centers, though the location and magnitude do not fit well with the reanalysis. The discrepancy in the exact location of the anomalous centers may be related with the coarse resolution of the model. We also compared the geopotential height and air temperature in different levels, and the IAP 9L-AGCM can also qualitatively reproduce the major anomaly centers, particularly over the Lake Baikal and western Bering Sea, responding to the SST anomalies in the Pacific Ocean. Take the surface air temperature as an example (Fig. 9), the model can simulate the warming to the south of the Lake Baikal, though the magnitude is also weaker. The warming signals in the North Pacific are in general consistence with the positive SST changes in this basin. Beside the above similarities in the large scale circulation between the model result and the reanalysis, there also exist some agreements in the local circulation in East Asia. The regional precipitation and 850 hPa wind field (which can approximately represent the low level moisture flux pattern) are shown in Fig. 10. In the IAP 9L-AGCM, the rainfall increases in the HR (Fig. 10a), which is related with the weakened westerly moisture flux out of this region across the eastern boundary, and the southerly anomalies into this region across the southern boundary (Fig. 10b). The easterly anomalies in the YR are reproduced, while the strong northerly anomalies in Fig. 4c are not captured by
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the model. This is probably due to the model’s failure in reproducing the eastward recession of the western Pacific subtropical high, which is quite difficult to simulate for the state-of-the-art AGCMs. At present, almost all the AGCMs, with the horizontal resolution of several degrees, have inadequate ability in simulating the regional precipitation anomalies even the observed global SST anomalies are given. That is why we focused on the discussion of the major circulation changes associated with the precipitation anomalies. Our AGCM experiments show that the major features of the associated circulation changes are qualitatively captured in the wholePacific SST anomalies experiment, suggesting that both the tropical and extra-tropical SST anomalies are responsible for the circulation changes.
4 Discussion and conclusion According to the above analyses, we find that new interdecadal changes in the summer precipitation pattern in East China occurs at about 1999, with more rainfall in the HR and less in the YR. The regional and large-scale atmospheric circulation changes associated with this decadal change of summer precipitation in East China are also analyzed. We find that the anomalous ascending (descending) motion and slightly increased (significantly decreased) water vapor content in the HR (YR) can directly account for the precipitation changes.
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Y. Zhu et al.: Recent changes in the summer precipitation pattern in East China
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This preliminary study also reveals that the interdecadal change in the summer precipitation pattern in East China is also related with some prominent features in the large scale circulation, though it is not as strong as the one during the late 1970s (until 2008). First, large warming occurred over the Lake Baikal during 2000–2008 relative to 1979–1999; thus, the meridional temperature gradient to the south decreased, and the westerly jet became weaker (via the thermal wind relation), which weakened the moisture flux out of the HR and finally caused an increased specific humidity in the HR. Second, the WPSH receded somewhat eastward during 2000–2008, the moisture flux into the YR
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was weakened, and the water vapor content in the YR decreased. Accompanying this interdecadal change, significant SST changes occurred in the Pacific Ocean, consistent with the PDO’s shift from positive to negative phase at about 1999. There has been solid observational evidence of the association between the PDO and East China climate at interdecadal timescale as mentioned in Sect. 3.4. Thus, we speculate that the atmospheric circulation changes in the mid-latitude Asia may be connected with the SST changes in the Pacific Ocean. This association is substantiated in this research by the observational evidence, and also
Y. Zhu et al.: Recent changes in the summer precipitation pattern in East China
supported by the AGCM simulations using the IAP 9LAGCM to some extent. So far, AGCM models’ capability in simulating East Asian summer monsoon climate is very limited. But the IAP 9L-AGCM can approximately reproduce the major features of the large scale circulation changes, and partly the regional circulation features, forced by the SST changes in the Pacific Ocean. However, the full story related to this new decadal change remains an open question, similar to the case of the interdecadal change of the East Asian summer monsoon in the late 1970s. In addition, the physical processes linking the PDO and East China climate are closely related with the causes and mechanisms of the PDO, which is still an unclear issue and being investigated. To date, several viewpoints about the PDO’s origin have been proposed. Results in Deser et al. (2004) support the notion that the tropics play a key role in the interdecadal climate variability in the North Pacific. Schneider and Cornuelle (2005) suggest that the PDO arises from the superposition of sea surface temperature fluctuations with different dynamical origins. Alexander (2005) show that the PDO in the Community Climate System Model version 3 is not strongly connected to tropical Pacific SSTs as found in nature. With the same model Zhong et al. (2007) highlight a mid-latitude origin of the multidecadal tropical–extratropical linkage in the Pacific. With a simple linear Rossby wave model, Kwon and Deser (2006) present that the wind stress curl forcing in Kuroshio Extension (one of the centers of PDO SST anomalies) is stronger than the wind stress curl response, and thus the simulated North Pacific decadal variability owes its existence to a two-way ocean–atmosphere coupling. Therefore, before the physical mechanisms behind the PDO are established, we cannot suggest in confidence any physical process controlling the linkage between the PDO and East China summer climate. The reason for the eastward recession of the WPSH also remains unclear. Our model results do not show evidence of the PDO’s significant contribution to the WPSH changes; however, the possibility cannot be ruled out (Hu 1997; Chang et al. 2000; Zhou et al. 2009), since the model’s capability in capturing the shift in the location of the WPSH (about 4° longitudes) may be highly associated with the AGCM resolution. Thus, simulations with higherresolution AGCMs or coupled models will be required to address the issue of the WPSH changes. Besides, the tropical circulations also exert significant impacts on the precipitation over East China. Previous studies showed that the spring Hadley circulation anomaly can result in the changes of the WPSH intensity and location, and thus influence the interdecadal changes of the East China summer rainfall (Zhou and Wang 2006; Zhou
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and Cui 2008). But the related issues are beyond the scope of this study, and need be investigated further. Additionally, the climate systems in the Southern Hemisphere can be also influential to the East Asian climate (Xue et al. 2004; Wang and Fan 2005; Fan and Wang 2004; Fan 2007). Boreal spring Antarctic oscillation (AAO) can affect the WPSH. As elucidated by Sun et al. (2009b), strong spring AAO is concurrent with strong convection over the Maritime Continent via anomalous meridional circulation along the central South Pacific and two meridional teleconnection wave train patterns, with one over the southern Indian Ocean at the lower level and the other along the central South Pacific at the upper level. The anomalous convection then propagates northward along the seasonal cycle, changes the summer WPSH. The boreal spring AAO shows a decreasing trend since the late 1990s (Fig. 11). But the spring AAO in the IAP 9L-AGCM does not show obvious difference between the control and sensitive experiment, suggesting that the AAO change may not be a result of the Pacific SST changes. Overall, the interdecadal change of the summer rainfall pattern in East China happened under the above significant changes in the background circulation and global SST changes. The negative-phase PDO probably provides favorable air–sea interaction and background circulation for the East China climate change, though the physical mechanisms remain unresolved. In this direction, more comprehensive AGCMs or coupled climate models should be employed for in-depth studies. The new interdecadal shift may be employed in the seasonal forecast to improve the predictive capabilities of the summer precipitation in East China due to the multiyear persistence of the interdecadal signals (Fan et al. 2007). However, there are still several questions that remain: how long will the precipitation over the HR (YR) stay above (below) normal, and will the positive precipitation anomalies keep moving northward towards North China? These questions are presumably related to the issues of global warming impacts and the natural fluctuation of the climate system at decadal timescales. 2 1 0 −1 −2
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Fig. 11 The 7 year smoothed spring Antarctic oscillation index during 1948–2008, defined as the time series of the leading principle component of the 700 hPa geopotential height anomalies. The thick vertical line approximates the changing point
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Acknowledgments We wish to acknowledge Prof. Shuanglin Li, Dr. Jianqi Sun, and Dr. Jianjian Fu for helpful discussions with them. Comments and helpful suggestions from the editor and two anonymous reviewers helped us to improve the presentation of our results. This research was jointly supported by the National Key Project for Basic Research under Grant No. 2009CB421406, National Natural Science Foundation of China under Grant 40875048, and the Chinese Academy of Sciences under Grants No. KZCX2-YW-Q1-02 and KZCX2-YW-Q11-00.
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