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China is located in the East Asian monsoon region, the variations of the amount and distribution of precipitation over China are strongly modulated by the East Asian monsoon. As the most important features of the East Asian monsoon is a significant multi-scale variation and thus the precipitation in China also showed great variations, persistent heavy rainfall over eastern China often results in hazardous climate events such as floods and droughts. According to statistics, since the 1980s, large-scale droughts and other weather disasters have brought severe damage to our industrial and agricultural production and the economic losses due to droughts and floods reach 200 billion RMB every year, accounting for about 3% to 6% of the national economic output. Thus, it is important to understand the variations of rainfall over eastern China and their associated physical processes (Zhai et al., 2005).
A lot of studies have revealed the climate characteristics, weather and circulation features, as well as the formation mechanisms of precipitation over China on different time scales, and carried out monthly, quarterly and annual precipitation forecasts. However, there are still significant inaccuracies on the current forecasts of precipitation. Precipitation is a complex meteorological phenomenon which exhibits oscillations at different time scales, including long-term trends, decadal oscillation, interannual oscillation, seasonal variation and high frequency oscillation. An important reason to these inaccuracies in forecasts is that the depth and completeness of the understanding to the changes and mechanisms of precipitation patterns on different time scales is still not enough.
Previous studies indicate that the decadal scale is a very important time scale, on one hand, it could be treated as a turbulence superimposed on the long-term trend, for example, the decline in global temperatures during the 1940s to the mid-1970s slowed the global warming since 20 century (Wang, 1999). One the other hand, the decadal variability could be treated as an important background for the interannual variability, which has some influence on the interannual variability. Therefore, understanding the characteristics and possible mechanisms of the decadal-scale climate changes, and determining their degree of predictability have become major global scientific issues (Jiang, 2000). Improving the understanding and knowledge of the decadal climate variability, and estimating the climate changes in the coming decades scientifically will provide important implications for water resources, agriculture, energy and the development of national long-term development plan.
1. Features of decadal precipitation variation in China
Since the beginning of 1980s, many researchers have noticed the decadal variations of precipitation in China (Wu et al., 1986; Zhang, 1989). Some studies have shown that there is a clear decadal variability of summer rainfall over eastern China during the second half of the 20th century (Zhao, 1999; Wang, 2001). Power spectrum analyses for summer rainfall showed a significant peak at 26.7 years. A drying trend was found in northern China and a wetting trend was found in central China in the last decades (Hu et al., 2003; Lei et al., 2010).
In eastern China, the precipitation showed a wetting trend over the middle-lower Yangtze River valley and a drying trend over the Yellow River valley during the past 40 years, which indicates a southward shift of the rain belt over eastern China. Interestingly, the summer rainfall over eastern China also showed a positive-negative-positive pattern on the decadal time scale, which means that when there is more rainfall over Yangtze River, there would be less rainfall over both northern China and southeastern China, this pattern could be related to the dipole pattern and the positive-negative-positive pattern of the meridional winds (Ren et al., 2000; Wang and Zhai, 2003; Liu et al., 2005; Zhao and Zhou, 2006; Ding et al., 2007).
The studies on summer rainfall over North China showed that the decadal variation is also evident (Chen, 1999; Huang et al., 1999; Li et al., 2002). From 1883 to 1898 and from 1949 to 1964 there is relatively abundant precipitation over North China, while from 1899 to 1920 and from 1965 to 1997 there is relatively less precipitation than normal over North China. This decadal variation caused severe climatic disasters, especially there was a persistent drought in northern China, resulting in the area of water resource scarcity (Huang et al., 1999; Zhang et al., 1999).
Precipitation variation over western China is quite different form eastern China. In eastern China, There was no linear trend of precipitation was dominant during last decades. On the contrary, the increasing trend of precipitation in west China was very noticeable especially during the last 30 years.
1.2 Possible forcing factors for the decadal precipitation variation in China
Since the 1980s, the atmospheric circulation, the weather and climate systems and the thermal forcing factor which affect the droughts and floods in China all exhibit an obvious 10-year interannual variation trend (Chen et al. 2002). To the decadal time scale forecast, the coupled air-sea interaction decadal mode is essential for the decadal precipitation forecast in East Asia. In contrast, the response of decadal climate change to human activities is not that important (Hawkins et al., 2009).
1.2.1 The decadal variation of East Asian monsoon
The variability of precipitation over eastern China is strongly modulated by the East Asian monsoon (e.g., Zhao and Xu 2002; Hu et al. 2003; Zhang et al. 2004). According to the studies, monsoon has significant variation on decadal time scale (Webster et al., 1998; Huang et al., 2003). The Indian summer monsoon circulation underwent two weakening processes in the last 50 years, one in the mid-1960s and the other in the late 1970s, the rain belt shift in East China was in great agreement with the later weakness of the monsoon. When the East Asian summer monsoon became weaker during 1970s, the location of rainy belt in eastern China also moved from North China to the Yellow River and the Huaihe River (Wang, 2001; Zhu et al., 2001; Guo, et al., 2004; Ding et al., 2007; Zeng et al., 2007). In associate with the variation of monsoon, the locations of subtropical High and Intertropical Convergence Zone (ITCZ) also have related changes, they move to the north if the summer monsoon is strong and active, and displace to the south when the summer monsoon is weaker. In connection with these changes, before 1976, the East Asian summer monsoon was in an active phase, the summer precipitation was stronger in North China, while after 1976, the North China suffered a drier period (Li et al, 2001).
1.2.2 The decadal variation of sea surface temperature
The decadal variations of the sea surface temperature (SST) in the Pacific and Indian Oceans play important roles in the variations of rainfall over eastern China (Ju and Slingo, 1995; Hu, 1997; Weng et al., 1999; Chang et al., 2000). Gong and Ho (2002) also proposed that, since 1980, the variations of SST over the tropical eastern Pacific and the tropical Indian Ocean were primarily responsible for the shift in summer rainfall over eastern China through their effects on the subtropical northwestern Pacific high. Further more, Yang and Lau (2004) have statistically obtained that a high negative relationship between the tropical central and eastern Pacific SSTs and the northern China precipitation had been confirmed, while over central eastern China, the inter-annual variation of precipitation is positively correlated with a north-south dipole mode of SST anomalies over the western North Pacific, the tropical Indian ocean and warm pool.
In the 1990s, the research on decadal variation originally focused on the oceanic state, because the oceanic variability was thought to be a slower process and its decadal features more evident. In the Pacific Ocean, decadal variation of the ENSO has been studied (Wang, 1995; Qian et al., 1998) and the EOF analysis of SST in the North Pacific still shows an decadal variation feature. The remaining part of the main EOF components, which is similar to the ENSO mode, was regarded as the decadal variation and named the “ENSO-like mode” (Zhang et al., 1997) or the Pacific Decadal Oscillation (PDO) (Mantua et al., 1997). PDO is one of the strongest and most important signals of global decadal variability, the relationship between PDO and the decadal climate variation in China is get great interest. Zhu et al. (2003) and Zhang et al. (2007) pointed out that when the PDO is in the positive phase, the East Asian summer monsoon is weaker, and results in the drought in North China and flood in Huai River Basin.
1.2.3 The decadal variation of snow depth
In contrast to the decreasing trend of the Eurasian winter and spring snow cover, the winter and spring snow cover, snow depth and number of snow days over the Tibetan Plateau (TP) have had an increasing trend during the last 45 years (1956–2000) (Li, 2002), with an abrupt increase occurring in the late 1970s (Liu et al., 2003; Zhang et al., 2004; Peng et al., 2005). Correlative relationship between the TP winter and spring snow and the Asian summer monsoon circulation and rainfall have been studied by a number of investigators with observed analyses (NCC, 1998; Qian et al., 2003; Wu and Qian, 2003; Liu et al., 2003; Zhang et al., 2004) and modelling simulations (Zwiers, 1993; Qian et al., 2003; Liu et al., 2004). Modelling results have shown the similar inverse relationship between excessive (deficient) TP winter and spring snow and a decreasing (increasing) intensity of the Asian summer monsoon (the South Asian and East Asian summer monsoon) through the snow monsoon mechanism. At the same time, they have further obtained a positive (negative) correlative relationship between the preceding winter and spring snow over TP and summer rainfalls in the Yangtze River basin (North China). This correlative relationship has been used in the seasonal prediction of the National Climate Center of China as a useful climate signal, and considerable success has been achieved, particularly for the seasonal prediction of the prolonged, excessively heavy rainfall and unprecedented flooding event in 1998 over the Yangtze River basin which was preceded by extremely excessive winter and spring snow over the TP.
1.2.4 Other forcing factors
Besides SSTs and snow depth, Yu et al. (2004) attributed the pattern of rainfall changes over eastern China to the summer cooling at the upper troposphere over extra-tropical East Asia, which was assumedly associated with stratosphere-troposphere interactions. Changes in both land temperature and SST modify land-ocean temperature gradients. When the temperature gradients became smaller, the southwesterly monsoon flow weakened and the moisture transported to southern China decreased, causing the local drying trend (Cheng et al. 2005). More recently, Ding et al. (2007) showed that the significant weakening of the tropical upper-level easterly jet, which could also be a result of the reduced temperature gradients, provided a dominant mechanism for the weakening of the Asian summer monsoon over the past 40 years. In addition, climate consequences of black and sulphate carbon aerosols are used to explain climate change in the Asian monsoon region in recent decades, including the tendency toward increased summer floods in South China and increased drought in North China (Ramanathan et al., 2001; Xu, 2001; Menon et al., 2002; Lau and Kim, 2006).
Conclusions as a result, It has been found that the decadal variability of precipitation in China for the past 50 years is very significant, with two major characteristic features identified: (1) occurrence of prolonged droughts in North China and, at the same time, marked flooding conditions in the Yangtze River basin and South China in the period from the end of the 1970s to the beginning of the 21stcentury, and (2) the rainfall regime has undergone an obvious abrupt shift or jump in the mid- and late 1970s. This precipitation regime shift is in good coincidence with a significant abrupt climate change or jump which has been extensively observed in other regions over the world as well as for other variables.
Several studies have indicated that the Asian summer monsoon has become weaker after the end of the 1970s. In connection with this change, the summer rainfall decreased over the lower reaches of the Yellow River and the Huaihe River. However, it is not clear yet how the weakening of the Asian (especially the East Asian) summer monsoon affects the significant southward shift of precipitation patterns in East China. Furthermore, the reason why the inter-decadal weakening of the Asian summer monsoon occurred remains an open question.
Studies also show that the inter-annual and decadal variations of the sea surface temperature in the Pacific and Indian Oceans have an impact on the variation of precipitation in China. When the SST in the North Pacific Ocean becomes cooler, the East Asian summer monsoon would be weaker, and results in the drought in North China and flood in Huai River Basin.
Although the influence of SSTs on the Asian summer monsoon and the precipitation in China has been recognized, not all the variances of precipitation can be explained by SSTs. Some researchers focused on the impact of Eurasian spring snow cover as well as the TP winter and spring snow. They found that the Asian summer monsoon circulation has a negative correlative relationship with the preceding winter and spring snow over TP, which means, when there is an increasing trend of the snow cover over the Tibetan Plateau, the summer rainfalls in the Yangtze River basin would increase too.
Although the features and possible forcing factors of the decadal variation of precipitation in China have been studied, there are still some key issues remained to be solved. For example, is there a coherent change in large-scale circulation features in the Asian region to correspond to the above inter-decadal variability of the summer precipitation in East China, especially the weakening of the Asian summer monsoon and its subsequent effect on significant shift of summer precipitation patterns in East Asia? Is the regime shift of summer precipitation in East China mainly a response to weakening in large-scale monsoon circulation systems in the Asian region? Further studies will be needed to solve these issues.