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RECENT ADVANCES IN THE STUDIES ON THE INTERACTION BETWEEN ASIAN SUMMER AND WINTER MONSOON CYCLE AND ENSO CYCLE

HUANG Ronghui1, CHEN Wen1 and ZHANG Renhe2

1. LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100080, China

2. Chinese Academy of Meteorological Sciences, Beijing 100081, China

ABSTRACT

Recent advances in the studies on the interaction between Asian monsoon and ENSO cycle are reviewed, especially the studies on the interaction between the East Asian winter and summer monsoons and ENSO cycle made by Chinese scholars are emphasized in this paper. Through the recent studies, Chinese scholars have proposed the East Asian summer monsoon system and the East Asian climate system affecting the interannual variability of this monsoon system and have revealed the responding features and processes of the East Asian winter and summer monsoon circulation anomalies and summer rainfall anomalies to ENSO cycle during its different stages. In recent years, the studies on the dynamical effect of East Asian monsoon on the thermal variability of the West Pacific warm pool and ENSO cycle have been greatly advanced. These studies not only demonstrated further that ENSO cycle originates from the tropical western Pacific but also pointed out that this dynamical effect may be through the impact on the atmospheric circulation and zonal wind anomalies over the tropical western Pacific, which can excite the oceanic Kelvin wave and Rossby waves in the equatorial Pacific. And it is also pointed in this paper that the warming phenomenon in the tropical central and eastern Pacific from the late 1970s of the 20th century may be one of the causes of the interdecadal variation of summer monsoon rainfall in East Asia. Besides, the scientific problems in the interaction between Asian monsoon and ENSO cycle, which should be studied further in the near future, are also pointed out in this paper.

Key words:  Asian summer monsoon, Asian winter monsoon, ENSO cycle  interacation

I.  INTRODUCTION

Asian monsoon and ENSO cycle are two important subsystems of the global climate system. As well known, monsoon is a kind of climatic phenomenon, in which dominant wind system changes with seasons. Many studies showed that Asian monsoon plays an important role in the global and regional climate variabilities (Yasunari, 1990; Ding, 1994; Huang and Fu, 1996b; Chang et al. 2000). Asian monsoon can bring a large amount of water vapor from the Pacific Ocean and the Indian Ocean to the mainland, as a consequence, a large amount of rainfall can be formed in the monsoon regions (Zhu, 1934; Tu and Huang, 1944; Huang et al., 1998; Huang et al., 1998). Because of the close relationship between monsoon and rainfall, monsoon variability influences economy, industry, agriculture and daily life of people in the monsoon regions, especially droughts and floods caused by monsoon may bring heavy economic losses in these regions. Every year of the 1980s and the 1990s, the climatic disasters caused the economic losses of about 200 billion RMB yuan, approximately 3%-6% of GDP of China, especially the extremely severe flood occurring in the summer of 1998 had caused the economic losses of about 260 RMB yuan in the Yangtze River Valley (Huang et al., 1998; Huang and Zhou, 2002).

Asian monsoon is a huge monsoon system including East and South Asian monsoon subsystems. Li and Yanai's (1996) investigation showed that the Asian summer and winter monsoons exhibit the characteristics of cycle in both the wind fields and temperature fields. And Chen et al. (2002) also pointed out that the East Asian summer and winter monsoons are a phenomenon of annual cycle in both the wind fields and rainfalls. In East Asia, there are many characteristic weather systems in different seasons, such as the Meiyu in China, the Changma in Korea and the Baiu in Japan in summer, persisting northwesterly winds and cold surges in winter. Moreover, the interannual and intraseasonal variabilities of East Asian monsoon are vary large, which can cause droughts and floods in the eastern part of China. Therefore, the different time-scale variabilities of East Asian monsoon have been an important scientific issue in China (Tao and Chen, 1987). Recently, studies on the interdecadal, interannual and intraseasonal variations and their causes, especially the physical mechanism of the interannual and intraseasonal variabilities of East Asian monsoon, have been greatly advanced. These recent studies on East Asian monsoon are systematically reviewed by Huang et al. (2003).

As another subsystem of the global climate system, ENSO cycle can be considered as the most important phenomenon in the tropical air-sea interaction. When an ENSO event occurs in the equatorial Pacific, severe climate anomalies will be caused in many regions of the world (Horel and Wallace, 1981; Rasmusson and Carpenter, 1982; Rasmusson and Wallace, 1983). Moreover, investigations have shown that ENSO cycle greatly influences South Asian monsoon (Mooley and Parthasarathy, 1983; Khandekar and Neralla, 1984). They found that a weak Asian summer monsoon tends to occur in an El Ni year. Similarly, ENSO event also has a large impact on the climate anomalies in East Asia (Wang, 1986; Fu and Teng, 1988; Huang and Wu, 1989).

Many studies showed that ENSO phenomenon is not only an event but also a cycle (Bjercknes, 1966; McCreay, 1983; Schopf and Suarez, 1988). Recently, the interaction between the Asian summer and winter monsoon cycle and ENSO cycle has become into an interesting scientific problem. The diagnostic and modeling studies have revealed that the Asian summer monsoon activities have a significant effect on the atmosphere/ocean coupled system in the equatorial Pacific (Yamagata and Matsumoto, 1989; Yasunari, 1990; Yasunari and Seki, 1992). Moreover, Li (1988,1990) pointed out that the strong East Asian winter monsoon activities play an important triggering effect on El Ni event. Because the study on the interaction between Asian winter and summer monsoon cycle and ENSO cycle is very important for the understanding of the physical mechanism of the formation of severe climatic disasters in China, recently, supported by the project “Research on the Formation Mechanism and Prediction Theory of Sever Climatic Disasters in China”, which has been started as the first batch of the National Key Program for Developing Basic Sciences, Chinese scholars made the deep research on the interactions between the East Asian summer and winter monsoon cycle and ENSO cycle. And many valuable advances have been achieved in the aspect. In order to summarize these progresses, the recent studies on the interaction between the Asian summer and winter monsoon cycle and ENSO cycle are systematically reviewed, especially the studies on East Asian monsoon made by Chinese scholars are emphasized in this paper.

 

II.  THE EAST ASIAN MONSOON SYSTEM AND ITS INTERANNUAL VARIABILITY

In order to understand the interaction between East Asian monsoon and ENSO cycle, firstly, it is necessary to realize the components of the East Asian monsoon system and the causes of the interannual variability of this system. However, up to now, only the understanding of the components of the East Asian monsoon system has been a little clear, so the studies on the East Asian summer monsoon system cannot but reviewed in the section.

1.  The East Asian Summer Monsoon System

It may be considered as an important progress in the study on Asian monsoon to realize the components of Asian monsoon circulation system. Krishnamurti (1982) proposed the Indian monsoon circulation system. Later on, Tao and Chen (1985) put forward the East Asian monsoon circulation system and pointed out that the East Asian summer monsoon circulation system includes: the monsoon trough over the South China Sea and the tropical western Pacific, the Indian SW monsoon flow, the cross-equatorial flow along the east to 100ºE, the western Pacific subtropical high and the tropical easterly flow, the disturbances in mid-latitudes, the Meiyu frontal zones, and the cold anticyclone in Australia.

2.  The East Asian Climate System Affecting the Interannual Variability of EASM

The interannual variability of the East Asian summer monsoon (EASM) is large. Recent studies have shown that due to the large interannual variability of the EASM, there is an obvious interannual variability of the summer monsoon rainfall in the Yangtze River valley and the Huaihe River valley, the Yellow River valley and North China, as shown in Figs.1a and 1b, respectively (Huang et al., 1998; Huang and Zhou, 2002). Due to this variability, drought and flood disasters frequently occur in East Asia, especially in the area from the Yangtze River valley to South Japan through South Korea. Moreover, Huang and Yan (1999) suggested that the interannual variability of the (EASM) can be well described by using the EAP index, which was defined with the summer (June-August) 500 hPa height anomalies according to the EAP (East Asia/Pacific) pattern teleconnection proposed by Nitta (1987), and Huang and Li (1987), respectively.

 

 

Fig.1.  Interannual variation of the monsoon rainfall anomaly percentage in the Yangtze River Valley and the Huaihe River Valley (a) and North China (b) for the summers from 1950 to 2000.

The causes of the interannual variability of the EASM are complex, as pointed out by Huang et al. (2003), it is influenced by many factors shown in Fig.2. These factors include the Indian monsoon, the western Pacific subtropical high, and the disturbances in mid-latitudes in the atmosphere, the West Pacific warm pool, and ENSO in the tropical Pacific, the Tibetan Plateau, the polar ice and the Eurasian snow cover and land-surface processes etc. Huang et al. (2003) suggested that these factors can be considered as the components of a system and the system consisting of these components may be called as the East Asian climate system. This East Asian climate system suggested by Huang et al. (2003) may be an extension of the East Asian monsoon circulation system proposed by Tao and Chen (1985). Therefore, the studies on the main components of the East Asian climate system affecting the interannual variability of the EASM will be simply reviewed as follows:

 

Fig.2.  Schematic map for the components of the East Asian climate system affecting the interannual variability of the EASM.

(1)  Dynamical and thermal effects of the Tibetan Plateau on the interannual variation of the Asian summer monsoon

The Tibetan Plateau has an important dynamical effect on the interannual variability of the Asian summer monsoon. Hahn and Manabe (1975) pointed out from the numerical simulations with the GFDL 9-layer GCM that due to the dynamical effect of the Tibetan Plateau, the strong southwesterly flow can extend to East Asia from the Bay of Bengal and the Indo-China Peninsula. And Ye and Gao (1979) first put forward the thermal effect of the Tibetan Plateau on Asian summer monsoon. Later on, Nitta (1983), Luo and Yanai (1984), and Huang (1984, 1985) pointed out that the heating anomaly over the Tibetan Plateau has a large impact on the summer atmospheric circulation anomalies over East Asia and South Asia. Recently, Wu and Zhang (1997), Wu et al. (2002) explained the air-pumping effect of the Tibetan Plateau on the Asian summer monsoon through the sensible heating. And Zhang et al. (2002) pointed out an important effect of the heating oscillation over the Tibetan Plateau on the east–west oscillation of the South Asian high, which has a significant influence on the EASM. Liu et al. (2002) pointed out the heating over the Tibetan Plateau can excite a Rossby wavetrain, thus the heating can influence the circulation anomalies over the Northern Hemisphere through the propagation of this wavetrain.

(2)  The thermal effect of the West Pacific warm pool on the interannual variability of the EASM

The thermal state of the West Pacific warm pool and the convective activities over the warm pool have an important effect on the EASM. The studies made by many scholars (Nitta, 1987; Huang and Li, 1987; Kurihara, 1989) showed that the thermal states of the West Pacific warm pool and the convective activities over the warm pool play an important role in the interannual variability of the EASM. And Nitta (1987), Huang and Li (1987), and Huang and Sun (1992,1994) analyzed systematically the influencing process of the thermal states of the warm pool and the convective activities around the Philippines on the interannual anomalies of East Asian monsoon circulation from observed data and dynamical theories, and they pointed out that there is a teleconnection pattern of the summer circulation anomalies over the Northern Hemisphere, i.e., the so-called East Asia/Pacific teleconnection pattern (or the EAP pattern). This teleconnection pattern can greatly influence the southward or northward shift of the western Pacific subtropical high and the interannual variability of the EASM. Recently, Lu (2001), and Lu and Dong (2001) also showed the significant impact of the convective activities over the tropical western Pacific on the east-west oscillation of the western Pacific subtropical high.

(3)  Impact of the Eurasian snow cover on the interannual variability of the Asian summer monsoon

The Eurasian snow cover and the land-sea thermal contrast influence the interannual variability of Asian monsoon. Hahn and Shukla (1976), and Dickinson (1984) investigated the relationship between the Indian monsoon rainfall and the Eurasian snow cover. Their investigations showed that there is an inverse relationship between these two quantities. Khandeker (1991) has demonstrated this inverse relationship with more new data.

Chen and Yan (1979, 1981), Wei and Luo (1996) showed that the snow cover on the Tibetan Plateau greatly influences the rainfall in the middle and lower reaches of the Yangtze River.

 

III.  INFLUENCE OF ENSO CYCLE ON THE SUMMER MONSOON RAINFALLS IN CHINA

In addition to the above-mentioned influencing factors, ENSO cycle in the tropical Pacific also has an important impact on the Asian summer monsoon. However, it has different impact on the summer monsoon anomalies over East Asia in different stages of ENSO cycle(Huang and Wu, 1989).

1.  Impact of ENSO Cycle on the Summer Monsoon Rainfall in China

Huang and Zhou (2002) analyzed in detail the impact of ENSO cycle on the interannual variability of the EASM during different stages of ENSO cycle. Figures 3a and 3b are the composite distributions of the monsoon rainfall anomaly percentage in China for the summers with the developing and decaying stages of El Ni events occurring in the period of 1951-2000, respectively. Figure 3a shows that positive rainfall anomalies are in the Huaihe River valley, and negative rainfall anomalies are located in the Yellow River Valley, North China and the area to the south of the Yangtze River. This explains that during the summer with the developing stage of ENSO event, hot and drought may occur in North China, but flood may be caused in the Huaihe River Valley and the lower reach of the Yangtze River. However, for the summers with decaying stage of ENSO events, the composite distribution of rainfall anomalies shown in Fig.3b is opposite to that shown in Fig.3a. This shows that during the decaying stage of ENSO event, severe flood may occur in the area to the south of the Yangtze River, especially in the Dongting Lake and the Boyang Lake Valleys, but drought may be caused in the Huaihe River Valley.

Figures 3c and 3d are the composite distributions of the monsoon rainfall anomaly percentage in China for the summers with the developing and decaying stages of La Ni events occurred in the period of 1951-2000, respectively. It may be seen from Fig.3c that the positive rainfall anomalies are in the area to the south of the Yangtze River, especially in the surroundings of the Dongting Lake and Boyang Lake and the upper reach of the Yellow River, but the negative rainfall anomalies are in South China and the Huaihe River Valley. Moreover, during the summer with the decaying of La Ni event, as shown in Fig.3d, the positive rainfall anomalies mainly distribute in the area between the Yangtze River and the Yellow River, especially in the Huaihe River Valley, while the negative rainfall anomalies appear in the area to the south of the Yangtze River and in North China.

The influence of ENSO cycle on the interannual variability of summer monsoon rainfall in East Asia is closely associated with the water vapor transport anomalies during different stages of ENSO cycle. The studies made by Zhang et al. (1996) and Zhang (2001) showed that the southerly wind anomalies can appear in the lower troposphere along the coast of East Asia during the mature phase of El Ni event, as shown in Figs. 7e-7h, and the intensified southerly winds are favorable for the enhancement of the water vapor transport from the Bay of Bengal and the tropical western Pacific to East Asia. Thus, after the mature phase of El Ni event, strong rainfall may be caused in the Yangtze River Valley, South Korea and Japan.

 

(a)

 

(b)

 

Fig.3.  Composite distributions of the summer (June-August) rainfall anomalies (percentage) for the summers with the developing stage. (a) and the summers with the decaying stage (b) of El Ni events, and for the summers with the developing stage (c) and the summers with the decaying stage (d) of La Ni events occurred in the period from 1951 to 2000, respectively. The shaded areas in figures indicate positive rainfall anomalies.

(c)

 

(d)

Fig.3. (to be continued)

From the above-mentioned analyses, it may be shown that drought disasters used to occur in North China in the summers with the developing stage of El Ni or the decaying stage of La Ni , but flood disasters frequently occur in the Yangtze River valley in the summers with the developing of La Ni event or the decaying of El Ni event. During the 20th century, the particularly serious flood disasters in the Yangtze River valley of China occurred in the summers of 1931, 1954 and 1998, respectively. These summers were in the decaying phase of El Ni event or the developing phase of La Ni event, respectively.

2. The Impact of the ENSO Cycle on the Monsoon Rainfall Anomalies in China during 1997-2001

Huang et al. (2002) analyzed the characteristics of the ENSO cycle including the El Ni event and the La Ni event and its impact on the summer rainfall anomalies in China during the period from 1997 to 2001 with the observed data. During the period from 1997 to 2001, the strong El Ni event and the prolonged La Ni event occurred in the tropical Pacific. Especially the El Ni event occurred in May, 1997 was the strongest El Ni event in the 20th century. It had serious impact on the global climate anomalies and brought huge economic losses into many countries and regions. The characteristics and the formation mechanism of this event have been investigated by many scholars.

 

 

Fig.4.  Distributions of the summer (June-August) monsoon rainfall anomaly percentage in 1998.

The studies made by Huang et al. (1998) and Huang et al. (2002) showed that the 1997/98 ENSO cycle had significant impact on the summer monsoon rainfall in East Asia, especially in China. In the summer of 1997 when the 1997/98 El Ni event was in its developing stage, the hod and drought summer appeared in North China. However, in the summer of 1998 when the El Ni event was in its decaying stage, the severe floods occurred in the Yangatze River Valley, especially in the Dongting Lake valley and the Boyang Lake valley, as shown in Fig.4. Moreover, in the summer of 1999 when the La Ni was in its developing stage, the flood disaster also appeared in the middle and lower reaches of the Yangtze River. And in the summer of 2001, influenced by the decaying of the 1999/2001 La Ni event the severe drought was also caused in North China and the southern part of Northeast China.

 

IV.  INFLUENCE OF ENSO CYCLE ON THE EAST ASIAN MONSOON CIRCULATION

1.  Response of the East Asian Winter and Summer Monsoon Cycle to El Ni Event

East Asia is not only a region of the strong summer monsoon, but also a region of the strong winter monsoon. As mentioned in the Introduction, the Asian winter and summer monsoons are a phenomenon of annual cycle (Li and Yanai, 1996; Tomas and Webster, 1997). However, the interannual variability of the cycle is greatly influenced by ENSO cycle.

As described in Section III, the East Asian summer monsoon features the strong southerly winds and strong rainfalls in East Asia. And the Asian winter monsoon features the strong northerly winds over East Asia, and strong cold surges can move southward along the coast of East Asia, the South China Sea to the Indo-China Peninsula (Staff members of Academia Sinica, 1957; Chen et al., 1991; Ding, 1994). Moreover, Chang et al. (1979), and Lau and Chang (1987) pointed out that the Asian winter monsoon can cause the strong convective activities over the maritime continent of Borneo and Indonesia.

Chen and Graf (1998), and Chen et al. (2000) systematically studied the interannual variability of the East Asian winter monsoon (EAWM). And according to the characteristics of the EAWM, Chen (2002) adopted the intensity of meridional wind representing the winter monsoon. Figure 5a is the composite distribution of the meridional wind anomalies at 850 hPa for the preceding winters of the occurrence of El Ni events. From Fig.5a it may be found that there are anomalous northerlies from the coastal area of China to the SCS, thus the EAWM is strong. Fig.5b presents the composite distribution of the wind anomalies at 850 hPa for the summers with the developing phase of El Ni . Obviously, there is an anomalous cyclonic circulation over the western Pacific, which indicates a weak western Pacific subtropical high in the summer with the developing stage of El Ni event. As shown in Fig.5b, anomalous northeasterlies are located over the area from the Yangtze-Huaihe River valley to the south of the Yangtze River. This indicates that the weak southwesterly monsoon flow in the summer with the developing phase of El Ni event. Following the developing stage, generally El Ni event can reach to its mature phase. As shown in Fig.5c, anomalous southerlies prevail in the southeastern coastal region of China and the SCS. This shows the weak EAWM may appear in the winter with the mature phase of El Ni . In the


 

 

Fig..5.  The composite distributions of wind field anomaly at 850 hPa for different phases of El Ni events occurring in the period from 1958 to 1998. (a) for the preceding winters of the occurrence of El Ni events; (b) for the summers with the developing stage of El Ni events; (c) for the winters with the mature phase of El Ni events; (d) for the summers with the decaying stage of El Ni events. There are only the meridional wind anomalies in (a) and (c), and the shaded areas in figures indicate the confidence level over 95%
     

 

Fig. 6.  Same as Fig.5 but for different phases of La Ni events occurring in the period from 1958 to 1998.

 

following summer, El Ni event may be in decay, as shown in Fig.5d, there is an anomalous anticyclonic circulation over the western Pacific, which respresents the strong western Pacific subtropical high. And anomalous southwesterlies distribute over the region from the southern part of China to the Yangtze River valley. This can explain that the strong EASM may appear in the summer with the decaying phase of El Ni event.


From the above-mentioned results, it may be seen that the strong or weak East Asian monsoon may be closely associated with different stages of evolution of ENSO cycle.


2. Responses of the East Asian Winter and Summer Monsoon Cycle to La Ni Event


Figures 6a-6d show the composite distributions of the wind fields at 850 hPa to different phases of the La Ni events occurred in the period from 1958 to 1998, respectively. As shown in Fig.6a, in the preceding winter of the occurrence of La Ni event, the southerly anomalies prevail along the coastal region of China. Thus, the EAWM is weak in the winter. In the summer with the developing phase of La Ni event, as shown in Fig.6b, there is an anomalous anticyclonic circulation over the western Pacific, which can represent the strong western Pacific subtropical high. And the anomalous southwesterlies shown in Fig.6b indicate that the southwest monsoon is strong over the eastern part of China. Thus, the strong EASM may appear over the eastern part of China during the developing stage of La Ni event. This characteristic corresponds well to the anomalous EASM during the decaying phase of El Ni . Generally, La Ni event may reach to its mature phase in the winter, as shown in Fig.6c, there are the anomalous northerlies over the coastal region of China and the SCS. In the following summer, La Ni event may be in decay, as shown in Fig.6d, there is an anomalous cyclonic circulation over the western Pacific. This represents that the western Pacific subtropical high is weaker. Therefore, the strong or weak East Asian monsoon is also in dependence on different stages of La Ni event. However, it is a matter worthy of note that the significance of these anomalies does not exceed the level of 95%. Therefore, the influence of La Ni event on the East Asian winter and summer monsoon cycle may be less significant as El Ni event.


As metioned above, in different stages of ENSO cycle including El Ni and La Ni events, ENSO cycle has different impact on the monsoon circulation and rainfall anomalies in East Asia. Why does ENSO cycle have an important influence on the monsoon circulation over the tropical western Pacific and East Asia? Ren and Huang (1999) suggested that it may be caused by the distribution of anomalous convections associated with the SSTA in the tropical Pacific. In the mature phase of El Ni event, the convections are enhanced over the equatorial central and eastern Pacific and weakened over the tropical western Pacific, respectively. Thus, a dipole pattern of anomalous convections can appear over the tropical Pacific, then the distribution of anomalous heat sources has also a dipole structure over the tropical Pacific. This anomalous thermal structure is favourable for the formation of a forced anticyclonic circulation over the tropical western Pacific and the South China Sea, and then the strong western subtropical high is located over the tropical western Pacific and the SCS. Thus, the southwest monsoon is enhanced over the Yangtze

River and Huaihe River Valley. Wang et al. (2001) also suggested that in the following summer after the mature phase of El Ni event, generally, the tropical western Pacific high shifts westward, which will enhance the monsoon circulation over the region of subtropical East Asia. This may explain further that strong rainfall in the Yangtze River Valley used to occur in the period with the decaying of El Ni event.


IV. EFFECT OF THE ASIAN WINTER AND SUMMER MONSOON CYCLE ON ENSO CYCLE


1. The Origin of ENSO Cycle


Since El Ni event can cause the severe climate anomalies in many regions of the world, especially in the Asian-Australian monsoon regions, many meteorologists and oceanologists in the world pay much efforts for the studies on the regularity and physical mechanism of ENSO cycle. Bjercknes (1969) first proposed a hypothesis that El Ni cycle may be a result of air-sea interaction over the equatorial eastern Pacific. However, after the implementation of TOGA experiment, scientists have gradually realized that ENSO cycle may originate from the tropical western Pacific. Philander (1981), McCreay (1983), McCreay and Anderson (1984), Yamagata (1985), Schopf and Suarez (1988), and Chao and Zhang (1988) systematically investigated the physical mechanism of ENSO cycle from the propagation of the equatorial oceanic waves or from the tropical oceanic coupling waves or from the unstable air-sea interaction, respectively. They pointed out that the warm state of the West Pacific warm pool is one of the necessary conditions for the occurrence of El Ni event.


Early in the 1990s, Chinese scholars also realized that ENSO cycle originates from the tropical western Pacific. Huang and Wu (1992) pointed out that the warming state of the West Pacific warm pool may provide a precondition for the occurrence of El Ni event. Recently, Li (2002), Li and Mu (2002) pointed out that the occurrence of El Ni event is due to the eastward propagation of the warm sea water in the subsurface layer of the equatorial Pacific from the West Pacific warm pool, and at the same time, there are the westward propagations of the warm sea water in the subsurface layer of the tropical Pacific along about 10oN and 10oS from the equatorial eastern Pacific, respectively. These propagations may provide a necessary condition of the thermal cycle of the warm pool. Moreover, Chao et al. (2002, 2003) investigated the origin of the warm sea water in the West Pacific warm pool from the observed data of the subsurface sea-temperature. Their results demonstrated further that ENSO cycle originates from the West Pacific warm pool.


2. The Triggering Effect of the Anomalous EAWM on El Ni Event


Many investigations showed that the interaction between Asian monsoon and ENSO cycle is very obvious (Ju and Slingo, 1995). The diagnostic and modelling studies have revealed that the variabilities of Asian monsoon activity have a significant effect on the atmosphere/ocean coupled system in the equatorial Pacific. Yamagata and Matsumoto (1989), Yasunari (1990), and Yasunari and Seki(1992) pointed out that a weaker (stronger) Asian summer monsoon seems to lead an anomalous state of the atmosphere/ocean system in the tropics, which is favorable for El Nino event (or anti-El Nino or La Nino event ) in the equatorial eastern Pacific.


Chinese scholars have made systematical studies on the dynamical effect of East Asian monsoon on ENSO cycle. These studies interpreted further that ENSO cycle originates from the West Pacific warm pool. Li (1988, 1990) studied the triggering effect of the anomalous EAWM on ENSO event in the equatorial Pacific and pointed out that the strong EAWM will intensify the convective activities over the equatorial western Pacific. This, in turn, may strengthen the 30-60 day oscillation in the atmosphere over the tropical western Pacific, and the intensified low-frequency oscillation may trigger an ENSO event. Recently, Li (1995,1996), Li (1998), Li and Li (1998), and Li and Liao(1998) studied further the process of the triggering effect of the anomalous EAWM on ENSO cycle and pointed out that the anomalous EAWM can intensify the westerly winds over the tropical western Pacific, and then can trigger the occurrence of El Ni event. This process has been well demonstrated by Li and Mu (1998) with the numerical simulations using the air-sea coupling model.


3. The Dynamical Effect of the Anomalous Monsoon Circulation on ENSO Cycle


Huang et al. (1992), and Huang and Fu (1996a,b) suggested from the analyses of observed data that the anomalous monsoon wind field over East Asia and the tropical western Pacific may play an important role in ENSO cycle. As shown in Fig.7, the circulation and zonal wind anomalies in the lower troposphere over the tropical western Pacific have a significant dynamical effect on the formation of ENSO cycle in the equatorial eastern Pacific. From Figs. 7a—7d, it may be seen that before the developing stage of El Ni events, there are cyclonic circulation anomalies in the lower troposphere over the tropical western Pacific, and the anomalies can cause the westerly anomalies over the Indonesia and the tropical western Pacific. However, when El Ni events develop to their mature phase, as shown in Figs.7e—7h there are anticyclonic circulation anomalies in the lower troposphere over the tropical western Pacific, and the anomalies can cause the easterly anomalies over the maritime continent and the tropical western Pacific. Moreover, Huang et al. (1998), and Huang et al. (2001) discussed theoretically the dynamical effect of the westerly anomalies over the tropical Pacific on the formation of El Ni event with a simple tropical air-sea coupling model and the observed anomalous wind stress near the Pacific surface. The theoretical result shows that this dynamical effect results from the exciting effect of the westerly wind anomalies on the equatorial oceanic Kelvin wave and Rossby waves in the tropical Pacific. As shown in Figs.8a—8d, during the developing stage of El Ni event, the westerly wind anomalies near the sea surface of the tropical western Pacific can excit the eastward-propagating warm Kelvin wave and the westward-propagating cold Rossby waves in the equatorial Pacific. However, since the easterly wind anomalies appear over the tropical western Pacific after El Ni event developed to its mature phase, the eastward-propagating cold Kelvin wave and the westward-propagating warm Rossby waves can be excited by the easterly wind stress anomalies near the sea surface of the tropical western Pacific, as shown in Figs.8e—8g, respectively.

 



Fig.7. Distributions of the circulation anomaly fields at 850 hPa over the tropical western Pacific before the developing stages (a-d) and in the mature phase (e-h) of the El Ni events occurring in the period of 1980-1998, respectively. (a) in the spring of 1980, (b) in the winter of 1985, (c) in the spring of 1991, (d) in the winter of 1996, (e) in the winter of 1982, (f) in the autumn of 1987, (g) in the spring of 1992, and (h) in the autumn of 1997.


Fig.8. The temporal and spatial distributions of the equatorial oceanic Kelvin wave and Rossby waves responding to the zonal wind stress anomalies near the sea surface of the equatorial Pacific (upper part) and the observed SST anomalies in the equatorial Pacific (lower part, solid line) and the observed zonal wind anomalies near the sea surface (lower part, dashed line) in March (a), May (b), July (c), and October (d), 1997, and in March (e), May (f), and August (g), 1998, respectively K, R2 and R4 in the upper part of Figs.9a-9g indecate the Kelvin wave, the two-order and four-order Rossby waves, respectively, and solid and dashed lines show the warm waves and the cold waves, respectively.

 

From the above-mentioned analyses, the zonal wind anomalies near the sea surface of the tropical western Pacific play significant dynamical effect on ENSO cycle. Therefore, many scholars pay attention to the investigation of the origin of the zonal wind anomalies over the sea surface of the tropical western Pacific, Philander (1981), Yamagata (1985), and Schopf and Suarez (1988), proposed that the zonal wind anomalies near the sea surface of the tropical Pacific may originate from the air-sea interaction over the tropical Pacific. Moreover, Fu and Huang (1996) showed that the westerly anomalies over the equatorial western Pacific originate not only from the anomalous South Asian monsoon but also from the anomalous East Asian monsoon in addition to


Fig.8. (to be continued)


the air-sea interaction over the tropical Pacific. The southward propagation of the westerly wind anomalies in the lower troposphere over the East Asian monsoon region may lead to the westerly wind anomalies over the tropical Pacific through the EU pattern teleconnection. Besides, Chao and Chao (2001) also pointed out that the westerly wind anomalies over the tropical western Pacific may result from the eastward propagation of the westerly wind anomalies over tropical eastern Indian Ocean.

 


V. INTERDECADAL VARIABILITY OF ENSO CYCLE AND ITS IMPACT ON EAST ASIAN MONSOON


1. Decadal El Ni –like Cycle


The observed facts have shown that the SST in the tropical Pacific has not only an obvious interannual variation, but also an significant decadal and interdecadal variations, Huang et al. (1999), Huang (2000), and Zhou and Huang (2003) systematically analyzed the interdecadal variability of the SST anomalies in the tropical Pacific and its impact on the summer monsoon


Fig.8. (to be continued)

 


rainfall and temperature anomalies in East Asia. The results showed that the SST anomalies in the equatorial central and eastern Pacific appear an obvious interdecadal variability. The equatorial central and eastern Pacific was cooling in the 1970s, but remarkable warming trend appeared from the late 1970s to the 1990s. As shown in Fig.9, an obvious El Ni –like SST anomaly pattern appeared in the tropical central and eastern Pacific from 1977 to 2000. This may explain that the “decadal El Ni –like event” occurred in the period from 1977 to 2000, while the decadal La Ni –like event occurred in the period of 1967-1976. Therefore, there may be the dedacal ENSO–like cycle in the decadal variability of SST anomalies in the tropical Pacific.


This decadal El Ni event occurring from the late 1970s have caused the severe Asian monsoon anomalies during the period from 1977 to 2000. As pointed out by Chen et al. (2000), and Wang (2002), both the EASM and the EAWM appeared a weakening trend from the late 1970s. As shown in Fig.10, there are large differences between the summer precipitation anomalies averaged for 1977-2000 and those averaged for 1967-1976 in North China and the Yangtze River valley. From 1977 to 2000, the summer precipitations obviously decreased in North China, and prolonged droughts occurring in this region, but in the Yangtze River Valley, the summer

 



Fig.8. (to be continued)

 


precipitations obviously increased and serious floods were frequently caused there. These can be also seen from Figs. 1a and 1b, respectively.

 


Fig.9. Difference between the winter SST anomalies averaged for 1977-2000 and those averaged for 1967-1976 in the Pacific. Units: oC. The shaded areas in figure indicate the SST anomalies over 0.5oC.

 



Fig..10. Difference between the summer (June-August) precipitation anomaly percentage averaged for 1977-2000 and those averaged for 1967-1976 in China. The shaded areas in figure indicate positive anomalies.

 


2. Interdecadal Variability of the Interacation between Asian Monsoon and ENSO Cycle


The above-mentioned analyses show that the interdecadal variability of summer monsoon rainfall in East Asia may be close association with the interdecadal variation of SST anomalies in the tropical Pacific. Since the equatorial central and eastern Pacific is in the warming episode on the decadal time-scale from the late 1970s to now, thus, the severe droughts occur in North China, but the floods frequently appear in the Yangtze River Valley. However, it should be pointed out that the interaction between Asian monsoon and ENSO cycle has not only an obvious interannual variation, but also an interdecadal variation. Shukla (1995) investigated the interdecadal variability of the relationship between the SOI and the South Asian summer monsoon rainfall and pointed out that the evolution of ENSO cycles occurred in the 1980s and the early 1990s is different from that in the period before the 1980s. Thus, as shown by Parthasarathy et al. (1992), the relationship between the South Asian summer monsoon and El Ni for the 1980s has had some changes. Wang (2002) also pointed out that the correlation between ENSO cycle and summer rainfall in China is instable in decadal time-scale, the correlations between the summer rainfall in China and SST anomaly in NINO.3 for the periods of 1975-1983 and 1992-1995 are lower than those for the periods of 1965 -1974 and 1983-1990.


VI. CONCLUSIONS AND PROBLEM TO BE STUDIED FURTHER


It is seen from the above-mentioned review that great progresses have been achieved in recent researches on the interaction between Asian monsoon and ENSO cycle. These progresses may be mainly summarized as follows:


(1) The studies have revealed that ENSO cycle seriously influences the strength of Asian summer and winter monsoons, and this is well reflected in both the interannual variations of seasonal-mean distributions and strengths and the interdecadal variability of the summer rainfall and temperature anomalies, and the summer and winter monsoon circulation anomalies caused by the SST anomalies during the warming or cooling episode of the tropical Pacific.


(2) Asian monsoon has an important dynamical effect on the thermal state of the West Pacific warm pool, and then on ENSO cycle. This is also a main reason for the origin of El Ni and La Ni events from the West Pacific warm pool.


(3) Not the interannual relationship between Asian monsoon and ENSO cycle is always constant, but it can change with decadal time-scale.


However, many problems on the interaction between Asian monsoon and ENSO cycle are still not clear. For example, the following problems.


(1) Most meteorologists have realized that the SST anomalies in the tropical Pacific have an important influence on Asian monsoon, and there is a good relationship between the Asian monsoon anomaly and ENSO cycle on the interannual time-scale. However, there appear to have some periods when the relationship between Asian monsoon and ENSO cycle is no so good. For example, in the early 1990s, the weak El Ni events contineoursly occurred in the equatorial central and eastern Pacific, there were no severe droughts in North China. Whether the signal of the SST anomaly in the equatorial central and eastern Pacific is the most important one among the factors affecting the interannual variability of Asian monsoon in those period or not? This problem should be studied further from analysis, diagnostics and numerical simulation.


(2) The interdecadal variation of ENSO cycle in the tropical Pacific can modulate the interannual variation of the interaction between Asian and ENSO cycle. Moreover, an interdecadal Asian monsoon anomaly also can influence an interdecadal ENSO cycle. What process these influences are through? This problem needs to be studied further from analysis of observed data, dynamical theory and numerical simulation in detail.


(3) Although the analyses of observed data show that there is a good relationship between Asian monsoon and ENSO cycle. However, it is difficult to simulate the well-known relationship between Asian monsoon and ENSO cycle with the current air-sea-land coupling climate models, especially there is a large difference between numerical simulation of the summer monsoon rain band and observed fact in the Yangtze River and Huaihe River Valley, Korea and Japan in the developing or decaying stages of El Ni event. Even if the prediction of the occurrence and evolution of an El Ni event is currect, it is also uncertain to make well the seasonal forecasting of summer monsoon rainfall anomalies in East Asia using coupling climate models. Thus, the study on predictability of Asian monsoon and ENSO cycle is still an important scientific issue for Chinese meteorologists in the near future.


The above-metioned problems are also the current important scientific issues in the International CLIVAR Programme. Thus, with the implementation of CLIVAR Programme, it will be possible to understand further the physical mechanism of the interaction between Asian monsoon and ENSO cycle. And we can believe that the seasonal and interannual predictions of ENSO cycle and Asian monsoon anomaly can be well improved in the future.


Finally, because authors's knowledge about the studies on the interaction between Asian monsoon and ENSO cycle is limited, this review can not but mainly emphasize the results studied by Chinese meteorologists and may involve only a part of these results.


Acknowledgements: This paper was supported by the Project 40231005, Major Research Program for Global Change and Regional Response, National Natural Science Foundation of China and the Project KZCX3-SW-218, Program for Knowledge Innovation Project, Chinese Academy of Sciences.


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