The East Asian Subtropical Summer Monsoon: Recent Progress

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1 NO.2 HE Jinhai and LIU Boqi 135 The East Asian Subtropical Summer Monsoon: Recent Progress HE Jinhai 1,3 (Û7 ) and LIU Boqi 2,4 (4ËÛ) 1 Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science & Technology, Nanjing State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing Laboratory of Research for Middle-High Latitude Circulation and East Asian Monsoon, Changchun State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing (Received November 9, 2015; in final form February 22, 2016) ABSTRACT The East Asian subtropical summer monsoon (EASSM) is one component of the East Asian summer monsoon system, and its evolution determines the weather and climate over East China. In the present paper, we firstly demonstrate the formation and advancement of the EASSM rainbelt and its associated circulation and precipitation patterns through reviewing recent studies and our own analysis based on JRA- 55 (Japanese 55-yr Reanalysis) data and CMAP (CPC Merged Analysis of Precipitation), GPCP (Global Precipitation Climatology Project), and TRMM (Tropical Rainfall Measuring Mission) precipitation data. The results show that the rainy season of the EASSM starts over the region to the south of the Yangtze River in early April, with the establishment of strong southerly wind in situ. The EASSM rainfall, which is composed of dominant convective and minor stratiform precipitation, is always accompanied by a frontal system and separated from the tropical summer monsoon system. It moves northward following the onset of the South China Sea summer monsoon. Moreover, the role of the land sea thermal contrast in the formation and maintenance of the EASSM is illustrated, including in particular the effect of the seasonal transition of the zonal land sea thermal contrast and the influences from the Tibetan Plateau and midlatitudes. In addition, we reveal a possible reason for the subtropical climate difference between East Asia and East America. Finally, the multi-scale variability of the EASSM and its influential factors are summarized to uncover possible reasons for the intraseasonal, interannual, and interdecadal variability of the EASSM and their importance in climate prediction. Key words: East Asian subtropical summer monsoon, rainbelt formation and advancement, precipitation property, zonal land sea thermal contrast seasonal transition, midlatitude influence, multiscale variability Citation: He Jinhai and Liu Boqi, 2016: The East Asian subtropical summer monsoon: Recent progress. J. Meteor. Res., 30(2), , doi: /s z. 1. Introduction Monsoon, as an ancient concept in climatology, is not only associated with the annual cycle of solar radiation, but also one of the most typical seasonal variations of the atmospheric circulation. The basic characteristics of monsoon is the seasonal reversal of the wind direction between winter and summer, which results in wet flow from the cold ocean to the warm land in summer and dry flow from the cold land to the warm ocean in winter. Halley (1686) was the first to treat monsoon as a scientific issue, pointing out that monsoon is the result of the land sea thermal contrast in response to the seasonal changes in solar radiation. He was also the first to consider monsoon as a planetary-scale land sea wind system. Later, Hadley (1735) introduced the effect of earth s rotation into Halley s monsoon model, suggesting that the south- Supported by the National (Key) Basic Research and Development (973) Program of China (2015CB453202), National Natural Science Foundation of China ( , , and ), Basic Research and Operation Fund of the Chinese Academy of Meteorological Sciences (2015Z001), Program for Changjiang Scholars and Innovative Research Team in Universities (PCSIRT), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Corresponding author: The Chinese Meteorological Society and Springer-Verlag Berlin Heidelberg 2016

2 136 JOURNAL OF METEOROLOGICAL RESEARCH VOL.30 westerly monsoon is formed by earth s rotation, which deflects the wind to its right-hand side after its arrival over warm land from the Southern Hemisphere. In fact, the generation of monsoon is also evidently influenced by condensation heating, and the rainfall and circulation possess a mutual cause-effect relationship (Eady, 1950). Thus, there are three factors driving the formation of monsoon: (1) seasonal changes in solar elevation angle; (2) non-uniform underlying conditions; and (3) moisture processes (i.e., feedback between monsoon precipitation and circulation) (He et al., 1989, 2004; Li and Zeng, 2003). The Asian monsoon is the strongest in the global monsoon system, and is composed of the South Asian monsoon (i.e., Indian monsoon) and the East Asian monsoon (Tao and Chen, 1987; Huang et al., 1998). The Asian monsoon can also be divided into tropical monsoon, subtropical monsoon, and temperate monsoon, based on climatic zones. Specifically, the tropical monsoon includes the Indian monsoon, the Bay of Bengal monsoon, and the South China Sea (SCS) western North Pacific (WNP) monsoon (Lau and Ploshay, 2009). The East Asian monsoon contains the tropical monsoon (including the SCS and WNP monsoon), the subtropical monsoon (including the rainbelt over South China, the Meiyu rainbelt over the Yangtze Huaihe River, and the Baiu or Changma rainbelt from Korea to Japan), and the temperate monsoon, according to geographic latitude (Wang et al., 2006; Tang et al., 2010). The East Asian subtropical summer monsoon (EASSM), as an important member of the East Asian summer monsoon, exhibits great effects on the weather and climate in China. Thus, studies on the dynamical mechanisms and multi-scale variability of the formation and development of the EASSM are essential for the improvement of weather forecasting and climate prediction in China. With the aim to provide a scientific basis and clues for further development in our understanding of monsoon dynamics, in this paper we comprehensively review recent research on the EASSM, alongside an analysis of the latest datasets, with particular focus on its characteristic nature, formation mechanisms, and multi-scale variability (i.e., intraseasonal, interannual, and interdecadal timescales), as well as their influences. 2. Data Daily Japanese 55-yr Reanalysis (JRA-55) data, provided by the Japanese Meteorological Agency (JMA; Kobayashi et al., 2015), are employed to describe the characteristics of the EASSM and its formation mechanisms. We also use pentad-scale CPC Merged Analysis of Precipitation (CMAP; Xie and Arkin, 1997) and Global Precipitation Climatology Project (GPCP; Adler et al., 2003) precipitation data, as well as daily Tropical Rainfall Measuring Mission (TRMM 3B42; Huffman et al., 2007) precipitation data, to validate the seasonal evolution of the EASSM rainbelt. The record length of the above datasets spans from 1981 to 2010, except for TRMM 3B42, which is from 1998 to Intrinsic characteristics of the EASSM 3.1 Formation and advancement of the EAS- SM rainbelt Chinese scientists proposed the concept of the EASSM in the mid 1980s and early 1990s (Yu and Mao, 1986; Yu and Yan, 1986; Zhu et al., 1986; Yu and Yang, 1991). They suggested that the East Asian monsoon could be divided into the SCS WNP monsoon (i.e., tropical East Asian monsoon) and the continent Japan subtropical monsoon (i.e., EASSM). Tao and Chen (1987) described the climate features of the East Asian monsoon, pointing out that the EASSM rainbelt was accompanied by the Meiyu front, along with prevailing southerly winds to its south, but northerly winds to its north where midlatitudinal influences were dominant, and also a typical feature of the EASSM is the northward movement of the Meiyu front to the Yangtze Huaihe River. Recent studies have argued that the EASSM begins with the generation of a rainbelt to the south of the Yangtze River (generally referred to as South China) in early April (Chi et al., 2005; He et al., 2007a; Zhao et al., 2007a; Zhu et al., 2011).

3 NO.2 HE Jinhai and LIU Boqi 137 Fig. 1. Climatological ( ) seasonal evolution of East Asian (averaged along E) precipitation (bars; mm day 1 ) and the pressure latitudinal cross-section of pseudo-equivalent potential temperature θ se (contours; K). Blue triangles indicate the position of frontal zones defined by the maximum meridional θ se gradient at 925 hpa. (a) Pentad 9, (b) Pentad 15, (c) Pentad 21, (d) Pentad 27, (e) Pentad 33, (f) Pentad 39, (g) Pentad 45, and (h) Pentad 51. Figure 1 presents the seasonal evolution of the EASSM rainbelt and its related front. The daily mean precipitation is weak over the region to the south of the Yangtze River in mid February, when the EASSM has yet to be established (Fig. 1a). The precipitation over this region enhances rapidly from March to April, along with a front over the southeastern coast of China settling to the south of the precipitation maximum (Figs. 1b and 1c). This indicates the beginnings of the formation of the EASSM rainbelt in situ. This rainbelt has been named the early rainy season rainfall over South China (Bao, 1980), the spring permanent rainfall (Tian and Yasunari, 1998), the early summer rainy season (Ding, 1994), and the spring rainfall over South China or Meiyu over South China and Taiwan (Chen, 2004). The precipitation strengthens from April to May, with the front located to the south of the Yangtze River, where

4 138 JOURNAL OF METEOROLOGICAL RESEARCH VOL.30 the EASSM rainbelt develops and lingers (Figs. 1c and 1d). Afterwards, when the SCS summer monsoon emerges in late May, the precipitation to the south of the Yangtze River further increases, accompanied by northward movement of the front to its south (Fig. 1e), indicating that the EASSM rainbelt starts to shift northward. During June July, when the Meiyu rainfall begins, the EASSM rainbelt and its associated front arrive at the Yangtze Huaihe River, which marks the formation of the typical pattern of the EASSM rainbelt (Figs. 1e and 1f). The EASSM rainbelt is then displaced further northward North and Northeast China, inducing the local rainy season, while the Meiyu season ends and the second flood season involving tropical cyclones and typhoons begins over South China (Fig. 1g). Subsequently, the EASSM rainbelt withdraws southward rapidly and reaches the southeast coast of China in mid September, marking the end of the EASSM (Fig. 1h). In addition, studies have reported noticeable interannual variation in the northward advancement of the EASSM rainbelt. For instance, Lian et al. (2007, 2009) defined an East Asian summer monsoon index based on NCEP reanalysis data and gauge-based observed rainfall data, and subsequently divided the northward advancement of the EASSM into a south path, middle path, and east path, according to their distinct features. Meanwhile, the low-level southerly flow has been found to be well correlated with the northward shift of the EASSM (Liu et al., 2008). Importantly, two separate rainbelts are apparent over East Asia. One is the tropical monsoon rainbelt to the south of 20 N, and the other is the EASSM rainbelt to the north of the Nanling Mountains near 25 N. The seasonal evolutions of these two well-organized rainbelts are isolated from one another (Fig. 2). Climatologically, the East Asian tropical summer monsoon starts with the onset of the Bay of Bengal summer monsoon in early May, and then propagates eastward to the SCS in mid May to result in the onset of the SCS summer monsoon in mid late May (Lau and Yang, 1997; Webster et al., 1998; Wang and LinHo, 2002; Liu et al., 2015a). However, the formation of the EASSM in early April is much earlier. Therefore, the formation of these two rainbelts over East Asia shows no direct association. 3.2 Properties of the EASSM rainbelt The EASSM rainfall is always associated with a large-scale frontal system. This is in contrast to the Fig. 2. Latitude time cross-section (averaged over E) of rainfall (mm day 1 ) in different datasets: (a) CMAP, (b) GPCP, (c) TRMM, and (d) JRA-55.

5 NO.2 HE Jinhai and LIU Boqi 139 tropical summer monsoon precipitation, which is largely composed of convective rainfall (Tao and Chen, 1987; Chi et al., 2005; Ren et al., 2010). The EASSM rainfall constitutes both stratiform and convective precipitation, and their ratio presents a conspicuous seasonal evolution (Fig. 3a). During March April, the EASSM rainbelt is preliminarily built up to the south of the Yangtze River, with the front located over the southeast coast of China (Fig. 1c). In the meantime, large-scale stratiform precipitation is dominant in the EASSM rainfall. Although the EASSM rain still lingers over South China in April, the lower troposphere of the East Asian subtropics becomes more convectively unstable in Pentad 19. Subsequently, this convective instability gradually develops during the early and mature phase of the EASSM rainy season, manifesting in the form of a gradual upward extension of a convectively unstable zone above 700 hpa (Fig. 3b). Thus, convective instability is a feature of the lower troposphere of the subtropics over East China during late March and early April, resulting in convective precipitation dominating the EASSM total rainfall (Fig. 3a). Therefore, the rainbelt located to the south of the Yangtze River in late March and early April is characterized by early summer monsoonal convection, and it can be treated as an indicator of the beginnings of the formation of the EASSM. 3.3 Atmospheric circulation associated with the formation and development of the EASSM The seasonal transition of the precipitation properties of the EASSM is closely associated with the general circulation. In late March, when the rainfall increases suddenly to the south of the Yangtze River, enhanced low-level in situ southerly merges with the southerly to the west of the western Pacific subtropical anticyclone, and northerly, due to the mechanical forcing of the Tibetan Plateau upon the westerly (Fig. 4a). The development of the southerly can be ascribed to the seasonal reversal of the zonal land sea thermal contrast. In the meantime, the strengthening and advancement of the EASSM is separate from that of the Asian tropical summer monsoon. As shown in the evolution of the 850-hPa relative vorticity (Fig. 4b), a positive vorticity belt of the EASSM appears to the south of the Yangtze River in early March, whereas the SCS is controlled by a negative vorticity belt. In mid May, the positive vorticity belt of the EASSM intrudes southward, followed by the onset of the SCS summer monsoon in Pentad 28, suggesting a possible triggering effect of the development of the EASSM on the onset of the SCS summer monsoon. The subtropical positive vorticity belt then becomes isolated from the tropical one over East Asia, presenting a separation of the circulation between the EASSM and the East Asian tropical summer monsoon (Fig. 5b). Similar characteristics can also be observed in the evolution of moisture convergence in the lower troposphere over East Asia (Fig. 5c). As a result, the evolution of the EASSM and the East Asian tropical summer monsoon should be considered as two relatively separate processes (He et al., 2007a, 2008). The intrinsic characteristics of the EASSM can be summarized as follows: (1) The formation of the EASSM is connected with the seasonal development Fig. 3. (a) Time series of the ratio between convective and total precipitation over the region to the south of the Yangtze River (24 29 N, E). (b) Vertical profile evolution of the convective instability ( θ se/ p) overthe region to the south of the Yangtze River (shading indicates the convectively unstable region; 10 2 KPa 1 ).

6 140 JOURNAL OF METEOROLOGICAL RESEARCH VOL.30 Fig. 4. Latitude-time cross-section (averaged over E) of (a) precipitation (shading; mm day 1 )andthe 850-hPa wind field (vectors; m s 1 ), (b) 850-hPa relative vorticity (10 6 s 1 ), and (c) moisture flux divergence (10 5 kg m 2 s 1 ) integrated from the surface to 700 hpa. Fig. 5. Time series of the climate-mean ( ) annual deviation of rainfall (bars; mm day 1 ), 850-hPa meridional wind (blue line; m s 1 ), zonal land sea thermal contrast ( T/ x) in the middle troposphere (red line; averaged between 600 and 400 hpa; Km 1 ), and the zonal sea level pressure gradient ( (SLP)/ x; green line; 10 6 hpa m 1 ) over the region to the south of the Yangtze River (24 29 N, E). of low-level southerly flow to the south of the Yangtze River, and its rainfall is always accompanied by a frontal system, which is absent in the tropical summer monsoon rainfall (Wang et al., 2006). (2) The EASSM rainfall contains both stratiform and convective precipitation, and their ratio has apparent seasonal evolution. Convective precipitation becomes the dominant component after the formation of the EASSM. (3) The evolution of the EASSM is isolated from that of the Asian tropical summer monsoon. The EASSM establishes in mid April, and its subsequent southward intrusion is beneficial for the onset of the SCS summer monsoon. As a result, the strengthening and northward advancement of the EASSM rainbelt and circulation does not result from the seasonal northward shifting of the Asian tropical summer monsoon. 4. Dynamical mechanisms involved in the formation and development of the EASSM As mentioned above, the formation and development of the EASSM is associated with the seasonal enhancement of low-level southerly flow to the south over the Yangtze River. Also, it is one of the important differences between the EASSM and the Asian tropical summer monsoon. According to the thermal wind relationship, the formation and strengthening of the low-level southerly is relevant to the seasonal transition of the zonal land sea thermal contrast over East Asia, which is a major forcing of the EASSM (He et al., 2008; Qi et al., 2008; Zhu et al., 2011).

7 NO.2 HE Jinhai and LIU Boqi Seasonal reversal of the zonal land sea thermal contrast over East Asia On the planetary scale, LinHo et al. (2008) linked the formation of the EASSM with the seasonal transition of global circulation from winter to summer, and argued that the meridional land sea thermal contrast decreases in February to weaken the Asian winter monsoon, which favors the commencement of summer-type circulation over South China. Previous studies have pointed out that the zonal land sea thermal contrast between the East Asian continent and the western Pacific reverses during late March and early April, when the low-level prevailing northerly turns to a southerly in situ (Fig. 5) (Zhao et al., 2007a; Qi et al., 2008). The seasonal reversal of the zonal land sea thermal contrast originates from the seasonal evolution of the thermal conditions over the Tibetan Plateau, the Asian continent, and the western Pacific. Zhu Z. W. et al. (2012) used the thermal wind relationship and thermal adaptation theory to investigate the physical mechanism through which the formation of the EASSM is induced by the seasonal changes in zonal land sea thermal contrast. They proposed that the land surface sensible heating enhances rapidly with the gradually northward shifting of the subsolar point, leading to a zonal asymmetric structure of the surface temperature field over East Asia involving a warm in the west but cold in the east pattern. The land surface sensible heating would weaken the winter cold high over land that moves eastward to the western Pacific. Meanwhile, the southerly to the east of the high prevailing over South China brings the lower latitudinal warm and wet air northward to strengthen the warm in the west but cold in the east pattern. Afterwards, the low-level southerly is able to strength the vertical northerly shear, resulting in ascending motion and rainfall locally. Furthermore, the condensation heating released by the rainfall can further strengthen the low-level southerly. Such positive feedback can ultimately lead to the formation of the EASSM. Recently, Qi et al. (2014) applied a high-resolution regional climate model in quantifying the influences of sensible heating over the Tibetan Plateau, the East Asian plain, and the northern West Pacific, on the formation of the EASSM. Their results suggested that the contribution of sensible heating over the WNP to the formation of the EASSM is small, whereas the sensible heating over the East Asian plain might enhance the low-level southerly over South China. However, their numerical experiment also validated the fact that the sensible heating over the Tibetan Plateau could prevent the establishment of the low-level southerly, thus inhibiting the formation of the EASSM. This finding is markedly distinct from those of other studies (e.g., Pan et al., 2013), which support the positive effect of sensible heating over the Tibetan Plateau on the pre-monsoonal rainfall over South China. Thus, further research is needed to further understand the dynamical mechanisms involved in the formation of the EASSM rainbelt. 4.2 Zonal land sea thermal contrast in the subtropics of East Asia On the global scale, the subtropical summer monsoon and its related precipitation firstly builds up to the south of the Yangtze River, but no monsoon is observed over East America at the same latitude. First of all, the formation of the EASSM is closely associated with the forcing of the Tibetan Plateau (Wan and Wu, 2007; Wu et al., 2007). In boreal winter, when the subtropical westerly meets the Tibetan Plateau, the climbing flow and rounded wind generate a large-scale anticyclone to the north, and a cyclone to the south, of the Tibetan Plateau. This pair of large-scale circulation systems can strengthen the southward intrusion of higher-latitudinal cold air and the northward transport of tropical warm air over East Asia. The cold air and warm air then merge together over South China to benefit the formation of the EASSM rainbelt. Li and Zhang (2012) also identified that the wind surrounding the Tibetan Plateau strengthens in March, giving rise to the spring permanent rainfall over Southeast China. Moreover, a recent study has revealed a distinction between the land sea thermal contrast of East Asia and East America (Chang et al., 2013). A seasonal reversal of the zonal and meridional temperature gradient is evident over East Asia (Figs. 6a, 7a, and 7b); but in contrast, no seasonal transition of the

8 142 JOURNAL OF METEOROLOGICAL RESEARCH VOL.30 Fig. 6. Hovmüeller diagram of the 500-hPa zonal deviation of air temperature (K) over (a) East Asia (averaged along N) and (b) East America (averaged along N). Panels (c) and (d) show the seasonal changes in the meridional wind (m s 1 ) over East Asia ( E) and East America (90 70 W), respectively. The seasonal change is defined as the difference in mean values between April September and October March of the following year. meridional temperature gradient exists over East America (Fig. 6b), and the seasonal reversal of the zonal temperature gradient is very weak (Figs. 7a and 7b). Consequently, the seasonal changes in low-level southerly flow over East America are much weaker than those over East Asia (Figs. 6c and 6d). Furthermore, the different seasonal change in the zonal land sea thermal contrast can be attributed to the difference between the thermal forcing of the Tibetan Plateau and that of the Rocky Mountains. In boreal spring, the sensible heating enhances rapidly over the Tibetan Plateau, strengthening the summer atmospheric heat source over the East Asian continent. The zonal land sea thermal contrast is then large

9 NO.2 HE Jinhai and LIU Boqi 143 Fig. 7. Pressure latitude cross-section of air temperature without its meridional mean (K) over (a, b) East Asia (averaged along E) and (c, d) East America (averaged along W) in (a, c) boreal winter and (b, d) summer. The arrows indicate the thermal gradient direction. enough to develop strong ascending motion and lowlevel southerly flow over East Asia to the east of the heating region. In contrast, the direct heating effect of the Rocky Mountains on the local atmosphere is much weaker over East America, related to a small zonal land sea thermal contrast and weak ascending motion and seasonal change in the low-level southerly flow. Therefore, the difference in the seasonal transition of the zonal land sea thermal contrast is an important factor resulting in the distinct spring subtropical climates of East Asia and East America. 4.3 Influence from the midlatitudes Another prominent characteristic of the EASSM is the influence from the midlatitudes, which is the cold air source that maintains the EASSM front. This

10 144 JOURNAL OF METEOROLOGICAL RESEARCH VOL.30 is also one of the key differences between the EASSM and the tropical summer monsoon. In the formation and development phase of the EASSM, the lowertropospheric cyclone associated with the cold vortex over Northeast China is active over the mid high latitudes of East Asia, suggesting a midlatitudinal influence on the seasonal advancement of the EASSM. The cold vortex over Northeast China occurs all year round, with high frequency from late spring to early summer in the Northern Hemisphere (Zhu et al., 1992), and can be ascribed to the mid high latitudinal Rossby wave activity (Lian et al., 2010). Previous studies have indicated that the strength of the cold vortex over Northeast China is positively correlated with the precipitation over South China during its preflood rainy season from May to June. Also, significant correlation exists between the strength of the cold vortex over Northeast China and the Meiyu rainfall, indicating that the Meiyu rainfall increases/decreases with a stronger/weaker cold vortex over Northeast China from June to July (Miao et al., 2006a, b; He et al., 2007b). Ding and Chan (2005) reported a number of possible mechanisms involved in the influence of the mid high latitudes on the EASSM. The mid high latitudinal circulation might instigate the subtropical convection and rainfall through the effect of uplift. It could enhance the meridional pressure gradient to increase the low-level northeasterly, leading to the shear vorticity and cyclone near the shear lines. It could not only provide the potential energy available to the turbulence and mesoscale systems in the frontal zone, but also strengthen the low-level trough to withdraw the western Pacific subtropical anticyclone southeastward. Furthermore, the midlatitudinal systems could affect the advancement of the EASSM. For instance, the formation of the Meiyu front is closely associated with the continuous southward intrusion of cold air, which is ascribed to the development and maintenance of a blocking high over the mid high latitudes of Eurasia. The most favorable circulation pattern for persistent Meiyu rainfall is characterized by the development of two blocking highs over the Ural Mountains and the Okhotsk Sea, respectively (Ding, 1991; Zhang and Tao, 1998; Wu, 2002). Thus, the midlatitudinal influence can be treated as a major difference between the East Asian monsoon and the South Asian monsoon, but it remains unclear as to how the synoptic systems over the mid-high latitudes modulate the advancement of the EASSM. In conclusion, the formation and development of the EASSM is closely associated with the seasonal reversal of the zonal land sea thermal contrast over East Asia. The particularity of the EASSM can also be attributed to the mechanical and thermal forcing of the Tibetan Plateau. In addition, synoptic systems (e.g., a cold vortex over Northeast China and a blocking high) can also influence the activity of the EASSM. 5. Multi-scale variability of the EASSM and relevant factors of influence The EASSM also exhibits multi-scale variability, and the relevant factors of influence are different on specific timescales. Thus, a summary of the multiscale variability of the EASSM and relevant factors of influence is of great scientific importance, both in a basic sense (for a deeper understanding of the nature of the EASSM) and in an applied sense (for developing seamless forecasting technologies). 5.1 Intraseasonal variability The intraseasonal oscillation (ISO) of the EASSM varies over different regions in an individual season. In the pre-flood rainy season over South China (March to May), the EASSM rainbelt is situated over South China with a significant day ISO, which can be attributed to the intraseasonal variability of surface sensible heating over the Tibetan Plateau with the same period in boreal spring (Pan et al., 2013). When the spring surface sensible heating over the Tibetan Plateau is stronger (weaker), the surrounding low-level air can converge (diverge) under the influence of the sensible heating air pump. Since South China is affected by large-scale converging circulation to the southeast of the Tibetan Plateau, the lowertropospheric convergence would enhance (weaken), corresponding to more (less) local rainfall. After the EASSM rainbelt moves northward to

11 NO.2 HE Jinhai and LIU Boqi 145 the Yangtze Huaihe River, both the bi-weekly and the day ISO are significant for the monsoonal precipitation with similar strength (Yang et al., 2010). Further studies have shown that the bi-weekly ISO is induced by the southward movement of vorticity anomalies in the midlatitudinal westerly jet, whereas the day ISO is associated with the westward extension of the western Pacific subtropical anticyclone in the lower troposphere (Yang et al., 2010). Moreover, Mao et al. (2010) pointed out that a day ISO exists in the Meiyu rainbelt of the EASSM, which is produced by the ISO of the western Pacific subtropical anticyclone via the northward or northwestward propagation of a Rossby-wave-like coupling system between convection and circulation. Recently, He et al. (2015) observed a day ISO related to the MJO over the EASSM region, and subsequently constructed a relevant monitoring index that demonstrates a certain amount of predictability. 5.2 Interannual variability As well as intraseasonal variability, the EASSM also demonstrates prominent interannual variability. The primary underlying conditions influencing the interannual variability of the EASSM include sea surface temperature anomalies (SSTAs; e.g., ENSO events and Indian Ocean SSTAs) and the land surface status (e.g., the snow anomalies over Eurasia and the Tibetan Plateau, and abnormal soil moisture over East Asia). On the interannual timescale, the EASSM is weaker (stronger) following an El Niño (La Niña) event, with a warmer (colder) SSTA in the equatorial tropical central eastern Pacific and an enhanced (weakened) western Pacific subtropical anticyclone. The summer rainfall then tends to increase (decrease) over the Yangtze Huaihe River. ENSO events in the previous winter may modulate the EASSM circulation and rainfall by changing the SST in the tropical Indian Ocean, the western Pacific, and the SCS in the following seasons (Klein et al., 1999; Alexander et al., 2002); also, the Walker circulation and the monsoonal cell will change subsequently (Webster and Yang, 1992; Ju and Slingo, 1995; Wu and Meng, 1998; Kawamura et al., 2003). Furthermore, ENSO events may modulate the monsoonal circulation and its onset process by simulating an anomalous anticyclone over the Philippine Sea (Zhang et al., 1996, 1999; Wang et al., 2000; Wang and Zhang, 2002; Zhang and Sumi, 2002) and changing the formation and evolution of the South Asian high (Liu et al., 2013, 2015b; Guo et al., 2014; He et al., 2014). The land surface conditions over Eurasia can also be affected by ENSO events to indirectly change the activity of the Asian summer monsoon (Meehl, 1994; Yang, 1996; Yang and Lau, 1998). Moreover, the role of ENSO in the EASSM depends on its specific phase. The decaying phase of ENSO is linked with the first mode of the EASSM, presenting out-of-phase rainfall anomalies between the northern SCS Philippine Sea and southern Japan. However, the second mode of the EASSM, with negative correlation of abnormal rainfall between South China and North Northeast China, is related to the developing phase of ENSO (Wang et al., 2008). Specifically, colder SST in the tropical WNP occurs in the developing phase of ENSO, suppressing local convection and leading to the southward movement of the western Pacific subtropical anticyclone. Thus, abnormally high rainfall appears over the middle and lower Yangtze River, while abnormally low precipitation exists over South China due to the effect of the Pacific Japan wave train (Huang and Wu, 1989). Moreover, local SSTAs, snow anomalies over the Tibetan Plateau, and abnormal soil moisture over East Asia, can also affect the interannual variability of the EASSM. The effects of local SSTAs on the EASSM are distinct among regions. For anomalies of the EASSM rainfall, a warm SSTA in the WNP is able to induce a westward extension of the western Pacific subtropical anticyclone, which results in persistently strong rainfall events over the middle and lower Yangtze River (Ren et al., 2013). Meanwhile, Indian Ocean SSTAs, including the Indian Ocean basin mode (IOBM) and the Indian Ocean dipole (IOD), can alter the rainfall and circulation of the Asian summer monsoon. The IOBM SSTAs act as a capacitor that prolongs the El Niño effect to enhance the subtropical anticyclone over the WNP via a Gill-type response and strengthened

12 146 JOURNAL OF METEOROLOGICAL RESEARCH VOL.30 South Asian high, corresponding to increased rainfall over the mid and lower Yangtze River and decreased precipitation over South China (Huang et al., 2011). Meanwhile, the East Asian jet is located to the north of its climate-mean position (Qu and Huang, 2012). Furthermore, in the summer following a positive IOD event, an anomalous low-level easterly occurs over South China, accompanied by an anticyclonic anomaly settling over the Bay of Bengal, and there is abnormally high rainfall over South China but abnormally low rainfall over southern India (Li and Mu, 2001; Qian and Guan, 2007; Yan et al., 2007). Guan and Yamagata (2003) proposed that the IOD SSTAs might influence the EASSM by inducing anomalous diabatic heating over the Bay of Bengal, stimulating a Rossby wave to affect the circulation over the Mediterranean to its west. Thereafter, the circulation anomalies would affect the EASSM via the silk road wave guide. Also, the IOD SSTAs could produce a northeastward-propagating Rossby wave train from the Indian Peninsula and Bay of Bengal. In addition, the interannual variability of sea ice could modulate the EASSM rainfall. Specifically, the sea ice over the Arctic and Greenland in spring could affect the EASSM rainfall via the Eurasian wave train at 500 hpa. More (less) spring sea ice over the Arctic and Greenland would correspond to increased (decreased) rainfall over northeastern and central China, and less (more) precipitation over South China (Wu et al., 2009a, b, 2013). Meanwhile, the sea ice over the Antarctic might affect the Mascarene and Australian high via anomalous Antarctic oscillation in the Southern Hemisphere, and then the cross-equatorial flow would change to influence the convection over the western Pacific and the westward extension of the western Pacific subtropical anticyclone, and ultimately the spatial distribution of the EASSM rainbelt would be modulated (Xue et al., 2003; Xue and He, 2005). Many data analyses and numerical simulations have been conducted on the effects of Tibetan Plateau snow on the EASSM and climate in China (e.g., Chen and Yan, 1981; Guo and Wang, 1986; Barnett et al., 1989; Luo, 1995; Wu et al., 1996; Zhang and Tao, 2001; Ding and Sun, 2003; Li, 2003; Qian et al., 2004; Zhao et al., 2007b; Wu et al., 2012). Most results have suggested that the thermal effect of the Tibetan Plateau would be damped to weaken the EASSM in years with more and deeper snow over the Tibetan Plateau. Meanwhile, there would be more flood events over the Yangtze River. A recent study validated that anomalies of surface sensible heating over the Tibetan Plateau in spring could influence the local summer heating source via land air interaction in situ, leading to interannual variability of the EASSM rainbelt (Wang et al., 2014). However, further investigation on the physical process through which Tibetan Plateau snow influences the EASSM is still required, because of the non-uniform spatial distribution of Tibetan Plateau snow (Wu and Qian, 2003; Qin et al., 2006) and the uncertainty in observations and numerical models. In addition, the East Asian summer monsoon rainfall can also be altered by soil moisture anomalies over East Asia in boreal spring. If the soil is wetter from the middle and lower Yangtze River to northern China, but drier over northeastern China, the land surface evaporation strengthens to cool the surface temperature over eastern China, which in turn decreases the land sea thermal contrast to weaken the EASSM. Thus, the EASSM rainbelt would be located to the south of its climate-mean position, presenting abnormally high precipitation over the Yangtze River and abnormally low precipitation over northern and southern China (Zuo and Zhang, 2007; Zhang and Zuo, 2011). 5.3 Interdecadal variability The EASSM has evidently weakened since the 1970s to aggravate the flood in the south but drought in the north rainfall pattern in summer over eastern China (Zhou et al., 2009b). The detailed features of this change include: (1) more extreme rainfall over the Yangtze River but less precipitation over North China (Hu, 1997); (2) weaker surface southerly flow in summer and a cooling trend of the air temperature in the mid and upper troposphere over East Asia (Yu et al., 2004; Yu and Zhou, 2007); (3) a southward extension of the WNP subtropical anticyclone and an increased

13 NO.2 HE Jinhai and LIU Boqi 147 zonal span of the South Asian high in the upper troposphere (Hu, 1997; Zhang et al., 2000; Gong and Ho, 2002); (4) southward movement of the 200-hPa East Asian subtropical westerly jet that induces the stronger westerly to its south but weaker one to its north (Yu et al., 2004; Schiemann et al., 2009); and (5) weakened land sea thermal contrast between the Asian continent and its surrounding oceans (Ding et al., 2007). However, rainfall has been decreasing evidently to the south of the Yangtze River in late spring since the 1970s, along with a significant cooling of the upper troposphere over central China (Xin et al., 2006). Studies show that the precipitation in the pre-flood season over South China and the rainy season over North China decreased from 1958 to 2000, when more Meiyu rainfall appeared over the Yangtze Huaihe River (Wu et al., 2006). During the late 1980s, summer rainfall increased over southern East China, which is closely associated with the decreased snow cover over the Eurasian continent in boreal spring and the warmer SSTAs in the WNP in boreal summer on the interdecadal timescale (Zhang et al., 2008; Wu et al., 2009c). In the early 1990s, the East Asian summer monsoon started to recover, corresponding to a northward shifting tendency of the monsoonal rainbelt (Liu et al., 2012). Meanwhile, the center of the WNP subtropical anticyclone started to withdraw eastward with a northward shifting of its ridgeline, which transported more moisture to the Huaihe River (Zhou and Yu, 2005). Also, the land sea thermal contrast over East Asia enhanced in this period due to the increased sea level pressure over the western Pacific, ascribed to a phase transition of the Pacific Decadal Oscillation (PDO; Zhu et al., 2012). The latest research suggests that the interdecadal variability of the EASSM over southern China in spring was opposite to that in summer in the 1990s, exhibiting more spring rainfall but less summer precipitation over southern China after 1994, which was attributed to the spring La Niña-like interdecadal cooling in the tropical eastern Pacific SST and the interdecadal summer warming in the IOBM SST (Zhu et al., 2014). Although we still have no consensus on the reason for the interdecadal weakening of the East Asian summer monsoon in the 1970s, mounting evidence suggests that the interdecadal warming in the tropical ocean may play a critical role. Such warming in the tropical ocean could decrease the meridional temperature gradient in the mid upper troposphere of the East Asian monsoon region to weaken the monsoonal circulation. The weakening of the East Asian summer monsoon in the last 50 years can be considered as one aspect of the interdecadal variability of the global monsoon, which is altered by interdecadal changes in the tropical ocean, especially the tropical central eastern Pacific and western Indian Ocean. Such interdecadal change in the tropical ocean is part of the PDO, belonging to natural variability (Zhou et al., 2008, 2009a; Li et al., 2010). Moreover, the more frequent summer floods over central eastern China are associated with the warming SST trend in the western Pacific warm pool (Yang and Lau, 2004). Nevertheless, one cannot neglect the role of air sea feedback in the interdecadal variability of the East Asian summer monsoon (Zhou et al., 2009a; Zhou and Zou, 2010). Aside from the influence of SST, research also stresses the effect of land air interaction on the interdecadal weakening of the East Asian summer monsoon. Specifically, the interdecadal decrease in spring sensible heating over the Tibetan Plateau and the increased snow cover or depth over Eurasia may attenuate the East Asian summer monsoon circulation, leading to a southward withdrawal of the monsoon rainbelt (Zhao and Chen, 2001; Wu and Qian, 2003; Zhao et al., 2007b; Gao et al., 2011; Duan et al., 2013). Since the warming in the higher latitudes of Eurasia is more rapid than that in the subtropics against the background of global warming, the East Asian westerly jet weakens, corresponding to a weaker EASSM (Molnar et al., 2010). 6. Conclusions and discussion The EASSM, as the main component of the Asian summer monsoon, determines the rainfall distribution over eastern China, and its multi-scale variability plays an important role in weather and climate anomalies over China as a whole. In the present paper, we revisit the nature of the EASSM by reviewing the literature

14 148 JOURNAL OF METEOROLOGICAL RESEARCH VOL.30 and analyzing the latest datasets. In particular, the formation and advancement of the EASSM is demonstrated, as well as the precipitation properties of the EASSM and the related circulation. Additionally, we discuss the characteristics of, and the dynamical mechanisms involved in, the seasonal transition of the land sea thermal contrast and midlatitudinal influence, as well as their roles in the formation and maintenance of the EASSM. The characteristics of the EASSM on different timescales and the related factors of influence can be summarized as follows: (1) The EASSM is characterized by the seasonal transition of zonal land sea thermal contrast in the subtropics of East Asia, which generates the EASSM rainbelt. The EASSM rainfall features frontal precipitation, which is absent in the tropical monsoon rainfall. The tropical monsoon rainfall is composed of convective precipitation, while both large-scale stratiform and convective precipitation constitute the EASSM rainfall, with a distinct seasonal variation in the ratio between the two precipitation types. Specifically, the convective precipitation becomes dominant in the EASSM rainfall after the EASSM becomes established. (2) The EASSM rainbelt firstly establishes over the region to the south of the Yangtze River from March to April, accompanied by seasonal enhancement of local low-level southerly flow associated with the seasonal transition of the zonal land sea thermal contrast between the East Asian continent and the western Pacific. In addition, the mechanical forcing of the Tibetan Plateau provides a favorable background for the enhancement of low-level southerly flow over the region to the south of the Yangtze River, which directly results from the seasonal evolution of the thermal conditions over the Tibetan Plateau, East Asian continent, and western Pacific. Since the seasonal transition of land sea thermal contrast gives rise to the monsoon, the establishment of the rainbelt over the region to the south of the Yangtze River in spring can be treated as the beginning of the EASSM, or at least its embryonic stage. (3) The seasonal evolution of the EASSM is separate to the formation and propagation of the tropical summer monsoon, and it is also influenced by midlatitudinal systems. The EASSM rainbelt builds up earlier than the onset of the tropical summer monsoon, while its southward intrusion in mid May favors the onset of the tropical summer monsoon, followed by a strengthening of low-level southerly flow that further advances the EASSM rainbelt northward. The occurrence of a cold vortex over Northeast China is most frequent during late spring and early summer, modulating the seasonal evolution of the EASSM rainbelt. Generally, the strength of such a cold vortex over Northeast China is significantly and positively correlated with the rainfall amount of the EASSM. (4) The spatial difference in the land sea thermal contrast leads to the distinct climates of East Asia and East America. The seasonal variation of rainfall is large in the subtropics of East Asia, exhibiting monsoonal precipitation characterized by a wet in the summer but dry in the winter pattern. Also, the low-level wind direction reverses from winter to summer, with northerly wind in winter but southerly wind in summer. Thus, a subtropical monsoon climate is observed over East Asia. In contrast, a non-monsoonal climate exists over East America, where the precipitation amount changes little all year round and there is a prevailing westerly in the lower troposphere. The differences originate from the distinct land sea thermal contrast over the two regions. A seasonal reversal of both the meridional and zonal temperature gradient is evident over East Asia, but the seasonal change in the zonal temperature gradient is weak over East America and the seasonal reversal of the meridional temperature gradient is absent. Moreover, the zonal cell related to the Tibetan Plateau is substantially different to that induced by the Rocky Mountains. As a result, the primary reason for the distinct climates of East Asia and East America is the different seasonal transition of the land sea thermal contrast and the differing roles of the Tibetan Plateau and Rocky Mountains. (5) Multi-scale variability exists in the EASSM circulation and rainfall, and the factors of influence on specific timescales are different. On the intraseasonal timescale, the non-uniform spatiotemporal distribu-

15 NO.2 HE Jinhai and LIU Boqi 149 tion of the EASSM ISO is ascribed to the intraseasonal variability of surface sensible heating over the Tibetan Plateau in spring, the movement of anomalous vorticity in the midlatitudinal westerly jet, the short-term oscillation of the western Pacific subtropical anticyclone, and the meridional propagation of MJO. On the interannual timescale, the major forcings are ENSO events, Indian Ocean SSTAs (including the IOBM an IOD SSTAs), snow anomalies over Eurasia and the Tibetan Plateau in the previous winter and spring, and anomalous soil moisture over East Asia in spring. Meanwhile, on the interdecadal timescale, the EASSM weakened from the 1970s, but began to recover from the 1990s. The interdecadal variability of the EASSM can be attributed to the anomalous tropical convection over the Indian Ocean and Maritime Continent, and the long-term changes in the thermal conditions of the Tibetan Plateau, which are affected by both the natural variability of climate systems and anthropogenic greenhouse gas and aerosol emissions. (6) The low-level southerly maximum is sometimes accompanied by the rainfall center in the EASSM, but it is separated from each other at other times. In the tropical summer monsoon, meanwhile, the wind maximum is always collocated with the rainfall center. In the formation and development stage of the EASSM, the southerly appears together with the rainfall, but it is isolated in the mature stage of the EASSM. This is because the EASSM rainfall is always closely associated with the large-scale frontal system, and the rainfall takes place in the convergence near the front. Therefore, the wind and rainfall emerge together over the region to the south of the Yangtze River, where the front is located, in the formation and development stage of the EASSM. Subsequently, the southerly is enhanced to shift the front northward, consistent with the northward movement of the rainfall region. The southerly is then located to the south of the front with no precipitation, exhibiting a separation of wind from rainfall. Consequently, a combined occurrence of wind and rainfall exists over the forward position of the EASSM, but they are separated over the EASSM hinterland. Since the tropical summer monsoon rainfall is ascribed to local convergence without a large-scale frontal system, it is always accompanied by the maximum low-level southerly. It is important to note that the EASSM rainfall is always accompanied by a frontal system. Since the position and strength of the front depend upon the multi-scale configuration between the warm-and-wet flow from lower latitudes and the cold-and-dry air from higher latitudes, it is possible to extract their activity and the related impact factors through scale separation technology. Ultimately, high-quality weather and climate prediction may then be achievable via hierarchical forecasting methods. It is clear that many achievements have been made through studies of the EASSM, providing important clues for furthering our understanding of the Asian monsoon systems. However, some key scientific questions remain open. For instance, most previous studies have focused on the formation mechanisms of the EASSM rainfall and circulation in boreal summer, but little attention has been paid to the process of EASSM establishment from winter to summer, especially the seasonal transition from winter monsoon to summer monsoon and the relevant contributing factors over the East Asian subtropics. Also, it should be noted that the EASSM is closely associated with the thermal forcing of the Tibetan Plateau. Thus, improvements in the observation network over the Tibetan Plateau is required to obtain accurate surface heating flux information, which is of great importance to future studies on the interaction between the subsystems of the Asian summer monsoon. Acknowledgments. We thank the two anonymous reviewers for their insightful comments. REFERENCES Adler, R. F., G. J. Huffman, A. Chang, et al., 2003: The version-2 global precipitation climatology project (GPCP) monthly precipitation analysis (1979 present). J. Hydrometeor., 4, Alexander, M. A., I. Bladé, M. Newman, et al., 2002: The atmospheric bridge: The influence of ENSO teleconnections on air sea interaction over the global oceans. J. Climate, 15, Bao Chenlan, 1980: Tropical Meterology. Science Press,

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