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Water Vapor Transport around the Tibetan Plateau and Its Effect on Summer Rainfall over the Yangtze River Valley



LI Chiqin1,2 (

), ZUO Qunjie1∗ (





), XU Xiangde3 (



), and GAO Shouting2,3 (

)

1 Key Laboratory of Cloud-Precipitation Physics and Severe Storms, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029 2 University of Chinese Academy of Sciences, Beijing 100049 3 State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081 (Received January 25, 2016; in final form May 13, 2016)

ABSTRACT The water vapor transport around the Tibetan Plateau (TP) and its effect on the rainfall in the Yangtze River valley (YRV) in summer are investigated by decomposing the moisture transport into rotational and divergent components. Based on the ERA-Interim and PREC/L (Precipitation Reconstruction over Land) data from 1985 to 2014, the vertically integrated features of the two components are examined. The results show that the divergent part dominates the western TP while the rotational part dominates the rest of the TP, implying that moisture may be mostly locally gathered in the western TP but could be advected to/from elsewhere over the rest of the TP. The divergent and rotational moisture fluxes exhibit great temporal variability along the southern periphery of the TP, showing sensitivity of water vapor to the steep topography there. Correlation analysis reveals that it is over the southeastern corner of the TP and to its south that a significant correlation between rotational zonal moisture transport and summer rainfall in the YRV appears, suggesting that the southeastern corner of the TP may serve as a moisture transport bridge between the South Asian (Indian) monsoon and the East Asian monsoon. Further composite analysis indicates that anomalous eastward (westward) zonal water vapor transport from the South Asian monsoon via the southeastern corner of the TP favors more (less) precipitation in the YRV in summer. Key words: moisture transport, Tibetan Plateau, rotational component, divergent component, Yangtze River valley, rainfall Citation: Li Chiqin, Zuo Qunjie, Xu Xiangde, et al., 2016: Water vapor transport around the Tibetan Plateau and its effect on summer rainfall over the Yangtze River valley. J. Meteor. Res., 30(4), 472–482, doi: 10.1007/s13351-016-5123-1.

1. Introduction The Tibetan Plateau (TP), often referred to as the world’s third pole, is of great importance to the global climate. Covering about a quarter of the Chinese land area and elevated up into the mid troposphere, the TP exerts huge impact on atmospheric circulation, precipitation, and ecological conditions. In China, summer precipitation over the Yangtze River valley (YRV) attracts considerable attention and bears enormous importance to people residing in and around this area. Given the large population and economic

activity of the region, people in the YRV suffer considerably from droughts and floods in summer (Ding and Hu, 2003; Arndt et al., 2014). The climate system is highly sensitive to hydrological processes (Webster, 1994). Water vapor transport and its anomalies are closely related to rainfall conditions. In the East Asian monsoon region, the summer moisture fluxes from the Somali jet, the Bay of Bengal, the South China Sea, and the western Pacific subtropical high bring water vapor from the ocean to East China. Analysis of these transport features has been quite extensive (e.g., Tao and Chen, 1987; Nino-

Supported by the National (Key) Basic Research and Development (973) Program of China (2012CB417201), China Meteorological Administration Special Public Welfare Research Fund (GYHY201406001), and National Natural Science Foundation of China (41130960 and 91437215). ∗ Corresponding author: [email protected]. ©The Chinese Meteorological Society and Springer-Verlag Berlin Heidelberg 2016

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miya and Kobayashi, 1999; Ding and Sun, 2001; Ding and Hu, 2003). Meanwhile, it is also very important to take into account the TP, owing to its considerable physical influence in the region. A large amount of water is preserved in the TP, which serves as a “global water tower” with respect to attracting and deflecting water vapor on the global scale (Xu et al., 2008a). In terms of general circulation, the East Asian monsoon, which predominately takes charge of the moisture transport to the YRV, is affected by the TP’s thermal and mechanical forcing (Wu and Zhang, 1998; Wu et al., 2007, 2012). Interacting with the tropical oceans, the TP plays a role as “transfer platform” of water vapor transport between East Asia and the Indian monsoon area (Xu et al., 2002). Diverted by the plateau, the water vapor transport accounts for prolonged rainy weather in the vicinity of East China in summer (Zhou et al., 2005; Shi and Shi, 2008). Around the TP, the components of the earth system show complex interactions with one another, and affect regional rainfall (Lau and Li, 1984; Liu and Yin, 2001; Sato and Kimura, 2007; Lau, 2016). The main purpose of this study is to investigate the role of moisture flux around the TP in modulating the YRV summer rainfall. The moisture flux is separated in two parts: rotational (non-divergent) and divergent (non-rotational). As indicated by the water vapor budget, the local hydrological balance (between evaporation and precipitation) is only affected by the divergent moisture flux. On the other hand, it is the rotational moisture transport that supplies water vapor to the rainy region from a distance. Previous studies of water vapor transport have mainly treated the situation as a whole (Simmonds et al., 1999; Zhou and Yu, 2005), i.e., without separating the two roles. Therefore, systematically studying the moisture flux around the TP in ways that distinguish these two components is a worthy avenue of research. In this study, reanalysis data are used to explore in detail the features of water vapor transport around the TP and its effect on the precipitation over the YRV. Both the climatological and anomalous states are examined, and the relative importance of the moisture supply over the southeastern corner of the TP is

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analyzed and discussed. 2. Data and methodology 2.1 Data The data used for calculating water vapor transport are from ERA-Interim (Dee et al., 2011; http: //apps.ecmwf.int/datasets/data/interim-full-daily/), and the monthly average vertically integrated water vapor flux is derived from the daily mean flux calculated by the ECMWF. For the mid troposphere, 500-hPa wind and specific humidity fields are collected at 0200, 0800, 1400, and 2000 BT (Beijing Time). The dataset is believed to represent a significant improvement compared with older generation reanalysis data, such as ERA-40 and NCEP–NCAR, especially for a number of parameters over the TP with respect to interannual variability (Wang and Zeng, 2012; Bao and Zhang, 2013; Lin et al., 2014). Moreover, the water vapor in ERA-Interim shows near realistic features to that observed by satellite and radiosonde between 50◦ S and 50◦ N (Kishore et al., 2011). The Precipitation Reconstruction over Land (PREC/L) product, which is produced by the Climate Prediction Center from gauge observations (Chen et al., 2004), is also used on the monthly scale for the summer season (June–August). This dataset is provided by NOAA/OAR/ESRL PSD, Boulder, Colorado, USA (http://www.esrl.noaa.gov/psd/). Both datasets are available at a resolution of 1◦ (latitude) × 1◦ (longitude) for the period 1985–2014. The YRV is represented by the grid area of (27.5◦ –32.5◦ N, 110◦ – 122.5◦ E). 2.2 Methodology A vertically integrated moisture balance equation depicts the atmospheric branch of the hydrological cycle, ∂W + ∇ · Q = E − P, ∂t p p where W = (1/g) pst qdp, Q = (1/g) pst V dp, E is the evaporation, q the specific humidity, g the gravity, P the precipitation, p the pressure, subscripts “t” and “s” mean top and surface, and V the velocity

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field. According to Chen (1985), the water vapor flux can be separated into rotational and divergent components, which can be expressed in forms of streamfunction (ψQ ) and velocity potential function (χQ ): Q = QR + QD = k × ∇ψQ + ∇χQ . The water-balance equation can then be written as ∂W + ∇ · QD = E − P, ∂t or ∂W + ∇2 χQ = E − P. ∂t As indicated by the equation, only the divergent part of water vapor flux modulates the balance of local precipitation and evaporation, and the rotational component accounts for supplying the water vapor that is essential to persistent rainfall. 3. Climatology of water vapor transport arou-

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as the “great moisture river” by He et al. (2007), originates from the Southern Hemisphere, rushes into the Northern Hemisphere via the Somali jet, and then consecutively flows over the Arabian Sea, the southern Indian Peninsula, and the Bay of Bengal. Another moisture band stems from the South China Sea and western tropical Pacific Ocean. The two branches compose the “big triangle”, whose dynamic and thermodynamic signals significantly affect Asian hydrology (Xu et al., 2002). A cyclonic circulation appears south of the Himalaya, corresponding to the Indian low vortex (He et al., 2007). The TP’s influence is clearly reflected by the deflection of southerly flow from the Bay of Bengal. After circling around the southeastern corner of the TP, the “big river” merges with the flux from the western Pacific subtropical high over the South China Sea, turning around almost orthogonally into the East Asian monsoon region, and going as far as 40◦ N or even farther north. The distinction between the South and East Asian monsoon is evident (Huang

nd the TP 3.1 Total vertically integrated flux To identify and verify the features of water vapor transport during seasonal transition, the climatological vertically integrated water vapor transport (hereafter total moisture flux) in April and July is first depicted in Fig. 1. The water vapor flux rotates around both sides of the TP, whose main body lies in the westerlies. The flux magnitude over the plateau itself is relatively weak due to its elevation and the resulting low specific humidity of air. In April (Fig. 1a), the western Pacific subtropical high is dominant over the South China Sea, resulting in comparatively heavy water vapor flux towards South China. The Somali jet has not gained its power, and the water vapor transport over the Arabian Sea and the Bay of Bengal is quite weak in April. The water vapor flux in the east front of the trough south of the Himalaya over Burma (Yin, 1949) is comparatively strong. An anticyclonic pattern controls the Arabian Sea. In July, flows in the mid and low latitudes get changed drastically (Fig. 1b). The transport on the whole intensifies. It is clear that a strong water vapor band, referred to

Fig. 1. Climatological distributions of the vertically integrated moisture flux (vectors; kg m−1 s−1 ) and its magnitude (shaded; kg m−1 s−1 ) in (a) April and (b) July, based on the ERA-Interim data from 1985 to 2014. The solid thick black curves indicate the area with elevation above 1500 m.

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et al., 1998); and since most water vapor is concentrated in the lowest 2–3 km of the atmosphere, lowlevel circulation (figure omitted) dictates the direction and magnitude of column-integrated water vapor transport. 3.2 Rotational and divergent part of the vertically integrated flux Viewed separately, the average rotational component of vertically integrated water vapor flux is generally similar to that of the whole part (Fig. 2). The magnitude of the rotational part is a little less than the total flux, indicating that it composes the majority of the total moisture flux. From the rotational part’s perspective, the TP’s influence on the vorticity of moist air motion is more representative. In July, when the northward flux over the Bay of Bengal encounters the steep orography of the Himalaya, it deflects in two directions (Fig. 2b). The westward branch hikes against the mountain range, resulting in a cyclonic path over the south of the TP (increase in relative vorticity in response to conservation of potential vorticity). The eastward branch flows through relatively lower mountain ranges, allowing it to continuously propagate through the southeastern TP and farther into East Asia. While the total transport over the west body of the plateau carries water eastward, moving downstream towards East China, the rotational flux brings water vapor westward out of the plateau. Rotational transport can only supply water vapor, and it is the divergent part that affects the local source and sink of water vapor. Therefore, the contrast in transport direction between total and rotational flux indicates that the flux over the western plateau only modulates the local precipitation and evaporation budget (where divergent flux plays a dominant role). These aspects can be seen in Fig. 3 more clearly: over the central and western TP, the rotational current flows in the direction opposite to that of the total moisture flux. As can be inferred from the water-balance equation, the divergent part thus flows eastward with stronger intensity than the total and rotational part. The water vapor transport south and east of the TP both supply remote precipitation and adjust their local hydrologi-

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cal conditions. As indicated in the following section, the rotational part not only transports water vapor, but also affects the YRV summer rainfall significantly. The divergent flux (Fig. 4) is generally one order of magnitude less than the rotational part, and strengthens from spring to summer. Indicative of the source and sink of water vapor, the divergent flux broadly reflects the land–sea configuration: the tropical oceans are the major evaporative supply, while the maritime and monsoonal continents act as water vapor sinks. Unlike the total moisture flux pattern, the divergent part’s intensity over the TP is comparable to that of the low latitudes. In July, the southeastern corner of the TP, together with the YRV and subtropical western Pacific, is the location of the convergent center, indicative of a water vapor sink. The seasonal transition is mainly demonstrated by the convergence center’s shifting from the western tropical Pacific to East China and its adjacent ocean, which, as pointed out by Chen et al. (1988), exhibits a 30–50-day oscillation associated with the Meiyu rainy season.

Fig. 2. Climatological distributions of vertically integrated rotational moisture flux (vectors; kg m−1 s−1 ), its magnitude (shaded; kg m−1 s−1 ), and corresponding streamfunction (contours; 106 kg s−1 ) in (a) April and (b) July. Contour interval: 108 kg s−1 .

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Fig. 3. Climatological summer distributions of (a) total moisture transport, its (b) rotational part and (c) divergent

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The climatological summer pattern (June– August; the same below) of the rotational component south of 30◦ N at 500 hPa is to a large extent similar to that of its vertically integrated counterpart (Fig. 5a). The major differences take place over the TP and to its north. This is consistent with the fact that the synoptic systems in the tropics are mainly barotropic, and the baroclinicity develops primarily in the mid and high latitudes. The TP and the area to its north witness eastward water vapor flux comparable to the magnitude of that in the low latitudes. The eastward moisture transport over the YRV consists of streams not only from the “big triangle”, but also from the westerlies (Wu and Zhang, 1998). A vortex (Lin, 2015) predominates over the central TP. Unlike the vertically integrated pattern, the divergent part at 500 hPa over the TP is not well distributed spatially (Fig. 5b). The convergence zone still prevails along the subtropical western Pacific. Along the periphery of the southern Himalaya, the divergent flux reaches its maximum. Intensive conver-

part (vectors; kg m−1 s−1 ), their magnitudes (shaded; kg m−1 s−1 ), and corresponding streamfunction/velocity potential (contours parallel or perpendicular to the vectors; 106 kg s−1 ) in the central and western TP region. Contour interval: 2 × 107 kg s−1 . The black curves delineate the Tibetan Plateau (elevation above 1500 m).

3.3 The rotational and divergent parts of water vapor flux in the mid troposphere While the vertically integrated water vapor flux denotes the total effect on the balance between column precipitation and evaporation, it is also of interest to investigate the major flow and convergence near the surface of the TP and their influence on downstream rainfall. Xu et al. (2003, 2008b) showed that the flux over the western boundary of the YRV, which largely comes from the TP in the mid troposphere, is also significant to downstream rainfall. In this section, 500 hPa is chosen as a representative level for the mid troposphere to investigate the condition and effect of water vapor transport over the TP and its surroundings. The integral interval is unit thickness (1 hPa) with the assumption of uniformity near this layer.

Fig. 4. As in Fig. 2, but for the vertically integrated divergent moisture flux (vectors; kg m−1 s−1 ), its magnitude (shaded; kg m−1 s−1 ), and corresponding velocity potential function (contours; 106 kg s−1 ). Contour interval: 4 × 107 kg s−1 .

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cioeconomics of the region (Lau and Li, 1984; Tao and Chen, 1987). In the following two sections, the linear trend is removed along the annually averaged summer data for both rainfall and moisture flux. By detrending, the interdecadal variation is taken out from the dataset, leaving only the interannual changes. The role and significance of moisture flux over the TP on the YRV summer rainfall on interannual timescale are discussed. 4. Summer water vapor transport around the TP in YRV flood/drought periods

Fig. 5. Climatological summer distributions of (a) rotational moisture flux (vectors; 10−4 kg m−1 s−1 ) and stream function (contours; kg s−1 ), and (b) divergent moisture flux (vectors; 10−4 kg m−1 s−1 ) and velocity potential (contours; kg s−1 ), along with their magnitudes (shaded; 10−4 kg m−1 s−1 ) for the 500-hPa unit thickness column. Contour interval: 400 kg s−1 .

gence covers central and western parts of the TP near the elevated surface, while significant mid-level divergence prevails over northern India and south of the Himalaya. Over the TP, the total moisture flux in the mid troposphere exhibits a similar pattern as the divergent part (figure omitted). The maximum against the southern periphery can possibly be traced back to the distinctive pool of water vapor maximum right over the TP (Xu et al., 2008b). It is also of interest that while the difference between the vertically integrated rotational and divergent parts is one order of magnitude, the two components at 500 hPa are roughly the same. It can be inferred that the inhomogeneity of specific humidity mainly contributes to the divergent component. The Asian monsoon exhibits great variability in terms of precipitation, which is a vital factor for the so-

By using the PREC/L dataset, the YRV flood/drought summers are selected based on the criterion that the detrended summer precipitation anomaly for the total YRV should be larger than one standard deviation in extreme years. In this way, 1985, 1990, 2001, and 2013 are chosen as YRV drought summers, while 1993, 1996, 1997, 1998, and 1999 are selected as YRV flood summers. In flood summers, anomalous water vapor rushes into the YRV from the western boundary (Fig. 6a). An anticyclonic anomaly prevails over the South China Sea with its northern periphery carrying abundant water to the YRV, which is related to the Pacific–Japan teleconnection pattern that develops in the southwesterly monsoon flow (Kosaka and Nakamura, 2006). Among the contributions to this anomaly, there are three dominant branches. The supply from the western tropical Pacific is the most significant one. The other two branches, originating from the former, are all related to the TP. One flows over the Bay of Bengal and turns around just south of the Himalaya. The other goes farther into the Arabian Sea and finally circles anticyclonically towards the south of the TP. All three branches move through ocean areas, travelling all the way along the anomalous transport path and turning around south of the TP to the YRV. The supply south of the TP is proven to be important to the downstream precipitation. When the YRV suffers deficient summer rainfall, the picture exhibits different characteristics (Fig. 6b). The intensity of the transport anomaly is, on the whole, approximately the same as in YRV flood sum-

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90% confidence level in both situations. From the perspective of divergent moisture transport, the TP and YRV are closely related to one another. 5. Significance of the water vapor supply from the southeastern TP for YRV summer rainfall 5.1 Correlation between rotational water vapor flux and YRV summer rainfall

Fig.

6. As in Fig. 2, but for the summer composite

anomaly during (a) YRV flood summers (1993, 1996, 1997,

Figure 8 shows the correlation distribution between the summer rotational component of vertically integrated zonal water vapor flux and total summer rainfall in the YRV. The moisture flux of this vast area in the low-latitude Pacific is negatively correlated to the YRV precipitation. This is corresponding to the cyclonic/anticyclonic anomaly pattern mentioned above related to the Pacific–Japan teleconnection. On the other hand, the zonal water vapor supply by the rotational part southeast of the TP is significantly in

1998, and 1999) and (b) YRV drought summers (1985, 1990, 2001, and 2013). Contour interval: 1.5 × 107 kg s−1 . The vectors exceed the 90% confidence level.

mers. Two abnormal vortexes control the north of the South China Sea and the northeast of the Bay of Bengal, respectively. The Indian monsoon low is therefore much stronger, especially along the southern periphery of the TP. An anticyclonic anomaly is situated over the Yellow Sea, bringing strong abnormal vapor from the ocean east of the YRV, and from there going on to carry it towards North China. Unlike the pattern in flood summers, a broad and much more intense cyclonic anomaly prevails north of the TP in drought summers, which brings a substantial amount of water vapor to Northeast China. Owing to the direct link with precipitation, the divergent component anomalous patterns in extreme summers are more clear-cut (Fig. 7). In flood summers, there is a convergence band extending from the YRV to North India and south of the Himalaya. In dry summers, however, a broad divergence band is situated from the YRV to eastern and central parts of the TP. However, only the abnormal divergent flux south of the YRV and in the southeastern TP exceeds the

Fig. 7. As in Fig. 6, but for the divergent part’s anomaly (vectors; kg m−1 s−1 ) and its velocity potential function (contours; 106 kg s−1 ). Contour interval: (a) 107 kg s−1 and (b) 2 × 106 kg s−1 . The vectors exceed the 90% confidence level. The thick dashed line in (a) indicates the convergence band.

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Fig. 8. Distribution of correlation coefficients between the summer rotational part of vertically integrated zonal moisture flux and summer rainfall in the YRV. The shaded area exceeds the 99% confidence level.

phase with the YRV rainfall, besides the YRV surrounding region. This further confirms that the water vapor transport at the southern periphery of the Himalaya is closely linked with the YRV precipitation. It is safe to say that the southeastern corner of the TP is a sensitive area with respect to the downstream rainfall.

the two components of the Asian monsoon, which display inverse proportions to one another in terms of moisture supply to the YRV (Zhang, 2001). The precipitation departure percentages in the

5.2 Effect of the water vapor flux anomaly over the southeastern corner of the TP In this subsection, the southeastern corner of the TP is represented by the area (20◦ –30◦ N, 90◦ –105◦ E), in which we sum the zonal rotational vertically integrated water vapor. The summers are chosen as the eastward/westward water vapor supply extremes using the same method as when filtering the YRV flood/drought years, mentioned above. The composite anomalies show a similar pattern as the YRV flood/drought summers, respectively. When water vapor supply is intense over the southeastern corner of the TP, more water vapor reaches the YRV from its western boundary (Fig. 9a). In these circumstances, the East Asian monsoon and South Asian monsoon are more closely related through the top of the “big triangle.” When the southeastern TP suffers weak water vapor flux, the YRV’s precipitation is supplied dominantly from its southern boundary, and the East Asian monsoon pushes farther north (Fig. 9b). The southeastern corner of the TP serves as a bridge between

Fig. 9. As in Fig. 2, but for (a) eastward moisture transport abnormal summers (1987, 1998, 2003, 2007, and 2008) and (b) westward moisture transport abnormal summers (1994, 2001, 2006, and 2013) for the southeastern corner of the TP. Contour interval: 108 kg s−1 .

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Fig. 10. As in Fig. 9, but for the composite percentages of anomalous precipitation of the Asian monsoon region. Note the different color scale for clarity. The solid thick black curves indicate the area with elevation above 1500 m.

eastward/westward water vapor anomalous summers are shown in Fig. 10. The pattern corresponds well with the moisture transport anomaly in the YRV. The YRV and central China gain above-normal rainfall when stronger eastward water vapor supply dominates the southeastern TP. However, drought takes place in the region when the elevated water vapor transport decreases. South China and the southern North China display opposite change trends with regard to the YRV, partly due to the different pace and progress of the East Asian monsoon in the two circumstances. A tripole also emerges in the South Asian monsoon region. The Himalayan foothills and southern India both co-vary with the YRV, while central India exhibits an opposite anomaly trend. The variation of moisture transport over the southeastern TP is believed to also be responsible for the rainfall oscillation in India, possibly through the cyclonic circulation south of the TP. These phenomena and proposed mechanism are described in Day et al. (2015). Due to its aridity, Northwest China shows large variability with respect to the anomalous percentage. The anomaly of divergent water vapor flux in the weak southeastern TP water supply summers is characterized by a strong convergence center over the South China Sea, and a divergence center over the main bodies of the TP, the East China Sea, and Japan (figure omitted). 6. Conclusions and discussion In this study, based on the ERA-Interim data for

the period 1985–2014, the characteristics of the rotational and divergent components of water vapor flux around the TP are investigated, in the climatology and in composite analysis. Their impacts on the YRV rainfall are also discussed. The following conclusions are drawn. In the climatology, the rotational part of vertically integrated water vapor flux around the TP contributes to the majority of total moisture flux. Therefore, the pattern of rotational water vapor flux is generally similar to that of the total. However, the opposite situation is true over the western TP, implying that the flux there mainly consists of atmospheric divergent flow just over the ground surface, modulating local rainfall, yet rarely providing water vapor downstream. The movement of the divergent part’s center is indicative of the seasonal transition. The TP also exerts significant influence near its surface. At 500 hPa, the whole plateau acts as a water vapor supplier for the YRV, with a parallel magnitude to that in the low latitudes. The divergent component of moisture flux reaches its maximum values and gradient over the southern TP, along the Himalayan mountains. Composite vertically integrated water vapor flux in YRV flood/drought summers shows that the water vapor supply changes most strikingly in the South China Sea and the south of the TP. The role of the TP as a transfer platform strengthens/weakens in YRV flood/drought summers. From the perspective of the divergent part, the anomaly in the YRV and the east-

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ern TP is closely related to one another. Correlation analysis verifies the importance of the water vapor supply from the southeastern corner of the TP. It acts as a bridge between two subsystems of the Asian monsoon, and exerts influence on both atmospheric circulation and precipitation. The separation of water vapor flux into rotational and divergent parts helps us to understand the different roles that they play. In both climatological and extreme states, the southern TP plays an important role in the hydrological cycle of Asia. The Asian monsoon consists of various components, and the interaction among them is complex. In this study, the TP shows its power both at the mid level and throughout the whole column. The hydrological cycle is closely related to, and imposes influence upon, atmospheric heating, and hence the thermal forcing of the TP. Upward transport supplying the wet pool of moisture over the TP and the onset order of Asian monsoon rainfall in early summer can both be traced back to the elevated heating structure of the TP (Yanai et al., 1992; Wu and Zhang, 1998; Xu et al., 2008a). Thus, it is necessary to investigate the relationship between thermal forcing and the two ingredients of moisture flux. Furthermore, how the “atmospheric water tower” (Xu et al., 2008a) and its amplifying effect (Liu et al., 2003) in association with other climate components will change is of great importance and interest with regards to the YRV rainfall, and much more. Acknowledgments. We would like to thank the two anonymous reviewers for their insightful and detailed comments and suggestions. We also extend our gratitude to the editors for their efforts in improving this paper.

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