• Bao, M., , and R. H. Huang, 2006: Characteristics of the interdecadal variations of heavy rain over China in the last 40 years (in Chinese). Chin. J. Atmos. Sci., 30, 10571067.

    • Search Google Scholar
    • Export Citation
  • Benton, G. S., , R. T. Blackburn, , and V. O. Snead, 1950: The role of the atmosphere in the hydrologic cycle. Eos, Trans. Amer. Geophys. Union, 31, 6173.

    • Search Google Scholar
    • Export Citation
  • Berbery, E. H., , and E. M. Rasmusson, 1999: Mississippi moisture budgets on regional scales. Mon. Wea. Rev., 127, 26542673.

  • Budyko, M. I., 1974: Climate and Life. Internal Geophysics Series, Vol. 18, Academic, 508 pp.

  • Chan, J. C. L., , and W. Zhou, 2005: PDO, ENSO and the early summer monsoon rainfall over south China. Geophys. Res. Lett., 32, L08810, doi:10.1029/2004GL022015.

    • Search Google Scholar
    • Export Citation
  • Chen, H. P., , J. Q. Sun, , X. L. Chen, , and W. Zhou, 2012: CGCM projections of heavy rainfall events in China. Int. J. Climatol., 32, 441450.

    • Search Google Scholar
    • Export Citation
  • Dee, D. P., and Coauthors, 2011: The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553597.

    • Search Google Scholar
    • Export Citation
  • Feng, J., , W. Chen, , C. Y. Tam, , and W. Zhou, 2011: Different impacts of El Niño and El Niño Modoki on China rainfall in the decaying phases. Int. J. Climatol., 31, 20912101.

    • Search Google Scholar
    • Export Citation
  • Fovell, R. G., , and M. Y. C. Fovell, 1993: Climate zones of the conterminous United States defined using cluster analysis. J. Climate, 6, 21032135.

    • Search Google Scholar
    • Export Citation
  • Gu, W., , C. Y. Li, , W. J. Li, , W. Zhou, , and J. C. L. Chan, 2009a: Interdecadal unstationary relationship between NAO and east China’s summer precipitation patterns. Geophys. Res. Lett., 36, L13702, doi:10.1029/2009GL038843.

    • Search Google Scholar
    • Export Citation
  • Gu, W., , C. Y. Li, , X. Wang, , W. Zhou, , and W. J. Li, 2009b: Linkage between mei-yu precipitation and North Atlantic SST on the decadal timescale. Adv. Atmos. Sci., 26, 101108.

    • Search Google Scholar
    • Export Citation
  • Huang, R. H., , Z. Zhang, , and G. Huang, 1998: Characteristics of the water vapor transport in East Asian monsoon region and its difference from that in South Asian monsoon region in summer (in Chinese). Sci. Atmos. Sin., 22, 460469.

    • Search Google Scholar
    • Export Citation
  • Huang, R. H., , Y. Liu, , L. Wang, , and L. Wang, 2012: Analyses of the causes of severe drought occurring in southwest China from the fall of 2009 to the spring of 2010 (in Chinese). Chin. J. Atmos. Sci., 36, 443457.

    • Search Google Scholar
    • Export Citation
  • Lau, N. C., , and M. J. Nath, 2000: Impact of ENSO on the variability of the Asian–Australian monsoons as simulated in GCM experiments. J. Climate, 13, 42874309.

    • Search Google Scholar
    • Export Citation
  • Li, X. Z., , and W. Zhou, 2012: Quasi-4-yr coupling between El Niño–Southern Oscillation and water vapor transport over East Asia–WNP. J. Climate, 25, 58795891.

    • Search Google Scholar
    • Export Citation
  • Li, X. Z., , Z. P. Wen, , and W. Zhou, 2011: Long-term change in summer water vapor transport over south China in recent decades. J. Meteor. Soc. Japan, 89A, 271282.

    • Search Google Scholar
    • Export Citation
  • Li, X. Z., , Z. P. Wen, , W. Zhou, , and D. X. Wang, 2012: Atmospheric water vapor transport associated with two decadal rainfall shifts over east China. J. Meteor. Soc. Japan, 90, 587602.

    • Search Google Scholar
    • Export Citation
  • Lin, A. L., , J. Y. Liang, , C. H. Li, , D. J. Gu, , and B. Zheng, 2007: Monsoon circulation background of ‘0506’ continuous rainstorm in south China. Adv. Water Resour., 18, 424432.

    • Search Google Scholar
    • Export Citation
  • McKee, T. B., , N. J. Doesken, , and J. Kleist, 1993: The relationship of drought frequency and duration to time scales. Preprints, Eighth Conf. on Applied Climatology, Anaheim, CA, Amer. Meteor. Soc., 179184.

  • Onogi, K. J. T., and Coauthors, 2007: The JRA-25 Reanalysis. J. Meteor. Soc. Japan, 85, 369432.

  • Schmitz, J. T., , and S. L. Mullen, 1996: Water vapor transport associated with the summertime North American monsoon as depicted by ECMWF analyses. J. Climate, 9, 16211634.

    • Search Google Scholar
    • Export Citation
  • Simmonds, I., , D. H. Bi, , and P. Hope, 1999: Atmospheric water vapor flux and its association with rainfall over China in summer. J. Climate, 12, 13531367.

    • Search Google Scholar
    • Export Citation
  • Sun, B., , and H. J. Wang, 2013: Water vapor transport paths and accumulation during widespread snowfall events in northeastern China. J. Climate, 26, 45504566.

    • Search Google Scholar
    • Export Citation
  • Tao, S. Y., , and Y. H. Ding, 1981: Observational evidence of the influence of the Qinghai-Xizang (Tibetan) Plateau on the occurrence of heavy rain and severe convective storms in China. Bull. Amer. Meteor. Soc., 62, 2330.

    • Search Google Scholar
    • Export Citation
  • Tao, S. Y., , and L. X. Chen, 1987: A review of recent research on the East Asian summer monsoon in China. Review of Monsoon Meteorology, C. P. Chang and T. N. Krishnamurti, Eds., Oxford University Press, 60–92.

  • Tibshirani, R., , G. Walther, , and T. Hastie, 2001: Estimating the number of clusters in a data set via the gap statistic. J. Roy. Stat. Soc., 63, 411423.

    • Search Google Scholar
    • Export Citation
  • Tong, K., , F. G. Su, , D. Q. Yang, , L. L. Zhang, , and Z. C. Hao, 2013: Tibetan Plateau precipitation as depicted by gauge observations, reanalyses and satellite retrievals. Int. J. Climatol., doi:10.1002/joc.3682, in press.

    • Search Google Scholar
    • Export Citation
  • Trenberth, K. E., 1991: Climate diagnostics from global analyses: Conservation of mass in ECMWF analyses. J. Climate, 4, 707722.

  • Trenberth, K. E., , A. Dai, , R. M. Rasmussen, , and D. B. Parsons, 2003: The changing character of precipitation. Bull. Amer. Meteor. Soc., 84, 12051217.

    • Search Google Scholar
    • Export Citation
  • Wang, B., , and Y. Q. Li, 2010: Relationship analysis between south branch trough and severe drought of southwest China during autumn and winter 2009/2020 (in Chinese). Plateau Mt. Meteor. Res., 30, 2837.

    • Search Google Scholar
    • Export Citation
  • Wang, B., , R. G. Wu, , and X. H. Fu, 2000: Pacific–East Asian teleconnection: How does ENSO affect East Asian climate? J. Climate, 13, 15171536.

    • Search Google Scholar
    • Export Citation
  • Wang, H. J., , and H. P. Chen, 2012: Climate control for southeastern China moisture and precipitation: Indian or East Asian monsoon? J. Geophys. Res., 117, D12109, doi:10.1029/2012JD017734.

    • Search Google Scholar
    • Export Citation
  • Wang, X., , C. Y. Li, , and W. Zhou, 2006: Interdecadal variation of the relationship between Indian rainfall and SSTA modes in the Indian Ocean. Int. J. Climatol., 26, 595606.

    • Search Google Scholar
    • Export Citation
  • Webster, P., , and S. Yang, 1992: Monsoon and ENSO: Selectively interactive systems. Quart. J. Roy. Meteor. Soc., 118, 877926.

  • Xie, Y. B., , and W. J. Dai, 1959: The calculation for some cases of the moisture transport over Yangtze River basin. J. Appl. Meteor. Sci., 13, 6777.

    • Search Google Scholar
    • Export Citation
  • Yi, L., , and S. Y. Tao, 1997: Role of the standing and the transient eddies in atmospheric water cycle in the Asian monsoon region (in Chinese). Acta Meteor. Sin., 55, 533544.

    • Search Google Scholar
    • Export Citation
  • Yihui, D., 1994: Monsoons over China. Springer, 432 pp.

  • Yuan, F., , W. Chen, , and W. Zhou, 2012: Analysis of the role played by circulation in the persistent precipitation over south China in June 2010. Adv. Atmos. Sci., 29, 769781.

    • Search Google Scholar
    • Export Citation
  • Yuan, Y., , H. Yang, , W. Zhou, , and C. Y. Li, 2008a: Influences of the Indian Ocean dipole on the Asian summer monsoon in the following year. Int. J. Climatol., 28, 18491859, doi:10.1002/JOC.1678.

    • Search Google Scholar
    • Export Citation
  • Yuan, Y., , W. Zhou, , J. C. L. Chan, , and C. Y. Li, 2008b: Impacts of the basin-wide Indian Ocean SSTA on the South China Sea summer monsoon onset. Int. J. Climatol., 28, 15791587, doi:10.1002/JOC.1671.

    • Search Google Scholar
    • Export Citation
  • Zhang, Q. L., , L. Wu, , and Q. Liu, 2009: Tropical cyclone damages in China 1983–2006. Bull. Amer. Meteor. Soc., 90, 485495.

  • Zhang, R. H., 2001: Relations of water vapor transport from Indian monsoon with that over East Asia and the summer rainfall in China. Adv. Atmos. Sci., 18, 10051017.

    • Search Google Scholar
    • Export Citation
  • Zhang, R. H., , A. Sumi, , and M. Kimoto, 1996: Impact of El Niño on the East Asian monsoon: A diagnostic study of the 86/87 and 91/92 events. J. Meteor. Soc. Japan, 74, 4962.

    • Search Google Scholar
    • Export Citation
  • Zhou, T. J., , and R. C. Yu, 2005: Atmospheric water vapor transport associated with typical anomalous summer rainfall patterns in China. J. Geophys. Res., 110, D08104, doi:10.1029/2004JD005413.

    • Search Google Scholar
    • Export Citation
  • Zhou, W., , and J. C. L. Chan, 2007: ENSO and South China Sea summer monsoon onset. Int. J. Climatol., 27, 157167.

  • Zhou, W., , J. C. L. Chan, , and C. Y. Li, 2005: South China Sea summer monsoon onset in relation to the off-equatorial ITCZ. Adv. Atmos. Sci., 22, 665676.

    • Search Google Scholar
    • Export Citation
  • Zhou, W., , C. Y. Li, , and J. C. L. Chan, 2006: The interdecadal variations of the summer monsoon rainfall over south China. Meteor. Atmos. Phys., 93, 165175, doi:10.1007/S00703-006-0184-9.

    • Search Google Scholar
    • Export Citation
  • Zhu, Y. L., , H. J. Wang, , W. Zhou, , and J. H. Ma, 2011: Recent changes in the summer precipitation pattern in east China and the background circulation. Climate Dyn., 36, 14631473, doi:10.1007/s00382-010-0852-9.

    • Search Google Scholar
    • Export Citation
  • View in gallery

    Cluster analysis based on the SPI_03 at 524 stations over China during 1979–2010: (a) curve of the variation of the sum of squares and (b) regional division over China. China is divided into nine climate zones that are identified by dots of different shading. The two rectangles indicate southwest China (21°–28.5°N, 97.5°–105°E) and southeast China (21°–28.5°N, 105°–120°E).

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    Time–latitude distributions of the climatological zonal averaged daily precipitation (mm day−1) over (a) southeast China (averaged between 105° and 120°E) and (b) southwest China (averaged between 97.5° and 105°E). Only amounts over 2 mm day−1 are shown.

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    Monthly evolution of the climatological observational precipitation (black line with crosses), ERA-Interim-derived net atmospheric moisture convergence (black line with dots), precipitation (black line with triangles), and evaporation (gray bars) averaged over (a) southeast China and (b) southwest China.

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    Monthly evolution of correlation coefficients between observational regional precipitation and ERA-Interim-derived atmospheric moisture convergence (black line with triangles), evaporation (gray line with crosses), and regional precipitation (gray line with dots) during 1979–2010 over (a) southeast China and (b) southwest China. The dashed lines show the crucial value of the correlation coefficient significant at the 99.9% confidence level by the Student’s t test.

  • View in gallery

    Spatial distribution of the vertical integral of seasonal stationary water vapor flux (vectors) and its divergence (shading) in (a) spring, (b) summer, (c) autumn, and (d) winter.

  • View in gallery

    As in Fig. 5, but for seasonal transient water vapor flux (vectors) and its divergence (shading).

  • View in gallery

    Boundary atmospheric fluxes (mm month−1) via four boundaries (grayscale lines), net zonal (solid black line) and meridional boundary atmospheric flux (dashed black line), and regional atmospheric moisture convergence (bars) over (a),(c) southeast China and (b),(d) southwest China. Boundary fluxes in (a),(b) are calculated based on the monthly stationary water vapor fluxes and in (c),(d) based on transient water vapor flux. The net zonal boundary atmospheric flux is calculated as subtracting moisture output from moisture input in the zonal direction. The meridional boundary atmospheric flux is calculated by subtracting moisture output from moisture input in the meridional direction. The regional atmospheric moisture convergence is the net moisture fluxes via four boundaries. (e),(f) The regions of southeast and southwest China; the vectors show the positive directions of water vapor transport via each boundary.

  • View in gallery

    Regressed vertical integral of seasonal stationary water vapor flux anomaly (vectors, kg m−1 s−1) and its divergence (shading, 10−5 kg m−2 s−1) based on the regional atmospheric moisture convergence over (left) southeast China and (right) southwest China in (a),(b) spring, (c),(d) summer, (e),(f) autumn, and (g),(h) winter. The black vectors represent the moisture fluxes significant at the 95% confidence level. Only the divergence with magnitude over 2.5 × 10−5 kg m−2 s−1 is shown.

  • View in gallery

    As in Fig. 8, but for the seasonal transient water vapor flux anomaly and its divergence.

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    Difference between climatological monthly precipitation (mm month−1) depicted by ERA-Interim and gauge observations over China (ERA-Interim minus observations).

  • View in gallery

    Climatological annual cycle of the individual terms of the regional moisture balance equation over (a) southeast China and (b) southwest China. Individual terms include precipitation (precip), evaporation (evapr), divergence of water vapor fluxes (DivQ), temporal variation of total precipitation (deltaq), and bias.

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Comparison of the Annual Cycles of Moisture Supply over Southwest and Southeast China

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  • 1 Guy Carpenter Asia-Pacific Climate Impact Centre, School of Energy and Environment, City University of Hong Kong, Hong Kong, China
  • 2 LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
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Abstract

The variation in regional precipitation over southeast and southwest China depends strongly on externally imported moisture rather than local evaporation. Associated with the different climate over the two regions, great discrepancies appear in the annual cycles of the moisture supply. Stationary moisture transport dominates externally imported moisture to a large extent, with transient transport being much weaker. The stationary moisture sink over southeast China is strong during spring and summer due to strong moisture input via the southern boundary and weak during fall and winter due to the offset between the output via the southern boundary and the net zonal boundary atmospheric flux. Zonal stationary moisture transport dominates the variation in moisture supply over southwest China. Negative net zonal boundary atmospheric flux countervails (collaborates) with positive meridional transport during the dry (wet) season.

Stationary moisture circulations dominate regional atmospheric moisture convergence anomalies over both southeast and southwest China. Weak cold air activity is favorable for a strong moisture sink over southeast China, while the reverse appears over southwest China in spring. The east-to-west location of the abnormal anticyclone determines whether strong moisture converges over southeast China or southwest China in fall. The anticyclonic circulation anomaly over the Philippine Sea, remotely forced by El Niño, is crucial to the strong moisture sink over southeast China from winter to spring, while it does not play a role in the abnormal moisture sink over southwest China.

Corresponding author address: Dr. Wen Zhou, Guy Carpenter Asia-Pacific Climate Impact Centre, School of Energy and Environment, City University of Hong Kong, Tat Chee Ave. 83, Kowloon, Hong Kong, 008852, China. E-mail: wenzhou@cityu.edu.hk

Abstract

The variation in regional precipitation over southeast and southwest China depends strongly on externally imported moisture rather than local evaporation. Associated with the different climate over the two regions, great discrepancies appear in the annual cycles of the moisture supply. Stationary moisture transport dominates externally imported moisture to a large extent, with transient transport being much weaker. The stationary moisture sink over southeast China is strong during spring and summer due to strong moisture input via the southern boundary and weak during fall and winter due to the offset between the output via the southern boundary and the net zonal boundary atmospheric flux. Zonal stationary moisture transport dominates the variation in moisture supply over southwest China. Negative net zonal boundary atmospheric flux countervails (collaborates) with positive meridional transport during the dry (wet) season.

Stationary moisture circulations dominate regional atmospheric moisture convergence anomalies over both southeast and southwest China. Weak cold air activity is favorable for a strong moisture sink over southeast China, while the reverse appears over southwest China in spring. The east-to-west location of the abnormal anticyclone determines whether strong moisture converges over southeast China or southwest China in fall. The anticyclonic circulation anomaly over the Philippine Sea, remotely forced by El Niño, is crucial to the strong moisture sink over southeast China from winter to spring, while it does not play a role in the abnormal moisture sink over southwest China.

Corresponding author address: Dr. Wen Zhou, Guy Carpenter Asia-Pacific Climate Impact Centre, School of Energy and Environment, City University of Hong Kong, Tat Chee Ave. 83, Kowloon, Hong Kong, 008852, China. E-mail: wenzhou@cityu.edu.hk

1. Introduction

South China, located in Southeast Asia, is bordered by the western North Pacific (WNP) and is influenced by a typical monsoon circulation. It experiences great variation in precipitation and is vulnerable to flooding and drought, which have brought great personal, agricultural, and economic losses (Tao and Chen 1987; Yihui 1994; Yuan et al. 2008a,b; Gu et al. 2009a,b). South China’s abundant annual precipitation sometimes comes in the form of heavy rainstorms (Tao and Ding 1981; Chan and Zhou 2005; Zhou et al. 2005, 2006; Zhou and Chan 2007; Bao and Huang 2006; Zhu et al. 2011; Chen et al. 2012). In June 2005, severe precipitation hit Guangdong and Guangxi provinces, and Longmen Station received around half of its annual precipitation in only three days (Lin et al. 2007). Besides heavy precipitation, south China also frequently experiences long-term precipitation deficits that result in large-scale, persistent droughts that may reduce crop productivity and rangeland water levels and increase the risk of wildfire. In the spring of 2004, a severe prolonged drought over southeast China reduced rainfall by 80% in some areas, lasted for three seasons, and caused serious socioeconomic problems. Poor rainfall from September 2009 to March 2010 led to the worst drought over southwest China in 60 years, causing millions of people and livestock to suffer from drinking water shortages (Huang et al. 2012; Wang and Li 2010). Because of the far-reaching influence of such extreme precipitation fluctuations, there is an urgent need to investigate the physical processes and mechanisms that govern precipitation variability over south China.

Sufficient moisture supply is necessary for precipitation generation. The precipitation amount over a region usually depends on the available moisture arising from two main sources: local evaporation and externally advective moisture. The latter is much more important than the former because, even on the most extensive continent where the relative role of local evaporation is maximized, the main portion of precipitation is contributed by externally advective moisture, rather than local evaporation (Benton et al. 1950; Budyko 1974). Moisture transport includes both vertical moisture transport, which is crucial to cloud formation and thus rain, and horizontal moisture transport, which advects moisture from source regions to sink regions. As estimated by Trenberth et al. (2003), a precipitation-producing weather system in a heavy rainstorm may reach out to about 3–5 times the radius of the sink region to capture the moisture evaporated there. Considering the importance of water vapor transported by atmospheric circulation, improved knowledge of it would likely contribute greatly to better insights into regional climate, the development of weather systems, and water balance in the atmosphere, on the earth’s surface, and even underground. It has been pointed out that an anomalous water vapor supply could directly affect rainfall over south China (Simmonds et al. 1999; Zhou and Yu 2005; Zhang et al. 2009; Chen et al. 2012). Xie and Dai (1959) stated that the two main sources of rainfall over China in summer are moisture from the western North Pacific, carried by the southeasterly flow at the edge of the western Pacific subtropical high (WPSH), and moisture from the Indian Ocean (IO), advected by the southwesterly flow at the edge of the Indian low. When comparing moisture transport in the East Asian and South Asian monsoon regions, Huang et al. (1998) pointed out an obvious difference between them: zonal water vapor transport is dominant in the Indian monsoon region, while meridional water vapor transport is crucial in the East Asian monsoon region. Further study found that water vapor transport from the Indian monsoon is the inverse of that over East Asia; more (less) Indian monsoon water vapor transport corresponds to less (more) water vapor transport over East Asia (Zhang 2001).

The climate over southeast and southwest China shows great east-to-west discrepancies. Because southwest China is located to the east of the Tibetan Plateau and close to the Indian Ocean, the moisture transport for this region is influenced not only by the high topography but also by the interaction of the Indian monsoon and the East Asian monsoon, which is quite different from the situation over southeast China. Climatically, in summer southeast China is influenced by three main branches of water vapor transport from lower latitudes: the cross-equator flow around 105°E, the southeasterly flow from the tropical Pacific Ocean to the west of the WPSH (southeast summer monsoon), and the southwesterly flow of the Indian summer monsoon (southwest summer monsoon) (Wang et al. 2006; Wang and Chen 2012; Sun and Wang 2013). The southwest summer monsoon from the Indian Ocean splits into two branches when it reaches the Indochinese Peninsula. One turns northwestward, crosses the northern part of the peninsula, and transports water vapor into southeast China via its southwestern boundary; the other moves to the east, converges with other flows over the South China Sea (SCS), and turns northward, bringing the majority of moisture into southeast China (Li et al. 2011; Feng et al. 2011; Yuan et al. 2012). In contrast, two main water vapor pathways are detected over southwest China. The moisture transported by the southwest summer monsoon is key. It originates from the southwestern Indian Ocean, crosses the equator, passes through the Arabian Sea and the Bay of Bengal (BOB), and moves northward into southwest China via the southwestern boundary. The other pathway involves the merging of the southwesterly flow with the cross-equator flow over the SCS, which then turns northward and affects southwest China via the southeastern boundary (Li et al. 2011). The moisture advected by the flow at the edge of the WPSH also contributes to the rainfall over southwest China (Chen et al. 2012). In the winter, as the summer monsoons retreat, moisture circulation over south China is affected mainly by the westerly flow to the south of the Tibetan Plateau and by the northeast winter monsoon (Zhang et al. 2009). In summary, the moisture regimes over southwest and southeast China have many features in common: though the combined flow over the SCS is the main path of moisture transport to southeast China, it also affects southwest China via the eastern boundary; the southwesterly moisture transported over southwest and southeast China originates from the same ocean source; and the westerly transport in winter over the two regions is governed by the same atmospheric circulation. However, many differences still exist. In contrast to southeast China, moisture circulation over southwest China is affected not only by the southeast summer monsoon but also by the southwest summer monsoon.

Though moisture circulation over south China has been widely examined by previous studies, most of the results are drawn subjectively and qualitatively from the general spatial distribution of the vertically integrated water vapor fluxes over East Asia. More objective and quantitative investigation into moisture convergence and rainfall variation over south China is necessary. Moreover, compared to massive studies on the moisture supply over southeast China, the situation over southwest China is rarely investigated, and there are not many comparisons between the two regions. In this study, the moisture supply to southwest and southeast China will be examined by looking at the moisture transported across each region’s boundary as well as by looking at the net regional atmospheric moisture convergence. The focus will be on comparing the moisture transport over the two regions in detail. The paper is organized as follows: section 2 describes the data and methodology. The west-to-east discrepancy between southwest and southeast China is investigated in section 3.

The determining role played by regional atmospheric moisture convergence in the variation of precipitation over southeast and southwest China is verified in section 4. Section 5 will describe the characteristics of the atmospheric moisture supply over southeast and southwest China by means of water vapor flux and boundary atmospheric flux. The moisture transport routines responsible for the moisture supply anomalies are examined in section 6, and the discussion and conclusions follow in section 7.

2. Data and methodology

a. Data

For the purpose of examining the character of the precipitation over southeast and southwest China and calculating the standard precipitation index, observed daily precipitation data over China are employed. Considering the missing values within the records, 524 stations are selected by the criterion that the proportion of missing values within each month is less than 20%. The locations of the stations are shown in Fig. 1b.

Fig. 1.
Fig. 1.

Cluster analysis based on the SPI_03 at 524 stations over China during 1979–2010: (a) curve of the variation of the sum of squares and (b) regional division over China. China is divided into nine climate zones that are identified by dots of different shading. The two rectangles indicate southwest China (21°–28.5°N, 97.5°–105°E) and southeast China (21°–28.5°N, 105°–120°E).

Citation: Journal of Climate 26, 24; 10.1175/JCLI-D-13-00057.1

The European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) is an interim reanalysis for the period 1979–present in preparation for the next-generation extended reanalysis that will replace the 40-yr ECMWF Re-Analysis (ERA-40) (Dee et al. 2011). ERA-Interim improves many of the problems experienced with ERA-40, especially those of the hydrological cycle, for example, excessive precipitation over the tropical oceans, extravagant total column water vapor, and the global imbalance of precipitation and evaporation. The time span of the ERA-Interim used in this study is from 1979 to 2010. Both monthly and 6-hourly data will be analyzed with a horizontal resolution of 1.5° × 1.5°.

In this study, the Japanese 25-yr Reanalysis (JRA-25) project dataset (http://jra.kishou.go.jp), which provides the foundation for a high-quality analysis of the Asian region, is also employed as a supplement to ERA-Interim. A detailed description can be found in Onogi et al. (2007).

b. Processing of water vapor flux and boundary atmospheric flux

The general balance equation for atmospheric water vapor, if the flux across the upper boundary, diffusion, and liquid and solid phase transport are neglected, can be expressed as (Schmitz and Mullen 1996)
e1
This means that the variation in total precipitable water, the amount of precipitation, the evaporation, and the convergence of water vapor flux should be balanced within a region. Here, E and P are evaporation and precipitation, respectively; W is the total precipitable water, defined by
e2
and Q is the vertically integrated water vapor flux:
e3
where g is the acceleration of gravity, q is the specific humidity, V is the horizontal wind vector, psurf is the surface pressure, and ptop is the pressure at the top of the troposphere, which is set at 200 hPa as the moisture content above this level is negligible (Trenberth 1991; Li et al. 2011). Compared to the water vapor flux over southeast China, that over southwest China is integrated within a thinner atmosphere, as the high topography over southwest China blocks moisture transport in the lower level.
The monthly vertical integral of water vapor flux can be decomposed into a stationary term (monthly mean moisture transported by the monthly mean flow) and a transient term (transient moisture transported by the transient flow):
e4
where the first term on the right-hand side is the stationary term and the second term is the transient term.
The atmospheric moisture transport via each boundary N is calculated by
e5
where L is the length of the boundary and is the inward-pointing normal vector of the boundaries of the target region (Schmitz and Mullen 1996). The regional atmospheric moisture convergence is calculated as the net effect of atmospheric moisture transport via each boundary: a positive regional atmospheric moisture convergence represents atmospheric water vapor that is transported from outside and converges within the region.

c. Cluster analysis

Cluster analysis (clustering) is also employed in this study to distinguish between the dry/wet conditions in southwest and southeast China and to help define the boundaries of the two regions. This analysis involves grouping a set of objects in such a way that objects in the same group are more similar to each other in some sense than to those in other groups (http://en.wikipedia.org/wiki/Cluster_analysis). To do this, some measure of distance between pairs of objects is needed. Here, the most widely used distance measure, “Euclidean distance,” is employed; that is, the distance between objects i and j in the n × p data matrix is calculated as the square root of the sum of the squared difference between them for each of the p variables:
e6

There are many clustering algorithms; in this study we use agglomerative hierarchical cluster analysis. It starts with n clusters, each containing only one object, and at each step fuses the cluster pair with the minimum separation distance to form a new cluster. With each step, the number of remaining clusters decreases by one until one all-inclusive cluster is created. The results are then inspected to determine which clustering level represents the “best” solution (Fovell and Fovell 1993).

A major challenge in cluster analysis is to decide the optimal number of “clusters.” Here, the typical plot of an error measure (the within-cluster sum of squares) versus the number of clusters k is employed. The error measure decreases monotonically as the number of clusters k increases, but from some k onward the decrease flattens. The location of such an “elbow” indicates the optimal number of clusters (Tibshirani et al. 2001).

3. West-to-east discrepancy over south China

Because they are dominated by different weather systems, southeast and southwest China differ greatly in climate even though they are at the same latitude. Before comparing the moisture supply over southeast and southwest China, it is reasonable to define the boundary between the two regions based on their climate diversity. We do this by applying cluster analysis to the 1979–2010 time series of the 3-month standardized precipitation index (SPI_03) (McKee et al. 1993) at 524 stations in China. The SPI is developed in order to flexibly monitor drought at different time scales. Based on its definition, it can represent not only dryness but also wetness anomalies over a region. The 3 months represent an arbitrary but typical time scale at which precipitation deficits affect soil moisture and harm agriculture directly. Results are shown in Fig. 1. The plot of the within-cluster sum of squares shows two main elbows, that is, where the number of clusters is four and nine. The east-to-west discrepancy over south China is not discernible with four clusters, which divide China only into the Tibetan Plateau climate zone and northern, middle, and southern regions; therefore, the nine clusters are selected. This is reasonable, as the Yangtze River valley region, where the climate differs greatly from that of north China, is merged into north China with 8 clusters, and only one extra single station is identified with 10 clusters. Hence, China is divided into nine climate zones based on the SPI_03, that is, northeast China, northwest China, north China, Tibetan Plateau, central China, Yangtze River basin, Hainan Island, southwest China, and southeast China. Within these, south China is divided into two regions, southwest China and southeast China. The boundary is located at 105°E, which is also widely accepted as the western boundary of the East Asian summer monsoon (Li et al. 2012). Hence, southwest China and southeast China are defined as 21°–28.5°N, 97.5°–105°E and 21°–28.5°N, 105°–120°E, respectively.

The intraannual evolution of the precipitation provides direct insight into the east-to-west discrepancy over southeast and southwest China. As shown in Fig. 2, the 2 mm day−1 rainfall intensity could act as a rough threshold for defining the wet/rainfall season (daily rainfall ≥2 mm day−1) and the dry season (daily rainfall <2 mm day−1) over southeast and southwest China. The commencements of the wet season over the two regions are not in sync. Moderate rainfall (≥2 mm day−1) over southeast China first takes place in February when southwest China is still dry. In May, especially after mid-May, rainfall over southeast China increases abruptly along with the onset of the East Asian summer monsoon, which advances northward in the following months. In contrast, rainfall over 2 mm day−1 emerges only after April in southwest China. In May, rainfall increases slightly. After June, consistent with the onset of the summer monsoon over southwest China, strong rainfall advances abruptly northward to 30°N and stabilizes until September. The maximum rainfall over southeast China is much greater than that over southwest China and displays a south-to-north advance that is connected to the step-by-step invasion of the East Asian summer monsoon, which is absent over southwest China. After October, rainfall over the two regions diminishes greatly and the dry season begins. Briefly, outstanding differences appear in the emergence, northward invasion, and strength of the rain belts over southeast and southwest China. Considering the determining role of moisture supply to regional rainfall, better knowledge of the moisture supply over southeast and southwest China should provide us with a better understanding of this east-to-west discrepancy over south China.

Fig. 2.
Fig. 2.

Time–latitude distributions of the climatological zonal averaged daily precipitation (mm day−1) over (a) southeast China (averaged between 105° and 120°E) and (b) southwest China (averaged between 97.5° and 105°E). Only amounts over 2 mm day−1 are shown.

Citation: Journal of Climate 26, 24; 10.1175/JCLI-D-13-00057.1

4. Determining the role of atmospheric moisture convergence

Local evaporation and externally imported moisture are two main sources of precipitation in a specific region. Benton et al. (1950) and Budyko (1974) stated that the majority of precipitation is derived from externally imported moisture rather than from local evaporation, even on the most extensive continent where the relative role of local evaporation is maximized. Over south China, is the regional moisture supply dominated by local evaporation or externally imported moisture? What roles do local evaporation and externally imported moisture each play in regional precipitation variation? To answer these questions, we investigate the annual cycle of the regional precipitation along with the regional atmospheric moisture convergence and evaporation (Fig. 3). In contrast to evaporation, the monthly variation in the regional atmospheric moisture convergence is consistent with that of precipitation not only in its amplitude but also in its monthly evolution. Over southeast China, both regional precipitation and atmospheric moisture convergence increase greatly at the beginning of the year, reach their maximum in June, and then decrease abruptly after June, possibly as a result of the northward shift in the monsoon rain belt. In August, the regional atmospheric moisture convergence increases briefly and then decreases again with the regional precipitation. It reaches its minimum at the end of the year. Over southeast China, along with the regional rainfall, the regional atmospheric moisture convergence increases just slightly in the first few months of the year and then enhances obviously after April and reaches its maximum in July. After July, both regional precipitation and atmospheric moisture convergence decrease continually until the end of the year. In contrast, the evaporation in both southeast and southwest China varies smoothly. Despite that its maximum and minimum levels coincide with those of precipitation, the amplitude of its monthly variation is much less than that of precipitation and atmospheric moisture convergence. It tends to make a limited contribution to the variation in precipitation, especially to heavy rainstorms during which abundant moisture is precipitated in only a few days. It is important to note that, though the variation in evaporation is much less than that of atmospheric moisture convergence, its magnitude is comparable to or even larger than that of atmospheric moisture convergence, especially from October to January over southeast China and from November to March over southwest China, when the atmospheric moisture convergence is close to or even below zero. The major source of precipitation in the dry season is local evaporation rather than externally imported moisture. Hence, local evaporation is also one of the key sources of precipitation, but it makes only a limited contribution to the variation in precipitation, as its variation is slow and of small amplitude. In contrast, atmospheric moisture convergence, which shows great variability, dominates the annual regional precipitation cycle.

Fig. 3.
Fig. 3.

Monthly evolution of the climatological observational precipitation (black line with crosses), ERA-Interim-derived net atmospheric moisture convergence (black line with dots), precipitation (black line with triangles), and evaporation (gray bars) averaged over (a) southeast China and (b) southwest China.

Citation: Journal of Climate 26, 24; 10.1175/JCLI-D-13-00057.1

It is important to note that both regional atmospheric moisture convergence and evaporation are derived based on reanalysis data rather than actual observation. Substantial errors may be introduced owing to the great dependence of reanalysis data on model physics and parameterizations, especially in the hydrological cycle. Figure 3 shows a comparison of regional precipitation calculated based on both observations and ERA-Interim. Over southeast China, the monthly precipitation derived from ERA-Interim shows great consistency with that from observations, indicating that ERA-Interim may be able to accurately simulate the hydrological cycle over southeast China. This is also why the sum of the regional evaporation (1102 mm yr−1) and atmospheric moisture convergence (591 mm yr−1) are in balance with the observed precipitation (1552 mm yr−1) over southeast China (less than 10% imbalance). Conversely, the ERA-Interim dataset overestimates the precipitation over southwest China, especially in the wet season; the yearly precipitation derived from the ERA-interim dataset is around 1951 mm day−1, which is nearly double the observed precipitation (1093 mm yr−1). This indicates that the ERA-Interim dataset cannot correctly simulate the hydrological cycle over southwest China, with its high-altitude topography (details will be discussed in section 7). This is a common deficiency in all reanalysis datasets (Dee et al. 2011). Although we recognize these deficits, the reanalysis dataset is still the only data source currently available for investigating the hydrological cycle over south China, as it guarantees spatial and temporal continuity.

The determining role played by regional atmospheric moisture convergence as opposed to evaporation in the variation of precipitation over southeast and southwest China appears not only at the intraannual scale but also at the interannual scale. Figure 4 shows the correlation coefficients between regional precipitation observations, regional atmospheric moisture convergence, and evaporation over southeast and southwest China. The atmospheric moisture convergences over both southeast and southwest China are highly positively correlated with regional precipitation throughout the year, with the coefficients significant at the 99.9% confidence level (except in February) over southwest China. This verifies the determining role played by externally imported moisture in the variation of precipitation over southwest and southeast China. In contrast, the variation in local evaporation tends to result from, rather than result in, precipitation anomalies. Generally, precipitation can influence local evaporation in a negative or positive direction. The negative effect is that lower (higher)-than-normal temperatures during high (low) precipitation may cause low (high) evaporation in spite of more (less) moisture being available on the surface; the positive effect is that high (low) precipitation can provide sufficient (insufficient) moisture for evaporation, and thus evaporation will be enhanced (depressed) even though the temperature is lower (higher) than normal. Evaporation over southeast and southwest China is significantly negatively correlated with precipitation in most months, except from November to February over southeast China and from December to April over southwest China. That is, during the rainfall season, the interannual variation of local evaporation is opposite to that of precipitation, indicating that the evaporation anomaly probably results from the negative effect of the precipitation. In contrast, the insignificant negative or even positive relationship during the dry season might be because the positive effect is comparable to or even larger than the negative effect, as the surface normally suffers a moisture deficit in the dry season. Hence, the main moisture source for precipitation variation over southeast and southwest China is externally imported moisture rather than local evaporation. Regional atmospheric moisture convergence plays a determining role in the variation of precipitation, while evaporation tends to be the result of, instead of the cause of, varying precipitation anomalies. The correlation between the regional precipitation derived from observations and from ERA-Interim is also examined. It is interesting to find that, though the reanalysis data exaggerate the magnitude of hydrological variables (e.g., the regional precipitation amount over southwest China), the data still capture their variation not only at the intraannual scale but also at the interannual scale. Hence, further comparison of the atmospheric moisture supply over southeast and southwest China based on ERA-Interim is reasonable.

Fig. 4.
Fig. 4.

Monthly evolution of correlation coefficients between observational regional precipitation and ERA-Interim-derived atmospheric moisture convergence (black line with triangles), evaporation (gray line with crosses), and regional precipitation (gray line with dots) during 1979–2010 over (a) southeast China and (b) southwest China. The dashed lines show the crucial value of the correlation coefficient significant at the 99.9% confidence level by the Student’s t test.

Citation: Journal of Climate 26, 24; 10.1175/JCLI-D-13-00057.1

5. Comparison of the moisture supply over southeast and southwest China

a. Water vapor flux and its divergence

Because externally imported moisture is the main source of the variation in precipitation over southeast and southwest China, it is meaningful to further investigate the features of this moisture transportation, including its spatial and temporal distributions, over south China and figure out what differences between southeast and southwest China are responsible for the east-to-west precipitation discrepancy. Based on the time scale of wind flow and moisture, the monthly vertical integral of water vapor flux can be decomposed into a stationary term (monthly moisture transported by monthly wind flow) and a transient term (transient moisture transported by transient wind flow). Stationary moisture transport tends to be large scale and persistent, such as monsoon flows, the WPSH, and the East Asian trough, while transient moisture transport is essentially accomplished by disturbances that develop over a frontal zone or the intertropical convergence zone, such as cyclonic activity, easterly waves, and westerly troughs and ridges (Yi and Tao 1997). The stationary and transient water vapor fluxes and their divergences that affect south China are displayed in Figs. 5 and 6, respectively. The stationary water vapor flux is nearly one order of magnitude larger than the transient water vapor flux, indicating that water vapor flux is dominated by the stationary term rather than the transient term. Climatologically, there are two crucial moisture transport routines affecting moisture convergence over southeast China in spring. One is from the west: moisture diverging over the Arabian Sea, Indian subcontinent, and Bay of Bengal is carried by the southern branch of the westerly to southeast China via the western boundary. The other is from the subtropical WNP: abundant moisture evaporating over the ocean is guided by the steering circulation of the WPSH, located to the south. These two moisture transport routines converge over southeast China and are responsible for spring rainfall over southeast China before the onset of the summer monsoon. In contrast, stationary water vapor transport results mainly in moisture divergence over southwest China, as southwest China is upstream of the westerly moisture transport to southeast China. This explains why southwest China remains dry in the spring. When summer arrives, both southeast and southwest China are within the vast moisture convergence zone over the Asian–Pacific monsoon region. Abundant moisture evaporating over the southern and western Indian Ocean is carried by the Indian summer monsoon flow to the Indian subcontinent and onward to East Asia. The moisture evaporated over the southern Pacific is advected by the cross-equator flow around 120°E, and the moisture evaporated over the southeast quadrant of the WPSH and guided by the steering circulation of the WPSH converges with that from the Indian Ocean over the East Asia–Pacific region, supplying abundant moisture to generate the monsoon rainfall. The moisture converging over southwest China comes mainly from the Indian Ocean and is imported via the southern and western boundaries by the southwest transport in front of the BOB monsoon trough, while the moisture converging over southeast China is a bit more complicated, as it is jointly influenced by the three aforementioned moisture transport routines. It is worth noting that the strongest moisture convergence appears over southern southwest China and at the southern edge of the Tibetan Plateau, though this might be due to the poor retrieval capability of ERA-Interim near the high topography (Dee et al. 2011). In fall, the summer monsoon subsystems retreat from south China and the winter monsoon invades the coastal region. At this stage, the northerly moisture transport advected by the anticyclonic circulation of the continental high over inland China dominates southeast China and induces strong moisture divergence in its southeastern region. In contrast, southwest China is affected by weak moisture convergence owing to the conflict of the westerly moisture transport and the southwesterly moisture transport from the BOB. However, this weak convergence might result from errors in the reanalysis data, as the actual precipitation is exaggerated over southwest China in the fall. In the following winter, in contrast to southwest China, southeast China is dominated by weak moisture convergence rather than divergence. Moisture evaporated over the coastal ocean is imported by the anticyclonic circulation over southern southeast China and the northern SCS and converges with the weak westerly transport over southeast China. In sum, the seasonal evolution of stationary moisture transported by stationary wind flow could represent the total moisture transport to a large extent. This evolution of the large-scale circulation dominates the moisture circulation over East Asia and the western North Pacific.

Fig. 5.
Fig. 5.

Spatial distribution of the vertical integral of seasonal stationary water vapor flux (vectors) and its divergence (shading) in (a) spring, (b) summer, (c) autumn, and (d) winter.

Citation: Journal of Climate 26, 24; 10.1175/JCLI-D-13-00057.1

Fig. 6.
Fig. 6.

As in Fig. 5, but for seasonal transient water vapor flux (vectors) and its divergence (shading).

Citation: Journal of Climate 26, 24; 10.1175/JCLI-D-13-00057.1

In contrast, the average effect of transient moisture transport, which is calculated as the average transient moisture transported by transient wind flow within a season, shows not only a much smaller magnitude than that of the stationary term but also a disparate spatial pattern (Fig. 6). From spring to winter, seasonal transient moisture transport shows a similar pattern; that is, moisture is transported poleward from extratropical regions to midlatitude regions, with an obvious south-to-north shift as the seasons change. The strongest average transient transport appears mainly over the western North Pacific at around 30°N in winter and 40°N in summer. The average effect of transient moisture transport represents mainly poleward moisture transport from low to high latitudes. Yi and Tao (1997) examined the moisture transported by transient eddies over the Asian monsoon region and pointed out that it plays an important role in maintaining the meridional moisture equilibrium between subtropical latitudes and mid to high latitudes. They also stated that the south-to-north seasonal shift in strong transient moisture transport is connected to the location of the frontal zone, which is to the south in winter and to the north in summer. Though the magnitude of the transient water vapor flux is much smaller, its moisture divergence is just slightly weaker than that of the stationary water vapor flux. The influence of the transient moisture transport over south China is strongest in spring, which shows a dramatic west-to-east difference between southwest and southeast China. The poleward transient water vapor flux dominates the northern part of southeast China; opposite to the stationary term, it induces moisture divergence rather than moisture convergence over southeast China as the strong output via the northern boundary. Over southwest China, the diverged westward transport from southeast China via the western boundary converges over southwest China. In summer, as the transient moisture transport shifts northward, its influence on the moisture supply over south China weakens. In the following fall and winter, the transient moisture transport shifts southward and affects southeast China; however, its divergence over southeast China is quite weak, as the poleward transport does not show a strong meridional gradient over southeast China. The transient moisture transport over southwest China in fall is relatively weak, and its impact is almost negligible. In winter, a portion of moisture over southeast China is exported to southwest China and results in enhanced moisture convergence. In sum, the impact of transient moisture transport on the moisture supply over south China takes place mainly in spring, with strong moisture divergence over southeast China and convergence over southwest China. Transient moisture convergence over southwest China is exported from southeast China, which first takes place in winter and is then enhanced the next spring. This offsets the stationary moisture divergence over southwest China in the dry season to a large extent.

b. Boundary atmospheric flux and regional atmospheric moisture convergence

The spatial distribution of water vapor flux and its divergence give us an illustration of the moisture transport routines and the direction of the water vapor transport via each boundary of our target regions. Nevertheless, this is a qualitative analysis, and any conclusions based on it are somewhat subjective; a more quantitative analysis is required to investigate the exact amount of moisture supplied to a specific region, as well as the moisture input via each boundary and its variation. To achieve this, the amount of stationary and transient moisture transport across each regional boundary and the regional atmospheric moisture convergence over southwest and southeast China are calculated based on the previously defined boundaries. The results are shown in Fig. 7.

Fig. 7.
Fig. 7.

Boundary atmospheric fluxes (mm month−1) via four boundaries (grayscale lines), net zonal (solid black line) and meridional boundary atmospheric flux (dashed black line), and regional atmospheric moisture convergence (bars) over (a),(c) southeast China and (b),(d) southwest China. Boundary fluxes in (a),(b) are calculated based on the monthly stationary water vapor fluxes and in (c),(d) based on transient water vapor flux. The net zonal boundary atmospheric flux is calculated as subtracting moisture output from moisture input in the zonal direction. The meridional boundary atmospheric flux is calculated by subtracting moisture output from moisture input in the meridional direction. The regional atmospheric moisture convergence is the net moisture fluxes via four boundaries. (e),(f) The regions of southeast and southwest China; the vectors show the positive directions of water vapor transport via each boundary.

Citation: Journal of Climate 26, 24; 10.1175/JCLI-D-13-00057.1

1) Stationary atmospheric moisture convergence over southeast China

Over southeast China, the most prominent feature is the moisture input via the southern boundary. This is the most important moisture input in the rainfall season and shows a peak during June and July, which is connected to strong southerly moisture transport by the joint monsoon systems. After that, the moisture input via the southern boundary decreases rapidly, and this boundary turns into an output boundary as the southerly transport is replaced by the northeasterly transport in fall and early winter. In spring, as the anticyclonic moisture transport over the subtropical WNP is established, the moisture input via the southern boundary increases gradually. The northern boundary acts as an output boundary, where the moisture output also shows a peak in July. Compared with the input via the southern boundary, it is far less and shows slow growth from January to June. Thus, the net meridional boundary atmospheric flux increases. In July, the moisture output via the northern boundary increases rapidly to its peak as it is guided by the steering circulation of the northward-shifted WPSH. It offsets a majority of the moisture input via the southern boundary, resulting in a sudden reduction in the net meridional boundary atmospheric flux. After July, along with the decrease in southerly moisture input, the moisture output via the northern boundary diminishes greatly and remains low from September to January. The net meridional boundary atmospheric flux is below zero, as it is dominated by the strong moisture output via the southern boundary during this period. The boundary atmospheric fluxes on the western and eastern boundaries also show striking monthly variation. The western boundary acts as an important input boundary from midfall to the next spring when the moisture input via the southern boundary is weak or negative. The moisture input via the western boundary increases gradually from fall to spring, which might be related to the establishment and intensification of westerly transport by the southern branch of the westerly to the south of the Tibetan Plateau. It weakens gradually after April and reaches its minimum in late summer and early fall. The moisture input via the western boundary is nearly offset by the output via the eastern boundary over southeast China; hence, the net zonal boundary atmospheric flux is much weaker than the meridional flux except in fall, when both western and eastern boundaries act as moisture input boundaries, and in early winter, when the output via the eastern boundary is much weaker than the input via the western boundary. The positive net zonal boundary atmospheric flux nearly counterbalances the negative net meridional flux during fall and early winter, resulting in weak regional atmospheric moisture convergence, and thus low precipitation over southeast China. Generally, stationary moisture transport results in a strong moisture sink over southeast China in the spring and summer, and the strong moisture input via the southern boundary results in a weak moisture sink in fall and winter as the net zonal boundary atmospheric flux is nearly offset by the output via the southern boundary.

2) Stationary atmospheric moisture convergence over southwest China

In contrast to southeast China, in southwest China the variation of boundary atmospheric fluxes via the eastern and western boundaries are more energetic than those via the southern and northern boundaries. The input via the southern boundary and the output via the northern boundary are quite stable, with the former larger than the latter; thus, the net meridional boundary atmospheric flux over southwest China is continually above zero throughout the year. The variation in moisture input via the western boundary shows a similar pattern to that in southeast China. It is the main input boundary over southwest China from November to July, which might connect to the strong westerly transport and the strong southwest monsoon transport. Its minimum occurs during August and September, when the southwest monsoon retreats and the south branch of the westerly flow is still not established. The moisture output via the eastern boundary is even larger than the moisture input via the western boundary from October to May; thus, most of the input moisture in the zonal direction is exported directly, without convergence over southwest China. During August and September, though the moisture transport in the zonal direction is at its weakest within the year, the net zonal boundary atmospheric flux is quite large as the eastern boundary changes into an input boundary; thus, the moisture inputs via the eastern and western boundaries converge over southwest China. This implies the importance of the interaction between the southwest and southeast summer monsoons to the moisture convergence and precipitation over southwest China from late summer to early fall. In sum, the intraannual variation in the moisture supply over southwest China is dominated by the moisture transport in the zonal direction. The moisture input via the southern boundary supplies stable moisture to southwest China. The monthly variation in the regional atmospheric moisture convergence is determined by the boundary atmospheric fluxes in the zonal rather than the meridional direction, as the negative net zonal boundary atmospheric flux countervails the positive meridional flux during the dry season and collaborates with it during the wet season.

3) Transient atmospheric moisture convergence over southeast China

The transient boundary atmospheric flux via each boundary of southeast China is much weaker than the stationary flux, which is consistent with the previous conclusion based on the water vapor flux. It also shows an energetic annual cycle. From February to May, the transient moisture output via the northern boundary increases, while the input via the southern boundary gradually decreases as the transient transport shifts northward from the winter half year to the summer half year. These are responsible for the rapid decrease in the net meridional transient boundary atmospheric flux. After June, as the transient transport does not significantly affect the moisture supply over southeast China, the boundary atmospheric fluxes via the southern and northern boundaries are the weakest of the year. After August, both the moisture input via the southern boundary and the output via the northern boundary increase again; however, they nearly counterbalance each other and result in weak net meridional boundary atmospheric flux. The boundary atmospheric fluxes via the western and eastern boundaries are much weaker than those via the southern and northern boundaries; however, as the output via the western boundary is large from late winter to spring and is not offset by the weak moisture transport via the eastern boundary, the net zonal boundary atmospheric flux is strongly negative from late winter to early summer. The joint negative net zonal and meridional boundary atmospheric fluxes result in moisture divergence over southeast China from late winter to early summer, with the peak from April to June, offsetting nearly half of the moisture convergence by the stationary moisture transport. After June, the regional transient atmospheric moisture convergence over southeast China decreases sharply and remains low during the rest of the year. In sum, the negative transient atmospheric moisture convergence, which results mainly from the intensive output via the northern boundary, strongly affects southeast China in the first half of the year. It is strongest in spring and offsets nearly half of the positive stationary atmospheric moisture convergence.

4) Transient atmospheric moisture convergence over southwest China

The impact of the transient atmospheric moisture convergence over southwest China is entirely different from that over southeast China. It causes strong moisture convergence from late fall to spring, which is the result of joint inputs via the southern and eastern boundaries. The moisture input via the southern boundary remains nearly the same from November to May, while that via the eastern boundary increases greatly after January, reaches its maximum in March and April, and then decreases. The moisture input via the northern boundary approximates zero during this period; the moisture input via the western boundary also approaches zero from November to February but decreases rapidly after that and reaches its minimum in April, whereas it still cannot counterbalance the strong moisture input via the eastern boundary. Hence, the regional transient atmospheric moisture convergence is strongly positive from late fall to spring, which balances the negative regional stationary atmospheric moisture convergence and results in nearly no moisture supply over southwest China. From summer to midfall, the net zonal atmospheric moisture flux is weak, as the decreasing moisture transport via the eastern boundary is offset by the increasing moisture transport via the western boundary, as is the net meridional atmospheric moisture flux. Thus, the net regional transient atmospheric moisture convergence is negligible. In sum, the positive transient atmospheric moisture convergence, which results from the intensive input via the southern and eastern boundaries, strongly affects southwest China from late fall to spring. It reaches its maximum in spring and counterbalances the negative stationary atmospheric moisture convergence.

6. Transport routines responsible for moisture supply anomalies

In the previous sections, moisture circulations, including the stationary term and the transient term, over southeast and southwest China are examined and compared by the means of both the water vapor flux pattern and the regional atmospheric moisture convergence. It is found that great discrepancies appear in the moisture supply over southeast and southwest China, which explains why the precipitation is diverse over these two regions even though they are at the same latitude. Considering the great impacts of anomalous precipitation and the determining role of the moisture supply in the regional precipitation over southeast and southwest China, it is worth further study to discover what causes the variation in moisture supply over southeast and southwest China. Do the responsible circulations differ between southeast and southwest China? Also, do they show any seasonal variation? Is it the stationary or the transient water vapor that dominates the moisture supply over southeast and southwest China? To answer these questions, the seasonal stationary and transient water vapor flux anomalies and their convergence are regressed based on the regional atmospheric moisture convergence over southwest and southeast China (Figs. 8 and 9).

Fig. 8.
Fig. 8.

Regressed vertical integral of seasonal stationary water vapor flux anomaly (vectors, kg m−1 s−1) and its divergence (shading, 10−5 kg m−2 s−1) based on the regional atmospheric moisture convergence over (left) southeast China and (right) southwest China in (a),(b) spring, (c),(d) summer, (e),(f) autumn, and (g),(h) winter. The black vectors represent the moisture fluxes significant at the 95% confidence level. Only the divergence with magnitude over 2.5 × 10−5 kg m−2 s−1 is shown.

Citation: Journal of Climate 26, 24; 10.1175/JCLI-D-13-00057.1

Fig. 9.
Fig. 9.

As in Fig. 8, but for the seasonal transient water vapor flux anomaly and its divergence.

Citation: Journal of Climate 26, 24; 10.1175/JCLI-D-13-00057.1

Figure 8 shows that the stationary moisture circulations responsible for the strong atmospheric moisture convergence show great seasonal variation. In spring, strong moisture convergence over southeast China is significantly connected to the convergence of abnormal westerly moisture transport from the west and southwesterly transport from the SCS and WNP. The latter is more intensive and widespread and is closely associated with the anticyclonic moisture circulation over the Philippine Sea–SCS. Abundant abnormal moisture diverging over southwest China and the Philippine Sea–SCS is transported to and converges over southeast China. The southwest moisture transport over southeast China and the continuing southeast moisture transport to the north indicate that weak or low-frequency East Asian winter monsoon activity also contributes to enhanced moisture convergence over southeast China. When summer arrives, an above-normal moisture sink in southeast China can be attributed mainly to a local stationary cyclonic moisture circulation. The climatological moisture transportation from the Indian Ocean and WNP, carried by the southwest and southeast monsoons, does not show significant variation. This might be explained by the fact that the trigger for moisture convergence, rather than abnormal moisture supply, is crucial to the enhanced moisture sink, as abundant moisture is transported across southeast China in summer climatologically (Fig. 7a). Hence, the convergence within the cyclone is crucial to the strong moisture sink over southeast China. Additionally, the easterly transport anomaly to the north of southeast China is the reverse of the climatic mean moisture transport, indicating that more moisture carried by the East Asian summer monsoon converges over southeast China instead of moving onward to the Yangtze–Huaihe River basin and southern Japan. In fall, two moisture transport routines converge over the northern SCS and southeast China. One is from the west, originally carried by the cross-equator flow at around 90°E, and it transports anticyclonically across the southern Indian subcontinent, BOB, and northern Indochina to southeast China; the other is from the subtropical WNP and is advected by the steering flow of the abnormal anticyclonic circulation, with a ridge at around 18°N over the WNP. In the following winter, the dominant feature is a vast anticyclonic moisture circulation over the southern South China Sea and Philippine Sea. Abundant moisture diverges within the anticyclone and is advected into southeast China by the outflow of the anticyclone. In addition, the abnormal westerly moisture transport over the northern Indian Ocean advects anomalous moisture from the Arabian Sea and Bay of Bengal into southeast China. Both moisture transport routines combine over the northern SCS and southeast China and then move northeastward to a vast region south of Japan. This abnormal southwest moisture transport over southeast China, on one hand, represents the stronger moisture transport from the lower-latitude oceans to southeast China and, on the other hand, indicates the weakening of the northeast winter monsoon. Hence, the activity of an anticyclone over the SCS–Philippine Sea and the enhanced westerly flow, together with the weakening of the East Asian winter monsoon, are the key systems dominating the moisture supply over southeast China in winter. It is important to note that the anticyclonic moisture circulation over the SCS–Philippine Sea is crucial to the strong moisture sink over southeast China from winter to the next spring, with its center shifting eastward and its strength slightly decreasing. This anticyclone can be attributed to the remote forcing of the sea surface temperature anomaly over the tropical eastern Pacific. Previous studies have stated that in late fall, when an El Niño–Southern Oscillation (ENSO) event matures, an anomalous anticyclone [termed the Philippine Sea anticyclone (PSAC)] is established over the Philippine Sea and persists until the following spring or early summer (Wang et al. 2000). It serves as a bridge between the ENSO events and the climate anomaly over East Asia, which not only weakens the East Asian winter monsoon (Webster and Yang 1992; Zhang et al. 1996; Lau and Nath 2000), but also transports abundant moisture into southeast China (Li and Zhou 2012). Hence, the ENSO signal may strongly affect the moisture supply over southeast China from midwinter until late spring via its remote forcing on the activity of the PSAC.

The stationary moisture circulations responsible for the strong moisture sink over southwest China show great differences from those over southeast China. In spring, opposite to that over southeast China, strong or frequent, rather than weak or infrequent, East Asian winter monsoon activity is favorable to an above-normal moisture supply over southwest China. On one hand, the northeasterly moisture inputs via the northern and eastern boundaries are crucial to the convergence of moisture over southwest China, as they conflict with the moisture inputs via other boundaries; on the other hand, the strong northeasterly winter monsoon over coastal southeast China helps bring abundant moisture evaporated over the adjacent ocean into southwest China via the southern boundary. The abnormal southerly moisture flow originating from the Indian Ocean and steered by the cyclone anomaly over the northern BOB also plays a key role in supplying moisture to southwest China. Hence, the conflict of the cold air from the north and the moisture-rich flow from the lower-latitude oceans is crucial to providing a strong moisture supply over southwest China. In summer, the strong moisture supply is connected to the convergence of two moisture transport flows. One originates from the northern BOB and the other originates from the WNP. The former is guided by a southwesterly flow downstream of the trough over the northern BOB. The latter enters southwest China through southeast China and may be associated with the southeast summer monsoon. Thus, the interaction of the southwest summer monsoon from the Indian Ocean and the southeast summer monsoon from the tropical western Pacific may play an important role in the strong moisture convergence over southwest China. However, these insignificant results should be viewed with caution. In fall, the strong moisture sink over southwest China is significantly related to the abnormal anticyclone over the northern SCS and Indochina, the peripheral circulation of which brings plenty of moisture from the SCS and WNP into southwest China. The trough over the northern BOB plays an additional role in transporting moisture, though it is much weaker and insignificant. When we compare the moisture circulations responsible for the strong moisture sinks over southeast and southwest China in fall, we find that the abnormal anticyclonic moisture circulation over the SCS–WNP plays a key role in the strong moisture convergence over both southeast and southwest China. However, its east-to-west location determines whether strong moisture convergence takes place in southeast China or southwest China. The regressed moisture circulation is also not significant in winter. No impact of the ENSO signal could be found in the moisture circulation over southwest China.

Compared to the stationary moisture circulation, the transient moisture circulation responsible for the variations in the regional atmospheric moisture convergence over southeast and southwest China is much weaker. The associated moisture divergence generally shows a pattern opposite to that of the stationary term but with a much smaller magnitude.

To further examine the diverse roles played by the stationary and transient terms in the regional atmospheric moisture convergence over southeast and southwest China, the correlations between the total atmospheric moisture convergence, the stationary term, and the transient term are calculated (Table 1). The stationary term is highly correlated with the total atmospheric moisture convergence in both southeast and southwest China in all seasons, verifying the dominant role played by the stationary term in the total atmospheric moisture convergence over both regions. In contrast, the transient term is negatively correlated with the total atmospheric moisture convergence, with the coefficients much weaker and insignificant except in spring over southwest China and in summer over southeast China. This indicates that the transient moisture circulation may play an additional role in the variation in the total atmospheric moisture convergence at certain times. However, its role is still much smaller than that of the stationary term, and the mechanism behind it is still not clear at this stage.

Table 1.

Correlation coefficients of regional atmospheric moisture convergence between the total term, the stationary term, and the transient term over southeast and southwest China. The boldface values denote correlation coefficients significant at the 99.9% confidence level by the Student’s t test.

Table 1.

7. Discussion and conclusions

In this study, the annual cycles of moisture supply over southeast and southwest China are investigated and compared. The variations in dry and wet conditions differ greatly over southwest and southeast China, according to the results of the cluster analysis based on the variation in the 3-month standardized precipitation index (SPI_03) (Fig. 1). Further analysis of the intraannual variation in precipitation reveals that outstanding differences appear in the emergence, northward invasion, and strength of the rain belts over southeast and southwest China (Fig. 2). As the variation in the regional precipitation over southwest and southeast China depends strongly on external imported moisture (Figs. 3 and 4) at both intraannual and interannual scales, this study focuses on investigating the characteristics of the moisture supply over southwest and southeast China and the comparison between them.

Based on the time scale of wind flow and moisture, the monthly vertical integral of water vapor flux is decomposed into a stationary term and a transient term, and their spatial pattern, seasonal variation, and contribution to the moisture supply over southeast and southwest China are studied by means of both water vapor flux and regional atmospheric moisture convergence. The seasonal evolution of the stationary moisture transport could represent the total moisture transport to a large extent, the evolution of which is greatly affected by the large-scale circulation over East Asia and the western North Pacific; it dominates the moisture transport to southeast and southwest China throughout the year (Fig. 5). The transient moisture transport is much weaker than the stationary transport; it transports moisture poleward from extratropical regions to midlatitude regions, with an obvious south-to-north shift as the seasons change, and its impact on the moisture supply over south China takes place mainly in spring, with strong moisture divergence over southeast China and convergence over southwest China (Fig. 6). The study of regional atmospheric moisture convergence gives a more quantitative insight into the moisture supply over southeast and southwest China. The stationary moisture transport results in a strong moisture sink over southeast China during spring and summer, as the strong moisture input via the southern boundary results in a weak moisture sink during fall and winter, and the net zonal boundary atmospheric flux is nearly offset by the output via the southern boundary. The moisture transport in the zonal direction dominates the variation in moisture supply over southwest China. The negative net zonal boundary atmospheric flux countervails (collaborates) with the positive net meridional boundary atmospheric flux in the dry (wet) season. The negative transient atmospheric moisture convergence over southeast China, mainly resulting from the intensive output via the northern boundary, is strongest in the spring and offsets nearly half of the positive stationary atmospheric moisture convergence, while positive transient atmospheric moisture convergence over southwest China, induced by intensive input via the southern and eastern boundaries, counterbalances the negative stationary atmospheric moisture convergence from late fall to spring (Fig. 7).

The moisture transport routines responsible for regional atmospheric moisture convergence anomalies over southeast and southwest China are also investigated (Figs. 8 and 9). The stationary moisture circulations dominate the regional atmospheric moisture convergence anomalies over both southeast and southwest China. Weak cold air activity is favorable for a strong moisture sink over southeast China, while energetic cold air activity is beneficial for strong moisture convergence over southwest China in spring. The east-to-west location of the abnormal anticyclone at around 18°N determines whether strong moisture convergence takes place in southeast China or southwest China in fall. The anticyclonic circulation anomaly over the Philippine Sea, which is attributed to the remote forcing of El Niño, is crucial to the strong moisture sink over southeast China from winter to spring. However, it does not play a key role in the moisture sink over southwest China. The role of transient moisture circulation is still much smaller and its variation is mainly opposite to that of the total atmospheric moisture convergence.

Unlike pressure, temperature, and wind, which are directly analyzed by the assimilation system, humidity, precipitation, and evaporation in the hydrological cycle are derived products involving many parameters that are constrained only indirectly by observation and depend on model physics and parameterizations (Berbery and Rasmusson 1999). Accurate representation of the hydrological cycle in the reanalysis dataset presents a special challenge, as approximations used in the model strongly affect the quality and consistency of the hydrological cycle. Though many aspects of the hydrological cycle in ERA-Interim are significantly improved compared to ERA-40 (Dee et al. 2011), it is necessary to examine the ability of ERA-Interim to represent the hydrological cycle over southwest and southeast China.

The monthly precipitation patterns depicted by the ERA-Interim reanalysis and observations are compared (Fig. 10). The precipitation depicted by the ERA-Interim over southwest China and the Tibetan Plateau is exaggerated during most months, with the largest difference over the southern edge of the plateau. The precipitation over the middle longitudes of east China and southeast China is also exaggerated from June to September. It is pointed out by Tong et al. (2013) that though the ERA-Interim has a better correspondence than ERA-40 with observation data at both annual and monthly scales, it still greatly overestimates observation data over regions with high terrain by 74%–290%. This great error in regions with high topography explains why the precipitation and other hydrological variables are overestimated over southwest China but not southeast China, as described in previous sections. The error in precipitation depicted by the JRA-25 reanalysis dataset is also estimated. Similar to ERA-Interim, the JRA-25 reanalysis dataset can also generate considerable error. The precipitation over the Tibetan Plateau derived from JRA-25 is exaggerated throughout the year. Compared to the errors from ERA-Interim, the magnitude of errors from JRA-25 over southeast China is much larger, especially over the coastal regions in the summer (figure not shown).

Fig. 10.
Fig. 10.

Difference between climatological monthly precipitation (mm month−1) depicted by ERA-Interim and gauge observations over China (ERA-Interim minus observations).

Citation: Journal of Climate 26, 24; 10.1175/JCLI-D-13-00057.1

Besides the discrepancies in precipitation between the reanalysis data and observations, the performance of the analysis data in the hydrological balance is also examined. The monthly variations of individual terms in the water balance equation over southwest and southeast China are presented in Fig. 11. Regional moisture convergence and local evaporation are the two main sources of moisture for precipitation. The variation in moisture divergence is highly consistent with that of precipitation. Not only is the great enhancement at the beginning and the decrease at the end of the rainfall season, but also the double peaks of rainfall in southeast China are well captured by the moisture convergence. Local change in precipitable water is much less than that of other terms, indicating its negligible role in the climatological hydrological cycle. It is important to note that, though the water balance should be close in theory, bias (which is calculated as ) can still be found in our regional atmospheric branch of the hydrological cycle. However, it is very small compared to the precipitation, evaporation, and moisture divergence. If we recognize this small bias within ERA-Interim in representing water balance, it is still reasonable to apply this reanalysis dataset to study the atmospheric branch of the hydrological cycle over south China.

Fig. 11.
Fig. 11.

Climatological annual cycle of the individual terms of the regional moisture balance equation over (a) southeast China and (b) southwest China. Individual terms include precipitation (precip), evaporation (evapr), divergence of water vapor fluxes (DivQ), temporal variation of total precipitation (deltaq), and bias.

Citation: Journal of Climate 26, 24; 10.1175/JCLI-D-13-00057.1

Acknowledgments

This research is sponsored by the Joint Project of the Natural Science Foundation of China and Yunnan Province U0833602, the National Nature Science Foundation of China Project 41175079, and the City University of Hong Kong Strategic Research Grants 7002917. We appreciate the constructive comments from three anonymous reviewers.

REFERENCES

  • Bao, M., , and R. H. Huang, 2006: Characteristics of the interdecadal variations of heavy rain over China in the last 40 years (in Chinese). Chin. J. Atmos. Sci., 30, 10571067.

    • Search Google Scholar
    • Export Citation
  • Benton, G. S., , R. T. Blackburn, , and V. O. Snead, 1950: The role of the atmosphere in the hydrologic cycle. Eos, Trans. Amer. Geophys. Union, 31, 6173.

    • Search Google Scholar
    • Export Citation
  • Berbery, E. H., , and E. M. Rasmusson, 1999: Mississippi moisture budgets on regional scales. Mon. Wea. Rev., 127, 26542673.

  • Budyko, M. I., 1974: Climate and Life. Internal Geophysics Series, Vol. 18, Academic, 508 pp.

  • Chan, J. C. L., , and W. Zhou, 2005: PDO, ENSO and the early summer monsoon rainfall over south China. Geophys. Res. Lett., 32, L08810, doi:10.1029/2004GL022015.

    • Search Google Scholar
    • Export Citation
  • Chen, H. P., , J. Q. Sun, , X. L. Chen, , and W. Zhou, 2012: CGCM projections of heavy rainfall events in China. Int. J. Climatol., 32, 441450.

    • Search Google Scholar
    • Export Citation
  • Dee, D. P., and Coauthors, 2011: The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Quart. J. Roy. Meteor. Soc., 137, 553597.

    • Search Google Scholar
    • Export Citation
  • Feng, J., , W. Chen, , C. Y. Tam, , and W. Zhou, 2011: Different impacts of El Niño and El Niño Modoki on China rainfall in the decaying phases. Int. J. Climatol., 31, 20912101.

    • Search Google Scholar
    • Export Citation
  • Fovell, R. G., , and M. Y. C. Fovell, 1993: Climate zones of the conterminous United States defined using cluster analysis. J. Climate, 6, 21032135.

    • Search Google Scholar
    • Export Citation
  • Gu, W., , C. Y. Li, , W. J. Li, , W. Zhou, , and J. C. L. Chan, 2009a: Interdecadal unstationary relationship between NAO and east China’s summer precipitation patterns. Geophys. Res. Lett., 36, L13702, doi:10.1029/2009GL038843.

    • Search Google Scholar
    • Export Citation
  • Gu, W., , C. Y. Li, , X. Wang, , W. Zhou, , and W. J. Li, 2009b: Linkage between mei-yu precipitation and North Atlantic SST on the decadal timescale. Adv. Atmos. Sci., 26, 101108.

    • Search Google Scholar
    • Export Citation
  • Huang, R. H., , Z. Zhang, , and G. Huang, 1998: Characteristics of the water vapor transport in East Asian monsoon region and its difference from that in South Asian monsoon region in summer (in Chinese). Sci. Atmos. Sin., 22, 460469.

    • Search Google Scholar
    • Export Citation
  • Huang, R. H., , Y. Liu, , L. Wang, , and L. Wang, 2012: Analyses of the causes of severe drought occurring in southwest China from the fall of 2009 to the spring of 2010 (in Chinese). Chin. J. Atmos. Sci., 36, 443457.

    • Search Google Scholar
    • Export Citation
  • Lau, N. C., , and M. J. Nath, 2000: Impact of ENSO on the variability of the Asian–Australian monsoons as simulated in GCM experiments. J. Climate, 13, 42874309.

    • Search Google Scholar
    • Export Citation
  • Li, X. Z., , and W. Zhou, 2012: Quasi-4-yr coupling between El Niño–Southern Oscillation and water vapor transport over East Asia–WNP. J. Climate, 25, 58795891.

    • Search Google Scholar
    • Export Citation
  • Li, X. Z., , Z. P. Wen, , and W. Zhou, 2011: Long-term change in summer water vapor transport over south China in recent decades. J. Meteor. Soc. Japan, 89A, 271282.

    • Search Google Scholar
    • Export Citation
  • Li, X. Z., , Z. P. Wen, , W. Zhou, , and D. X. Wang, 2012: Atmospheric water vapor transport associated with two decadal rainfall shifts over east China. J. Meteor. Soc. Japan, 90, 587602.

    • Search Google Scholar
    • Export Citation
  • Lin, A. L., , J. Y. Liang, , C. H. Li, , D. J. Gu, , and B. Zheng, 2007: Monsoon circulation background of ‘0506’ continuous rainstorm in south China. Adv. Water Resour., 18, 424432.

    • Search Google Scholar
    • Export Citation
  • McKee, T. B., , N. J. Doesken, , and J. Kleist, 1993: The relationship of drought frequency and duration to time scales. Preprints, Eighth Conf. on Applied Climatology, Anaheim, CA, Amer. Meteor. Soc., 179184.

  • Onogi, K. J. T., and Coauthors, 2007: The JRA-25 Reanalysis. J. Meteor. Soc. Japan, 85, 369432.

  • Schmitz, J. T., , and S. L. Mullen, 1996: Water vapor transport associated with the summertime North American monsoon as depicted by ECMWF analyses. J. Climate, 9, 16211634.

    • Search Google Scholar
    • Export Citation
  • Simmonds, I., , D. H. Bi, , and P. Hope, 1999: Atmospheric water vapor flux and its association with rainfall over China in summer. J. Climate, 12, 13531367.

    • Search Google Scholar
    • Export Citation
  • Sun, B., , and H. J. Wang, 2013: Water vapor transport paths and accumulation during widespread snowfall events in northeastern China. J. Climate, 26, 45504566.

    • Search Google Scholar
    • Export Citation
  • Tao, S. Y., , and Y. H. Ding, 1981: Observational evidence of the influence of the Qinghai-Xizang (Tibetan) Plateau on the occurrence of heavy rain and severe convective storms in China. Bull. Amer. Meteor. Soc., 62, 2330.

    • Search Google Scholar
    • Export Citation
  • Tao, S. Y., , and L. X. Chen, 1987: A review of recent research on the East Asian summer monsoon in China. Review of Monsoon Meteorology, C. P. Chang and T. N. Krishnamurti, Eds., Oxford University Press, 60–92.

  • Tibshirani, R., , G. Walther, , and T. Hastie, 2001: Estimating the number of clusters in a data set via the gap statistic. J. Roy. Stat. Soc., 63, 411423.

    • Search Google Scholar
    • Export Citation
  • Tong, K., , F. G. Su, , D. Q. Yang, , L. L. Zhang, , and Z. C. Hao, 2013: Tibetan Plateau precipitation as depicted by gauge observations, reanalyses and satellite retrievals. Int. J. Climatol., doi:10.1002/joc.3682, in press.

    • Search Google Scholar
    • Export Citation
  • Trenberth, K. E., 1991: Climate diagnostics from global analyses: Conservation of mass in ECMWF analyses. J. Climate, 4, 707722.

  • Trenberth, K. E., , A. Dai, , R. M. Rasmussen, , and D. B. Parsons, 2003: The changing character of precipitation. Bull. Amer. Meteor. Soc., 84, 12051217.

    • Search Google Scholar
    • Export Citation
  • Wang, B., , and Y. Q. Li, 2010: Relationship analysis between south branch trough and severe drought of southwest China during autumn and winter 2009/2020 (in Chinese). Plateau Mt. Meteor. Res., 30, 2837.

    • Search Google Scholar
    • Export Citation
  • Wang, B., , R. G. Wu, , and X. H. Fu, 2000: Pacific–East Asian teleconnection: How does ENSO affect East Asian climate? J. Climate, 13, 15171536.

    • Search Google Scholar
    • Export Citation
  • Wang, H. J., , and H. P. Chen, 2012: Climate control for southeastern China moisture and precipitation: Indian or East Asian monsoon? J. Geophys. Res., 117, D12109, doi:10.1029/2012JD017734.

    • Search Google Scholar
    • Export Citation
  • Wang, X., , C. Y. Li, , and W. Zhou, 2006: Interdecadal variation of the relationship between Indian rainfall and SSTA modes in the Indian Ocean. Int. J. Climatol., 26, 595606.

    • Search Google Scholar
    • Export Citation
  • Webster, P., , and S. Yang, 1992: Monsoon and ENSO: Selectively interactive systems. Quart. J. Roy. Meteor. Soc., 118, 877926.

  • Xie, Y. B., , and W. J. Dai, 1959: The calculation for some cases of the moisture transport over Yangtze River basin. J. Appl. Meteor. Sci., 13, 6777.

    • Search Google Scholar
    • Export Citation
  • Yi, L., , and S. Y. Tao, 1997: Role of the standing and the transient eddies in atmospheric water cycle in the Asian monsoon region (in Chinese). Acta Meteor. Sin., 55, 533544.

    • Search Google Scholar
    • Export Citation
  • Yihui, D., 1994: Monsoons over China. Springer, 432 pp.

  • Yuan, F., , W. Chen, , and W. Zhou, 2012: Analysis of the role played by circulation in the persistent precipitation over south China in June 2010. Adv. Atmos. Sci., 29, 769781.

    • Search Google Scholar
    • Export Citation
  • Yuan, Y., , H. Yang, , W. Zhou, , and C. Y. Li, 2008a: Influences of the Indian Ocean dipole on the Asian summer monsoon in the following year. Int. J. Climatol., 28, 18491859, doi:10.1002/JOC.1678.

    • Search Google Scholar
    • Export Citation
  • Yuan, Y., , W. Zhou, , J. C. L. Chan, , and C. Y. Li, 2008b: Impacts of the basin-wide Indian Ocean SSTA on the South China Sea summer monsoon onset. Int. J. Climatol., 28, 15791587, doi:10.1002/JOC.1671.

    • Search Google Scholar
    • Export Citation
  • Zhang, Q. L., , L. Wu, , and Q. Liu, 2009: Tropical cyclone damages in China 1983–2006. Bull. Amer. Meteor. Soc., 90, 485495.

  • Zhang, R. H., 2001: Relations of water vapor transport from Indian monsoon with that over East Asia and the summer rainfall in China. Adv. Atmos. Sci., 18, 10051017.

    • Search Google Scholar
    • Export Citation
  • Zhang, R. H., , A. Sumi, , and M. Kimoto, 1996: Impact of El Niño on the East Asian monsoon: A diagnostic study of the 86/87 and 91/92 events. J. Meteor. Soc. Japan, 74, 4962.

    • Search Google Scholar
    • Export Citation
  • Zhou, T. J., , and R. C. Yu, 2005: Atmospheric water vapor transport associated with typical anomalous summer rainfall patterns in China. J. Geophys. Res., 110, D08104, doi:10.1029/2004JD005413.

    • Search Google Scholar
    • Export Citation
  • Zhou, W., , and J. C. L. Chan, 2007: ENSO and South China Sea summer monsoon onset. Int. J. Climatol., 27, 157167.

  • Zhou, W., , J. C. L. Chan, , and C. Y. Li, 2005: South China Sea summer monsoon onset in relation to the off-equatorial ITCZ. Adv. Atmos. Sci., 22, 665676.

    • Search Google Scholar
    • Export Citation
  • Zhou, W., , C. Y. Li, , and J. C. L. Chan, 2006: The interdecadal variations of the summer monsoon rainfall over south China. Meteor. Atmos. Phys., 93, 165175, doi:10.1007/S00703-006-0184-9.

    • Search Google Scholar
    • Export Citation
  • Zhu, Y. L., , H. J. Wang, , W. Zhou, , and J. H. Ma, 2011: Recent changes in the summer precipitation pattern in east China and the background circulation. Climate Dyn., 36, 14631473, doi:10.1007/s00382-010-0852-9.

    • Search Google Scholar
    • Export Citation
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