Abrupt Flood–Drought Alternation in Southern China during Summer 2019

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  • Author Bio: Ding, Ting dingting@cma.gov.cn
  • Corresponding author: Hui GAO, gaohui@cma.gov.cn
  • Funds:

    Supported by the National (Key) Basic Research and Development Program of China (2018YFC1505603) and the National Natural Science Foundation of China (U1902209, 41776039, and 41205039).

  • doi: 10.1007/s13351-021-1073-3
  • Note: This paper will appear in the forthcoming issue. It is not the finalized version yet. Please use with caution.

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  • We investigated the abrupt alternation from flood to drought in southern China during summer 2019 using multiple datasets. Positive anomalies of precipitation occurred in southern China in the summer of 2019 and the daily precipitation in the Jiangnan area south of the mid- and lower reaches of the Yangtze River valley showed an abrupt change from flood to drought conditions around mid-July. The highest precipitation in 39 years was recorded between 1 June and 14 July, 2019. The circulation systems affecting this high precipitation included a persistent deepened East Asian trough, the southward location of the western Pacific subtropical high, an intensified East Asian subtropical jet, an anomalous low-level cyclone from southern Japan to southern China and extremely strong positive vorticity over the Jiangnan area. Completely different atmospheric circulation anomalies from 15 July to 31 August caused continuously high temperatures, below-normal precipitation and severe drought in Jiangnan. Further investigations showed that the sudden change in atmospheric circulation around mid-July started in the mid and lower troposphere and was influenced by the northward track of tropical cyclone Danas in the northwestern Pacific.
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  • Fig. 1.  (a) Daily variation of precipitation (units: mm) averaged in four provinces (Hunan, Jiangxi, Fujian, and Zhejiang) of the Jiangnan area, with the blue line representing 2019 and the red line for the climatological mean. (b) Time–latitude profile of the station maximum temperature averaged in eastern China (110–120° E). The dashed box indicates the Jiangnan area. Only values ≥35oC are shown (units: °C).

    Fig. 2.  Distribution of the anomalous percentage of precipitation in (a) 1 June–14 July (P1) and (c) July 15–August 31 (P2) 2019 (units: %). Time series of the mean precipitation in the four provinces of Jiangnan in (b) P1 and (d) P2 from 1981 to 2019, with blue bars for each year (red bar for 2019) and red line representing the climatological mean (units: mm).

    Fig. 3.  Daily running long-cycle drought–flood abrupt alternation index (LDFAI, according to equation 1) averaged in the four provinces (Hunan, Jiangxi, Fujian, and Zhejiang) of the Jiangnan area during June–July–August of each year from 1981 to 2019, with yellow (blue) indicating drought–flood alternation.

    Fig. 4.  Circulations in P1 of summer 2019. (a) Mean (contours) and anomalous (shading) 500 hPa geopotential height (red solid contour for the 5880 gpm contour in 2019 and red dashed contour for the climatological mean; units: gpm). (b) 850 hPa anomalous winds (vectors; units: m s−1) and vorticity (shading; units: 10−6 s−1). (c) Anomalous integrated moisture flux (vectors; units: kg (s m)−1) and divergence (shading; units: 10−5 kg (s m2)−1 from the surface to 300 hPa. (d) Mean (contours) and anomalous (shading) 200 hPa zonal winds (red dashed contour for the maximum zonal wind speed center (≥30 m s−1) of the climatological mean; units: m s−1), with the black box for the Jiangnan area (25–30° N, 105–122° E).

    Fig. 5.  (a) Time–latitude profile of the mean (contours) and anomalous (shading) 500 hPa geopotential height averaged over 125–135° E from June 1 to August 31 in 2019. The dashed box indicates the persistently intensified trough within 30–40° N. (b) Time–longitude profile of the 500 hPa geopotential height averaged over 25–30° N. (c) Time–latitude profile averaged over 110–130° E in summer 2019 (blue shading with black contours) and the climatological mean (red contours). Only values ≥5860 gpm are displayed to show the eastward and westward movement (in part (b)) and the northward and southward swing (in part (c)) of the WPSH (units: gpm).

    Fig. 6.  (a) Daily variation of the anomalous precipitation in the four provinces (highlighted in Fig. 2a) of Jiangnan (bars; units: mm) and 850 hPa anomalous vorticity in Jiangnan (black box in Fig. 3b; black line; units: 10−6 s−1). (b) Time series of the normalized precipitation averaged in the four provinces of Jiangnan (bars) and the normalized 850 hPa vorticity averaged in Jiangnan (black line) in P1 from 1981 to 2019.

    Fig. 7.  Circulations in P2 in summer 2019. (a) Mean (contours) and anomalous (shading) 500 hPa geopotential height (red solid contour for the 5880 gpm contour in 2019 and red dashed contour for the climatological mean; units: gpm). (b) 850 hPa anomalous winds (vectors; units: m s−1) and vorticity (shading; units: 10−6 s−1). (c) Anomalous integrated moisture flux (vectors; units: kg (s m)−1) and divergence (shading; units: 10−5 kg (s m2)−1 from the surface to 300 hPa. (d) Mean (contours) and anomalous (shading) 200 hPa zonal winds (red dashed contour for the maximum zonal wind speed center (≥30 m s−1) of the climatological mean; units: m s−1), with the black box for the Jiangnan area (25–30° N, 105–122° E).

    Fig. 8.  (a) Time–height profile of the normalized geopotential height averaged over eastern East Asia (30–40° N, 125–135° E) from June 1 to August 31, 2019. (b) Daily variation of the 850 hPa (black line; left-hand y-axis), 500 hPa (red line; left-hand y-axis); and 200 hPa (blue line; right-hand y-axis) geopotential height anomaly averaged in eastern East Asia (30–40° N, 125–135° E) in July 2019. Units: gpm.

    Fig. 9.  Anomalous 850 hPa winds (vectors; units: m s−1) and mean (contours) and anomalous (shading) 500 hPa geopotential height (units: gpm) on (a) July 13, (b) July 14, (c) July 15, and (d) July 16 2019. The red line represents the 5880 gpm contour and the red dot is roughly the center of tropical cyclone Danas.

    Fig. 10.  Daily locations of tropical cyclone Danas (dots) and the daily mean 5880 gpm contours representing the WPSH (a) from July 14 to 17, 2019 and (b) from July 18 to 20, 2019.

  • [1]

    Chang, C. P., and G. T. J. Chen, 1995: Tropical circulations associated with Southwest monsoon onset and westerly surges over the South China Sea. Mon. Wea. Rev., 123, 3254–3267. doi: 10.1175/1520-0493(1995)123<3254:tcawsm>2.0.co;2.
    [2]

    Chen, G. T. J., 1994: Large-scale circulations associated with the East Asian summer monsoon and the Mei-Yu over South China and Taiwan. J. Meteor. Soc. Japan, 72, 959–983. doi: 10.2151/jmsj1965.72.6_959.
    [3]

    Chen, L. J., Y. Yuan, M. Z. Yang, et al., 2013: A review of physical mechanisms of the global SSTA impact on EASM. J. Appl. Meteor. Sci., 24, 521–532. doi: 10.3969/j.issn.1001-7313.2013.05.002. (in Chinese)
    [4]

    Chen, X. F., and Z. G. Zhao, 2000: Study on Precipitation Forecast in China during Flood Season and its Applications. China Meteorological Press, Beijing. (查阅所有网上资料,未找到本条文献英文翻译,请联系作者确认). (in Chinese)
    [5]

    Dai, A. G., K. E. Trenberth, and T. R. Karl, 1998: Global variations in droughts and wet spells: 1900–1995. Geophys. Res. Lett., 25, 3367–3370. doi: 10.1029/98GL52511.
    [6]

    Ding, T., Y. Yuan, J. M. Zhang, et al., 2019: 2018: The hottest summer in China and possible causes. J. Meteor. Res., 33, 577–592. doi: 10.1007/s13351-019-8178-y.
    [7]

    Ding, Y. H., 1992: Summer monsoon rainfalls in China. J. Meteor. Soc. Japan, 70, 373–396. doi: 10.2151/jmsj1965.70.1B_373.
    [8]

    Dou, J., Z. W. Wu, and J. P. Li, 2020: The strengthened relationship between the Yangtze River Valley summer rainfall and the Southern Hemisphere annular mode in recent decades. Climate Dyn., 54, 1607–1624. doi: 10.1007/s00382-019-05078-4.
    [9]

    Feng, G. L., H. W. Yang, S. X. Zhang, et al., 2012: A preliminary research on the reason of a sharp turn from drought to flood in the middle and lower reaches of the Yangtze River in late spring and early summer of 2011. Chinese J. Atmos. Sci., 36, 1009–1026. doi: 10.3878/j.issn.1006-9895.2012.11220. (in Chinese)
    [10]

    Hirata, H., and R. Kawamura, 2014: Scale interaction between typhoons and the North Pacific subtropical high and associated remote effects during the Baiu/Meiyu season. J. Geophys. Res. Atmos., 119, 5157–5170. doi: 10.1002/2013JD021430.
    [11]

    Hu, Z. Z., 1997: Interdecadal variability of summer climate over East Asia and its association with 500 hPa height and global sea surface temperature. J. Geophys. Res. Atmos., 102, 19403–19412. doi: 10.1029/97JD01052.
    [12]

    Hu, Z. Z., S. Yang, and R. G. Wu, 2003: Long-term climate variations in China and global warming signals. J. Geophys. Res. Atmos., 108, 4614. doi: 10.1029/2003JD003651.
    [13]

    Huang, R. H., Q. Y. Guo, and A. J. Sun, 1997: Seasonal Charts of Climate Disasters in China (1951~1990). Ocean Press, Beijing, 124 pp. (in Chinese)
    [14]

    Jin, R., Z. W. Wu, and P. Zhang, 2018: Tibetan Plateau Capacitor Effect during the summer preceding ENSO: From the Yellow River climate perspective. Climate Dyn., 51, 57–71. doi: 10.1007/s00382-017-3906-4.
    [15]

    Kalnay, E., M. Kanamitsu, R. Kistler, et al., 1996: The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteor. Soc., 77, 437–471. doi: 10.1175/1520-0477(1996)077<0437:tnyrp>2.0.co;2.
    [16]

    Kistler, R., E. Kalnay, W. Collins, et al., 2001: The NCEP-NCAR 50-year reanalysis: Monthly means CD-ROM and documentation. Bull. Amer. Meteor. Soc., 82, 247–268. doi: 10.1175/1520-0477(2001)082<0247:TNNYRM>2.3.CO;2.
    [17]

    Lau, K. M., and S. Yang, 1997: Climatology and interannual variability of the Southeast Asian summer monsoon. Adv. Atmos. Sci., 14, 141–162. doi: 10.1007/s00376-997-0016-y.
    [18]

    Lau, K. M., G. J. Yang, and S. H. Shen, 1988: Seasonal and intraseasonal climatology of summer monsoon rainfall over East Asia. Mon. Wea. Rev., 116, 18–37. doi: 10.1175/1520-0493(1988)116<0018:SAICOS>2.0.CO;2.
    [19]

    Li, J., Y. Yuan, Z. Y. Wang, et al., 2020: Evolution characteristics of continuous drought in late summer and autumn in the middle and lower reaches of Yangtze River Valley in 2019. Meteor. Mon., 46, 1641–1650. doi: 10.7519/j.issn.1000-0526.2020.12.011. (in Chinese)
    [20]

    Li, M., C. W. Zhu, and Y. S. Pang, 2014: Possible causes of abrupt turning from drought to flooding in the middle and lower reaches of Yangtze River Valley during spring to summer of 2011. J. Meteor. Environ., 30, 70–78. doi: 10.3969/j.issn.1673-503X.2014.03.010. (in Chinese)
    [21]

    Li, W. J., 1999: General atmospheric circulation anomaly in 1998 and their impact on climate anomaly in China. Meteor. Mon., 25, 20–25. doi: 10.3969/j.issn.1000-0526.1999.04.004. (in Chinese)
    [22]

    Miao, R., M. Wen, and R. H. Zhang, 2017: Persistent precipitation anomalies and quasi-biweekly oscillation during the annually first rainy season over South China in 2010. J. Trop. Meteor., 33, 155–166. doi: 10.16032/j.issn.1004-4965.2017.02.002. (in Chinese)
    [23]

    Nan, S. L., and J. P. Li, 2003: The relationship between the summer precipitation in the Yangtze River valley and the boreal spring Southern Hemisphere annular mode. Geophys. Res. Lett., 30, 2266. doi: 10.1029/2003GL018381.
    [24]

    Qiu, S., and W. Zhou, 2019: Variation in summer rainfall over the Yangtze River region during warming and hiatus periods. Atmosphere, 10, 173. doi: 10.3390/atmos10040173.
    [25]

    Ren, Z. H., Y. Yu, F. L. Zou, et al., 2012: Quality detection of surface historical basic meteorological data. J. Appl. Meteor. Sci., 23, 739–747. doi: 10.3969/j.issn.1001-7313.2012.06.011. (in Chinese)
    [26]

    Shen, B. Z., S. X. Zhang, H. W. Yang, et al., 2012: Analysis of characteristics of a sharp turn from drought to flood in the middle and lower reaches of the Yangtze River in spring and summer in 2011. Acta Phys. Sinica, 61, 109201. doi: 10.7498/aps.61.109202. (in Chinese)
    [27]

    Shen, H. B., S. P. He, and H. J. Wang, 2019: Effect of summer Arctic sea ice on the reverse August precipitation anomaly in eastern China between 1998 and 2016. J. Climate, 32, 3389–3407. doi: 10.1175/JCLI-D-17-0615.1.
    [28]

    Si, D., Y. J. Liu, L. J. Ma, et al., 2012: Climatic characteristics and cause analysis of precipitation over the middle and lower reaches of the Yangtze River Valley during early summer of 2011. Meteor. Mon., 38, 601–607. doi: 10.7519/j.issn.1000-0526.2012.5.011. (in Chinese)
    [29]

    Tao, S. Y., and L. X. Chen, 1987: A review of recent research on the East Asian summer monsoon in China. Monsoon Meteorology, C.–92.
    [30]

    Tong, J., and H. M. Xu, 2014: Effects of multi-scale low frequency oscillations on the drought-flood abrupt transition over the middle and lower reaches of the Yangtze River. J. Trop. Meteor., 30, 707–718. doi: 10.3969/j.issn.1004-4965.2014.04.011. (in Chinese)
    [31]

    Wang, B., 1994: Climatic regimes of tropical convection and rainfall. J. Climate, 7, 1109–1118. doi: 10.1175/1520-0442(1994)007<1109:CROTCA>2.0.CO;2.
    [32]

    Wang, B., and X. H. Xu, 1997: Northern Hemisphere summer monsoon singularities and climatological intraseasonal oscillation. J. Climate, 10, 1071–1085. doi: 10.1175/1520-0442(1997)010<1071:NHSMSA>2.0.CO;2.
    [33]

    Wang, H. J., 2001: The weakening of the Asian monsoon circulation after the end of 1970’s. Adv. Atmos. Sci., 18, 376–386. doi: 10.1007/BF02919316.
    [34]

    Wei, W., R. H. Zhang, M. Wen, et al., 2015: Interannual variation of the South Asian High and its relation with Indian and East Asian summer monsoon Rainfall. J. Climate, 28, 2623–2634. doi: 10.1175/JCLI-D-14-00454.1.
    [35]

    Wen, M., J. J. Luo, and J. H. He, 1997: Air-sea interaction and dry and wet years. J. Nanjing Inst. Meteor., 20, 341–347. (in Chinese)
    [36]

    Wen, Y. R., L. Xue, Y. Li, et al., 2015: Interaction between Typhoon Vicente (1208) and the western Pacific subtropical high during the Beijing extreme rainfall of 21 July 2012. J. Meteor. Res., 29, 293–304. doi: 10.1007/s13351-015-4097-8.
    [37]

    Wu, B. Y., R. H. Zhang, and B. Wang, 2009: On the association between spring Arctic sea ice concentration and Chinese summer rainfall: A further study. Adv. Atmos. Sci., 26, 666–678. doi: 10.1007/s00376-009-9009-3.
    [38]

    Wu, Z. W., J. P. Li, J. H. He, et al., 2006a: Occurrence of droughts and floods during the normal summer monsoons in the mid- and lower reaches of the Yangtze River. Geophys. Res. Lett., 33, L05813. doi: 10.1029/2005GL024487.
    [39]

    Wu, Z. W., J. P. Li, J. H. He, et al., 2006b: Large-scale atmospheric singularities and summer long-cycle droughts-floods abrupt alternation in the middle and lower reaches of the Yangtze River. Chinese Sci. Bull., 51, 2027–2034. doi: 10.1007/s11434-006-2060-x.
    [40]

    Xie, J., and N. F. Zhou, 2019: Analysis of the July 2019 atmospheric circulation and weather. Meteor. Mon., 45, 1494–1500. doi: 10.7519/j.issn.1000-0526.2019.10.016. (in Chinese)
    [41]

    Xie, S. P., K. M. Hu, J. Hafner, et al., 2009: Indian Ocean capacitor effect on Indo-western Pacific climate during the summer following El Niño. J. Climate, 22, 730–747. doi: 10.1175/2008JCLI2544.1.
    [42]

    Yang, J. L., Q. Y. Liu, S. P. Xie, et al., 2007: Impact of the Indian Ocean SST basin mode on the Asian summer monsoon. Geophys. Res. Lett., 34, L02708. doi: 10.1029/2006GL028571.
    [43]

    Yang, S. Y., B. Y. Wu, R. H. Zhang, et al., 2013: Relationship between an abrupt drought-flood transition over mid-low reaches of the Yangtze River in 2011 and the intraseasonal oscillation over mid-high latitudes of East Asia. Acta Meteor. Sinica, 27, 129–143. doi: 10.1007/s13351-013-0201-0.
    [44]

    Ye, X. C., and Z. W. Wu, 2018: Contrasting impacts of ENSO on the interannual variations of summer runoff between the upper and mid-lower reaches of the Yangtze River. Atmosphere, 9, 478. doi: 10.3390/atmos9120478.
    [45]

    Ye, X. C., Z. W. Wu, Z. M. Wang, et al., 2018: Seasonal prediction of the Yangtze River runoff using a partial least squares regression model. Atmos.–Ocean, 56, 117–128. doi: 10.1080/07055900.2018.1448751.
    [46]

    Ying, M., W. Zhang, H. Yu, et al., 2014: An overview of the China Meteorological Administration tropical cyclone database. J. Atmos. Oceanic Technol., 31, 287–301. doi: 10.1175/JTECH-D-12-00119.1.
    [47]

    Yuan, Y., S. Yang, and Z. Q. Zhang, 2012: Different evolutions of the Philippine Sea anticyclone between the Eastern and Central Pacific El Niño: Possible effects of Indian Ocean SST. J. Climate, 25, 7867–7883. doi: 10.1175/JCLI-D-12-00004.1.
    [48]

    Yuan, Y., H. Gao, X. L. Jia, et al., 2016: Influences of the 2014–2016 super El Niño event on climate. Meteor. Mon., 42, 532–539. doi: 10.7519/j.issn.1000-0526.2016.05.002. (in Chinese)
    [49]

    Yuan, Y., H. Gao, W. J. Li, et al., 2017: The 2016 summer floods in China and associated physical mechanisms: A comparison with 1998. J. Meteor. Res., 31, 261–277. doi: 10.1007/s13351-017-6192-5.
    [50]

    Yuan, Y., H. Gao, and T. Ding, 2020: The extremely north position of the western Pacific subtropical high in summer of 2018: Important role of the convective activities in the western Pacific. Int. J. Climatol., 40, 1361–1374. doi: 10.1002/joc.6274.
    [51]

    Zhai, P. M., R. Yu, Y. J. Guo, et al., 2016: The strong El Niño of 2015/16 and its dominant impacts on global and China's climate. J. Meteor. Res., 30, 283–297. doi: 10.1007/s13351-016-6101-3.
    [52]

    Zhang, J. M., T. Ding, and H. Gao, 2021: Record-breaking high temperature in Southern China in 2017 and influence from the middle-latitude trough over the East of Japan. Atmos. Res., 258, 105615. doi: 10.1016/j.atmosres.2021.105615.
    [53]

    Zhang, P., Z. W. Wu, and R. Jin, 2021: How can the winter North Atlantic Oscillation influence the early summer precipitation in Northeast Asia: Effect of the Arctic sea ice. Climate Dyn., 56, 1989–2005. doi: 10.1007/s00382-020-05570-2.
    [54]

    Zhang, Q. Y., S. L. Xuan, and S. Q. Sun, 2018: Anomalous circulation characteristics of intraseasonal variation of East Asian subtropical westerly jet in summer and precursory signals. Chinese J. Atmos. Sci., 42, 935–950. doi: 10.3878/j.issn.1006-9895.1803.18107. (in Chinese)
    [55]

    Zhao, Z. G., 1999: Summer Drought and Flood in China and the Circulation Patterns. China Meteorological Press, Beijing, 297 pp. (查阅所有网上资料,未找到本条文献英文翻译,请联系作者确认). (in Chinese)
    [56]

    Zhou, G. B., and S. Z. Gao, 2019: Analysis of the August 2019 atmospheric circulation and weather. Meteor. Mon., 45, 1621–1628. doi: 10.7519/j.issn.1000-0526.2019.11.012. (in Chinese)
    [57]

    Zhou, W., C. Li, and J. C. L. Chan, 2006: The interdecadal variations of the summer monsoon rainfall over South China. Meteor. Atmos. Phys., 93, 165–175. doi: 10.1007/s00703-006-0184-9.
    [58]

    Zhu, C. W., B. Q. Liu, Z. Y. Zuo, et al., 2019: Recent advances on sub-seasonal variability of East Asian summer monsoon. J. Appl. Meteor. Sci., 30, 401–415. doi: 10.11898/1001-7313.20190402. (in Chinese)
    [59]

    Zhu, X. Y., J. H. He, and Z. W. Wu, 2007: Meridional seesaw-like distribution of the Meiyu rainfall over the Changjiang-Huaihe River Valley and characteristics in the anomalous climate years. Chinese Sci. Bull., 52, 2420–2428. doi: 10.1007/s11434-007-0280-3.
    [60]

    Zong, H. F., and Q. Y. Zhang, 2011: A New precipitation index for the spatiotemporal distribution of drought and flooding in the reaches of the Yangtze and Huaihe Rivers and related characteristics of atmospheric circulation. Adv. Atmos. Sci., 28, 375–386. doi: 10.1007/s00376-010-9223-z.
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Abrupt Flood–Drought Alternation in Southern China during Summer 2019

    Corresponding author: Hui GAO, gaohui@cma.gov.cn
  • National Climate Center, China Meteorological Administration, Beijing 100081
Funds: Supported by the National (Key) Basic Research and Development Program of China (2018YFC1505603) and the National Natural Science Foundation of China (U1902209, 41776039, and 41205039).

Abstract: We investigated the abrupt alternation from flood to drought in southern China during summer 2019 using multiple datasets. Positive anomalies of precipitation occurred in southern China in the summer of 2019 and the daily precipitation in the Jiangnan area south of the mid- and lower reaches of the Yangtze River valley showed an abrupt change from flood to drought conditions around mid-July. The highest precipitation in 39 years was recorded between 1 June and 14 July, 2019. The circulation systems affecting this high precipitation included a persistent deepened East Asian trough, the southward location of the western Pacific subtropical high, an intensified East Asian subtropical jet, an anomalous low-level cyclone from southern Japan to southern China and extremely strong positive vorticity over the Jiangnan area. Completely different atmospheric circulation anomalies from 15 July to 31 August caused continuously high temperatures, below-normal precipitation and severe drought in Jiangnan. Further investigations showed that the sudden change in atmospheric circulation around mid-July started in the mid and lower troposphere and was influenced by the northward track of tropical cyclone Danas in the northwestern Pacific.

    • The mid- and lower reaches of the Yangtze River valley (MLRYRV for short) and the regions to the south, generally referred to as southern China, are an important food-producing area and the center of economic development in China. Drought and flood disasters there can often cause major losses to people’s lives and the economy (Huang et al., 1997). As one of the worst areas for drought and flood disasters, the causes and prediction of droughts and floods in southern China are important topics in short-term climate research (Wen et al., 1997; Dai et al., 1998; Wu et al., 2006a, b).

      Influenced by the advance and retreat of the East Asian Summer Monsoon (EASM; e.g. Tao and Chen, 1987; Lau et al., 1988; Ding, 1992; Wang, 1994; Chang and Chen, 1995; Lau and Yang, 1997; Zhu et al., 2007), severe droughts and floods in southern China are associated with anomalies in the large-scale atmospheric circulation (e.g. Chen, 1994; Wang and Xu, 1997; Nan and Li, 2003; Wei et al., 2015; Qiu and Zhou, 2019). Important external forcing factors for these droughts and floods include air–sea interactions in the tropical Indian Ocean (e.g. Yang et al., 2007; Xie et al., 2009; Yuan et al., 2012; Chen et al., 2013) and the El Niño Southern Oscillation (e.g., Zhao, 1999; Chen and Zhao, 2000; Zhou et al., 2006; Yuan et al., 2016, 2017; Zhai et al., 2016; Ye and Wu, 2018; Ye et al., 2018), as well as land surface thermal processes, such as sea ice cover in the Arctic and snow cover over the Qinghai–Tibetan Plateau or the Eurasian continent (e.g. Li, 1999; Zhao, 1999; Chen and Zhao, 2000; Jin et al., 2018; Dou et al., 2020, Zhang P. et al., 2021). These studies have significantly improved our understanding of the causes and mechanisms of droughts and floods in the MLRYRV and its southern area and are of great importance in short-term climate predictions.

      The majority of previous researches on severe droughts and floods in this region have emphasized the seasonal mean rainfall. However, the sub-seasonal variation of summer rainfall is also important (Wu et al., 2006a, b; Zhang et al., 2018) and the recent rapid developments in society have led to disasters caused by uneven seasonal precipitation, which received increasing attention. The coexistence of droughts and floods over a period of time or a rapid change between drought and flood within a single season can have serious adverse societal effects (Wu et al., 2006a, b; Tong and Xu, 2014).

      A recent example is the sharp change from drought to flood in the MLRYRV in early June 2011. Precipitation in the MLRYRV was persistently below normal from January to May 2011, leading to the worst winter–spring drought in recent 60 years. However, several heavy precipitation processes in the MLRYRV starting in early June caused rapid changes from drought to flood in some areas. More than 10 rivers experienced floods with an excess water level (Si et al., 2012). Many researchers investigated the possible causes for this sharp change from drought to flood and found that it was mainly influenced by the summer monsoon in the South China Sea and the EASM, an abrupt northward shift of the western Pacific subtropical high (WPSH), enhancement of the East Asian trough (EAT), the thermal effect of Qinghai–Tibetan Plateau, weakening of the blocking high over the Okhotsk Sea, and 10–20 and 30–60 day low-frequency oscillations (e.g. Feng et al., 2012; Shen et al., 2012; Si et al., 2012; Yang et al., 2013; Li et al., 2014; Tong and Xu, 2014). Zhu et al. (2019) summarized the advances in understanding the sub-seasonal variability of the EASM and suggested that its potential predictability depends on phase-locking between the sub-seasonal variability and the seasonal cycle of the EASM.

      Drought–flood alternations are not uncommon, but usually have a different intensity. However, a sharp change from flood to drought is very rare. In the summer of 2019, an abrupt flood–drought change occurred in the Jiangnan area south of the MLRYRV. From June to the first half of July 2019, five heavy precipitation processes affected southern China and, as a result, at least eight provinces (including Zhejiang, Fujian, Jiangxi, Guizhou, and Hunan) were affected by floods, hail, landslides, debris flows and other disasters. However, from mid-July until November 2019, a severe meteorological drought occurred in several provinces (Li et al., 2020), leading to significant adverse impacts on agriculture, ecology and water resources.

      We therefore analyzed the characteristics of the abrupt flood–drought alternation in southern China during the summer of 2019 and investigated the possible effects of the atmospheric circulation on this sharp change. This study will help our understanding of abrupt alternations between floods and droughts and their physical mechanisms.

    2.   Data and methodology
    • Daily station precipitation data from 1951 to the present day were extracted from the China National Surface Weather Stations Basic Meteorological Observation Dataset (V3.0) released by the National Meteorological Information Center of China (Ren et al., 2012). A description of the dataset is given in Ding et al. (2019). We focused on the daily station temperature and precipitation in southern China south of MLRYRV (also called the Jiangnan area). This region covers four provinces (Hunan, Jiangxi, Fujian, and Zhejiang) and 324 stations based on the location of the maximum precipitation in summer (June–July–August) 2019.

      The atmospheric circulation data were obtained from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) daily reanalysis product (Kalnay et al., 1996; Kistler et al., 2001) and included the geopotential height, wind, relative humidity, and vertical velocity at various vertical levels. The horizontal resolution of the data is 2.5° latitude × 2.5° longitude.

      The tracks of tropical cyclones in 2019 were obtained from the datasets of the China Meteorological Administration tropical cyclone optimal tracks (Ying et al., 2014) and the National Meteorological Center (http://typhoon.nmc.cn/web.html).

      Considering the quality of the data and the interdecadal scale transition since the late 1970s (Hu, 1997; Wang, 2001; Hu et al., 2003), we mainly focused on the time period 1981–2019 for all the datasets. However, the correlation and composite analyses were computed over the time period 1981–2018 to avoid the impact of the special case in 2019. According to the World Meteorological Organization standard, the climate normal is the average of a climatological variable over the latest three decades (i.e., 1981–2010). The anomalies of all the variables were therefore derived based on the climate mean of 1981–2010. Radiosonde data were also used in the daily synoptic analysis for comparison with the NCEP/NCAR dataset and similar results were obtained.

    3.   Features of summer precipitation in southern China
    • Positive anomalies of precipitation occurred in southern China in the summer of 2019. The precipitation was 20%–50% above normal, especially in the Jiangnan area, with precipitation in some regions >50% above normal (figures not shown). The daily precipitation in four provinces (Hunan, Jiangxi, Fujian and Zhejiang) in Jiangnan showed significant sub-seasonal variations (Fig. 1a). Five heavy precipitation processes occurred in Jiangnan in June and the first half of July ( 6–13 June, 20–25 June, 3 June– 5 July, 6–9 July and 12–14 July), which led to a mean precipitation 20%–100% above normal and even >100% above normal in some regions (Fig. 2a). The area-averaged precipitation in Jiangnan from 1 June to 14 July was 539.27 mm in 2019, 65.5% more than the climatological mean (325.78 mm) and the highest since 1981 (Fig. 2b; also the highest since 1951).

      Figure 1.  (a) Daily variation of precipitation (units: mm) averaged in four provinces (Hunan, Jiangxi, Fujian, and Zhejiang) of the Jiangnan area, with the blue line representing 2019 and the red line for the climatological mean. (b) Time–latitude profile of the station maximum temperature averaged in eastern China (110–120° E). The dashed box indicates the Jiangnan area. Only values ≥35oC are shown (units: °C).

      Figure 2.  Distribution of the anomalous percentage of precipitation in (a) 1 June–14 July (P1) and (c) July 15–August 31 (P2) 2019 (units: %). Time series of the mean precipitation in the four provinces of Jiangnan in (b) P1 and (d) P2 from 1981 to 2019, with blue bars for each year (red bar for 2019) and red line representing the climatological mean (units: mm).

      However, from mid-July until the end of August, the precipitation in Jiangnan was mostly below normal and almost no strong precipitation processes were observed (Fig. 1a), except one on 10 August caused by the super typhoon Lekima (Zhou and Gao, 2019). This resulted in precipitation >50% below normal (Fig. 2c). The area-averaged precipitation in the four provinces of Jiangnan was only 159.06 mm from 15 July to 31 August, 33.03% less than the climatological mean (237.51 mm) and the third lowest since 1981 (Fig. 2d). Large-scale high temperatures occurred continuously in southern China over this time period. The maximum temperature was mostly <35°C in June and early July, especially in the Jiangnan area, but increased abruptly to ≥35oC from mid-July to the end of August (Fig. 1b).

      To quantify this type of abrupt-turning phenomenon, we adapted the long-cycle drought–flood abrupt alternation index (LDFAI) in summer (June–July–August) according to Wu et al. (2006b):

      $$\rm{LDFAI}=\left({\it{R}}_{p2}-{R}_{p1}\right)\times (│{R}_{p1}│+│{R}_{p2}│)\times {1.8}^{-│{R}_{p1}+{R}_{p2}│}$$ (1)

      where Rp1 and Rp2 refer to the normalized precipitation averaged in the Jiangnan area in P1 (1 June–14 July) and P2 (15 July–31 August), respectively. (Rp2Rp1) represents the intensity term of the events, (|$ {R}_{p1} $| + |$ {R}_{p2} $|) denotes as the magnitude of the droughts and floods, and $ {1.8}^{-|{R}_{p1}+{R}_{p2}|} $ is the weight coefficient.

      Fig. 3 shows the daily running LDFAI in the Jiangnan area in the summer of each year from 1981 to 2019. The date on the x-axis indicates that the summer is divided into P1 and P2 at this day. For example, 6 June means that P1 is from 1 June to 6 June and P2 is from 7 June to 31 August. The absolute value of LDFAI in the Jiangnan area is not always strong in every year and a large absolute value of LDFAI occurs at different times of the summer in different years. The dark blue color in Fig. 3 indicates a strong flood–drought transition and it is clear that only 2019 and 1998 show a large negative LDFAI in summer, with the former in mid-July and the latter in late July. The abrupt flood–drought alternation event in Jiangnan in mid-July 2019 therefore ranked as one of the top two strongest alternation events in the last 39 years.

      Figure 3.  Daily running long-cycle drought–flood abrupt alternation index (LDFAI, according to equation 1) averaged in the four provinces (Hunan, Jiangxi, Fujian, and Zhejiang) of the Jiangnan area during June–July–August of each year from 1981 to 2019, with yellow (blue) indicating drought–flood alternation.

      We next consider which circulation features in these two different periods caused the very different precipitation anomalies in southern China during the summer of 2019. We investigate how the circulation suddenly changed and the possible reasons for this sudden change.

    4.   Differences in circulation between P1 and P2
    • During P1 (1 June–14 July2019), the EAT showed a northeast–southwest extension from the Okhotsk Sea to the northwestern Pacific south of Japan. A negative anomaly of 500 hPa geopotential height dominated the eastern part of East Asia (Fig. 4a), indicating that the EAT was much stronger than normal. Accompanied by an intensified high ridge around Lake Baikal, the meridional gradient was increased over East Asia, indicating that cold air activity from higher latitudes to southern China was stronger and more frequent than normal. The time–latitude profile of the daily 500 hPa geopotential height (mean and anomalies) averaged over 125–135° E shows that the intensified EAT persistently dominated over eastern East Asia from June 5 until mid-July (Fig. 5a), with continuously negative anomalies within latitudes 30–40° N.

      Figure 4.  Circulations in P1 of summer 2019. (a) Mean (contours) and anomalous (shading) 500 hPa geopotential height (red solid contour for the 5880 gpm contour in 2019 and red dashed contour for the climatological mean; units: gpm). (b) 850 hPa anomalous winds (vectors; units: m s−1) and vorticity (shading; units: 10−6 s−1). (c) Anomalous integrated moisture flux (vectors; units: kg (s m)−1) and divergence (shading; units: 10−5 kg (s m2)−1 from the surface to 300 hPa. (d) Mean (contours) and anomalous (shading) 200 hPa zonal winds (red dashed contour for the maximum zonal wind speed center (≥30 m s−1) of the climatological mean; units: m s−1), with the black box for the Jiangnan area (25–30° N, 105–122° E).

      Figure 5.  (a) Time–latitude profile of the mean (contours) and anomalous (shading) 500 hPa geopotential height averaged over 125–135° E from June 1 to August 31 in 2019. The dashed box indicates the persistently intensified trough within 30–40° N. (b) Time–longitude profile of the 500 hPa geopotential height averaged over 25–30° N. (c) Time–latitude profile averaged over 110–130° E in summer 2019 (blue shading with black contours) and the climatological mean (red contours). Only values ≥5860 gpm are displayed to show the eastward and westward movement (in part (b)) and the northward and southward swing (in part (c)) of the WPSH (units: gpm).

      According to our statistical analysis, the strength of the negative anomalies at 500 hPa over eastern East Asia (30–40° N, 125–135° E) averaged in P1 was below 1.5 times the standard deviation (SD) and was the fourth strongest during the last 39 years (1981–2019). The accumulated number of days with negative anomalies in eastern East Asia were up to 36 days from a total of 44 days in P1, which also ranked the third longest period since 1981 (figures omitted). The persistently intensified EAT is therefore an important factor in the extremely high precipitation in Jiangnan in P1 of 2019.

      Influenced by the intensified EAT, an anomalous low-level cyclone prevailed from southern Japan to southern China. The associated anomalous northerly winds largely prevented the northward invasion of the anomalous southerly winds in the South China Sea. Positive anomalies of the 850 hPa vorticity dominated over the Jiangnan area (Fig. 4b). The daily variation of the 850 hPa vorticity averaged in Jiangnan (25–30° N, 105–122° E; highlighted by the box in Fig. 4) corresponded well with that of the daily precipitation anomaly in the region.

      The five strong rainfall processes with above-normal rainfall in P1 all occurred during the period with intensified 850 hPa vorticity (Fig. 6a). The interannual variation of the mean 850 hPa vorticity in Jiangnan in P1 was also consistent with the interannual variation of the mean precipitation in the four provinces of Jiangnan during 1981–2019 (Fig. 6b). Among the last 39 years, 30 years had positive (negative) precipitation with positive (negative) vorticity, indicating that the probability of the same sign between the two variables was as high as 76.9%. The linear correlation was 0.66, exceeding the 99% confidence level. The 850 hPa vorticity in Jiangnan in 2019 was above 3 SD, much larger than that of all the other years (mostly within 2 SD). The precipitation in Jiangnan in 2019 was not only the highest since 1981, but also reached nearly 3 SD (Fig. 6b). This indicates that the strongest 850 hPa vorticity also had an important role in the high precipitation in Jiangnan in P1 of 2019.

      Figure 6.  (a) Daily variation of the anomalous precipitation in the four provinces (highlighted in Fig. 2a) of Jiangnan (bars; units: mm) and 850 hPa anomalous vorticity in Jiangnan (black box in Fig. 3b; black line; units: 10−6 s−1). (b) Time series of the normalized precipitation averaged in the four provinces of Jiangnan (bars) and the normalized 850 hPa vorticity averaged in Jiangnan (black line) in P1 from 1981 to 2019.

      The WPSH was also intensified, with a larger area of the 5880 gpm contour and its ridge line extending more westward and southward than the climatological mean (Fig. 4a). Influenced by the more southward WPSH, more moisture was transported from the western Pacific to southern China (Fig. 4c). In co-operation with the anomalous northerly moisture flux caused by the intensified meridional gradient at mid- to high latitudes and the low-level anomalous cyclone in eastern East China, this resulted in anomalous convergence of the moisture flux in southern China (Fig. 4c, green shading in the Jiangnan area). The area-averaged moisture convergence in Jiangnan (25–30° N, 105–122° E) in P1 of 2019 was the third largest since 1981 and its daily variation was consistent with the daily precipitation in Jiangnan (figures not shown), similar to the variation of the 850 hPa vorticity (Fig. 6a).

      In the upper troposphere, the maximum zonal wind center of the East Asian subtropical jet (EASJ) was located within latitudes 30–40° N in P1 of 2019. Positive anomalies of the 200 hPa zonal wind prevailed from eastern China to southern Japan (Fig. 4d), indicating that the westerly jet was stronger and more southward than normal. Because the Jiangnan area is located on the right-hand side of the entrance of the EASJ, the high-level divergence and low-level convergence intensified the vertical velocity at 500 hPa in the Jiangnan area (figure not shown). As a result, the anomalous upward motion in 2019 was the second largest since 1981 (figure not shown), which provided an important dynamic condition for the higher precipitation in Jiangnan.

      The extremely strong precipitation in Jiangnan during P1 (1 June–14 July) of summer 2019 therefore resulted from the combined effects of several circulation anomalies: (1) a larger meridional gradient with frequent cold air activity caused by the persistently intensified EAT; (2) an anomalous low-level cyclone from southern Japan to southern China with a positive anomaly of the 850 hPa vorticity; (3) anomalous convergence of the moisture flux induced by the intensified and southward WPSH; and (4) the stronger and more southward 200 hPa westerly jet and the anomalous 500 hPa upward motion in Jiangnan. These four important circulation anomalies were also observed in each of the five heavy rainfall processes in P1 of 2019 (figures omitted), further confirming our conclusions.

      The atmospheric circulations had totally changed by the second period (P2) from 15 July to 31 August, 2019. At 500 hPa, the WPSH moved northward and the 5880 gpm contour controlled the northwestern Pacific south of Japan, which also displayed a larger area, a greater intensity, and more westward and northward extension than the climatological mean. The northward-shifting WPSH weakened the previously strong EAT and caused a positive geopotential height anomaly over the northwestern Pacific (Fig. 7a). The time–latitude profile of the daily 500 hPa geopotential height averaged over 125–135° E shows that the persistent trough with negative anomalies in P1 abruptly changed to a high-pressure ridge and positive anomalies of the geopotential height after about 15 July (Fig. 5a). From late July to early August, an intensified high-pressure center dominated over 30–40° N, caused by the northward jump of the WPSH (see Section 5). The meridional gradient at mid- to high latitudes was largely reduced and the cold air activity influencing southern China was also significantly decreased.

      Figure 7.  Circulations in P2 in summer 2019. (a) Mean (contours) and anomalous (shading) 500 hPa geopotential height (red solid contour for the 5880 gpm contour in 2019 and red dashed contour for the climatological mean; units: gpm). (b) 850 hPa anomalous winds (vectors; units: m s−1) and vorticity (shading; units: 10−6 s−1). (c) Anomalous integrated moisture flux (vectors; units: kg (s m)−1) and divergence (shading; units: 10−5 kg (s m2)−1 from the surface to 300 hPa. (d) Mean (contours) and anomalous (shading) 200 hPa zonal winds (red dashed contour for the maximum zonal wind speed center (≥30 m s−1) of the climatological mean; units: m s−1), with the black box for the Jiangnan area (25–30° N, 105–122° E).

      The time–longitude profile of the daily 500 hPa geopotential height averaged over 25–30° E shows that the WPSH (5860 gmp contours, blue shading) extended more westward than normal (red contours) after mid-July and covered the Jiangnan area almost continuously, except for a break in early August (Fig. 5b). The previously low-level anomalous cyclone from southern Japan to southern China in P1 disappeared and was replaced by an anomalous anticyclone in the northwestern Pacific south of Japan (Fig. 7b). Anomalous northerly winds prevailed and the 850 hPa vorticity also reduced to near-normal or slightly below normal in the Jiangnan area (Fig. 7b). The northward-shifting WPSH resulted in anomalous divergence of the integrated moisture flux in Jiangnan (Fig. 7c, yellow shading). All these circulation anomalies led to high temperatures and less precipitation in Jiangnan during P2 of 2019.

      In the upper level, the maximum zonal wind center of the EASJ moved northward to 40–50° N and westward to western China in P2, where it was a little weaker than the climatological mean (Fig. 7d). Because the Jiangnan area is located on the right-hand side of the exit area of the EASJ, the high-level convergence and low-level divergence led to an anomalous 500 hPa sinking motion over the Jiangnan area (figure omitted). The strength of the sinking motion in southern China in P2 of 2019 was the third strongest since 1981 (figure omitted), favoring high temperatures, anomalous below-normal precipitation and drought.

      All the atmospheric circulation conditions in P2 had changed completely from those in P1: (1) a reduced meridional gradient at mid- to high latitudes caused by the weakened EAT and northward-jumping WPSH; (2) an anomalously negative 850 hPa vorticity and westward-extending WPSH over Jiangnan; (3) anomalous divergence of the moisture flux; and (4) a weakened and northward-moving EASJ and anomalous 500 hPa sinking in the Jiangnan area. Together, this resulted in continuously high temperatures, below-normal precipitation and severe drought in the Jiangnan area.

    5.   Possible reasons for the sudden intra-seasonal change in the circulation
    • Our analysis showed that crucial changes in the large-scale atmospheric circulation were responsible for the abrupt transition from flood to drought in the Jiangnan area during the summer of 2019, including the disappearance of the intensified EAT, the northward jump of the WPSH and weakening of the EASJ. The difference figures (omitted) between P1 (Fig. 4) and P2 (Fig. 7) are similar to Fig. 4, further confirming our conclusions.

      This section focusses on the first two important circulation systems due to their close interactions. Before mid-July, the persistently stronger EAT resulted in an extremely southward WPSH with delayed northward-jumping. After mid-July, the WPSH jumped northward over 35° N and was continuously further north than normal until the end of August (Fig. 5c), which caused the previously persistent EAT to be suddenly replaced by a high-pressure ridge (Fig. 5a).

      We examined the geopotential height at different levels and found that the intensified trough over Japan was observed in each vertical level from the lower to upper troposphere. The abrupt change in the geopotential height from negative to positive anomalies was clearly identified around mid-July (Fig. 8a). Before that time, negative anomalies prevailed over eastern East Asia from 1000 to 100 hPa and continuously from 5 June to 15 July, except for a short break in late June. The minimum centers of the negative geopotential height corresponded fairly well with the five rainfall processes. For example, the geopotential height from 500 to 200 hPa was mostly below −2 SD during the first two rainfall processes (June 6–13 and 20–25 June) and below −1 SD from 1000 to 200 hPa during the later three rainfall processes (3–5 July, 6–9 July and 12–14 July). This feature further confirmed that the continuously intensified EAT played an important part in the extremely high precipitation in Jiangnan during P1 of 2019.

      Figure 8.  (a) Time–height profile of the normalized geopotential height averaged over eastern East Asia (30–40° N, 125–135° E) from June 1 to August 31, 2019. (b) Daily variation of the 850 hPa (black line; left-hand y-axis), 500 hPa (red line; left-hand y-axis); and 200 hPa (blue line; right-hand y-axis) geopotential height anomaly averaged in eastern East Asia (30–40° N, 125–135° E) in July 2019. Units: gpm.

      After mid-July, positive anomalies of the geopotential height dominated the troposphere from the lower to upper levels and continued until the end of August. From mid-July to mid-August, the maximum positive centers over 2 SD mainly dominated the upper troposphere above 400 hPa. Although the geopotential height anomalies were much larger in the upper level than in the lower level, the change in the geopotential height from negative to positive anomalies around mid-July showed a slightly oblique trend from the lower to upper levels, possibly indicating an earlier occurrence of this sudden change in the lower level than in the upper level.

      To prove this speculation, we calculated the daily variation of the area-averaged geopotential height at different levels in eastern East Asia (30–40° N, 125–135° E). The geopotential height anomalies at 850, 500, and 200 hPa levels all showed an apparent negative to positive transition around mid-July (Fig. 8b). Before mid-July, the negative anomalies seemed to be more continuous at 850 and 500 hPa than at 200 hPa. After mid-July, however, the positive anomalies were more stable at 200 hPa than at 500 and 850 hPa. More importantly, the previously negative anomalies of the geopotential height during P1 in 2019 began to weaken on 14 Julyat 850 and 500 hPa, about two days earlier than those at 200 hPa (on 16 July). The sudden change from negative to positive anomalies occurred exactly one day earlier at 850 and 500 hPa (on 16 July) than at 200 hPa (on 17 July). This confirms that the sudden change in the atmospheric circulation around mid-July started in the lower and mid-troposphere.

      We inspected the daily circulation anomalies from July 13–16, 2019. On July 13, the atmospheric circulation at mid- to high latitudes (Fig. 9a) was similar to that during P1 in 2019 (see Fig. 4a). Both periods were characterized by an intensified high-pressure ridge near Lake Baikal, an enhanced EAT and a resultant strong meridional gradient, as well as a large anomalous low-level cyclone from southern Japan to southern China. However, the WPSH was slightly further to the north than the mean situation in P1, probably because of the anomalous low-level cyclone in the tropical western Pacific (Fig. 9a).

      Figure 9.  Anomalous 850 hPa winds (vectors; units: m s−1) and mean (contours) and anomalous (shading) 500 hPa geopotential height (units: gpm) on (a) July 13, (b) July 14, (c) July 15, and (d) July 16 2019. The red line represents the 5880 gpm contour and the red dot is roughly the center of tropical cyclone Danas.

      On 14 July, the atmospheric circulation at mid- and high latitudes showed few changes, with the EAT becoming a little weaker and the meridional gradient becoming a little smaller. The tropical cyclone became stronger and the WPSH retreated eastward and moved slightly northward (Fig. 9b), with its area shrinking greatly compared with the previous day.

      On July 15, the tropical cyclone became stronger and was named Danas, the fifth tropical cyclone in the northwestern Pacific in 2019. It moved northwestward with the center of the cyclone near (19° N, 130° E). The WPSH moved northward and westward, and the shape of its western part became smaller and narrower, which was clearly influenced by the peripheral wind of tropical cyclone Danas (Fig. 9c). The low-pressure trough still controlled eastern East Asia, but its strength was decreased compared with previous days. The low-level anomalous cyclone over southern China almost disappeared.

      On July 16, tropical cyclone Danas moved westward and its peripheral cyclonic flow prevailed over the southeast coast of China. As a result, the WPSH shifted northward, with its western part even narrower and controlling southern China, and its eastern part located to the southeast of Japan. The previously persistent EAT disappeared and positive anomalies of the 500 hPa geopotential height dominated eastern East Asia (Fig. 9d). Figures 9a–9d show that positive anomalies of the 500 hPa geopotential height upstream of the EAT gradually became stronger and moved slightly southward. We examined the 500 hPa geopotential height upstream of the EAT. The high ridge at mid- to high latitudes showed a much weaker change around mid-July than the circulation in the tropical and subtropical regions (figure not shown). The activity of tropical cyclone Danas was therefore an important trigger for the abrupt change in circulation around mid-July in 2019.

      Figure 10 confirms the impact of tropical cyclone Danas on the northward movement of the WPSH. From July 15 to 17, Danas showed a westward and then northward track. Influenced by its peripheral cyclonic flow, the WPSH also gradually moved westward and northward. Its western part became smaller and narrower and controlled southern China, before breaking from its eastern main part. The eastern part of the WPSH shifted northward to the south of Japan and replaced the previous trough over eastern East Asia (Fig. 10a). During the period July 18–20, Danas moved northward along the east coast of China. As a result, the area of the WPSH became larger. It retreated eastward, moved even more northward and eventually controlled the southern part of Japan (Fig. 10b).

      Figure 10.  Daily locations of tropical cyclone Danas (dots) and the daily mean 5880 gpm contours representing the WPSH (a) from July 14 to 17, 2019 and (b) from July 18 to 20, 2019.

      The generation and northward movement of tropical cyclone Danas is therefore an important trigger for the northward-jump of the WPSH around mid-July, which also played a key part in the sudden change in the atmospheric circulation in the mid- and lower troposphere. However, it was not responsible for the persistence of the northward position of the WPSH from mid-July until the end of August 2019, which is attributed to the seasonal northward migration of the WPSH (Tao and Chen, 1987) and the active tropical cyclone activities with northward tracks in P2 of 2019 (Xie and Zhou, 2019; Zhou and Gao, 2019). Similar physical processes were also observed in the extreme rainfall event in Beijing, China on 21 July, 2012 (Wen et al., 2015) and during the Baiu season in Japan (Hirata and Kawamura, 2014).

      We used radiosonde data of the 500 hPa geopotential height in the daily synoptic analysis of the weather forecast from the China Meteorological Administration and compared the deep cyclonic circulation with the tropical cyclones observed in the summer of 2019. The results were very similar to our conclusions based on the NCEP/NCAR dataset.

    6.   Discussion and Conclusions
    • We investigated the abrupt flood–drought alternation in the Jiangnan area during summer 2019, in contrast with previous research on the sharp turn from drought to flood (Wu et al., 2006a, b; Si et al., 2012; Tong and Xu, 2014). Five heavy rainfall processes caused serious floods in Jiangnan from June 1 to July 14 (P1) 2019. The area-averaged precipitation in four provinces (Hunan, Jiangxi, Fujian and Zhejiang) in the Jiangnan area was the largest since 1981 and the anomalies were nearly 3 SD. By contrast, precipitation in Jiangnan decreased sharply from July 15 to August 31 (P2) to the third smallest since 1981. Persistently below-normal precipitation and high temperatures resulted in severe drought The daily running LDFAI in the summer of each year indicated that this abrupt flood–drought alternation event in Jiangnan during the summer of 2019 was one of the top two most severe events since 1981.

      The completely different atmospheric circulation anomalies were the reason for the different precipitation anomalies in the two time periods. In P1, there was a larger meridional gradient caused by persistently intensified EAT. An anomalous low-level cyclone with positive anomalies of the 850 hPa vorticity prevailed from southern Japan to southern China. Anomalous convergence of the moisture flux dominated over Jiangnan because of the intensified and southward location of the WPSH and there was an enhanced and southward 200 hPa westerly jet with anomalous 500 hPa upward motion in Jiangnan. These important circulation anomalies together resulted in increased extreme precipitation in the Jiangnan area.

      We compared this 2019 event with other seven years (1992, 1993, 1995, 1997, 1998, 2010, and 2017) in the time period 1981–2018 for which the precipitation anomalies in Jiangnan during P1 were all above 1 SD. The composite events and the 2019 event had many similarities: a stronger high-pressure ridge over Lake Baikal; a deepened EAT; and a southward EASJ with intensified rising motion in southern China. However, the EAT in 2019 was much stronger and showed a more northeast–southwest oblique feature than the composite events. The accumulated days with an intensified EAT in 2019 also exceeded most of the composite events during P1. The WPSH in 2019 was located more southward and the anomalous low-level cyclone with a positive vorticity in southern China was more evident than the composite events. The anomalous 850 hPa vorticity in Jiangnan area was above 3 SD and was the largest in the past 39 years, much stronger than all the composite events.

      In P2 of 2019, the previously persistent EAT was replaced by a high-pressure ridge because of the northward-jumping WPSH. An anomalously negative 850 hPa vorticity and divergence of the moisture flux dominated the Jiangnan area. The EASJ moved northward and largely decreased, along with an anomalous 500 hPa sinking movement in the Jiangnan area. The precipitation in Jiangnan therefore abruptly changed to below normal and, accompanied by large-scale high temperatures in southern China, led to a severe drought in this region. The drought in Jiangnan continued until the end of November 2019 and was the most severe midsummer–autumn continuous drought in the last 50 years (Li et al., 2020). The continuously intensified WPSH, which was further north than normal, and the decreased sea ice concentration in the previous spring may have led to this circulation pattern of less rainfall in this region (Wu et al., 2009; Shen et al., 2019).

      By comparing P1 and P2, we concluded that both the EAT and the WPSH had a key role in this abrupt flood–drought change in precipitation in the Jiangnan area. The geopotential height over eastern East Asia changed from persistently negative anomalies to large positive anomalies, whereas the WPSH changed from its location far to the south to a location much further north than normal. These two abrupt changes completely altered the anomalies in the transport of moisture into southern China, as well as the low-level vorticity and the 500 hPa vertical velocity in the Jiangnan area. These two important circulation systems interacted with each other: the long-lasting deepened EAT in P1 of 2019 prevented the WPSH from jumping north and resulted in the delayed northward invasion of the EASM. By contrast, the northward-jumping of the WPSH in P2 destroyed the EAT and led to the replacement of a high-pressure ridge. Composite analysis of similar years confirms this suggestion (figures omitted), although the detailed interaction between the EAT and WPSH in other years requires further research.

      Further investigation of the daily variation of the geopotential height at different vertical levels showed that the sudden change in the atmospheric circulation around mid-July started in the mid- and lower troposphere and was mainly caused by tropical cyclone activity. The northward track of tropical cyclone Danas in the northwestern Pacific adjusted the large-scale circulation in East Asia around mid-July and became an important trigger for the northward jump of the WPSH. However, the system did not return to normal after the passage of the cyclone as a result of the long-lasting persistence of the WPSH (references cited in Yuan et al., 2020) and also because most of the tropical cyclones generated in the northwestern Pacific showed northward tracks in P2 of 2019 (Xie and Zhou, 2019; Zhou and Gao, 2019).

      We analyzed the abrupt change in the atmospheric circulation anomalies from flood to drought in Jiangnan during the summer of 2019. Further analyses are still required to explore the possible mechanisms. The intra-seasonal change in the positions of the dominant factors should be considered. Zong and Zhang (2011) showed that different locations of the influencing systems may lead to different distributions of flood or drought in the Yangtze and Huai river valleys. Zhang J. M. et al. (2021) also showed the importance of the shape of the WPSH in causing the drought and hot weather in southern China. This abrupt change may also be influenced by some external forcing factors, such as the variation in the heating flux over the Qinghai–Tibetan Plateau (Li et al., 2014) or low-frequency oscillations in tropical and mid- to high latitudes (Yang et al., 2013; Tong and Xu, 2014; Miao et al., 2017; Yuan et al., 2020). This will be the focus of future research.

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