# Diagnosing the Dynamic and Thermodynamic Effects for the Exceptional 2020 Summer Rainy Season in the Yangtze River Valley

## 长江流域2020年夏异常雨季的动力和热力效应诊断分析

• Author Bio: Wen, Na wenna@nuist.edu.cn LIU, Shujie liushujie96@163.com LI, Laurent laurent.li@lmd.jussieu.fr
• Corresponding author: Laurent Z. X. LI, laurent.li@lmd.jussieu.fr
• Funds:

Supported by the National Key Research and Development Program of China (2018YFC1507704) and National Natural Science Foundation of China (42088101)

• doi: 10.1007/s13351-022-1126-2
• An exceptional rainy season occurred in the Yangtze River valley of eastern China in June–July 2020. The relative importance of the dynamic and thermodynamic effects on this unusual event is evaluated through the budget equations of moisture and moist static energy (MSE). The moisture budget analysis suggests that the thermodynamic effect contributes to the precipitation anomaly by 8.5% through the advection of abnormal water vapor by mean verti-cal motion, while the dynamic effect, related to water vapor advection by anomalous vertical motion, has the dominant contribution. The MSE budget analysis further reveals that the anomalous vertical motion is both constrained by the dynamic effect related to changes in atmospheric circulation and the thermodynamic effect related to changes of the atmospheric thermal state, with a ratio of thermodynamic versus total effects estimated at 45.3%. The dynamic effect is linked to the advection of warm and humid air by the abnormal southwesterly wind, which is related with an anomalous anticyclone over the Philippine Sea. The thermodynamic effect is partly induced by the positive advection of anomalous MSE (mainly latent energy) by the mean vertical motion. This analysis of the dynamic and thermodynamic effects is useful to understand the underlying physical mechanisms leading to the unusual rainy season in the Yangtze River valley in summer 2020. It is also helpful to put forward a few speculations on the potential role of global warming whose primary effect is, after all, to change the thermal state of the atmosphere.
长江中下游2020年6和7月遭受异常降水。本文使用水汽和湿静力能平衡方程研究动力和热力效应对这次异常梅雨季的贡献。对于水汽，热力效应主要是气候态垂直上升运动对异常水汽含量的平流，但对降水异常的贡献只有8.5%，而通过异常上升运动对气候态水汽的平流来实现的动力效应则是降水异常的主要贡献因子。异常上升运动通过湿静力能方程也可分解为动力和热力效应，热力效应占到总效应的45.3%。动力效应是异常西南风对暖湿空气的平流，热力效应是平均垂直运动对异常湿静力能的平流。本文结果有助于理解导致2020年夏季长江流域异常雨季的基本物理机制，同时对全球变暖的潜在作用提出一些推测，因为全球变暖毕竟首先改变大气的热状态，直接和间接地影响降水。
• Fig. 1.  Monthly-mean anomalies of moisture budget (mm day−1) for June–July 2020 averaged over the YRV (27°–34°N, 107°–122°E) as indicated by the black box in Fig. 2a.

Fig. 2.  Spatial distributions of each term (as indicated in the upper-right corner of each panel) of the moisture budget equation during June–July 2020. (a) Precipitation anomalies over East China, (b) evaporation anomaly, (c) horizontal advection of anomalous moisture by climatological wind, (d) horizontal advection of climatological moisture by anomalous wind, and (e) and (f) as in (c) and (d), but for vertical advection.

Fig. 3.  Vertical profiles of (a) anomalous vertical velocity ${\omega}'$ (blue line; 10−2 Pa s−1) and climatological MSE $\overline{h}$ (red line; 103 J kg−1), and (b) climatological vertical velocity $\overline{\omega}$ (blue line; 10−2 Pa s−1), anomalous MSE h' (red solid line; 103 J kg−1), anomalous enthalpy cpT' (red dashed line; 103 J kg−1), anomalous latent energy ${L}_{v}{q}'$ (red short dashed line; 103 J kg−1), and anomalous geopotential $gz'$ (red dotted line; 103 J kg−1) averaged over the YRV (black rectangle in Fig. 2a) during June–July 2020.

Fig. 4.  Different terms of the MSE budget (W m−2) averaged over the YRV.

Fig. 5.  Spatial distributions of each term of the MSE budget equation during June–July 2020. Panels (a) and (b) show the anomalous vertical advection decomposed into anomalous vertical motion and anomalous MSE. Panels (c) and (d) are similar to (a) and (b), but for the anomalous horizontal advection (for which moist enthalpy replaces MSE in the calculation). Panel (e) is the net energy flux (at surface and top of atmosphere) into the atmospheric column.

Fig. 6.  Horizontal advection of (a–c) climatological dry enthalpy and (d–f) latent energy by anomalous wind, designated as dynamic effect during June–July 2020. (a, d) Total advection, (b, e) zonal component, and (c, f) meridional component.

Fig. 7.  As in Fig. 6, but for the thermodynamic effect (horizontal advection of anomalous dry enthalpy and latent energy by climatological wind) during June–July 2020.

Fig. 8.  Mean anomalies for June–July 2020 of GPCP (Global Precipitation Climatology Project) precipitation (shading; mm day−1), vertically-integrated (1000–500 hPa) specific humidity (contour; g kg−1), and wind (vector; m s−1).

•  [1] Bi, B. G., X. L. Zhang, and K. Dai, 2017: Characteristics of 2016 severe convective weather and extreme rainfalls under the background of super El Niño. Chinese Sci. Bull., 62, 928–937.. [2] Chen, J. Q., and S. Bordoni, 2014: Orographic effects of the Tibetan Plateau on the East Asian summer monsoon: An energetic perspective. J. Climate, 27, 3052–3072.. [3] Chen, T., H. F. Zhang, C. Yu, et al., 2020: Synoptic analysis of extreme Meiyu precipitation over Yangtze River Basin during June–July 2020. Meteor. Mon., 46, 1415–1426. . (in Chinese) [4] Chen, Y., and P. M. Zhai, 2014: Two types of typical circulation pattern for persistent extreme precipitation in Central–Eastern China. Quart. J. Roy. Meteor. Soc., 140, 1467–1478. doi: 10.1002/qj.2231. [5] Chen, Y., and P. M. Zhai, 2015: Synoptic-scale precursors of the East Asia/Pacific teleconnection pattern responsible for persistent extreme precipitation in the Yangtze River Valley. Quart. J. Roy. Meteor. Soc., 141, 1389–1403. doi: 10.1002/qj.2448. [6] Chen, Y., and P. M. Zhai, 2016: Mechanisms for concurrent low-latitude circulation anomalies responsible for persistent extreme precipitation in the Yangtze River Valley. Climate Dyn., 47, 989–1006.. [7] Chou, C., L.-F. Huang, J.-Y. Tu, et al., 2009: El Niño impacts on precipitation in the western North Pacific–East Asian sector. J. Climate, 22, 2039–2057.. [8] Chou, C., J. C. H. Chiang, C.-W. Lan, et al., 2013a: Increase in the range between wet and dry season precipitation. Nat. Geosci., 6, 263–267. doi: 10.1038/ngeo1744. [9] Chou, C., T.-C. Wu, and P.-H. Tan, 2013b: Changes in gross moist stability in the tropics under global warming. Climate Dyn., 41, 2481–2496.. [10] Ding, Y. H., Y. Y. Liu, and Z.-Z. Hu, 2021: The record-breaking Mei-yu in 2020 and associated atmospheric circulation and tropical SST anomalies. Adv. Atmos. Sci., 38, 1980–1993.. [11] Ebita, A., S. Kobayashi, Y. Ota, et al., 2011: The Japanese 55-year Reanalysis “JRA-55”: An interim report. SOLA, 7, 149–152.. [12] Huang, S. N., and F. Huang, 2012: Spatial-temporal variations of dominant drought/flood modes and the associated atmospheric circulation and ocean events in rainy season over the east of China. J. Ocean Univ. China, 11, 137–146.. [13] IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, 1535 pp. [14] Kane, R. P., 1999: Some characteristics and precipitation effects of the El Niño of 1997–1998. J. Atmos. Sol.-Terr. Phys., 61, 1325–1346.. [15] Katz, R. W., and B. G. Brown, 1992: Extreme events in a changing climate: Variability is more important than averages. Climatic Change, 21, 289–302.. [16] Kundzewicz, Z. W., 2005: Flood risk in the changing world—Yangtze floods. Climate Change and Yangtze Floods, T. Jiang, L. King, M. Gemmer, et al., Eds., Science Press, Beijing, 63–72. [17] Liu, S. J., N. Wen, and L. Li, 2021: Dynamic and thermodynamic contributions to northern China dryness in El Niño developing summer. Int. J. Climatol., 41, 2878–2890. doi: 10.1002/joc.6995. [18] Liu, Y. Y., and Y. H. Ding, 2020: Characteristics and possible causes for the extreme Meiyu in 2020. Meteor. Mon., 46, 1393–1404. . (in Chinese) [19] Neelin, J. D., 2007: Moist dynamics of tropical convection zones in monsoons, teleconnections and global warming. The Global Circulation of the Atmosphere, T. Schneider, and A. H. Sobel, Eds., Princeton University Press, Princeton, 267–301. [20] Oueslati, B., P. Yiou, and A. Jézéquel, 2019: Revisiting the dynamic and thermodynamic processes driving the record-breaking January 2014 precipitation in the southern UK. Sci. Rep., 9, 2859.. [21] Rong, X. Y., R. H. Zhang, and T. Li, 2010: Impacts of Atlantic sea surface temperature anomalies on Indo-East Asian summer monsoon–ENSO relationship. Chinese Sci. Bull., 55, 2458–2468.. [22] Shepherd, T. G., 2016: A common framework for approaches to extreme event attribution. Curr. Clim. Change Rep., 2, 28–38.. [23] Si, D., Z.-Z. Hu, A. Kumar, et al., 2016: Is the interdecadal variation of the summer rainfall over eastern China associated with SST? Climate Dyn., 46, 135–146.. [24] Su, B. D., M. Gemmer, and T. Jiang, 2008: Spatial and temporal variation of extreme precipitation over the Yangtze River Basin. Quatern. Int., 186, 22–31.. [25] Sun, Y., T. J. Zhou, G. Ramstein, et al., 2016: Drivers and mechanisms for enhanced summer monsoon precipitation over East Asia during the mid-Pliocene in the IPSL-CM5A. Climate Dyn., 46, 1437–1457.. [26] Sun, Y., G. Ramstein, L. Z. X. Li, et al., 2018: Quantifying East Asian summer monsoon dynamics in the ECP4.5 scenario with reference to the mid-Piacenzian warm period. Geophys. Res. Lett., 45, 12,523–12,533.. [27] Takaya, Y., I. Ishikawa, C. Kobayashi, et al., 2020: Enhanced Meiyu-Baiu rainfall in early summer 2020: Aftermath of the 2019 super IOD event. Geophys. Res. Lett., 47, e2020GL-090671.. [28] Trenberth, K. E., J. T. Fasullo, and T. G. Shepherd, 2015: Attribution of climate extreme events. Nat. Climate Change, 5, 725–730.. [29] Wang, B., and Q. Zhang, 2002: Pacific–East Asian teleconnection. Part II: How the Philippine Sea anomalous anticyclone is established during El Niño development. J. Climate, 15, 3252–3265.. [30] Wang, B., R. G. Wu, and X. H. Fu, 2000: Pacific–East Asian teleconnection: How does ENSO affect East Asian climate? J. Climate, 13, 1517–1536.. [31] Wang, B., J. Li, and Q. He, 2017: Variable and robust East Asian monsoon rainfall response to El Niño over the past 60 years (1957–2016). Adv. Atmos. Sci., 34, 1235–1248.. [32] Wang, L. C., X. G. Sun, X. Q. Yang, et al., 2021: Contribution of water vapor to the record-breaking extreme Meiyu rainfall along the Yangtze River valley in 2020. J. Meteor. Res., 35, 557–570.. [33] Wen, N., Z. Y. Liu, and Y. H. Liu, 2015: Direct impact of El Niño on East Asian summer precipitation in the observation. Climate Dyn., 44, 2979–2987.. [34] Wen, N., Z. Y. Liu, and L. Li, 2019: Direct ENSO impact on East Asian summer precipitation in the developing summer. Climate Dyn., 52, 6799–6815.. [35] Wen, N., L. Li, and J.-J. Luo, 2020: Direct impacts of different types of El Niño in developing summer on East Asian precipitation. Climate Dyn., 55, 1087–1104.. [36] 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.. [37] Xie, S.-P., Y. Kosaka, Y. Du, et al., 2016: Indo-western Pacific Ocean capacitor and coherent climate anomalies in post-ENSO summer: A review. Adv. Atmos. Sci., 33, 411–432.. [38] 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.. [39] Yao, J. C., T. J. Zhou, Z. Guo, et al., 2017: Improved performance of high-resolution atmospheric models in simulating the East Asian summer monsoon rain belt. J. Climate, 30, 8825–8840.. [40] Ye, Y. B., and C. Qian, 2021: Conditional attribution of climate change and atmospheric circulation contributing to the record-breaking precipitation and temperature event of summer 2020 in southern China. Environ. Res. Lett., 16, 044058.. [41] 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.. [42] Zhang, F. H., T. Chen, F. Zhang, et al., 2020: Extreme features of severe precipitation in Meiyu period over the middle and lower reaches of Yangtze River Basin in June–July 2020. Meteor. Mon., 46, 1405–1414. . (in Chinese) [43] Zhao, W., S. F. Chen, W. Chen, et al., 2019: Interannual variations of the rainy season withdrawal of the monsoon transitional zone in China. Climate Dyn., 53, 2031–2046.. [44] Zheng, J. Y., and C. Z. Wang, 2021: Influences of three oceans on record-breaking rainfall over the Yangtze River Valley in June 2020. Sci. China Earth Sci., 64, 1607–1618.. [45] Zhou, Z.-Q., S.-P. Xie, and R. H. Zhang, 2021: Historic Yangtze flooding of 2020 tied to extreme Indian Ocean conditions. Proc. Natl. Acad. Sci. U. S. A., 118, e2022255118..
###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## Diagnosing the Dynamic and Thermodynamic Effects for the Exceptional 2020 Summer Rainy Season in the Yangtze River Valley

###### Corresponding author: Laurent Z. X. LI, laurent.li@lmd.jussieu.fr
• 1. College of Atmospheric Sciences, Nanjing University of Information Science & Technology, Nanjing 210044, China
• 2. Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science & Technology, Nanjing 210044, China
• 3. Qingdao Engineering Technology Research Center for Meteorological Disaster Prevention, Qingdao Meteorological Bureau, Qingdao 266003, China
• 4. Laboratoire de Météorologie Dynamique, Centre National de la Recherche Scientifique, Sorbonne Université, Ecole Normale Supérieure, Ecole Polytechnique, Paris 75252, France
Funds: Supported by the National Key Research and Development Program of China (2018YFC1507704) and National Natural Science Foundation of China (42088101)

Abstract: An exceptional rainy season occurred in the Yangtze River valley of eastern China in June–July 2020. The relative importance of the dynamic and thermodynamic effects on this unusual event is evaluated through the budget equations of moisture and moist static energy (MSE). The moisture budget analysis suggests that the thermodynamic effect contributes to the precipitation anomaly by 8.5% through the advection of abnormal water vapor by mean verti-cal motion, while the dynamic effect, related to water vapor advection by anomalous vertical motion, has the dominant contribution. The MSE budget analysis further reveals that the anomalous vertical motion is both constrained by the dynamic effect related to changes in atmospheric circulation and the thermodynamic effect related to changes of the atmospheric thermal state, with a ratio of thermodynamic versus total effects estimated at 45.3%. The dynamic effect is linked to the advection of warm and humid air by the abnormal southwesterly wind, which is related with an anomalous anticyclone over the Philippine Sea. The thermodynamic effect is partly induced by the positive advection of anomalous MSE (mainly latent energy) by the mean vertical motion. This analysis of the dynamic and thermodynamic effects is useful to understand the underlying physical mechanisms leading to the unusual rainy season in the Yangtze River valley in summer 2020. It is also helpful to put forward a few speculations on the potential role of global warming whose primary effect is, after all, to change the thermal state of the atmosphere.

Reference (45)

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