Historic Yangtze Flooding of 2020 Tied to Extreme Indian Ocean Conditions
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Historic Yangtze flooding of 2020 tied to extreme Indian Ocean conditions Zhen-Qiang Zhou (周震强)a,b,c, Shang-Ping Xie (谢尚平)d,1, and Renhe Zhang (张人禾)a,b,c,1 aDepartment of Atmospheric and Oceanic Sciences and Institute of Atmospheric Sciences, Fudan University, Shanghai 200438, China; bCMA-FDU Joint Laboratory of Marine Meteorology, Shanghai 200438, China; cInnovation Center of Ocean and Atmosphere System, Zhuhai Fudan Innovation Research Institute, Zhuhai 518057, China; and dScripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093 Edited by John M. Wallace, University of Washington, Seattle, WA, and approved January 26, 2021 (received for review October 24, 2020) Heavy monsoon rainfall ravaged a large swath of East Asia in warm advection from Tibet (Fig. 1D) adiabatically induce up- summer 2020. Severe flooding of the Yangtze River displaced ward motions (Fig. 1C) to enhance Mei-yu rainfall. The resultant millions of residents in the midst of a historic public health crisis. anomalous diabatic heating reinforces the anomalous vertical This extreme rainy season was not anticipated from El Niño con- motion, forming a positive feedback (8–12). This is consistent ditions. Using observations and model experiments, we show that with the empirical relationship known to Chinese forecasters be- the record strong Indian Ocean Dipole event in 2019 is an impor- tween the 500 hPa geopotential height and the Mei-yu rain band. tant contributor to the extreme Yangtze flooding of 2020. This On the interannual timescale, El Niño-Southern Oscillation Indian Ocean mode and a weak El Niño in the Pacific excite (ENSO) has been identified as the dominant forcing of Mei-yu downwelling oceanic Rossby waves that propagate slowly west- rainfall variability (3–5, 13). Mei-yu rainfall in the Yangtze Basin ward south of the equator. At a mooring in the Southwest Indian tends to increase (decrease) in post-El Niño (La Niña) summer. Ocean, the thermocline deepens by a record 70 m in late 2019. The A Northwest Pacific anomalous anticyclone often develops rap- deepened thermocline helps sustain the Indian Ocean warming idly during an El Niño winter (14), interacting with local sea through the 2020 summer. The Indian Ocean warming forces an surface temperature (SST) (15, 16) and modulated by the back- anomalous anticyclone in the lower troposphere over the Indo- ground annual cycle (17). The anomalous anticyclone cools the Northwest Pacific region and intensifies the upper-level westerly tropical Northwest Pacific on the southeastern flank by strength- jet over East Asia, leading to heavy summer rainfall in the Yangtze ening the northeast trade winds and surface evaporation. The Basin. These coupled ocean-atmosphere processes beyond the EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES ocean cooling suppresses atmospheric convection, reinforcing equatorial Pacific provide predictability. Indeed, dynamic models initialized with observed ocean state predicted the heavy summer the anomalous anticyclone with a Rossby wave response. El Niño rainfall in the Yangtze Basin as early as April 2020. also causes the tropical Indian Ocean to warm. The Indo-western Pacific Ocean capacitor refers to the following interbasin posi- East Asia flooding | anomalous anticyclone | Indian Ocean | El Niño tive feedback in summer between the Indian Ocean warming and the Northwest Pacific anomalous anticyclone. The tropical In- dian Ocean warming excites a Matsuno-Gill-type (18, 19) re- ummer is the rainy season for East Asia. A northeastward- sponse in tropospheric temperature, with a Kelvin response that slanted rain band—called Mei-yu in China and Baiu in S penetrates eastward and induces northeasterly surface wind Japan—extends from the Yangtze River valley of China to the anomalies in the tropical Northwest Pacific. The resultant Ekman east of Japan during early summer (early June to mid-July). The divergence suppresses convection and induces the anomalous Yangtze is the longest river of Asia, flowing from the eastern Tibetan Plateau and exiting into the ocean in Shanghai. Ap- anticyclone (20). The anomalous anticyclone in turn feeds back to proximately one-third of the population of China live in the river basin. The Mei-yu rain band displays marked interannual vari- Significance ability with great socioeconomic impacts on the densely popu- lated region, including agriculture production, water availability, Summer rainfall along the Yangtze River in 2020 was the food security, and economies (1–5). heaviest since 1961, with devastating socioeconomic impacts. During June through July 2020, the Mei-yu rain band inten- While official forecasts based on tropical Pacific state failed, we sified markedly, with rainfall exceeding the 1981 to 2010 mean of show that dynamic models, when initialized with ocean ob- ∼300 mm by ∼300 mm over the Yangtze River valley (Fig. 1A). servations globally, succeed in predicting the extreme rainfall. This corresponds to an excess of up to 4 SDs (Fig. 1B). By 12 July Slowly propagating oceanic Rossby waves in the South Indian 2020, the Yangtze floods caused 141 deaths, 28,000 homes were Ocean are the source of predictability, which are in turn tied to flattened, and 3.53 million hectares of crops were affected, with the record-breaking Indian Ocean Dipole in late 2019. The the direct economic loss at 82.23 billion yuan (11.76 billion US identification of antecedent subsurface conditions of the In- dollars) (6). South of the Mei-yu rain band, negative rainfall anom- dian Ocean as a key predictor represents an important con- alies (−60 mm/month) extended over a broad region from the Bay of ceptual advance in Asian summer monsoon dynamics, helping Bengal to tropical Northwest Pacific (Fig. 1A). This meridional di- improve disaster preparation that saves lives and properties. pole of rainfall anomalies is known as the recurrent Pacific–Japan Author contributions: Z.-Q.Z., S.-P.X., and R.Z. designed research; Z.-Q.Z., S.-P.X., and R.Z. pattern (7). performed research; Z.-Q.Z. analyzed data; and Z.-Q.Z., S.-P.X., and R.Z. wrote the paper. In June through July 2020, an anomalous anticyclone with The authors declare no competing interest. depressed rainfall dominates the lower troposphere over the This article is a PNAS Direct Submission. tropical and subtropical Northwest Pacific through the South A This open access article is distributed under Creative Commons Attribution-NonCommercial- China Sea (Fig. 1 ). The easterly wind anomalies on the south NoDerivatives License 4.0 (CC BY-NC-ND). flank of the anomalous anticyclone extend into the North Indian 1To whom correspondence may be addressed. Email: [email protected] or rhzhang@fudan. Ocean, while the anomalous southwesterlies on the northwest edu.cn. flank transport water vapor from the south to feed the enhanced This article contains supporting information online at https://www.pnas.org/lookup/suppl/ Mei-yu rainband (Fig. 1C). In the mid-troposphere, the westerlies in- doi:10.1073/pnas.2022255118/-/DCSupplemental. tensify over midlatitude East Asia, and the anomalous mid-tropospheric Published March 8, 2021. PNAS 2021 Vol. 118 No. 12 e2022255118 https://doi.org/10.1073/pnas.2022255118 | 1of7 Downloaded by guest on September 26, 2021 AB CD Fig. 1. Atmospheric dynamics of the Yangtze flooding of 2020. June through July averaged anomalies of (A) rainfall (shading, mm/month), SLP (contours at ±0.3, ±0.6, ±1.2, and ±1.8 hPa), and 850 hPa wind (vector, displayed with speed > 0.3 m/s); (C) column-integrated moisture transport (vector, displayed with − − magnitude > 15 kg · m 1 · s 1) and 500 hPa omega (shading, Pa/s, and negative values for ascent motions); and (D) 500 hPa horizontal temperature advection (shading, K/s) and wind (vector, displayed with speed > 0.5 m/s). Blue solid curves denote the Yangtze and Yellow Rivers. Black dashed curves (2,000 m isoline of topography) denote the Tibetan Plateau and surrounding mountains. SLP anomalies over and north of Tibetan Plateau are masked out for clarity. (B) June through July averaged rainfall anomalies (mm/month) over the Yangtze River Valley (26° to 33°N, 105° to 122°E) during 1979 to 2020, with one SD of 35 mm/ month. Major El Niño events are marked. the North Indian Ocean warming by weakening the background are largely forced by SST anomalies instead of due to internal southwest monsoon and suppressing surface evaporation (21–23). atmospheric dynamics. The above ocean-atmospheric coupling processes work well The SST anomalies are positive over the tropical Indian for major El Niño events and provide predictability for Mei-yu Ocean and a weak La Niña developed over the tropical Pacific in rainfall over East Asia (Fig. 1B). A robust Northwest Pacific June 2020 (Fig. 2A). To evaluate relative importance of Pacific anomalous anticyclone developed during the summers of 1998 and Indian Ocean SST anomalies, we conduct another two ex- and 2016, each following a major El Niño. A strong anomalous periments forced separately by tropical Pacific (referred to as anticyclone and excessive Mei-yu rainfall were not expected for PACIFIC) and Indian Ocean (referred to as INDIAN) SST 2020 summer, however, since in the 2019/20 winter (November anomalies. In June 2020, the tropical Indian Ocean is anomalously A to January), the Niño3.4 index was marginal at only 0.5 °C (SI warm (Fig. 2 ). The INDIAN run largely reproduces the results Appendix, Fig. S1) as compared to 2.4 °C and 2.6 °C in 1997/98 of the GLOBAL run, including positive Mei-yu rainfall anoma- lies over the Yangtze Basin and negative rainfall anomalies to and 2015/16 winters, respectively. SST anomalies in the equa- D torial central Pacific (Niño4) were positive and nearly constant in the south with the anomalous anticyclone (Fig. 2 ). In compar- ison, tropical Pacific anomalies only play a limited role (Fig.