Anomalous Features of Extreme Meiyu in 2020 over the YangtzeHuai River Basin and Attribution to Large-Scale Circulations Ruoyun Niu, Panmao ZHAI, Guirong TAN Citation: Niu, R. Y., P. M. Zhai, G. R. Tan, 2021: Anomalous features of extreme Meiyu in 2020 over the YangtzeHuai River basin and attribution to large-scale circulations. J. Meteor. 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Part I: Data Analysis Journal of Meteorological Research. 2019, 33(3), 400 https://doi.org/10.1007/s13351-019-8604-1 The Record-Breaking Extreme Drought in Yunnan Province, Southwest China during Spring–Early Summer of 2019 and Possible Causes Journal of Meteorological Research. 2020, 34(5), 997 https://doi.org/10.1007/s13351-020-0032-8 Volume 35 Journal of Meteorological Research OCTOBER 2021 ● Forecasting Forum ● Anomalous Features of Extreme Meiyu in 2020 over the Yangtze–Huai River Basin and Attribution to Large-Scale Circulations Ruoyun Niu1, Panmao ZHAI2*, and Guirong TAN3 1 National Meteorological Center, China Meteorological Administration, Beijing 100081 2 Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing 100081 3 Nanjing University of Information Science &Technology, Nanjing 210044 (Received January 24, 2021; in final form June 6, 2021) ABSTRACT Extremely anomalous features of Meiyu in 2020 over the Yangtze–Huai River basin (YHRB) and associated causes in perspective of the large-scale circulation are investigated in this study, based on the Meiyu operational monitoring information and daily data of precipitation, global atmospheric reanalysis, and sea surface temperature (SST). The main results are as follows. (1) The 2020 YHRB Meiyu exhibits extremely anomalous characteristics, which are the most prominent since the 1980s. The 2020 Meiyu season features the fourth earliest onset, the third latest retreat, the longest duration, the maximum Meiyu rainfall, the strongest mean rainfall intensity, and the maximum number of sta- tions/days with rainstorm. (2) The extremely long duration of the 2020 Meiyu season lies in the farily early onset and late retreat of Meiyu in this particular year. The early onset of Meiyu is due to the earlier-than-normal first northward shift and migration of the key influential systems including the northwestern Pacific subtropical high (NWPSH) and the South Asian high (SAH) along with the East Asian summer monsoon, induced by weak cold air activities from late May to early mid-June. However, the extremely late retreat of Meiyu is because of later-than-normal second northward shift of the associated large-scale circulation systems accompanied with strong cold air activities, and ex- tremely weak and southward located ITCZ over Northwest Pacific in July. (3) The extremely more than normal Meiyu rainfall is represented by its long duration and strong rainfall intensity. The latter is likely attributed to ex- treme anomalies of water vapor convergence and vertical ascending motion over the YHRB, resulting from the com- pound effects of the westward extended and enlarged NWPSH, the eastward extended and expanded SAH, and the strong water vapor transport associated with the low-level southerly wind. The extremely warm SST in the tropical Indian Ocean seems to be the key factor to induce the above-mentioned anomalous large-scale circulations. The res- ults from this study serve to improve understanding of formation mechanisms of the extreme Meiyu in China and may help forecasters to extract useful large-scale circulation features from numerical model products to improve me- dium-extended-range operational forecasts. Key words: extreme Meiyu, anomalous feature, large-scale circulation, cause Citation: Niu, R. Y., P. M. Zhai, G. R. Tan, 2021: Anomalous features of extreme Meiyu in 2020 over the Yangtze–Huai River basin and attribution to large-scale circulations. J. Meteor. Res., 35(5), 1–16, doi: 10.1007/s13351-021-1018-x. 1. Introduction fects the droughts and floods in the YHRB, but also asso- ciates with precipitation patterns throughout China. Stud- Meiyu in China refers to the persistent rainy weather ies have shown that Meiyu, as a product of the seasonal happening mainly over the Yangtze–Huai River basin transition of the East Asian atmospheric circulation, is (YHRB) in early summer, with occurrence of frequent modulated by the interaction among multiple members of and concentrated regional rainstorms. Under the back- the Asian summer monsoon system. Moreover, the ground of climate warming, the number of days and thermal conditions in the tropical oceans and other ex- amount of heavyPaper rainfall in most parts of China showin in- ternal Press factors can also exert influences on Meiyu (Niu creasing trends (Jiang et al., 2014). Meiyu not only af- and Jin, 2009; Yuan et al., 2017). Supported by the National Key Research and Development Program of China (2018YFC1507703). *Corresponding author: [email protected] © The Chinese Meteorological Society and Springer-Verlag Berlin Heidelberg 2021 2 Journal of Meteorological Research Volume 35 Around the time when Meiyu starts, atmospheric cir- Asia, and the extremely anomalous characteristics of the culation changes significantly over the Indian Peninsula, key influential systems in the upper, middle, and lower the YHRB and east coast of China, and the central North troposphere. The purpose of this study is to obtain a fur- Pacific (Liu et al., 2011). In the years of Meiyu early on- ther understanding on the formation mechanism of the set, the South Asian high (SAH) and East Asian subtrop- extreme Meiyu in 2020 in China, providing scientific ical westerly jet establish earlier and the South China Sea basis for better Meiyu forecast in the future. summer monsoon also bursts earlier (Zhao et al., 2018a). When the “ + − + ” wave train appears in the region from 2. Data and methods the Ural Mountain to the Okhotsk Sea in the mid–high latitudes of Asia and from the low to high latitudes of 2.1 Data East Asia, the rainfall in the Meiyu season is more Data used in this paper are as follows: the Meiyu on- abundant than normal (Zhang and Tao, 1998). The two set and retreat dates for three sub-regions (i.e., the re- seasonal northward shifts of the northwestern Pacific gions to the south, along, and north of the Yangtze River) subtropical high (NWPSH) are closely related to the of the YHRB (28°–34°N, 110°–123°E), released by the Meiyu onset and retreat. The active convection of the in- National Climate Center of China Meteorological Ad- tertropical convergence zone (ITCZ) can be regarded as ministration (CMA) (National Climate Center, 2018; the precursory signal for the strengthening and north- Wang and Zheng, 2018; Chen et al., 2019; Ding et al. ward shift of the NWPSH, and can impact the northward , 2020; Dai et al., 2021); the daily precipitation data from shift and westward extension of the NWPSH (Xu et al., National Meteorological Information Center of CMA; 2001). Weaker monsoon troughs along with less fre- the NCEP daily global atmospheric reanalysis data on a quency of tropical cyclones are found to be responsible horizontal resolution of 2.5° × 2.5° and 17 layers from for stronger intensity of the NWPSH and Meiyu rainfall 1000 to 10 hPa (Kanamitsu et al, 2002); and the daily (Zhu et al., 2017). The distinctive tropospheric warming data of optimal interpolation SST (Reynolds et al., 2007) and stratospheric cooling in the midlatitudes can lead to and interpolated outgoing longwave radiation (OLR) elevated tropopause in the subtropics, widening of the (Liebmann and Smith, 1996) from NOAA with horizon- subtropics over East Asia, and the northward shift of the tal resolution of 0.25° × 0.25° and 2.5° × 2.5°, respect- Meiyu belt (Si et al., 2009). ively. Since the beginning of the 21st century, Meiyu in The data period in this study is defined as from 1981 China has been featured with late onset and early retreat, to 2020, and the climatological mean is the average from along with short duration and weak rainfall intensity (Ji- 1981 to 2010. However, the SST data are available from ang and Gao, 2013). In particular, the Meiyu in 2016 and 1982 to 2020; due to limitation in the starting time, the 2020 was characterized by historically rare early onset, climatology mean of the SST data is the average from long duration, and strong rainfall intensity (Zhao and 1982 to 2010. Niu, 2019; Zhang et al., 2020; Li et al., 2021), which 2.2 Methods brought about serious flooding disasters to the YHRB. The study on Meiyu has once again become a hot topic. 2.2.1 Statistics of Meiyu features Wang et al. (2020) and Liu et al. (2021) analyzed the in- According to GB/T 33671-2017 Meiyu Monitoring In- fluence of the atmospheric circulation anomalies from dices (General Administration of Quality Supervision, June to July in summer 2020 and the external forcing Inspection, and Quarantine of the People’s Republic of factors such as sea surface temperature (SST) on Meiyu.