Anomalous Features of Extreme Meiyu in 2020 over the YangtzeHuai River Basin and Attribution to Large-Scale Circulations Ruoyun Niu, Panmao , 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. Res., 35(5), 1-16, doi: 10.1007/s13351-021-1018-x

View online: http://jmr.cmsjournal.net/article/doi/10.1007/s13351-021-1018-x

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● 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 , 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 ( 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 ( 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 ( 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 ( 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 (- 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; 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. China, 2017), the onset (retreat) date of the Meiyu sea- Ding et al. (2021) pointed that the record-breaking Meiyu son over the YHRB is the earliest (latest) one in the three over the YHRB in 2020 is closely related to the typi- sub-regions. The Meiyu rainfall (mm) is the accumula- cal quasi-biweekly oscillation of the East Asian summer tion of the daily area averaged precipitation over the monsoon (EASM) circulation. YHRB (277 meteorological stations in total) during the In the present study, we focus on sorting out the ex- Meiyu season, and the mean rainfall intensity (mm day−1) tremely anomalous features of Meiyu over the YHRB in is the mean of the daily area averaged precipitation over 2020, and explorePaper the causes of the extreme Meiyu in in as- the PressYHRB during the Meiyu season. In addition, identi- sociation with the anomalous evolution of the large-scale fication of the regional rainstorm process is based on circulations, which is suspected to lead to the extreme weather events with a combination of subjective and ob- anomalies in Meiyu onset/retreat over the YHRB in jective methods (Niu et al., 2018). 2020. Especially, we discuss from the perspective of the 2.2.2 Indices and circulation characteristic indicators seasonal transition of the atmospheric circulation in East The area index, westward ridge point (WRP), and OCTOBER 2021 Niu, R. Y., P. M. Zhai, G. R. Tan 3 ridge line position (RLP) of the NWPSH at 500 hPa, and south of the θse isolines of 335 K and 340 K are basic- the area index, eastward ridge point (ERP), and RLP of ally consistent, with a very small interval. However, the SAH at 200 hPa are calculated by using the defini- there is sometimes a large interval between the two θse tions of Niu and Zhai (2013). The western RLP of the isolines in the late Meiyu season and before and after the

NWPSH refers to the average of the PLP of the NWPSH Meiyu retreat date. In such a case, the θse isoline of 340 over 110°–130°E, and the eastern RLP of the SAH refers K is closer to the strong gradient zone of southerly wind to the average of the PLP of the SAH over 110°–125°E. associated with the southwesterly flows and the northern With reference to previous studies (Gao and Xue, edge of the Meiyu belt than that of 335 K. For this reas- 2006; Li and Li, 2014) in combination with the climato- on, the north boundary of the EASM is determined by the logy distribution characteristics of the meridional wind latitude location of the north edge of the southwesterly reflected in the atmospheric reanalysis data, the Philip- flow with θse exceeding 340 K. pines and New Guinea cross-equatorial flow (CEF) in- Among the Meiyu monitoring indices, the north and dices are defined as the area averaged meridional wind south boundaries of the western RLP of the NWPSH are over the region 2.5°S–2.5°N, 125°–135°E and the region defined to correspond to the Meiyu onset (≥ 18°N) and 2.5°S–2.5°N, 142.5°–152.5°E at 925 hPa, respectively. retreat (≥ 27°N) over the YHRB. However, the north and The Indian Ocean basin-wide mode (IOBW) index is south boundaries of the eastern RLP of the SAH are not the area averaged SST anomaly in the tropical Indian clearly related to the Meiyu onset and retreat, thus they Ocean (IO) (20°S–20°N, 40°–110°E; Yuan et al., 2017). need to be further standardized. Firstly, the daily western The ITCZ intensity index over the Northwest Pacific RLP of the NWPSH and the daily eastern RLP of the is the number of grid points with OLR ≤ 220 W m−2 over SAH at 200 hPa in the Meiyu season over the YHRB the region 5°–20°N, 110°–140°E. The larger the index from 1981 to 2020 are sorted from south to north, re- value is, the stronger the intensity of the ITCZ and the spectively, to obtain their respective RLPs at different more active the tropical convection would be in the quantiles in the season (Fig. 1a). Secondly, the number of Northwest Pacific. days when the RLP falls in the latitude range correspond- The north boundary of the EASM is determined based ing to each percentile interval among the nine percentile on both the distribution and the thermal properties of air intervals is calculated during the maximum possible dur- flows, in which southwesterly flows with potential ation period of Meiyu over the YHRB. The 9 percentile pseudo-equivalent temperature (θse) exceeding a certain intervals are cut from the two tail ends (5% and 95%) to threshold value are examined, and two thresholds of 335 the middle at interval of every 5 percentages, and the and 340 K are adopted according to Li et al. (2013). In maximum possible duration period of Meiyu is from the the context, the daily evolution of θse, wind vector, meri- earliest onset date of 25 May (1995 and 2016) to the dional wind, and precipitation averaged over 110°–123°E latest retreat date of 7 August (1993) during 1981 to during the Meiyu season and its adjacent periods in the 2020. Finally, the correlation coefficients between the past 40 years are analyzed and compared. The results number of days of the RLP falling in the latitude range show that in most cases the movement to the north and corresponding to each percentile interval and the dura-

(a) (b) 32 Western RLP Western RLP 30 Eastern RLP 0.6 Eastern RLP 28 26 0.5

24 r

RLP (°N) 22 0.4 20 18 0.3 16 0.2 5 Paper10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 in85 90 95 Press 5−95 10−90 15−85 20−80 25−75 30−70 35−65 40−60 45−55 Percentile (%) Percentile (%)

Fig. 1. (a) The western RLP of the NWPSH at 500 hPa and the eastern RLP of the SAH at 200 hPa at the different percentile in the Meiyu sea- son and (b) correlation coefficient (r) between the number of days of the RLP falling in the latitude range corresponding to each percentile inter- val during maximum possible duration period of Meiyu and the duration days of the Meiyu season over the YHRB from 1981 to 2020. 4 Journal of Meteorological Research Volume 35 tion days of the Meiyu season over the YHRB during Meiyu and the longer the duration of the Meiyu season 1981 to 2020 are calculated. would be. The eastern RLP of the SAH at 200 hPa cor- The results show that the correlation is the highest responding to 10% and 90% quartile is 21.2 and 30.2°N, between the number of days when the western RLP of respectively. Therefore, when the north and south bound- the NWPSH falls in the latitude range corresponding to aries of the eastern RLP of the SAH at 200 hPa is farther 15%–85% and the duration days of the Meiyu season, north of 21.2 and 30.2°N respectively, it is more condu- with a correlation coefficient of 0.48 (passing the t-test at cive to early onset and late retreat of Meiyu over the

the significance level of α = 0.05; Fig. 1b). This indic- YHRB. ates that the more the number of days that the western RLP of the NWPSH falls in the latitude range corres- 3. Extremely anomalous features of Meiyu in ponding to 15%–85%, the more favorable the occur- 2020 rence and maintenance of Meiyu and the longer the dura- tion of the Meiyu season would be. The western RLP of According to the monitoring information, the onset the NWPSH at 15% and 85% quartile is 18.4 and 26.6°N, date of the Meiyu season over the YHRB in 2020 is May respectively, closest to the south and north boundaries of 29 with a standardized anomaly of −1.18σ, and is the the western RLP of the NWPSH in the obtained Meiyu fourth earliest Meiyu season since 1981, but later than monitoring indices. that in 2016, 1995, and 2015. The Meiyu retreat date is 2 By analogy, the number of days that the eastern RLP August with a standardized anomaly of 1.44σ, the third of the SAH at 200 hPa falls in the latitude range corres- latest since 1981, but only earlier than that in 1993 and ponding to 10%–90% has the highest correlation to the 1998. The number of duration days of 2020 Meiyu is 65 duration days of the Meiyu season, with a correlation days with a standardized anomaly of 2.03σ, the longest coefficient of 0.54, indicating that the more the number duration since 1981 (Fig. 2a). The 2020 Meiyu rainfall of days that the eastern RLP of the SAH at 200 hPa is reaches 780.9 mm with a standardized anomaly as high located in the latitude range corresponding to 10%–90%, as 3.08σ, the maximum since 1981. The mean rainfall in- the more favorable the occurrence and maintenance of tensity of 2020 Meiyu is 12.1 mm day−1 with a standard-

(a) (b)

3 Onset date r = −0.62 3 Duration days r = 0.83 Duration days Meiyu rainfall r = 0.67 Retreat date Mean rainfall intensity r = 0.73 2 2

1 1

0 0 −1 −1 −2 −2 1981 1986 1991 1996 2001 2006 2011 2016 1981 1986 1991 1996 2001 2006 2011 2016

(c) (d) Rainstorm 60 1200 Heavy rainstorm

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0 Paper in Press0 1981 1986 1991 1996 2001 2006 2011 2016 1981 1986 1991 1996 2001 2006 2011 2016

Fig. 2. (a, b) Normalized and (c, d) original time series of the extreme properties of heavy rain in the Meiyu season over the YHRB from 1981 to 2020: (a) the Meiyu onset date, retreat date, and duration days, (b) the Meiyu rainfall, mean rainfall intensity, and duration days, (c) the num- ber of stations/days of rainstorm and heavy rainstorm, and (d) the number of stations with daily precipitation ranking the top five. Note that r in (a) and (b) denotes correlation coefficient. OCTOBER 2021 Niu, R. Y., P. M. Zhai, G. R. Tan 5 ized anomaly of 1.84σ, which is the strongest since 1981 ity. This extremely long Meiyu season over the YHRB in (Fig. 2b). 2020 is due to the extremely early onset and late retreat It is evident in Fig. 3a that heavy rainfall processes oc- of Meiyu. Statistical results from 1981 to 2020 show that curred frequently in the Meiyu season of 2020, with 15 the duration of the Meiyu season is highly positively cor- processes identified. Although the heavy rainfall belt os- related with the onset date but is highly negatively correl- cillated from north to south, it is still relatively stable ated with the retreat date, with correlation coefficient of within the latitude range of the YHRB. The accumulated −0.62 and 0.67, respectively. The Meiyu rainfall amount precipitation over the YHRB exceeded 500 mm in gene- is related to both the Meiyu duration and the rainfall in- ral, and reached 900–1700 mm (1–2 times more than tensity. The correlation coefficient between the Meiyu normal) in the center area, in eastern Hubei Province, rainfall and the duration (mean rainfall intensity) in the southern Anhui Province, northeastern Jiangxi Province, Meiyu season from 1981 to 2020 is as high as 0.83 and northwestern Zhejiang Province (Figs. 3b, c). The (0.73). This indicates that the longer the duration of the number of stations (days) of rainstorm or heavy rain- Meiyu season and the stronger the rainfall intensity, the storm is as high as 1275 (285), ranking the maximum in greater the Meiyu rainfall. Next, we focus on analysis the Meiyu season since 1981. The number of stations that and discussion of the causes of the extremely anomalous experienced the top 5 heaviest daily precipitation ranks features of the onset date, retreat date, and rainfall intens- the second in the Meiyu season since 1981, next to 2016 ity of the Meiyu season over the YHRB in 2020. (Figs. 2c, d). To sum up, the Meiyu season over the YHRB in 2020 4. Large-scale circulations associated with exhibits extremely anomalous features in many aspects the extremely anomalous Meiyu in 2020 such as the Meiyu onset (retreat) date, duration, rainfall amount, and rainfall intensity. The 2020 Meiyu is charac- 4.1 Causes for the extremely early onset of Meiyu terized by extremely early onset, late retreat, long dura- The early onset of Meiyu is related to the early seasonal tion, abundant Meiyu rainfall, and strong rainfall intens- adjustment and migration of the East Asian atmospheric

(a)

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105E 110E 115E 120E 105E 110E 115E 120E

Fig. 3. (a) Time–latitude evolution of daily precipitation averaged over 110°–123°E from May to August, and distributions of (b) accumulated precipitation and (c) its percentage anomalies during the Meiyu season over the YHRB in 2020. 6 Journal of Meteorological Research Volume 35 circulation from winter to summer (Zhao et al., 2018b). and southward retreat of the NWPSH, SAH, EASM, and From Fig. 4, it is seen that the western RLP of the NWP- other key influence systems are affected by the cold air SH is obviously northward than normal from late May to carried by the westerly trough moving southward from early mid-June, of which the 5-day sliding average is the mid–high latitudes, and by the intensity and location stably located to the north of 18°N since 30 May. This of the ITCZ (Zhang and Tao, 1999; Tao and Wei, 2006). indicates that the western ridge of the NWPSH has fin- From late May to early mid-June in 2020, the Arctic re- ished its first seasonal northward shift, which is 7 days gion was dominated by polar vortex, the cold air from the earlier than normal (6 June). The eastern RLP of the polar region to the south was significantly weakened, and SAH at 200 hPa also migrated northward to the north of the front zone at mid-high latitudes contracted north- 21.2°N from 31 May and stably located in the latitude ward. Although the low troughs appeared frequently range conducive to the Meiyu onset, which is 11 days within the latitude range of the YHRB, temperature an- earlier than normal (11 June). Under the guidance of the omalies at 850 hPa were positive, suggesting that the mid–upper level circulation system, the north boundary cold air activities over the YHRB are frequent but weak- of the EASM at the low level (the north edge of south- er than normal (Fig. 5c). Weaker-than-normal cold air westerly winds with θse more than 340 K at 850 hPa) mi- activities are favorable for seasonal northward shift and grated northward into the YHRB at an earlier time than the norther-than-normal location of the NWPSH. Ac- normal. Afterwards, the EASM continuously poured in cording to the climatological mean, the date when the and transported abundant water vapor and energy to the northern boundary of OLR less than 220 W m−2 aver- YHRB, and frequently converged with cold air in the re- aged over 110°–140°E in the Northwest Pacific extends gion, resulting in frequent heavy rainfall processes over to the north of 10°N is basically consistent with the the YHRB (Figs. 5a, b) and the Meiyu was initiated on Meiyu onset date (8 June) over the YHRB (the gray line 30 May. Thus, the extremely early Meiyu onset is mainly in Fig. 6), which is very close to normal situation (Fig. due to the early first seasonal northward shift and migra- 6a). It can be seen from the above analysis that the early tion of the key influence systems, such as the NWPSH, first seasonal northward shift and migration of the key in- SAH, and EASM, which are responsible for the earlier fluence systems including the NWPSH, SAH, and EASM transition of the East Asian atmospheric circulation pat- in 2020 are mainly affected by the weak cold air activit- tern from winter to early summer. ies from late May to early mid-June. Previous studies have shown that the northward shift 4.2 Causes for the extremely late retreat of Meiyu

(a) The second northward shift of the NWPSH is closely related to the Meiyu retreat, implying establishment of 36 32 the midsummer atmospheric circulation pattern over East 28 27°N Asia. After the Meiyu onset in 2020, although the west- 24 ern RLP of the NWPSH at 500 hPa displayed a tendency RLP (°N) 20 18°N of northward migration slowly, it was oscillating from 16 north to south in the latitude range of 18°–27°N, condu- 12 cive to the maintenance of Meiyu. Until 29 July, the 16 May 1 June 16 June 1 July 16 July 1 August western RLP of the NWPSH settled north of 27°N and (b) finished the second seasonal northward shift, 8 days later

36 than normal (21 July). Similarly, the eastern RLP of the 32 30.2°N SAH at 200 hPa was relatively stable in the latitude 28 range of 21.2°–30.2°N, conducive to the maintenance of 24 21.2°N Meiyu after the Meiyu onset. It was not until 1 August RLP (°N) 20 that it moved to the north of the northern boundary 16 (30.2°N), also later than normal (Fig. 4). Regulated by 12 the mid–upper level circulation system, the north bound- 16 May 1Paper June 16 June 1 July 16 July 1 August in Press ary of the low-level EASM was relatively stable in the Fig. 4. Evolution of the daily (a) western RLP of the NWPSH at 500 latitude range of the YHRB after the Meiyu onset, and hPa and (b) eastern RLP of the SAH at 200 hPa from May to August in strong meridional gradient of the southerly flows over 2020. Red lines denote the original time series, black lines represent the 5-day sliding average, and green lines represent the climatological the south side of the north EASM boundary also main- mean. tained in the latitude range of the YHRB, leading to con- OCTOBER 2021 Niu, R. Y., P. M. Zhai, G. R. Tan 7

(a) 45N

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℃ −4 −2 0 2 4

Fig. 5. Time–latitude cross-sections averaged over 110°–123°E of the daily (a) wind vector (arrow), meridional wind (brown line and shading), and θse isoline of 340 K (bold black line) at 850 hPa; (b) water vapor flux vector (arrow), and its meridional component (brown line) and diver- gence (shading) at 850 hPa; and (c) geopotential height at 500 hPa (bold black line denotes the 588-dagpm isoline of the NWPSH while black dot–dashed line showsPaper the ridge line of the NWPSH) and temperaturein anomalies Press at 850 hPa (shading) from May to August in 2020. In (a)–(c), green dashed lines denote the latitude range of the YHRB. tinuous moisture convergence for frequent heavy rain late July that the north edge of the low-level EASM and long duration of the Meiyu season. It was not until crossed the north flank of the YHRB, and then the Meiyu 8 Journal of Meteorological Research Volume 35

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16 May 1 June 16 June 1 July 16 July 1 August

W m−2 160 180 200 220

Fig. 6. Time–latitude cross-sections averaged over 110°–140°E of daily geopotential height at 500 hPa above 586 dagpm (bold dark-red solid line shows the 588-dagpm isoline of NWPSH, dark-red dot–dashed line shows the ridge line of NWPSH, and black lines show the climatic NWPSH and its ridge line) and OLR below 220 W m−2 (blue shading; gray line denotes the climatic isoline of 220 W m−2) from May to August: (a) 2020, (b) composite for the Meiyu season with anomalously late retreat date and long duration, and (c) composite for the Meiyu season with anomalously early retreat date and short duration. retreated on 2 August (Figs. 5a, b). Obviously, the ex- NWPSH delayed? Why was the establishment of the tremely late MeiyuPaper retreat is mainly caused inby the midsummer Press atmospheric circulation pattern over East delayed second northward shift and migration of the Asia postponed? Compared with June, the Arctic region NWPSH, SAH, and EASM, which contributes to the in July 2020 turned to be dominated by the Polar high, delayed setup of the midsummer atmospheric circulation and the region near the Okhotsk Sea was affected fre- pattern over East Asia. quently by a strong blocking high, forcing the cold air to Why was the second seasonal northward shift of the invade the lower latitudes. Though low troughs still ap- OCTOBER 2021 Niu, R. Y., P. M. Zhai, G. R. Tan 9 peared frequently, temperature anomalies at 850 hPa alous characteristics of the ITCZ in Northwest Pacific turned negative in the latitude range of the YHRB (Fig. and the NWPSH for the composite of the Meiyu season 5c). This indicates that the cold air activities over the with anomalously late retreat and long duration are very YHRB were more frequent and stronger than normal. similar to those in 2020. That is to prove, the ITCZ con- The stronger-than-normal cold air is bound to suppress vection over Northwest Pacific is obviously weaker than the seasonal northward shift of the NWPSH. normal, the date of the northern boundary of OLR less Climatologically, the northern boundary of OLR less than 220 W m−2 averaged over 110°–140°E extending to than 220 W m−2 averaged over 110°–140°E in Northw- north of 17°N is later (in late July), and the western RLP est Pacific is south of 17°N during the Meiyu season, and of the NWPSH is obviously more southward (Fig. 6b). steadily crosses 17°N until about the retreat date of The reverse is true for the composite Meiyu season with Meiyu. The date of crossing 17°N of the above northern anomalously early retreat and short duration (Fig. 6c). boundary of OLR less than 220 W m−2 was 7 days later The intensity and location of the ITCZ over North- than normal in 2020. In addition, the ITCZ intensity over west Pacific can be affected by the CEFs from the South- Northwest Pacific during most time of July (Fig. 7a) was ern Hemisphere. The weaker the CEFs in the east of weaker than normal. The ITCZ intensity index over 100°E, the weaker the intensity of the ITCZ and the more Northwest Pacific was the third lowest in July since 1981 southward its location would be ( et al., 2004; Liu et (Fig. 8a), with a normalized anomaly as low as −1.45σ. al., 2009; Sui and , 2017). In July 2020, the Philip- The ITCZ intensity index of Northwest Pacific in July is pines and New Guinea CEFs were continuously weaker positively correlated with the western RLP of the NW- than normal (Fig. 7b), and their indices ranked the fourth PSH, with a correlation coefficient of 0.59 (significant at lowest and the lowest since 1981 with normalized anom- 0.05). This means that the weaker the ITCZ, the more alies of −1.85σ and −1.49σ, respectively (Figs. 8b, c). In southward the western ridge of the NWPSH would be. summary, the late second northward shift of the NWP- Furthermore, the years for the Meiyu season with an- SH and the other key influence systems in 2020 are also omalously late retreat (σ more than 1.0) and long dura- influenced by the extremely weak and further southward tion (σ more than 1.0) over the YHRB were selected, in- ITCZ over Northwest Pacific, as well as the extremely

cluding 1993, 1998, 1999, and 2020. For comparison, the weak Philippines and New Guinea CEFs in July 2020. years for the Meiyu season with anomalously early re- treat (σ less than −1.0) and short duration (σ less than 4.3 Causes of the extremely strong rainfall intensity −1.0) were also extracted, which are 2001, 2002, and During the Meiyu season over the YHRB, the range 2013. The composite results show clearly that the anom- and intensity of the key influential systems such as the NWPSH have a significant impact on the rainfall intens- (a) ity. The WRP and area index of the NWPSH are closely 2.5 2.0 related to the mean rainfall intensity (with correlation 1.5 1.0 coefficient of −0.49 and 0.46, respectively). Table 1 in- 0.5 0 dicates that the more westward the WRP and the larger −0.5 the coverage of the NWPSH, the stronger the rainfall in- −1.0 −1.5 tensity tends to be in the Meiyu season. In the Meiyu sea- −2.0 −2.5 son of 2020, the WRP of the NWPSH is 108°E, about 16 March 1 June 16 June 1 July 16 July 1 August 20° more westward than normal (Fig. 9a). It is the second westernmost, just eastward to that in 2010 since 1981. (b) The range of the NWPSH is the third largest and only 1.5 1.0 smaller than that in 2010 and 2016 since 1981 (Fig. 10a). 0.5 0 It can be inferred that the extremely westward extended −0.5 western ridge and extremely large coverage of the NWP- −1.0 −1.5 SH are the key factors leading to the extremely strong −2.0 rainfall intensity of the Meiyu season in 2020. −2.5 Paper in Press −3.0 It is found that the extreme anomalies of the NWPSH 16 March 1 June 16 June 1 July 16 July 1 August are linked to the extreme anomalies of the SAH and the

warm SST in the tropical IO. Studies have shown that the Fig. 7. Normalized time series of the daily (a) ITCZ intensity index over Northwest Pacific and (b) Philippines CEF index (red line) and SAH can affect the development of the NWPSH by dy- New Guinea CEF index (green line) from May to August in 2020. namic and thermal mechanisms, and they have a tend- 10 Journal of Meteorological Research Volume 35

(a)

2

1

0

−1

1981 1986 1991 1996 2001 2006 2011 2016

(b) (c)

1 2

0 1

−1 0

−2 −1

1981 1986 1991 1996 2001 2006 2011 2016 1981 1986 1991 1996 2001 2006 2011 2016

Fig. 8. Normalized time series of (a) ITCZ intensity index over Northwest Pacific, (b) Philippines CEF index, and (c) New Guinea CEF index, in July from 1981 to 2010. ency to move towards (backwards) each other (Ren et al., ernmost since 1981, and the coverage of the SAH is the 2007). In the Meiyu season, there is a significant negat- fourth largest (Fig. 10b). On the other hand, the warm ive correlation (with correlation coefficient of −0.59) SST anomalies over the tropical IO can trigger an anom- between the ERP of the SAH at 200 hPa and the WRP of alous anticyclone circulation over Northwest Pacific the NWPSH at 500 hPa, and a significant positive correl- through the Matsuno–Gill response (Matsuno, 1966; Gill, ation (with correlation coefficient of 0.81) between the 1980), which facilitates the westward extension and area indices of the two system (Table 2). Meanwhile, the strengthening of the NWPSH ( et al., 2009; Yuan et ERP and area index of the SAH are significantly correl- al., 2012, 2017), further enhancing the rainfall intensity. ated with the mean rainfall intensity (with correlation The IOBW index in the Meiyu season is significantly coefficients of 0.31 and 0.33, respectively) (Table 1). negative and positive correlated with the WRP and area This suggests that the more eastward the eastern ridge index of the NWPSH (correlation coefficients are −0.53 and the larger the coverage of the SAH, the more west- and 0.63, respectively), and is significantly positive cor- ward the western ridge and the larger the coverage of the related with the mean rainfall intensity (correlation coef- NWPSH, and the stronger the rainfall intensity would be. ficient is 0.44). Thus, when the SST over the tropical IO In the Meiyu season of 2020, the ERP of the SAH de- is uniformly warmer than normal, the western ridge of noted by the 1252-dagpm was 136°E, about 27° more the NWPSH tends to be more westward and its coverage eastward than normal (Fig. 9a), ranking the fourth east- would be larger than normal, and the rainfall intensity

Table 1. Correlation coefficients between the mean rainfall intensity and averaged circulation indices, related physical quantities, and the IOBW index in the Meiyu season from 1981 to 2020 NWPSH SAH at 200 hPa Average over the YHRB IOBW index WRP Area index ERP Area index Water vapor flux divergence at 850 hPa Omega at 500 hPa −0.49# 0.46# 0.31* 0.33# 0.44# −0.60# −0.67# Note: # (*) denotesPaper the value passing the t-test at the significance in level of α = 0.05Press (0.10). Table 2. Correlation coefficients between averaged circulation characteristic indices and IOBW index in the Meiyu season from 1981 to 2020 SAH at 200 hPa IOBW index ERP Area index WRP of NWPSH −0.59# −0.62# −0.53# Area index of NWPSH 0.75# 0.81# 0.63# Note: # denotes the value passing the t-test at the significance level of α = 0.05. OCTOBER 2021 Niu, R. Y., P. M. Zhai, G. R. Tan 11

(a) (b)

0 0 8 6 8 4 3 2 6

30E 1 30E −1 s

0 4 −6

−1 10 −2 2 −3 −4 −6 0 −8 60E 60E Dagpm m s−1

90E 120E 150E 90E 120E 150E

(c) (d)

40N −2 40N 6 4 30N −4 30N 2 −1 1 −1 s −6 s −2 0 −2 20N 20N −8 −1 kg m −2 kg m 10N −10 10N −4 −6 −12 0 0 −8

60E80E 100E 120E 140E 15 kg m−1 s−1 60E80E 100E 120E 140E 5 kg m−1 s−1

Fig. 9. Mean large-scale circulations for the Meiyu season of 2020: (a) geopotential height (black solid lines) and its anomalies (shading) at 500 hPa, (b) geopotential height (black solid lines), zonal wind greater than 30 m s−1 (blue lines), and divergence (shading) at 200 hPa, (c) water va- por flux (vector) and its divergence (shading) at 850 hPa, (d) anomalies of water vapor flux (vector) and its divergence (shading) at 850 hPa. In (a) and (b), the bold black solid lines show the range of the subtropical highs at 500 hPa and the SAH at 200 hPa denoted by the 588- and 1252- dagpm isolines respectively; the black dashed lines show the ridge line of the highs; and the green lines show the climatic range and ridge line of the highs. The rectangle in (c) and (d) denotes the region 20°–33°N, 105°–123°E. would resultantly be more intense. In the Meiyu season water vapor was transported (Figs. 9c, d) and the area av- of 2020, the SSTs over the tropical IO were extremely eraged meridional water vapor flux was the fifth warm than normal and the IOBW index ranked the strongest since 1981 (Fig. 10d). Meanwhile, the circula- highest since 1981 (Fig. 10c). tion over the mid–high latitudes in Asia displayed a pat- The low-level anomalous anticyclone circulation in tern of double blocking highs, with the ridges near the the Northwest Pacific triggered by the warm SST in the Ural Mountains and Okhotsk sea, and the low troughs tropical IO is conducive to the strengthening of the near Lake Balkhash, Lake Baikal, and the East China southerly flow to its west side, and the westward anom- Sea, respectively (Figs. 9a, b). The cold air carried by the aly of the western ridge of the NWPSH is favorable for low troughs frequently invaded the YHRB from the west, guiding the low-level southerly flow to turn along its northwest, and east. The confluence of the cold air and western edge into China, providing abundant moisture the extremely strong warm–wet southerly flows pro- and energy for the occurrence of heavy rainfall. In the duced obviously stronger than normal moisture conver- 2020 Meiyu season, the western ridge of the NWPSH ex- gence at 850 hPa over the YHRB (Figs. 9c, d); the area hibited extremely westward anomaly, the water vapor averaged moisture convergence (divergence) over the transport by thePaper southerly flow turning along the westerninYHRB Press is the third strongest (lowest) in the Meiyu sea- edge of the NWPSH also displayed extremely strong an- son since 1981 (Fig. 10d). The low-level moisture con- omaly. The shear line formed by the confluence of cold vergence is crucial to the formation of heavy rainfall. and warm air flows was located near 33°N, and the re- Analysis suggests that the area averaged water vapor flux gion south of the shear line (20°–33°N, 105°–123°E) was divergence at 850 hPa is highly pertinent to the mean dominated by southerly flows, in which more abundant rainfall intensity in the Meiyu season with a correlation 12 Journal of Meteorological Research Volume 35

(a) (b)

WRP 3 ERP Area index Area index 2 2

1 1

0 0

−1 −1

−2 −2

1981 1986 1991 1996 2001 2006 2011 2016 1981 1986 1991 1996 2001 2006 2011 2016

(c) (d) 3 3 Omega Meridional component IOBW index Divergency 2 2

1 1 0 0 −1 −1 −2 −2 1981 1986 1991 1996 2001 2006 2011 2016 1981 1986 1991 1996 2001 2006 2011 2016

Fig. 10. Standardized time series of the averaged circulation characteristic indices, relative physical quantities, and the IOBW index in the Meiyu season of the YHRB from 1981 to 2020: (a) the WRP and area index of the NWPSH at 500 hPa, (b) the ERP and area index of the SAH at 200 hPa, (c) the IOBW index and area averaged vertical motion (omega) over the YHRB at 500 hPa, (d) area averaged divergence over the YHRB and meridional component of the water vapor flux in the region 20°–33°N, 105°–123°E at 850 hPa. coefficient of −0.60. This indicates that stronger low- between the area averaged vertical motion at 500 hPa and level moisture convergence is prone to cause stronger the mean rainfall intensity over the YHRB in the Meiyu rainfall intensity. Thus, another key factor inducing the season with correlation coefficient of −0.67, indicating extremely strong rainfall intensity is the extremely strong that the stronger the vertical ascending motion, the moisture convergence resulting from the extremely stronger the rainfall intensity would be. Therefore, the strong low-level water vapor transport associated with extremely strong vertical ascending motion is also one of the southerly flow turning along the western edge of the the key factors resulting in the extremely strong rainfall NWPSH and the confluence of cold and warm air flows. intensity in the Meiyu season of 2020. When the SAH extends eastward, the upper-level di- To sum up, the extremely strong Meiyu rainfall intens- vergence located at its northeastern quadrant also moves ity in 2020 is a result of the extremely strong water va- eastward, providing necessary dynamic conditions for the por convergence and vertical ascending motion over the formation of heavy rainfall over the YHRB. In the Meiyu YHRB, which occurred due to the combined effects of season of 2020, the eastern ridge of the SAH was ex- the extremely westward extended and enlarged NWPSH, tremely anomalously eastward. The YHRB was con- extremely eastward extended and expanded SAH, and trolled by strong upper-level divergence over the north extremely strong water vapor transport associated with side of the SAH ridge line and the south side of the west- the low-level southerly wind. The extremely warm SST erly jet belt (Fig. 9b). The intensity of the divergence is in the tropical IO is supposed to have exerted important stronger than normal (figure omitted). Due to the com- influences on the formation of the extremely anomalous bined effects of the extremely strong moisture conver- large -scale circulations mentioned above. gence at the low level and the stronger-than-normal di- vergence at thePaper upper level, extremely strong vertical in as- 5. Press Conclusions and discussion cending motion was induced over the YHRB, where the area averaged vertical ascending motion is the second The extremely anomalous features of Meiyu over the strongest (i.e., the second lowest omega) in the Meiyu YHRB in 2020 and associated large-scale circulation an- season since 1981 (Fig. 10c). The statistical results in omalies are investigated in this paper, based on a com- Table 1 show that a highly negative correlation exists parative analysis of the features of the Meiyu in 2020 and OCTOBER 2021 Niu, R. Y., P. M. Zhai, G. R. Tan 13

NAO NAO Extremely positive phase negative phase weak CEFs (Liu et al., 2020) (Liu et al., 2020)

Extremely weak Weak cold Strong cold and southward air activities air activities located ITCZ

Early first seasonal north- Late second seasonal north- ward migration of the NWPSH ward migration of the NWPSH and the other key systems and the other key systems

Extremely Extreme early Extreme late long duration of onset of Meiyu retreat of Meiyu Meiyu season

Extremely more than normal Meiyu rainfall

Extremely strong rainfall intensity of the Meiyu season

Extremely strong Extremely strong Stronger vertical ascending water vapor divergence at the motion at the middle- convergence at the upper level lower levels low level

Extremely strong Extremely Extremely westward water vapor trans- eastward extended extended and enlarged port by the low- and expanded SAH NWPSH level southerly wind

Previous warm Extremely warm SST in the western SST in the tropical IO North Atlantic (Zheng and Wang, 2021)

Super IOD episode in the preceding autumn (Takaya et al., 2020; Zhou et al., 2021)

Fig. 11. Schematic diagram showing large-scale circulation anomalies responsible for the extremely anomalous features of Meiyu in 2020 over the YHRB. the climatologicalPaper features of the Meiyu season in during third Press latest of the retreat date, the longest duration, the 1981–2020. The main conclusions are summarized as maximum amount of Meiyu rainfall, the strongest mean follows (Fig. 11). rainfall intensity, and the maximum number of stations/ (1) The extremely anomalous features of the Meiyu days of rainstorm and heavy rainstorm in the Meiyu sea- season in 2020 are very prominent. The Meiyu in 2020 is son since 1981. characterized by the fourth earliest of the onset date, the (2) The extremely long duration of the Meiyu season 14 Journal of Meteorological Research Volume 35 in 2020 is represented by the extremely early onset and by the atmospheric wave train cross the European contin- late retreat of Meiyu. The former is mainly induced by 1) ent. The preceding positive SST anomaly in the western the early first northward shift and migration of the key North Atlantic is probably an important factor for the ex- influence systems including the NWPSH, SAH, and tremely large coverage of the SAH in the early summer EASM; and 2) the corresponding earlier transition of the of 2020. It is also worth further in-depth study on the East Asian atmospheric circulation pattern from winter to modulation and influence mechanisms of the SST anom- early summer under the influence of the weak cold air alies and the role of circulation systems in the Southern activities from late May to early mid-June. On the other Hemisphere in the formation of the extremely anomal- hand, the extremely late Meiyu retreat is mainly due to ous Meiyu in 2020. the late second northward shift and migration of the The conclusions obtained in this study are expected to NWPSH and the other key influence systems, and the support the medium-to-extended-range forecast. This can corresponding delayed establishment of the midsummer be achieved through extracting large-scale circulation atmospheric circulation pattern in East Asia, induced by features from the numerical model products. For ex- strong cold air activities and the extremely weak and ample, the ECMWF ensemble mean forecast initiated at southward located ITCZ over Northwest Pacific in July. 1200 UTC 22 May and 20 July in 2020 revealed that the The phased anomalous cold air activities are likely re- cold air activities would be weakened and the front zone lated to the downstream dispersion of the wave energy at the mid–high latitudes would shrink and move north- associated with the positive and negative phases of the ward from 26 May, the RLP of the NWPSH would also North Atlantic Oscillation (NAO) (Liu et al., 2020). The shift northward and be stably located to the north of extremely weak and further southward ITCZ over the

Northwest Pacific are related to the extremely weak Phil- (a) ippines and New Guinea CEFs in July. 45N

(3) The extremely more than normal Meiyu rainfall is 40N an inevitable result of the extreme long duration and strong rainfall intensity in 2020. The latter is resulted 35N from the extreme strong water vapor convergence and 30N vertical ascending motion over the YHRB, due to the ex- 25N tremely westward extended and enlarged NWPSH, the 20N extremely eastward extended and expanded SAH, and the extremely strong water vapor transport associated 15N with the low-level southerly wind turning along the west- 11 May 16 May 21 May 26 May 1 June 6 June ern edge of the NWPSH. The extremely warm SST in the (b) tropical IO is the key factor to induce the above-men- 45N tioned extremely anomalous large-scale circulations. 40N Takaya et al. (2020) and Zhou et al. (2021) pointed out 35N that the extreme warm anomaly over the tropical IO in the early summer of 2020 was originated from the super 30N Indian Ocean dipole (IOD) episode in the autumn of 25N 2019. In the present study, we also examined the correla- 20N tion between the preceding autumn (August–October) IOD index and the early summer (June–July) IOBW in- 15N dex since 1982. The results showed that there was in- 6 July 11 July 16 June 21 July 26 July 1 August deed a positive correlation between the preceding au- ℃ −4 −2 0 2 4 tumn IOD index and the early summer IOBW index, but the correlation coefficient (0.31) failed to pass the t-test Fig. 12. Time–latitude cross-sections averaged over 110°–123°E of at the significancePaper level of α = 0.05. Zheng andin Wang geopotential Press height at 500 hPa (bold black line shows the 588-dagpm (2021) have proved that the positive SST anomalies in isoline of NWPSH and black dot−dashed line shows the NWPSH ridge the western North Atlantic in May can induce positive line) and temperature anomalies at 850 hPa (shading) based on the en- geopotential height anomalies in the midlatitudes of semble mean forecast of ECMWF initialized at 1200 UTC of (a) 22 May and (b) 20 July with lead time of 15 days and initial fields in the North Atlantic in June, and then lead to positive geopo- previous 15 days, respectively, in 2020 (purple vertical line indicates tential height anomalies in the midlatitudes of East Asia initial time; green dashed lines show the latitude range of the YHRB). OCTOBER 2021 Niu, R. Y., P. M. Zhai, G. R. Tan 15

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