Large-scale background and role of quasi-biweekly moisture transport in the extreme Yangtze River rainfall in summer 2020

A severe flooding hit southern China along the Yangtze River in summer 2020. The floods were induced by extreme rains, and the associated dynamic and thermodynamic conditions are investigated using daily gridded rainfall data of China and NCEP-NCAR reanalysis. It is found that the June–July rainfall over the Yangtze River Basin (YRB) experienced pronounced subseasonal variation in 2020, dominated by a quasi-biweekly oscillation (QBWO) mode. The southwestward-moving anomalous QBWO circulation was essentially the fluctuation of cold air mass related to the tropospheric polar vortex or trough-ridge activities over the mid-high latitude Eurasian in boreal summer. The southwestward-transport of cold air mass from mid-high latitudes and the northeastward-transport of warm and moist air by the strong anomalous anticyclone over the western North Pacific provided important large-scale circulation support for the extreme rainfall in the YRB. The analysis of streamfunction of water vapor flux demonstrates that a large amount of water vapor eastward zonal transport from the Bay of Bengal and Indo-China and northward transport from the South China Sea provided the background moisture supply for the rainfall. The quasi-biweekly anomalies of potential and divergent component of vertically integrated water vapor flux played an important role in maintaining the subseasonal variability of rainfall in June–July of 2020. The diagnosis of moisture tendency budget shows that the enhanced moisture closely related to the quasi-biweekly fluctuated rainfall was primarily attributed to the moisture convergence. Further analysis of time-scale decomposition in the moisture convergence indicates that the convergence of background mean specific humidity by the QBWO flow and convergence of QBWO specific humidity by the mean flow played dominant roles in contributing to the positive moisture tendency. In combination with adiabatic ascent over the YRB induced by the warm temperature advection, the boundary layer moisture convergence strengthened the upward transport of water vapor to moisten the middle troposphere, favoring the persistence of rainfall during June–July. The vertical moisture transport associated with boundary layer convergence was of critical importance in causing low-level tropospheric moistening. By comparison, the horizontal moisture advection played a secondary important role in the quasi-biweekly oscillation of rainfall in June–July 2020.


Introduction
In summer 2020, record-breaking severe floods hit southern China from the middle to lower reaches of the Yangtze River valley. The floods were induced by extreme rains from June to August and affected 54.8 million people in 27 province. About 368,000 houses were destroyed, and direct economic losses amounted to 144.43 billion yuan (20.66 billion US dollars) by 29 July 2020 (http:// www. xinhu anet. com/ engli sh/ 2020-07/ 29/c_ 13924 7689. htm). Some of towns in the central and southern provinces received record-breaking precipitation in June and July (https:// mp. weixin. qq. com/s/ SScXr BYG2T JfLKc 0BmIp 2Q). In the continuous heavy rainfall 1 3 periods, eastern China, especially the lower Yangtze River Basin, suffered flooding and severe damage after multiple rainstorms. The accumulative rainfall during summer 2020 set the highest record since 1961. The floods were reminiscent of the 1998 China floods that caused life and economic losses. In fact, the summer floods in 2020 were actually more impactful as the floods hit the populated regions not only in eastern China but also in southern Japan.
The interannual variability of East Asian summer monsoon is characterized by rainfall anomalies from the middlelower reaches of the Yangtze River in China to southern Japan. A series of studies have investigated the record-breaking summer rainfall in 2020 using numerical model simulations or/and observations. Although operational forecast models successfully captured the enhanced rainfall in the Yangtze River Basin (YRB) in summer 2020, the magnitude of rainfall anomalous was underestimated (Li et al. 2021a). The factors responsible for the extreme rainfall were associated with the large-scale atmospheric circulation anomalies related to tropical and extratropical forcing. Zhou et al. (2021) proposed that the slowly westward propagation of oceanic Rossby waves in the South Indian Ocean induced by a strong Indian Ocean Dipole event in the late 2019 was the key process. The oceanic Rossby waves help sustain the Indian Ocean warming and intensify the anomalous anticyclone over the western North Pacific (WNP) through the Indo-western Pacific Ocean capacitor mechanism (Xie et al. 2009), and further leads to the extreme rainfall in the YRB in summer 2020. A similar conclusion was reached through climate model simulations using the coupled seasonal prediction system of Japan Meteorological Agency (Takaya et al. 2020). Pan et al. (2021) further demonstrated that the extremely strong anomalous anticyclone over the WNP resulted from a combined effect of a quick El Niño to La Niña phase transition besides the effect of strong Indian Ocean warming. In addition to the tropical Indo-Pacific large-scale thermal condition, the positive sea surface temperature anomalies in the Atlantic Ocean contributed to the extreme rainfall through an atmospheric wave train across Europe, including an anomalous anticyclone over the WNP (Wang 2019;Zheng et al. 2021). Zhang et al. (2021) emphasized the important role of the Madden-Julian Oscillation (MJO), with an extraordinary long-lasting and quasi-stationary active phase over the Indian Ocean during June-July 2020, contributing to the extreme rainfall. Li et al. (2021b) illustrated that the eastward moving vortices from Tibetan Plateau may trigger the heavy Meiyu rainfall in summer 2020. Ding et al. (2021) pointed out that the evolution and configuration of the East Asian summer monsoon circulation subsystems, including quasi-biweekly oscillation (QBWO) of the western North Pacific subtropical high, the blocking highs and troughs in mid-high latitudes over Eurasia, the upper-level westerly jet, and low-level southwesterly flow, played an important role in triggering and maintaining the excessive and persistent rainfall along the YRB.
Statistical analysis shows that the summer rainfall in the Yangtze River basin exhibits not only interannual variability but also intraseasonal variability (Mao et al. 2006(Mao et al. , 2010Hsu et al. 2016;Qi et al. 2019;Yan et al. 2019;Liu et al. 2020b;Ding et al. 2020). The biweekly perturbation of subtropical westerly jet over East Asia shows a significant impact on the variation of summer rainfall over YRB (Yang et al. 2010;Jin et al. 2020). In the case study, the subseasonal perspective of the record-long 2020 summer rainfall was examined by Qiao et al. (2021), who suggested that subseasonal variations of the long-lasting floods were modulated by the tropical and extratropical teleconnections. The extreme rains can be attributed to different systems at the different stages, including the East Atlantic/West Russia teleconnection, the Pacific-Japan pattern, and the combined effect of tropical forcing and mid-latitude teleconnection. The purpose of this study is to investigate the subseasonal variations of dynamic and thermodynamic conditions contributing to the extreme rains in summer 2020. We will mainly focus on large-scale circulations and the role of moisture transport in the occurrence and maintenance of the extreme rainfall. Further the scale interactions between the background mean state and subseasonal variability in the moisture convergence and moisture advection are investigated to examine the relative contributions among the decomposed component terms to the excessive rainfall along the Yangtze River basin in summer 2020.
Data and analysis methods are introduced in Sect. 2. The quasi-biweekly rainfall fluctuation in summer 2020 and the associated large-scale moisture transport are described in Sect. 3. In Sect. 4, we discuss the contributions of the moisture convergence and moisture advection to the extreme rainfall by diagnosing the moisture budget. Conclusions and discussions are given in Sect. 5.

Data and method
The daily gridded rainfall data with a resolution of 0.5° × 0.5° from the National Meteorological Information Center of China Meteorological Administration are used to depict the spatial distribution and subseasonal variations of rainfall in eastern China in summer 2020. This dataset is based on the daily rain gauge measurements over 2400 stations in the Chinese national dense observational network. The National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis at a 2.5° × 2.5° spatial resolution (Kalnay et al. 1996) is utilized. Daily mean winds, pressure vertical velocity, geopotential height, air temperature and specific humidity on standard pressure levels are used to analyze large-scale atmospheric circulation and diagnose moisture budget associated with extreme rainfall.
The wavelet spectrum analysis (Torrence and Compo 1998) is used to extract the dominant signal of summer rainfall in 2020 over the YRB. Before performing the wavelet analysis, the annual cycle and its first three Fourier harmonics are removed from the daily mean rainfall. To isolate subseasonal signals, daily rainfall and other atmospheric variables including zonal and meridional winds, vertical velocity, geopotential height, specific humidity, and temperature at each standard pressure level are subjected to a quasi-biweekly time-scale bandpass filtering based on the harmonic decomposition (Kemball-Cook et al. 2002;Teng and Wang 2003;Jiang et al. 2004;Qi et al. 2008).
The total water vapor flux Q can be written as where g is the gravity, wind vector V = uî + vĵ , q is the specific humidity. Following Rosen et al. (1979), Salstein et al. (1980) and Chen (1985), the water vapor transport is decomposed into the contribution by the non-divergent (rotational) and divergent (irrotational) components based on the Helmholtz flow theorem decomposition, where Q ψ and Q χ denote the non-divergent (rotational) and divergent (irrotational) components of water vapor flux, respectively. and χ satisfy the following equations, respectively, The above equations are used to display the relationship between the large-scale atmospheric circulation and water vapor transport.
The decomposition of the quasi-biweekly perturbation and the low-frequency background state associated with moisture budget is described in Sect. 4.
3 Quasi-biweekly fluctuation of extreme summer rainfall and moisture transport

Features of summer rainfall and associated atmospheric circulation
In the summer of 2020, the central-southern part of China was hit by the heavy rainfall as shown in Fig. 1a. The regions around the north and the south of middle to lower reaches (2.4) ∇ 2 = k∇ × , and ∇ 2 χ = ∇Q along the Yangtze River in Hubei, Anhui, Jiangsu and Zhejiang provinces were hit by floods with a historical record since 1998. Some of these areas experienced strong rainstorms with the maximum precipitation more than 1200 mm during June-July (JJ). The anomalous JJ rainfall in 2020 departed from climatology shows pronounced positive rainfall anomalies along the Yangtze River basin from upper to lower reaches (Fig. 1b). The maximum precipitation center was located over the eastern part along the River, which led to floods over there.
Previous studies (e.g. Tao and Chen 1987;Lau et al. 1988) showed that the prominent feature of the major rainband over eastern China appears to move northward from southern to northern China from May to August in association with the evolution of subtropical high over the western Pacific. Figure 2 displays the time-latitude section of daily rainfall in eastern China during summer of 2020. On average, the northward migration of 2020 summer rain belt exhibits the similar intraseasonal space-time structures compared with the climatology except for the rainfall intensity and duration along the Yangtze River basin. The pre-Meiyu front developed in mid-May in the southern China and the rain belt shifted northward to the Yangtze River basin  (units: mm) around 30° N in early June, persisting until late July with a couple of short break periods. It was the long-lasting Meiyu front rainfall with its excessive heavy rains during June to July that led to the disastrous floods along the YRB. By the end of July, the continuous heavy rainfall along the Yangtze basin decreased drastically, the rainfall belt split into two with one over northern China and one over the southeastern part of China in August. The subseasonal variations of the summer rainfall in eastern China are highly correlated with the northward shift and southward withdrawal of the western Pacific subtropical high from May to August (Tao and Chen 1987;Lau et al. 1988;Chang et al. 2000).
The large-scale atmosphere circulation associated with the extreme strong rainfall developed meridionally and transported the cold air southward from the mid-high latitudes to eastern China. The 500 hPa geopotential height field shows an anomalous low over the sea of Okhotsk during June-July of 2020 ( Fig. 3b). It has a barotropic structure with a low-level anomalous cyclonic circulation, which is an anomalous northeasterly flow along inland near the coast. Such vertical structure accompanied by the anomalous cyclonic vorticity favors the advection of dry and cold air southward. Meanwhile, an unusually strong subtropical high in the WNP extended further westward to the southern China in 2020 compared with the climatological mean (Fig. 3a). The southwesterlies in the northwest flank of subtropical high transported the warm and moist air from the tropics toward the Yangtze basin and converged with the northeasterlies, yielding an anomalous convergence zone along the Yangtze River and leading to the increase of rainfall. The distribution of meridional shear vorticity at 850 hPa with positive vorticity anomaly along Yangtze River and negative anomaly over southern China, which was first proposed by Wang and Fan (1999) in defining a shear vorticity index to measure the East Asian summer monsoon, provided the dynamic conditions for enhancing the rainfall in the Yangtze basin ( Fig. 3b). This configuration of the middle and lower tropospheric circulations favored the occurrence of rainfall over the YRB. The anomalously strong western Pacific subtropical high played a critical role in enhancing rainfall over the Yangtze River basin.

Quasi-biweekly fluctuations
The daily rainfall in Fig. 2 is characterized by strong subseasonal variability. The maximum variation center of June-July rainfall in 2020 was located along the middle to lower reaches of the Yangtze River ( Fig. 4), which is consistent with the climatological mean (Qi et al. 2019).
The key region of Yangtze River basin, marked with a box (114°-118° E, 28.5°-32.5° N) in Fig. 4, is subject to severe floods frequently in summer, such as the extreme floods in 1991 and 1998 (Mao et al. 2006;Qi et al. 2016).
To investigate the dominant signal of the subseasonal variability in summer rainfall in 2020, the wavelet analysis (Torrence and Compo 1998) is applied to daily rainfall in the key region (indicated by the box in Fig. 4) in 2020, with the first four harmonics removed. Figure 5 displays the normalized wavelet power spectrum for the time series of area-averaged rainfall in the key region. It is noted that a significant quasi-biweekly mode with the period of 8-18-day during June-July 2020. The result is consistent with Ding et al. (2021) who presented that the quasi-biweekly oscillation (QBWO) was associated with the onset, northward shift, and retreat of the rain belt during the Meiyu season of 2020.
The time series of summer rainfall in 2020 over the key region is applied to 8-18-day bandpass filtering based on the harmonic decomposition. It is shown that the rainfall anomalies in the YRB exhibited several remarkable quasi-biweekly cycles from June to July (Fig. 6). These filtered cycles corresponded well with the rainfall anomalies except the one at the end of June. The peak wet and dry phases represent the during summer of 2020 (unit: mm/day). The rectangles correspond to the rainfall areas in the summer season maximum positive and negative rainfall anomalies in the cycles, respectively. Four cycles with the amplitude exceeding one and minus one standard deviation, respectively, were selected for inclusion in constructing composite for the wet and dry phases. The peak wet and dry phases are marked with solid black triangles in Fig. 6.
To reveal the impacts of large-scale atmosphere circulation from the mid-high latitudes on the QBWO of rainfall in the YRB, we constructed lead/lag composites of the 8-18-day filtered geopotential height and horizontal winds based on the selected four QBWO cycles. Day 0 corresponds to the maximum positive rainfall anomaly, i.e. the peak of the QBWO wet phase. The time sequence of the composite from day-8 to 6 (with a 2-day interval) in QBWO geopotential height and winds anomalies at 200 hPa is illustrated in Fig. 7. At day-8, a strong positive height anomaly accompanied by an anomalous anticyclonic circulation appeared to the Central Siberian plateau and a negative height anomaly was on the eastern side of the plateau. In the ensuing days, the positive height anomalies and the corresponding anticyclonic circulation weakened and expanded westward and southward, while the negative height anomaly and corresponding cyclonic circulation intensified as it moved southwestward. At day 0, an anomalous cyclone extends from east Fig. 3 (a) 500-hPa geopotential height averaged of June-July in 2020 (orange contours with intervals of 30 gpm) and in climatology (blue contours with intervals of 60 gpm). The shading shows temperature anomalies at 500-hPa in June-July 2020 (unit: K). (b) Anomalies of 850-hPa wind (vectors, unit: m/s) and vorticity (shading, unit: 10 -5 /s), and 500-hPa geopotential height (contours, unit: gpm) in June-July 2020 of the Urals to the Sea of Okhotsk with the center over the Lake Baikal. After that, it began to weaken as continuing to move westward (Fig. 7). This westward propagating QBWO geopotential height anomaly was part of the westward circulation of polar vortex with a clockwise rotation in the eastern hemisphere in boreal summer. The southwestward-moving quasi-biweekly oscillation anomalous circulation was essentially the fluctuation of cold air mass which was closely related to the polar vortex or trough-ridge system activities over the Eurasian mid-high latitudes in boreal summer (Yang et al. 2013). The vertical distribution of the evolution of QBWO geopotential height anomaly had a barotropic structure (Figures not shown), suggesting that the strong cold air affecting the extreme summer rainfall in eastern China in 2020 originated from the polar vortex.

Water vapor transport and maintenance
The relationship between the atmospheric circulation and the water vapor transport can be illustrated by means of the streamfunction and potential (Chen 1985). Figure 8 shows the streamfunction field with non-divergent component and potential field with irrotational (divergent) component of the vertically integrated (1000-300 hPa) water vapor flux in June-July of 2020. The spatial pattern and intensity of largescale water vapor transport can be delineated by the streamfunction and nondivergent component as shown in Fig. 8a. It exhibited a three-cell structure in the Northern Hemisphere and a two-cell in the Southern Hemisphere. Each cell corresponded to an anticyclonic transport of water vapor in the oceans or monsoon region. The zonal characteristics of the nondivergent field indicated a strong westward transport of water vapor in the tropics and strong eastward transport in the mid-latitude of both hemispheres. During June-July in 2020, the strong three cells of anticyclonic transport in the Northern Hemisphere were centered over the Pacific, The Morlet wavelet power spectrum of rainfall with 1-4th harmonics removed over the Yangtze River Basin in 2020. The abscissa is time. The ordinate is the period in days. The regions enclosed by the solid black contours are the areas greater than 99% confidence Fig. 6 Time series of daily rainfall anomalies (left axis, bars) and the anomalous of 8-18-day filtered rainfall (solid line) over the Yangtze River region (114°-118° E, 28.5°-32.5° N) in summer 2020 (both units: mm/day). Thin dashed lines denote one and minus one standard deviation of the filtered time series. The solid black triangles represent the peak wet or peak dry phases Atlantic and equatorial Indian Ocean to South Asian monsoon region, respectively. The intensity of the Pacific Ocean cell was the strongest, may be attributed to the water vapor transported by the intensification of low-level tropical circulation over the Pacific Ocean (Chen 1985). As shown in Fig. 8a, there were two sources of water vapor transport that led to the heavy rainfall in the YRB. One is the northeastward water vapor transport from the Indian Ocean. Noted that this transport originated from the westward transport of easterly in the south of equator and turned to eastward when it reached the east coast of Africa. Another transport pathway was the northward transport associated with the strong anticyclonic circulation over the South China Sea to the WNP (Fig. 8a).
To illustrate how the high water vapor content which transported toward the heavy rainfall area was maintained, the potential and divergent component of vertically integrated water vapor flux were computed to present the maintenance of moisture during the period of heavy rainfall (Fig. 8b). The magnitude of potential is smaller than that of streamfunction, indicating that the divergent component of the water vapor flux is smaller than the non-divergent component according to Eqs. (2.3) and (2.4). The zonal and meridional components of the divergent water vapor flux are comparable in contrast to the dominated zonal component in the non-divergent water vapor flux. It is consistent with the climatology result from Chen (1985). The maximum convergence center of water vapor flux appeared over the Southeast Asia in June-July of 2020. The moisture air was converged toward Southeast Asia to increase and maintain the high content of water vapor. The converged water vapor would be transported upward to moisten the atmosphere of mid-troposphere and, in turn, to enhance condensation and precipitation. At this point, the large-scale rainfall should be consistently distributed with the significant convergence of water vapor flux and the moisture convergence played a critical role in providing moisture supply for the extreme rainfall in 2020, though the magnitude of divergent component of water vapor flux was smaller than that of non-divergent component. The eastern China around YRB was controlled by the robust moisture transport and strong elongated moisture convergence zone to southern Japan during June-July 2020 (figure not shown), suggesting that it was not only the sufficient water vapor supply but also the moisture convergence to strengthen and maintain the heavy rainfall.
In order to investigate the quasi-biweekly fluctuations in the large-scale transport and maintenance of water vapor flux, the difference of QBWO streamfunction field with nondivergent component and potential field with divergent component of vertically integrated water vapor flux between the composite peak wet and dry phase is displayed in Fig. 9. The positive QBWO anomalous streamfunction cells associating with anticyclonic water vapor transport appeared over the Northwestern Pacific and Indochina monsoon region including South China Sea, corresponding with those streamfunction cells in the summer mean (June-July) (Figs. 8a and 9a). From the nondivergent component, noted that the strong anomalous westerlies and southwesterlies transported positive moisture anomalies toward eastern China along the Yangtze River (approximately 30°N) to enhance the water vapor content favoring the QBWO rainfall. It is found that there was an anomalous cyclonic transport to the north of the Yangtze River, tending to transport water vapor northward guided by the anomalous southerlies along the east coast of China. In Fig. 7 Composite evolution of QBWO anomalies in geopotential height (shading, unit: gpm) and wind (vectors, unit: m/s) at 200-hPa from day-8 to day + 6. Day 0 represents the time of peak wet phase the QBWO divergent component of the water vapor flux, it appeared an elongated narrow moisture convergence zone from southwest to northeast toward the key region, corresponding the rainfall belt along the Yangtze River. The strong moisture convergence favors the maintenance of the anomalous heavy rainfall (Fig. 9b). This can be proved by the result of the divergence of the vertically integrated water vapor flux (figure not shown). This implies that the QBWO convergence of the water vapor flux played an important role in maintaining the heavy rainfall fluctuations during June to July in 2020. It is noted that the northward moisture transport by the QBWO perturbations in the mid-latitude was largely carried out by the eddy divergent component. This is consistent with the study by Chen (1985), who concluded that the transient mode is an effective factor transporting water vapor poleward in midlatitude to reduce the north-south gradient of water vapor content, and the water vapor flux of the transient mode is mostly described by the divergent component.
The vertical structures in vertical motion, specific humidity and wind divergence further illustrate the thermal and dynamic conditions contributing to heavy rains. Figure 10 displays the vertical structures of quasibiweekly specific humidity and vertical motion between 110° E and 120° E for the composite peak dry and peak wet phase. In the peak dry phase, the YRB was characterized by the subsidence associated with low-level divergence, negative vorticity, and negative specific humidity anomalies (Figs. 10a, 11a), which were not conductive to precipitation over the YRB. In contrast, during the peak wet phase, the strong ascending motions associating with the low-level convergence and upper-level divergence transported moisture upward from the boundary layer and increased the water vapor content in the middle troposphere. The cyclonic vorticity and boundary layer moisture convergence strengthened the rainfall in the Yangtze basin (Figs. 10b, 11b).

Contributions of the moisture convergence and advection to the extreme rainfall
To assess the contribution of moisture advection and convergence related to the heavy rainfall in summer 2020, moisture tendency budget was analyzed. According to Yanai et al. (1973), the moisture tendency at each constant pressure level is determined by the sum of horizontal and vertical moisture advections and the atmospheric apparent moisture sink Q 2 as shown in Eq. (4.1): where q is the specific humidity, t is the time, V is the horizontal wind vector, ∇ is the horizontal gradient operator, is the vertical pressure velocity, p is the pressure, Q 2 is the atmospheric apparent moisture sink, and L is the latent heat of condensation. The vertical advection term may be further decomposed into the horizontal moisture convergence term ( −q∇ • V ) and the vertical flux term ( − q/ p). Based on diagnosis of the above moisture budget, Maloney (2009), Hsu et al. (2012 and Li et al. (2015) analyzed the atmospheric moisture dynamic in relation to the MJO initiation, development and propagation processes. To explore the effect of moisture transport on the quasibiweekly fluctuated rainfall in summer 2020, we applied a QBWO-filtering operator (denoted by a prime) to the above moisture tendency Eq. (4.1). The anomalous moisture budget equation may be derived as follows: The first term in the right-hand side in Eq. (4.2) represents the horizontal advection of moisture, the second term the horizontal convergence of moisture, the third term the flux form of vertical moisture advection, and the fourth term moisture loss (gain) due to the condensational heating (raindrop-induced evaporation in the unsaturated atmosphere and surface evaporation) process. The combination of the second and third term represents the vertical moisture advection (Hsu et al. 2012).
To illustrate the role of quasi-biweekly perturbation in enhancing and maintaining the moisture during the period of extreme rainfall over the region of YRB, both specific humidity and winds fields are decomposed into two components: where an overbar and a prime denote the low-frequency background state (LFBS, with a period longer than 18 days) and QBWO component, respectively. The decomposed LFBS component includes an annual and semi-annual cycles, and seasonal to subseasonal mean state with the period longer than 18 days. The QBWO component is referred to 8-18day perturbation, the component of synoptic time-scale less than 8 days is ignored. In order to examine the contributions to the QBWO moisture tendency by the low-frequency background state, the horizontal moisture convergence and moisture advection in Eq. (4.2) can be written as: and By applying an 18-day low-pass filter and an 8-18-day band-pass filter to each variable in Eqs. (4.4) and (4.5), respectively, one may extract the LFBS and QBWO signals from the raw data.

(a) (b)
shows the contributions of each term from the moisture tendency Eq. (4.2) during the peak wet phase. The positive contributions to the QBWO moisture tendency come from the anomalous horizontal moisture advection and moisture convergence. The positive moisture advection corresponds to the water vapor transport toward the YRB represented by the streamfunction of water vapor flux, providing the moisture supply (Fig. 9a). Although the anomaly of moisture advection is positive, it played a minor role in contributing to the QBWO moisture tendency. The most significant term to the QBWO moisture is the horizontal moisture conver- , which together with vertical flux term [ − ( q) � ∕ p ] to combine the vertical advection of moisture.
It is noted that the vertical flux term with a small magnitude is negative. The vertical moisture advection played a critical role in enhancing moisture content, even though the two terms are offset with each other. In the vertical profiles of QBWO anomalous temperature advection, vertical velocity, wind divergence and specific humidity, it can be found that the positive (negative) anomaly of temperature advection increased (decreased) with height, resulting in the ascending (descending) motion over the region of YRB (Fig. 13a, b) (Qi et al. 2019). The boundary layer moisture convergence associating with the ascending motion induced by the warm temperature advection transported the moisture upward to moisten the middle troposphere (Fig. 13c, d). The negative anomaly of term [ −Q 2 � ∕L ] indicates the loss of moisture in the lower troposphere, which is resulted from the condensation and the reduction of the surface evaporation (Fig. 12). This result is consistent with Benedict and Randall (2007) and Hsu and Li (2012), who both showed the increased moisture convergence and advective processes are crucial in the analyzing the development and eastward propagation of MJO convection. The sum of all anomalous terms in the right-hand side of Eq. (4.2) is close to the observed QBWO moisture tendency.
To reveal the contribution of interactions between the LFBS and QBWO perturbation to the moisture related to the extreme rainfall in June-July 2020, each individual decomposed term with different time-scale in moisture convergence and advection is examined to show the specific processes that give rise to the positive moisture tendency. Figure 14 displays the budget difference of vertical integral (from 1000 to 300 hPa) of moisture convergence and moisture advection between the peak wet and peak dry composite phase over the YRB for the three individual terms in both Eqs. (4.4) and (4.5). In the decomposed component of moisture convergence, all the three terms had positive contributions (Fig. 14a). The leading term (−q∇ • V � ) , which denotes the convergence of background mean specific humidity by the QBWO anomalous flow, was dominant in the peak wet phase. To illustrate the contribution of boundary layer moisture convergence to the QBWO rainfall, the spatial distributions of the mean specific humidity and QBWO flows in the lower troposphere in association with (−q∇ • V � ) are shown in Fig. 15a. Both mean specific humidity and anomalous flow are integrated vertically from 1000 to 700 hPa and composited at the peak wet phase. It is noted that a large amount of boundary layer moisture accumulated in latitude of the Yangtze River extended from eastern China to southern Japan with the maximum mean specific humidity center located over the YRB during the peak wet phase. The low-level cyclonic QBWO anomalous flow over the YRB associating with the lower tropospheric positive vorticity induced the intensive moisture convergence in situ (Figs. 11b, 15a). The resultant low-level moisture convergence transported the boundary layer moisture upward to the middle troposphere with the help of large-scale ascending motion adiabatically induced by the warm temperature sum advection during the peak wet phase (Fig. 13a, b), leading to the occurrence and intensification of rainfall over the YRB. The second leading term (−q � ∇ • V) in the moisture convergence, representing LFBS mean convergence of the QBWO anomalous specific humidity, also shows a pronounced contribution to produce positive moisture. Comparing these two leading moisture convergence terms, the much greater contribution of (−q∇ • V � ) than (−q � ∇ • V) is primarily attributed to the large difference between the mean and intraseasonal moisture amplitude. The mean moisture ( q) is about 10 times larger than the QBWO ( q ′ ) over the YRB. At the same region, however, the mean convergence ( ∇ • V ) in the lower troposphere is only about 2 ~ 3 times larger than the QBWO convergence ( ∇ • V � ). This result is consistent with Hsu et al. (2012) who found that the convergence of the mean moisture by the MJO flows is crucial for supporting the moisture asymmetry during the eastwardpropagation of MJO convection. The QBWO eddy-eddy term (−qʹ∇•Vʹ), denoting the perturbation convergence of the QBWO moisture by the anomalous flow, also played a minor but positive role in moistening over the YRB. The decomposed analysis of the moisture convergence shows that the LFBS mean moisture converged by the QBWO flow played a more dominant role in contributing to the positive wind divergence (unit: 10 -6 /s) and (d) specific humidity (unit: g/kg) for peak wet (blue) and dry (red) phase averaged over the YRB Fig. 14 Difference of each individual decomposed term in (a) moisture convergence (unit: 10 −5 kg/(m 2 s)) and (b) moisture advection (unit: 10 −9 kg/(m 2 s)) by calculating vertically integrated (1000-300 hPa) water vapor flux between peak wet and peak dry phase over the YRB moisture tendency than the convergence of QBWO moisture, providing the continuous moisture support and favoring the intensification of quasi-biweekly anomalous rainfall. Among the decomposed components in the horizontal moisture advection (Fig. 14b), the advection of the mean moisture by the QBWO flow (−V � • ∇q) and advection of anomalous QBWO moisture by the LFBS mean flow (−V • ∇q � ) are shown to contribute positively to the moisture tendency. This is attributed to the anomalous QBWO southwesterlies or southerlies advected northward the background high moisture from WNP and South China Sea to increase the lower-tropospheric moisture over the YRB (Fig. 15a) and the strong LFBS winds associated with the East Asian summer monsoon advected the fluctuated moisture to the region of YRB (Fig. 15b). The quasi-biweekly eddy-eddy advection (−V � • ∇q � ) did not show a positive contribution to the moisture transport. By comparing the contribution of moisture advection with moisture convergence, note that the magnitude of QBWO horizontal moisture advection term was about 10 times smaller than that of QBWO moisture convergence (Fig. 12), and even 10 4 times smaller than the moisture convergence in their decomposed component terms (Fig. 14), indicating that the horizontal moisture advection was of the secondary importance in contributing to the boundary layer moistening. Based on the above case analysis of extreme rainfall in 2020, it implies that the QBWO moisture convergence associating with the vertical moisture advection played the critical role in increasing the local moisture from lower to middle troposphere and in turn in generating the quasi-biweekly fluctuated rainfall over the YRB during June-July 2020.

Conclusions and discussions
In the summer of 2020, the severe flooding in decades hit central-southern China from the middle to lower reaches of the Yangtze River valley. The subseasonal variations of the extreme rainfall and the associated large-scale circulations with moisture transport during summer 2020 are investigated. The maximum subseasonal variability in rainfall appeared in the middle and lower reaches of Yangtze River basin, where dominated by a significant quasi-biweekly oscillation mode in the June-July rainfall. It is found that the southwestward-moving anomalous quasi-biweekly circulation was essentially the fluctuation of cold air mass which is closely related to the polar vortex or trough-ridge system activities over the Eurasian mid-high latitudes in boreal summer. In the low latitude, the western North Pacific subtropical high was extraordinarily strong and extended further westward in June-July 2020 compared with the climatological mean. The warm air with intensive moisture transported by the southwesterlies west of the anomalous strong subtropical high converged with the cold air from mid-high latitudes over the YRB, providing the background favorable large-scale circulation and moisture conditions for the occurrence of extreme rainfall. The analysis of streamfunction of water vapor flux further shows that the enhanced water vapor transported to the YRB came from the eastward zonal transport in the latitudes of the Bay of Bengal and Indo-China and northward transport from the South China Sea. The large amount of water vapor was then converged and accumulated over the YRB and transported vertically upward to moisten the middle troposphere by the strong ascending motion which induced by the warm advection. The quasi-biweekly anomalies of the potential and divergent component of water vapor flux played a critical role in maintaining the subseasonal variability of June-July rainfall in 2020. The diagnosis of moisture tendency budget indicates that both the moisture advection and moisture convergence Fig. 15 Vertically integrated (1000-700 hPa) LFBS specific humidity ( q ) and QBWO anomalous wind fields ( V � ) (a) and LFBS wind ( V ) and QBWO specific humidity ( q � ) (b) in the composite peak wet phase. Blank areas in (a) and (b) denote the Tibetan Plateau. Shading denotes the specific humidity in unit of g/kg and vector denotes horizontal wind in unit of 10 3 m/s made positive contribution to the moisture supply for the quasi-biweekly oscillation of June-July rainfall in 2020, with the moisture convergence dominating the contribution to moisture tendency. Among the three time-scale decomposed terms of moisture convergence, the largest contribution to the positive moisture was the term (−q∇ • V � ) , which denotes the convergence of the background mean moisture by the QBWO flow. The abundant moisture in the boundary layer was transported upward by the strong anomalous ascending motion associated with the cyclonic vorticity to help enhance and maintain the QBWO rainfall. The second leading term (−q � ∇ • V) in the moisture convergence, the mean convergence of the QBWO moisture, also shows a pronounced contribution to produce positive moisture tendency. The horizontal moisture advection provided continuously the moisture supply for the extreme rainfall during June-July of 2020. In the horizontal moisture advection processes, the decomposed terms (−V • ∇q � ) and (−V � • ∇q) had positive contributions to the moisture, although it played a minor role compared with the moisture convergence. The magnitude of decomposed terms in moisture advection was 10 4 times smaller than that of moisture convergence. It suggests that the QBWO vertical advection of moisture is more important than the horizontal moisture advection in determining the quasi-biweekly fluctuated rainfall in this case study. The boundary layer moisture convergence intensified the air moisture upward with the ascending motion which induced by the warm temperature advection. The significant contribution to the QBWO rainfall over the YRB was primarily attributed to the moisture convergence. This result is a little different from the climatology statistical study of Qi et al. (2019), in which the moisture advection and moisture convergence are equally important for the occurrence of intraseasonal oscillation of rainfall in the YRB. As one of the important large-scale circulation factors, the anomalous anticyclone over the WNP links closely East Asian climate variations with central-eastern Pacific sea surface temperature (SST) anomalies (Zhang et al. 2017;Li et al. 2017). The anomalous anticyclone strengthens the summer Meiyu/Baiu rainfall through the northward transport of high moisture from the tropics by anomalous southerlies in the western flank of the anomalous anticyclone (Zhang 2001). According to the previous studies, the dominant anomalous anticyclone is primarily due to the warm SST anomalies in the tropical eastern Pacific associated with El Niño event (Zhang et al. 1996(Zhang et al. , 1999Wang et al. 2000), Indian Ocean basin-wide warming mode effect (Xie et al. 2009;Wu et al. 2009) and the warming in the tropical north Atlantic Ocean (Rong et al. 2011). The pronounced anomalous anticyclone in the WNP, which closely related to the severe floods along the Yangtze River during the summer of 2020, was not caused directly by the weak El Niño in the Pacific, but by the basin-wide warming in the Indian Ocean induced by the strong Indian Ocean Dipole event in 2019 (Zhou et al. 2021;Takaya et al. 2020) and positive SST anomalies in May over the North Atlantic Ocean (Zheng and Wang 2021). What is the relative role of tropical boreal summer intraseasonal oscillation (BSISO) in the formation and maintenance of the strong anticyclone over the WNP? Whether are there interactions between the tropical BSISO and the QBWO circulation from mid-high latitudes? The structural evolution of BSISO related to the anticyclonic circulation and the extreme rainfall need to be further examined, and the possible impacts of the local SST and Indo-Pacific SST anomalies on the BSISO activities also need to be investigated in the future. In addition, the other interannual variabilities, such as East Asian/Pacific pattern (EAP), Silk Road pattern, North Atlantic Oscillation (NAO), and western North Pacific High, are all related to the 2020 extreme Meiyu (Ding et al. 2021;Liu et al. 2020a, b). How importance of these climate factors in controlling the subseasonal components is also worthy of further investigation.