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

长江流域2020年夏异常雨季的动力和热力效应诊断分析

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

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

  • doi: 10.1007/s13351-022-1126-2

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

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

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

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

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

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

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

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

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

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

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

长江流域2020年夏异常雨季的动力和热力效应诊断分析

长江中下游2020年6和7月遭受异常降水。本文使用水汽和湿静力能平衡方程研究动力和热力效应对这次异常梅雨季的贡献。对于水汽,热力效应主要是气候态垂直上升运动对异常水汽含量的平流,但对降水异常的贡献只有8.5%,而通过异常上升运动对气候态水汽的平流来实现的动力效应则是降水异常的主要贡献因子。异常上升运动通过湿静力能方程也可分解为动力和热力效应,热力效应占到总效应的45.3%。动力效应是异常西南风对暖湿空气的平流,热力效应是平均垂直运动对异常湿静力能的平流。本文结果有助于理解导致2020年夏季长江流域异常雨季的基本物理机制,同时对全球变暖的潜在作用提出一些推测,因为全球变暖毕竟首先改变大气的热状态,直接和间接地影响降水。
    • The East Asian summer monsoon rainfall along a front extending from the mid–lower reaches of the Yangtze River valley (YRV), China to Japan was particularly strong in June–July 2020, causing disastrous floods in both eastern China and Japan. By its intensity and persistency, the rainy season in June–July 2020 was the strongest one since the last decades, and it attracted large public awareness and media attention. Qualified as super-Meiyu (plum rain), the 2020 rainy season also stimulates scientific interests (Chen et al., 2020; Liu et al., 2020; Zhang et al., 2020). An important question arises on the causality of this event, in particular, its relation and potential attribution to the anthropogenic warming of global climate.

      Many studies have shown that the intensification of regional precipitation can be a response to global climate warming (Katz and Brown, 1992; Kundzewicz, 2005; IPCC, 2013). In fact, a warmer atmosphere is more likely to hold more water vapor, which creates the necessary condition for the water cycle to be intensified (Su et al., 2008; Ye and Qian, 2021).

      However, the response of regional atmospheric circulation may also increase precipitation anomalies (Huang and Huang, 2012; Chen and Zhai, 2014, 2015, 2016; Si et al., 2016; Takaya et al., 2020), and most of these atmospheric conditions favorable for a wet YRV have their origin in relation to tropical ocean conditions (Wang et al., 2000, 2017; Wang and Zhang, 2002; Yang et al., 2007; Xie et al., 2009; Rong et al., 2010; Wen et al., 2015, 2019, 2020; Zhao et al., 2019). It is widely recognized that the years with heavy precipitation in the YRV in the past were mostly during strong El Niño decaying summer, such as 1983, 1998, and 2016 (Kane, 1999; Zhai et al., 2016; Bi et al., 2017; Wang et al., 2017). But El Niño in 2019/2020 was not strong. Why is there such an exceptional rainy season in the YRV in a weak El Niño decaying summer? It is necessary to understand the driving process of this exceptional precipitation and its any potential link to climate warming.

      A few studies on the 2020 super-Meiyu have been reported in the paper, but most of them were limited to a qualitative inspection on atmospheric circulation, water vapor transport (Wang et al., 2021), and sea surface temperature (SST). There is a general lack of quantitative assessment of the relevant dynamic and thermodynamic effects on this event. However, a fruitful method of precipitation event attribution is often used to separate the dynamic and thermodynamic contributions (Trenberth et al., 2015; Shepherd, 2016; Oueslati et al., 2019). It is based on the moisture budget analysis to realize the attribution of the precipitation anomalies, as practiced in Chou et al. (2013a) and Oueslati et al. (2019). The moist static energy (MSE) equation is an effective tool for diagnosing vertical velocity and quantifying the influence or contribution of atmospheric humidity, temperature, large-scale circulation, and radiation (Neelin, 2007; Chou et al., 2013b; Chen and Bordoni, 2014). It is also widely used in various applications to understand regional climate variation in China for future evolution (Yao et al., 2017), El Niño events (Liu et al., 2021), and paleoclimate (Sun et al., 2016, 2018). Thus, in this study, we use moisture and MSE diagnostic equations to quantify the contribution of dynamic and thermodynamic processes to the exceptional rainfall in the mid–lower reaches of YRV.

      The remaining of the paper is organized as follows. Section 2 introduces the observational and reanalysis datasets, and analysis methods. Section 3 presents the dynamic and thermodynamic processes in controlling the budget equations of moisture and MSE, which can help us to understand the underlying physical mechanisms leading to the exceptional rainfall in the mid–lower reaches of YRV in 2020. Section 4 summarizes a few major conclusions and discussion on the potential role of global warming for this case of summer 2020.

    2.   Data and method
    • Multiple datasets are used in this study from June to July 2020. The monthly precipitation data of 160 weather stations are obtained from the National Climate Center, China Meteorological Administration (CMA). The study area is east of 100°E and focuses on the YRV (33 stations included). Monthly variables, including atmospheric wind, temperature, humidity, evaporation, geopotential height, and surface heat fluxes, are from the Japanese 55-yr Reanalysis (JRA-55; Ebita et al., 2011) with a 1.25° × 1.25° horizontal resolution. Anomalies of all variables are obtained after removing their 30-yr means of 1981–2010.

      Our analysis methodology is identical to what described in Liu et al. (2021). It is based on the moisture budget equation for diagnosing precipitation anomalies and on the MSE budget equation for diagnosing anomalies in vertical motion.

    • The budget equation controlling the moisture variation for a tropospheric air column, in its vertically-integrated form, can be expressed as:

      $$ \left\langle {{\partial _t}q} \right\rangle + \left\langle {{\boldsymbol{V}} \cdot {\nabla _h}q} \right\rangle + \left\langle {{\omega }} \cdot {\partial _p}q} \right\rangle = E - P, $$ (1)

      where $ q $ (kg kg−1) represents specific humidity; $\boldsymbol{V}$ (m s−1) and ${\omega }$ (Pa s−1) denote horizontal wind and vertical pressure velocity, respectively; $ E $ (mm day−1) is surface evaporation into the atmosphere; and $ P $ (mm day−1) denotes precipitation. Angle brackets “$ \left\langle{}\right\rangle $” indicate a mass-weighted vertical integral from the surface (ps = 1000 hPa) to the top of atmosphere (pt = 100 hPa).

      The time tendency term $ {\partial }_{t}q $ can be neglected since it is generally much smaller than the other terms at monthly or longer timescales. The derivative form in Eq. (1), with the horizontal and vertical advection terms both decomposed into their linear terms and a residual, can be written as:

      $$ {P^\prime } = {E^\prime } - \left\langle {\overline {\omega }} {\partial _p}{q^\prime }} \right\rangle - \left\langle {{{\omega }}^\prime }{\partial _p}\overline q} \right\rangle - \left\langle {\overline {\boldsymbol{V}} \cdot \nabla {q^\prime }} \right\rangle - \left\langle {{{\boldsymbol{V}}^\prime } \cdot \nabla \overline q} \right\rangle + \rm{Res}, $$ (2)

      where the overbar (–) indicates the 1981–2010 climatology monthly mean; the prime (′) represents monthly anomaly, i.e., the deviation from the climatology; and Res includes non-linear terms due to simultaneous changes in both circulation and water vapor content, noted as a residual to measure the accuracy of the linear decomposition. In Eq. (2), the two advection terms related to water vapor change, $\left\langle{-\overline{\boldsymbol{V}}\cdot \nabla {q}'}\right\rangle$ and $\left\langle{-\overline{{\omega }}{\partial }_{p}{q}'}\right\rangle$, are considered as thermodynamic contribution (or effect) to precipitation changes, since only the thermal state of the atmosphere is involved. Similarly, the two terms related to atmospheric circulation change, $\left\langle{-{{\omega }}'{\partial }_{p}\overline{q}}\right\rangle$ and $\left\langle{-\boldsymbol{V}' \cdot \nabla \overline{q}}\right\rangle$, are dynamic contribution (or effect) to precipitation changes. The thermodynamic effect can thus be considered as a direct contribution of water vapor anomaly to precipitation anomaly, the dynamic effect being the consequence of other factors, including circulation anomalies and indirect contribution of the thermodynamics.

    • Due to the fact that water vapor decreases rapidly with altitude, creating a remarkable vertical gradient, the vertical advection term in the moisture budget equation is the most important one in controlling changes of precipitation. The conservation of MSE in the atmosphere allows us to have a powerful tool to constrain anomalous vertical motions. The MSE budget equation can be written as:

      $$\left\langle {{\partial _t}\left( {{c_v}T + {L_v}q} \right)} \right\rangle + \langle {\boldsymbol{V}} \cdot \nabla M\rangle + \left\langle {{{\omega }}{\partial _p}h} \right\rangle = {F_{\rm{net}}}, $$ (3)

      where $ M $ (W m−2) $ = {c}_{p}T+{L}_{v}q $ is moist enthalpy (sum of dry enthalpy and latent energy); $ h $ (W m−2) $ = {c}_{p}T+{L}_{v}q+\phi$ is MSE; $ T $ (K) and $\phi$ (m2 s−2) denote air temperature and geopotential, respectively; $ {c}_{p} $ ($ {c}_{v} $) (J kg −1 K −1) and $ {L}_{v} $ (J kg−1) denote the specific heat at constant pressure (volume) and the latent heat of vaporization, respectively; and $ {F}_{\rm{net}} $ (J kg−1) is the net energy entering the atmospheric column at the surface and top of atmosphere (sum of sensible heat, latent heat, and shortwave and longwave radiative fluxes).

      Similar to manipulations applied to the moisture budget equation, we can ignore here for the MSE equation the time tendency term, simply perform the derivative to obtain anomalous variables, and linearly decompose the advection terms. The MSE budget equation, in its form useful for our diagnostic purpose of vertical motion, can be expressed as:

      $$ \left\langle {{{{\omega }}^\prime }{\partial _p}\bar h} \right\rangle = F_{\rm{net}}^\prime - \left\langle {\overline {\boldsymbol{V}} \cdot \nabla {M^\prime }} \right\rangle - \left\langle {{{\boldsymbol{V}}^\prime } \cdot \nabla \overline M} \right\rangle - \left\langle {\overline {{\omega }} {\partial _p}{h^\prime }} \right\rangle + \rm{Res}.$$ (4)

      This equation is a powerful diagnostic tool to deduce the anomalous vertical motion ${{\omega }}'$ with a given profile of $ \overline{h} $. Equation (4) separates $\left\langle{\boldsymbol{V}\cdot \nabla M}\right\rangle$ and $\left\langle{{\omega }{\partial }_{p}h}\right\rangle$ into thermodynamic (related to water vapor content and temperature change) effect ($\left\langle{-\overline{\boldsymbol{V}}\cdot \nabla {M}' }\right\rangle$ and $\left\langle{- \overline{{\omega }}{\partial }_{p}{h}'}\right\rangle$) and dynamic (related to atmospheric circulation) effect ($\left\langle{-{\boldsymbol V}' \cdot \nabla \overline{M}}\right\rangle$). The thermodynamic effect can be used to measure the enhancement of vertical air ascents, attributable to changes of atmospheric thermal state. It describes the feedback between the atmospheric circulation and its thermal state that can ultimately increase precipitation. It provides a measure of (vertical-motion-related) indirect effect of the atmospheric thermal state on precipitation changes.

    3.   Results
    • Firstly, we assess the different terms of the moisture budget equation [Eq. (2)] to show their anomalies in the mid–lower reaches of YRV in 2020. Our goal is to decompose local changes of precipitation into changes in atmospheric water vapor and changes in atmospheric circulation, and ultimately to assess the physical drivers of this exceptional rainy season. Results from the moisture budget analysis [Eq. (2)] are shown in Fig. 1 for areal averages in the domain 27°–34°N, 107°–122°E and in Fig. 2 for geographic distribution. The evaporation term (−0.33 mm day−1) contributes negatively to the anomalous precipitation (3.93 mm day−1). It is an unusual situation that we observe negative local feedback between the evaporation and precipitation, certainly due to the particular conditions in the region with soil moisture already saturated and reduced available solar radiation energy for evaporation (or more precisely, evapotranspiration) to occur with rainfall and cloudiness. It is also to be noted that the residual term, for a value of −1.65 mm day−1, is larger than expected. The explanation resides in the fact that the whole period of June and July 2020 is not a continuous one, but filled with several intra-seasonal oscillatory weather systems (Ding et al., 2021). At the present stage, we believe that the other linearly-decomposed terms are not fundamentally impacted, especially for our purpose of investigating their relative proportionality.

      Figure 1.  Monthly-mean anomalies of moisture budget (mm day−1) for June–July 2020 averaged over the YRV (27°–34°N, 107°–122°E) as indicated by the black box in Fig. 2a.

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

      Let us now examine the thermodynamic and dynamic contributions deduced from the horizontal and vertical moisture advection. The two terms of the dynamic effect ($\left\langle{-\boldsymbol{V}'\cdot \nabla \overline{q}}\right\rangle$ and $\left\langle{-{{\omega }}'{\partial }_{p}\overline{q}}\right\rangle$) are 0.96 and 4.45 mm day−1, while those associated with the thermodynamic effect ($\left\langle{-\overline{\boldsymbol{V}}\cdot \nabla {q}'}\right\rangle$ and $\left\langle{-\overline{{\omega }}\cdot \nabla {q}'}\right\rangle$) are only 0.09 and 0.41 mm day−1. The thermodynamic contribution for the precipitation anomaly is only about 8.5% of the total effect (the sum of thermodynamic and dynamic contributions). The relationship between strong rainfall anomalies and anomalous vertical advection is the most significant. Similarly, Fig. 2f also suggests that there is a prominent anomalous vertical advection in the YRV, and its intensity is equivalent to that of precipitation, up to 6.5 mm day−1. These positive advection anomalies lead to moistening of the troposphere through vertical transport of water vapor, and maintain the low-level water vapor convergence, such as shown in Fig. 2d—the positive moisture advection by anomalous horizontal wind to south of the Yangtze River. Therefore, mainly the dynamic process controls the exceptional YRV precipitation in the rainy season of 2020.

      It is worth noting that the advection of anomalous water vapor by climatological vertical motion $\left\langle{-{\overline { \omega}}\cdot \nabla {q}'}\right\rangle$, although weak (Figs. 1, 2e), reflects the contribution of atmospheric thermal state changes to the rainfall anomaly of summer 2020 in the YRV. This thermodynamic effect can help us to speculate on the potential role of global warming on the exceptional precipitation anomaly, because the most prominent manifestation of the anthropogenic global warming is the increase of the tropospheric temperature and the enhancement of its water-vapor holding capacity. Since the part attributed to the thermodynamics is only 8.5%, we can thus conclude that the direct effect of anthropogenic global warming on the disastrous flooding of 2020 Meiyu is very weak. A similar conclusion was also obtained by Oueslati et al. (2019) who used the same diagnostic tool to study the flooding episode in South England in January 2014.

    • It is clear that the advection of mean humidity by anomalous vertical motion is the most important term of dynamic effect leading to rainfall anomaly in the YRV in June–July 2020. As a diagnostic tool, the MSE budget analysis can help to understand how anomalous vertical motion is created and maintained. We can decompose it into a part depending on the thermal state of the atmosphere and another part depending on the atmospheric circulation. The thermodynamic component is a surrogate of the contribution of atmospheric thermal state changes to vertical motion anomalies. Similar to the thermodynamic effect of the water vapor budget analysis, we can use the MSE thermodynamic component to analyze the potential role of global warming to vertical motion anomalies. It can be compared to the dynamic effect related to changes of horizontal wind divergences.

      Figure 3a shows that $ {\mathrm{\omega }}{\boldsymbol'} $ is negative throughout the troposphere in the mid–lower reaches of YRV, which means that there is a deep ascending motion. The vertical profile of $ {{\omega }}' $ presents a bottom-heavy bow structure with maximum at about 500 hPa (Fig. 3a). The variable $\overline{h} $ also presents a bottom-heavy bow structure with a maximum situated at about 600 hPa (Fig. 3a), indicating that generally $ \left\langle{{\partial }_{p}\overline{h}}\right\rangle < 0 $. Thus, positive $\left\langle{{{\omega }}'\partial _p}{\overline{h}}\right\rangle$ represents anomalous vertical ascending motions in the YRV.

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

      From the regional average MSE analysis (Fig. 4), the largest contribution to the ascending anomaly comes from the dynamic term ($ \left\langle{-\boldsymbol{V}' \cdot \nabla \overline{M}}\right\rangle $), which brings moist enthalpy into the domain. It can be seen that its areal mean, about 45.0 W m−2, is close to the advection of mean MSE by anomalous vertical motion. The two thermodynamic terms ($ \left\langle{-\overline{\boldsymbol{V}}\cdot \nabla {M}'}\right\rangle $ and $ \left\langle{-\overline{{\omega }}{\partial }_{p}{h}'}\right\rangle $) are 23.4 and 13.9 W m−2 (37.3 W m−2 in total, i.e., 82.9% of the dynamic effect, or 45.3% of the total effect). The horizontal and vertical combined thermodynamic contribution for the vertical motion anomaly is almost equivalent to the dynamic contribution. This means the thermodynamic effect is important in controlling the vertical motion in the case of the exceptional 2020 rain in the YRV. Specifically, the horizontal thermodynamic term, which is the anomalous moist enthalpy advected by climatological wind $ \left\langle{-\overline{\boldsymbol{V}}\cdot \nabla {M}'}\right\rangle $ (Fig. 5d), is similar to $ \left\langle{- \overline{\boldsymbol{V}}\cdot \nabla {q}'}\right\rangle $ (Fig. 2c), the moist enthalpy is accumulated on the north side of YRV, and lost on the south. As a whole, there is energy gain in the YRV (Fig. 4), which is conducive to air ascending. Because of the offset of energy loss, this effect is not as strong as the dynamic contribution. In addition, there is also a weak energy loss in radiation and turbulent heat fluxes entering into the atmospheric column in the YRV (Figs. 4, 5b). This weak negative feedback is largely attributable to augmented cloudiness impacting short wave radiation, reducing energy absorption in the air column, which is inhibitory for ascending motion.

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

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

      The advection of anomalous MSE by climatological vertical motion $ \left\langle{-\overline{{\omega }}{\partial }_{p}{h}'}\right\rangle $ has a weaker positive contribution in the YRV (Figs. 4, 5c), about 10–30 W m−2. The variable $\overline{{\omega }} $ is negative in the entire troposphere, corresponding to ascending motion, and shows a top-heavy bow pattern with maximum at about 400 hPa (Fig. 3b). The variable $ {h}' $ is increasing in the troposphere, with a bottom-heavy bow pattern, and the maximum is situated at about 750 hPa (Fig. 3b). The anomalous MSE, $ {h}' $, is composed of three parts: $ {c}_{p}{T}' $, $ {L}_{v}{q}' $, and $ g{z}' $. The term $ g{z}' $ is very small in the whole troposphere, $ {h}' $ is close to $ {L}_{v}{q}' $ in low level, but to $ {c}_{p}{T}' $ above 300 hPa. Ascending motion is favorable for low-level water vapor to rise to upper level and then condense, which results in energy input and therefore ascending motion in the YRV. From the perspective of global warming that affects the thermal state of the atmosphere, temperature and water vapor, the vertical motion can be impacted, which indirectly promotes precipitation anomalies through dynamic processes.

      In general, from each term in MSE budget, the circulation anomaly is still a key factor affecting the exceptional rainfall in the YRV during the Meiyu period in 2020. However, the positive contribution of thermodynamic effect cannot be ignored. In other words, feedbacks between the atmospheric dynamics and thermodynamics can exert obvious indirect effects on the exceptional precipitation of 2020 summer in the YRV.

    • Moist enthalpy $ M={c}_{p}T+{L}_{v}q $ is the sum of dry enthalpy and latent energy, so the horizontal advection of climatological moist enthalpy by anomalous wind can be therefore decomposed into two parts in terms of dry enthalpy $ \left\langle{-\boldsymbol{V}' \cdot \nabla {c}_{p}\overline{{T}}}\right\rangle $ (Fig. 6a) and latent energy $\left\langle{-\boldsymbol{V}' \cdot \nabla {L}_v\overline{q}}\right\rangle$ (Fig. 6d). The latter is more important (Figs. 6a vs. 6d) and shows a positive area covering YRV and the south bank. Further decomposition of the horizontal advection into meridional and zonal components (as in Figs. 6e, f) shows that both have their effect. In the meridional direction, the climatological specific humidity is generally wet in the south and dry in the north, creating a positive meridional gradient in the mid–lower reaches of YRV. The anomalous southerlies favor the transport of humid air to the rainy area. In the zonal direction, it is possible that the anomalous westerlies bring relatively humid air here. Both zonal and meridional components increase the energy and create conditions for the ascending motion. Figures 6b and 6c are zonal and meridional advection of the climatological dry enthalpy by anomalous wind, respectively. Different from the zonal component, the meridional abnormal advection $\left\langle{-{v}' \cdot {\partial }_{y}{c}_{p}\overline{T}}\right\rangle$ has a more conspicuous effect. The climatological field of temperature is generally warm in the south and cold in the north, creating a meridional enthalpy gradient in the YRV. The presence of anomalous southerlies is favorable for transporting warm air, causing energy increase and ascending motion.

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

    • Similar to the dynamic term, the thermodynamic term (i.e., the horizontal advection of anomalous moist enthalpy by climatological wind) also can be decomposed into two terms related to dry enthalpy $\left\langle{- \overline{\boldsymbol{V}}\cdot \nabla {c}_{p}{T}'}\right\rangle$ (Fig. 7a), and latent energy $\left\langle{-\overline{\boldsymbol{V}}\cdot \nabla {L}_{v}{q}'}\right\rangle$ (Fig. 7d). Again, the latent energy term is the largest. Latent energy is accumulated on the north bank but lost on the south, making the areal average along the YRV slightly positive. Figures 7e and 7f furthermore show the zonal and meridional components, with a clear dominance from the meridional one. This may be due to a conspicuous increase in water vapor in the YRV, which leads to a negative meridional gradient in the south of YRV and a positive one in the north. Under the influence of southerlies, there is energy output (input) to the south (north) of the Yangtze River (as indicated in Fig. 7f). But the total energy gain is positive for the main precipitation area in the YRV (as indicated by the black box in Fig. 2a), which implies net latent energy input and then ascending motion. The asymmetric energy input on both sides of the Yangtze River is probably due to the background wind. As shown in Fig. 8, to the south of the Yangtze River, the southwesterly wind is strong, and gradually converges over the YRV. As a result, the maximum evaporation is slightly southward shifted relative to the anomalous precipitation center, leading to a larger convergence of the anomalous water vapor in the north of the Yangtze River compared to the divergence in the south.

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

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

      Figures 7b and 7c show the decomposed zonal and meridional components of the total advection of anomalous dry enthalpy by climatological wind. Both components have weak contributions, with the zonal one slightly larger. The zonal southwest wind transports warmer air to this area, causing energy gain and promoting ascent motion.

      In short, the abnormal precipitation over the YRV changes the moisture conditions of its local underlying surface. Through the transport of anomalous water vapor by southwesterly wind, it leads to the net moist enthalpy into the YRV, and as a result it reinforces the original precipitation anomaly. This forms a positive feedback process, which could further amplify the effect of the atmospheric thermal state (water vapor) on the exceptional case of 2020 precipitation in the YRV.

    4.   Summary and discussion
    • This study analyzed the dynamic and thermodynamic processes controlling the exceptional rainy season in the mid–lower reaches of YRV in June–July 2020. Our diagnostics explored the budget equation of moisture and MSE. The dynamic processes are those related to changes of atmospheric circulation, while the thermodynamic processes are related to the thermal state (consequently the water vapor content) of the atmosphere. Such a decomposition allows us to have a better understanding of the underlying mechanisms that lead to rainfall anomalies. The thermodynamic effect, in particular, can be used to make a few speculations for the potential role of global warming in the exceptional precipitation anomalies in summer 2020. Actually, the most prominent manifestation of the anthropogenic global warming is the increase of the tropospheric temperature and the enhancement of its water-vapor holding capacity.

      From the moisture budget equation analysis, we can conclude that the precipitation anomaly of summer 2020 was mainly controlled by the dynamic term in reference to the advection of climatological water vapor by anomalous vertical motion. However, the positive contribution of the thermodynamic term, which is the advection of abnormal water vapor by climatological vertical motion, was only 8.5% of the total effect. That is, the exceptional 2020 rainy season in the YRV was mainly caused by atmospheric circulation anomalies. It can be hardly explained by the argument in relation to global warming. Our analysis does not fully support the intuitive argument saying that “the global warming enhances water vapor content in the atmosphere, which enhances the amount of rainfall.” Such a statement would be very partial in the case of the exceptional 2020 rainy season, since the direct effect of water vapor enhancement did not exceed 8.5%.

      Further MSE analysis indicates that the anomalous vertical motion (mainly responsible for the 2020 event) was almost equally constrained by the dynamic and thermodynamic effects controlling the MSE budget. The dynamic effect was from the advection of warm and humid air by anomalous southwesterly winds. This resulted in an increase of moist static energy over the YRV and therefore ascending motion to balance the energy gain. The abnormal southwesterly winds were associated with an anomalous anticyclone over the Philippine Sea, which is consistent with the conclusions of Zhou et al. (2021) showing that the tropical Indian Ocean warming-induced anticyclone was a major player for the excessive precipitation in the summer of 2020. The thermodynamic effect, which accounts about 45.3% of the total effect (thermodynamic and dynamic effects as a sum), is the advection of anomalous MSE (mainly for latent energy) by mean climatological wind. It is actually the indication of a positive feedback between the atmospheric circulation and the MSE. We believe that the thermodynamic effect could amplify the potential role of global warming in creating vertical motion anomalies.

      To further assess the causality of the exceptional precipitation in 2020, we evaluated the similarities and differences between the exceptional precipitation in 2020 and other super El Niño decaying years (such as 1983, 1998, and 2016; figures omitted). Similar to the 2020 event, these heavy precipitation events are also mainly attributed to the dynamic effect, related with the advection of warm and humid air by anomalous southwesterlies over South China. They are all associated with the Philippine anticyclone, largely caused by intermediate SST anomalies of the tropical Indian Ocean in the decaying summer of the super El Niño (Chou et al., 2009; Xie et al., 2016). Although the 2020 case is not related to any known super El Niño, it shows some similarities to the El Niño cases in terms of their dynamic processes. It is worthy of noting that the thermodynamic effect has a general positive contribution to precipitation anomalies in all the three super El Niño cases. Furthermore, we also calculate the dynamic and thermodynamic effects, for June and July, separately. It shows a stronger dynamic effect in June, while a stronger thermodynamic effect in July. This seems to echo previous studies (Liu and Ding, 2020; Zheng and Wang, 2021) and will be reported in the future.

      Acknowledgments. The precipitation data of meteorological stations are available from the National Climate Center/CMA at https://cmdp.ncc-cma.net/. The JRA-55 reanalysis data are available from JMA public datasets at https://jra.kishou.go.jp/JRA-55/.

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