3.1 The distribution and evolution of rainfall
The extreme Meiyu in 2020 is produced under specific circulation conditions (Ding et al. 2021). Figure 3 shows the observed circulation distribution of the 30-year climatological mean state from 1991 to 2020, the state in 2020, and the bias between them. The southwesterly jet at 850 hPa in June-July 2020 is weaker than the climatological mean state, which only reaches the YHRV (Figs. 3a and 3b). The southerly anomalies converge with the northerly anomalies in YHRV, favoring the convergence at lower levels and the long-term maintenance of Meiyu (Fig. 3c). In addition, the upper-level westerly jet at 200 hPa in the summer of 2020 is also stronger than the climatological mean state (Figs. 3g, 3h and 3i), which means large horizontal temperature gradient with strong cold air from the north and warm air from the south meet in YHRV (Li and Zhang 2014; Xu et al. 2022), and this favors the occurrence of the 2020 extreme Meiyu. In the following part, the effects of different lateral boundary forcings on the extreme 2020 Meiyu in the YHRV are specifically analyzed through comparisons between observations and different sensitivity experiments.
From the observed precipitation distribution in June-July 2020 (Fig. 4a), the precipitation intensity of the 2020 Meiyu appears to be very strong and the maximum value rainfall center exceeds 20 mm per day. The main rainband is located along the lower and middle reaches of the Yangtze River near 30°N, with an east-west zonal distribution. Comparing the simulated precipitation of Exp_Control (Fig. 4b) and the observations, it can be found that the Exp_Control can well reproduce the characteristics of the Meiyu rainfall distribution, but the simulated precipitation intensity is weaker than the observations. The weak rainfall bias may due to the hydrostatic dynamic core of RegCM4.6 (Giorgi et al. 2011), it cannot well describe the intense precipitation produced by local non-hydrostatic meso-small scale system in Meiyu front.
Driven by the 30-year climatological average reanalysis data, the summer rainfall distribution simulated by Exp_30ave (Fig. 4d) is relatively similar to the observed 30-year climatological averaged condition (Fig. 4c), that is, there is more precipitation in South China and North China but less precipitation in the YHRV. The precipitation simulated by Exp_30ave shows a "+-+" wavelike pattern.
To avoid the impacts of the systematic weak bias simulated by RegCM4.6, the simulated rainfall of Exp_Control is assumed to be accurate. The differences between Exp_Control and other sensitivity experiments can be obtained by subtracting the Exp_Control results from sensitivity experiments results to quantify the contributions of the lateral boundary forcings in different directions. From the precipitation bias between Exp_30ave and Exp_Control (Fig. 5a), it can be found that the precipitation bias shows a "+-+" pattern, with a strong negative deviation center in the YHRV and a positive center in North China and South China. These results indicate that the extreme Meiyu is produced under special atmospheric forcings in 2020, and if these anomalous atmospheric signals are removed, heavy rainfall will not occur in the YHRV.
Summer rainfall in China is closely related to the atmospheric anomalies from the south ocean, such as the summer monsoon and the water vapor transported by the monsoon, convection over the tropical ocean, and the boreal summer intraseasonal oscillation (BSISO) transmitted from south to north (Kemball-Cook and Wang 2001, Zhang and Sumi 2002, He et al. 2007, Chen et al. 2015, Song et al. 2016, Ding et al. 2020). After adding the 2020 realistic boundary forcings to the south boundary, it can be found that little precipitation occurs over most parts of China (Fig. 4e). The bias between Exp_South and Exp_Control shows large negative deviations, especially in the YHRV (Fig. 5b). The southwesterlies from the south ocean can transport moisture and heat to YHRV. However, without the cold air from the north, the water vapor in the air cannot be lifted to produce precipitation.
Frequent trough and ridge activities in upper-level westerlies north of YHRV can bring cold air to the YHRV, lift the moist air, and result in thermal instability for the occurrence of precipitation, which is important for the maintenance of Meiyu in the YHRV (Li et al. 2004, Sampe and Xie 2010). Therefore, the results of Exp_North show that after adding the realistic atmospheric forcing in the north boundary, the precipitation increases significantly compared to both Exp_South and Exp_30ave, with large precipitation center locates in South China, the lower reach of the Yangtze River, and parts of North China (Fig. 4f). From the precipitation bias between Exp_North and Exp_Control (Fig. 5c), it can be found that the distribution of precipitation bias shows a "+-+-+" pattern from south to north in the meridional direction, indicating that when the cold air from the north is active, it also makes the teleconnection pattern similar to the distribution of summer precipitation in East Asia.
If the 2020 realistic south and north anomalous atmospheric signals are added to the lateral boundary condition at the same time (Exp_South&North), large amount of precipitation occurs in North China, and some precipitation appears in the Yangtze River basin (Fig. 4g). The bias between Exp_South&North and Exp_Control further shows that the precipitation caused by the combined effect of 2020 realistic south boundary forcing and north boundary forcing is still weak in the YHRV. The location of the main rainband is in North China, not in the YHRV (Fig. 5d). Both the Exp_South and Exp_South&North add the 2020 realistic south boundary forcing, which is characterized as weaker southwesterlies than the climatological mean state (Fig. 3). However, the weak southwesterlies alone does not result in heavy rainfall in the YHRV but rather results in less precipitation in both the YHRV and South China.
The WPSH is located in east China and is one of the important systems influencing the Meiyu rainfall (Lu et al. 2005, Yang et al. 2010, Liu et al. 2012, Sun et al. 2021). After adding the 2020 realistic east boundary forcing, it can be found that the Exp_East can well simulate the location and intensity of the WPSH as well as the rainband. The northeast-southwest rainband is mainly located in South China and the middle and lower reaches of the Yangtze River (Fig. 4h). This result indicates that the WPSH is very important for anchoring the Meiyu rainband location in 2020. Strong intensity and south location of WPSH often correspond to the strong Meiyu in YHRV (Fig. 3). From the distribution of the bias between Exp_East and Exp_Control (Fig. 5e), positive bias can be seen in the lower reach of the Yangtze River, south China, and North China, indicating that the Exp_East produce excessive precipitation in South China and North China. Negative bias still exists in the middle reach of the Yangtze River.
Exp_South&East uses both the realistic south boundary forcing and east boundary forcing. From the results of Exp_South&East (Fig. 4i), it can be found that the precipitation in South China is significantly reduced and the northeast-southwest rainband appears in the middle and lower reaches of the Yangtze River. The distribution of precipitation bias between Exp_South&East and Exp_Control (Fig. 5f) that the south boundary forcing can decrease the precipitation in South China, and produce a northeast-southwest rainband rather than an east-west rainband as the observation and Exp_Control.
After adding both the 2020 realistic north and east boundary forcing (Exp_North&East), the simulated precipitation in South China is further reduced and the rainband shifts northwards. In addition, the simulated rainband shows the same east-west zonal distribution characteristics as the observations and Exp_Control (Figs. 4i and 5g). These results indicate that the combination of the active cold air from the north and strong WPSH can generate an east-west zonal rainband in the Yangtze River basin. Therefore, the cold air from the north is important for the formation of precipitation, and the location of WPSH determines the location of the rainbands.
By comparing the precipitation bias between sensitivity experiments and Exp_Control, for the extreme Meiyu in summer 2020, the different lateral boundary forcings from the south, north and east sides are key factors for the formation of 2020 Meiyu. The weak warm moist southwesterlies can reduce precipitation in South China and provide moisture and heat for heavy rainfall. However, the south boundary forcing alone cannot generate much rainfall without the lift by cold air and thermal instability conditions. In addition, the active troughs and ridges as well as cold air activities in the north provide lifting and instability conditions for the occurrence of precipitation. The WPSH in the east can anchor the position of the rainband.
We can also infer the contributions of different lateral boundary forcings in the precipitation evolution by comparing the simulated precipitation evolutions of sensitivity experiments with the Exp_Control. Figure 6 shows the time-latitude profiles of the zonal-averaged precipitation in 107–122°E during June and July. From the observed evolution of precipitation (Fig. 6a), an obvious QBWO of Meiyu is presented in the Yangtze River basin, and the rainband position oscillates north and south around 30°N. By comparing Exp_Control (Fig. 6b) and the observations, it can be found that the Exp_Control can basically simulate the evolution of rainband location and intensity under the 2020 realistic atmospheric forcings, but the precipitation simulated by Exp_Control is weaker than the observations after July 15, indicating that the physical processes causing the strong rainfall in YHRV after July 15 are poorly simulated by the model.
By comparing the Exp_30ave with Exp_Control, there are no significant QBWO of the rainfall in the Yangtze River basin until July 10 (Fig. 6c). Between June 13 and July 5, precipitation is concentrated in South China and South China Sea. However, Exp_30ave can reproduce the rainband evolution after July 10, with two northward movements and one southward movement, indicating that the atmospheric conditions after July 10 in 2020 are similar to the climatological mean state, resulting in the similar rainband evolution in 2020 to the climatological mean state. However, the two northward shifts simulated by Exp_30ave reach further north than the observations, indicating that the southwesterlies from the south in the climatological mean state is stronger than that in 2020, and that the WPSH can jump to a more northerly position than that in 2020 (Fig. 3).
The results of Exp_South (Fig. 6d) shows that if only the realistic south boundary forcing is added, there is nearly no precipitation occurring in the Yangtze River basin, and the precipitation does not have characteristics of QBWO before July 15. The rainband moves southward during 15–20 July, then moves northward after 20 July, which is consistent with the Exp_30ave (Fig. 6c), indicating that this precipitation process can be generated even without the atmospheric anomalies in 2020.
After adding the realistic north boundary forcing, the precipitation simulated by Exp_North significantly increases and shows an obvious QBWO characteristics. However, the location of the rainbands is more northward than the observation before June 20, and the oscillation feature is different from those of Exp_Control and the observations. The southward shift of the rainband simulated by Exp_North from July 3 to July 10 is nearly the same as Exp_Control, indicating that the anomalous atmospheric forcing in the north plays an important role in the precipitation evolution during this period.
If both the realistic north and south boundary forcings are added at the same time, it can be found that the simulated precipitation by Exp_South&North is larger than the Exp_North, and the north-south oscillation of the precipitation is closer to the observations than the Exp_North (Fig. 6f). The pattern correlation coefficient between the simulated precipitation evolution of Exp_South&North and Exp_Control is the highest among all experiments (Fig. 7), so it can be inferred that the north-south oscillation of the precipitation is mainly caused by the interplay of warm moist air from the south and the cold air from the north. Many studies have demonstrated the combined effect of the warm moist air from the south and the cold air from the north is important for the QBWO of summer precipitation (Chen et al. 2015, Ding et al. 2020). However, due to the lack of the realistic WPSH’s forcing in 2020, the rainband simulated by Exp_South&North locates more northward than observation, with precipitation mainly concentrated in north of 30°N (Fig. 6f).
By comparing the Exp_East (Fig. 6g) with Exp_Control (Fig. 6b) and the observations (Fig. 6a), it can be found that the southward movement of the rainband is consistent with the observation before June 10. This result suggests that the WPSH plays a major role in the precipitation evolution before June 10. However, with only the east boundary forcing, Exp_East simulates no precipitation oscillations in YHRV from June 11 to July 3. The north-south oscillation of the rainband after July 10 simulated by Exp_East is similar to Exp_30ave, further indicating that the atmospheric conditions during this period are close to the 30-year climatological mean state and no atmospheric forcing in any one direction plays a dominant role.
After adding both the south and east boundary forcings, the precipitation oscillations simulated by Exp_South&East (Fig. 6h) are very close to those simulated by Exp_Control (Fig. 6b), and it has the second highest pattern correlation coefficient (Fig. 7). The pattern correlation coefficients of Exp_South&East and Exp_South&North are high because both the two experiments have realistic south boundary forcing, indicating that the south atmospheric signals are crucial for the QBWO of the extreme Meiyu.
Figure 6i shows that the simulated rainband by Exp_North&East experiences a southward process and a northward process before June 17, and the intensity of precipitation is strong in the YHRV, suggesting that the cold air from the north together with the southeasterly warm and moist air from the south of the WPSH are the main reasons for the occurrence of strong rainfall during this period. However, Exp_North&East fails to simulate the strong rainfall in YHRV and its QBWO from June 18 to July 3, and the simulated rainband is more northward, indicating that the anomalous rainfall in this period must be produced by the combined effect of anomalous atmospheric signals from the north, south and east.
In a whole, Fig. 7 shows that the contributions of the anomalous atmospheric forcings in different directions change with time. Among them, the east boundary forcing contributes to the precipitation evolution before June 10. The precipitation oscillation process from June 11 to July 3 is mainly caused by the combined effect of south and north atmospheric anomalies forcings. From July 3 to July 10, the north anomalous atmospheric forcings are important in the southward movement of the rainband. The atmospheric conditions in 2020 are similar to the climatological mean state after July 10, so the rainband evolution in 2020 is also similar to the climatological mean state, and no anomalous atmospheric forcing in any direction plays a leading role during this period.
3.2 Relationship between southwesterly jet and Meiyu
The summer rainfall in East China is closely related to the EASM, and the evolution and strength of EASM can lead to precipitation anomalies (He et al. 2007; Ding et al. 2021). Figure 8 shows the time-latitude profiles of the 850 hPa meridional wind, horizontal wind and divergence of the vertically integrated water vapor transported from the surface to 300 hPa averaged over 107°-122°E during June-July. From the observations (Fig. 8a), it can be seen that the large value center of the southerlies is stable in the south of 30°N. Its intensity and evolution show an obvious characteristic of QBWO during June and July 2020. The convergence of water vapor, which is located in the north of the large value area of the southerly wind, provides favorable dynamic lifting conditions for heavy rainfall. The monsoon southwesterlies from the south and the cold air from the north keep converging near 30°N, leading to the super Meiyu in 2020. By comparing Exp_Control (Fig. 8b) with the observations, it can be found that the Exp_Control can basically reproduce the evolution of the southwesterlies, but the large value region of the southerlies extends more northward than observations after July 15, resulting in a more northward rainband and less Meiyu rainfall during this period.
The monsoon southwesterlies simulated by Exp_30ave (Fig. 8c) is stronger and extends further more northward than that simulated by Exp_Control, resulting in the convergent zone of water vapor and the rainband move northward of 30°N by June 21. From June 21 to July 5, the southerlies intensity simulated by Exp_30ave is weak, making the rainband move southward. After July 10, the wind evolution simulated by Exp_30ave is similar to Exp_Control, so the simulated rainband evolutions are also consistent with the Exp_Control, further indicating that the atmospheric circulation evolution after July 10 in 2020 is similar to the climatological mean state.
Adding the realistic boundary forcings in south or north direction alone cannot reproduce the evolution of the monsoon southwesterlies. For example, the Exp_South simulates strong southerlies, which extends northerly, resulting in a more northerly rainband than observation. After July 15, the monsoon southwesterlies evolution of Exp_South is close to Exp_Control, indicating that the atmospheric anomalies are also close to the climatological mean state during this period (Fig. 8d). The simulated southwesterlies by Exp_North extend more northward and has weaker intensity than Exp_Control in June (Fig. 8e), resulting in a north rainband in June (Fig. 6e). The evolution of monsoon southwesterlies in July simulated by Exp_North is similar to the Exp_Control, thus the rainband evolution is also be well reproduced by Exp_North. The realistic south and north boundary forcings together help the Exp_South&North to produce a reasonable evolution of the southwesterlies, which makes the simulated evolution of the rainband by Exp_South&North the best in all the sensitivity experiments. Thus, the interaction between cold air from the north and the warm moist air from the south is very important for the maintenance and oscillation of rainband.
Exp_South&East (Fig. 8h) and Exp_North&East (Fig. 8i), Exp_East (Fig. 8g) all have realistic east boundary forcing. From the results of the three experiments, it can be found that the east boundary forcing can help control the strong southerlies stay south of the 30ºN and the convergence zone of the northerlies and southerlies maintained near the 30ºN. However, without the north boundary forcing and the south boundary forcing, the simulated evolution of the southwesterlies by Exp_East deviate from the Exp_Control in June, further indicating that both the north boundary forcing and south boundary forcing are very important to the intraseasonal oscillation of monsoon southwesterlies.
By analyzing the 850 hPa atmospheric circulation deviations between sensitivity experiments and the Exp_Control (Fig. 9), it can be found that the circulation deviation has a south-to-north distribution of "cyclone", "anticyclone", and "cyclone", thus causing the precipitation deviation to show "+-+" characteristics from south to north (Fig. 5). The anomalous atmospheric forcings can meridionally distribute the anomalous precipitation in 2020, similar to the teleconnection pattern of the summer precipitation in East Asia (figure omitted).
3.4 Relationship between the upper-level westerly jet and Meiyu
Figure 11 shows the time-latitude profiles of the zonal-averaged 200 hPa zonal winds, geopotential height and divergence in June-July. As shown in the observations (Fig. 11a), the subtropical westerly jet is maintained between 30°N and 40°N from the beginning of June to July 20, with a constant strong intensity. The strong subtropical westerly jet is related with the continuous convergence of cold air from the north and warm air from the south, which intensify the south-north temperature gradient. The south side of the jet is the positive divergent zone which can provide good lifting conditions for precipitation, and the evolutions of both the positive divergence and the rainband (Fig. 6a) are basically consistent. Moreover, the position of the South Asia High is relatively stable and maintained south of 30°N. The weak jet simulated by Exp_Control makes the total precipitation simulated by Exp_Control also has a weak bias in intensity (Fig. 11b).
If only the realistic south boundary forcing is added, the westerly jet simulated by Exp_South is weak before June 20, so there is no precipitation occurring in the YHRV during this period. Between June 20 and July 10, the simulated westerly jet by Exp_South is stronger but located more northward than observation, resulting in no precipitation in YHRV. According to the previous analysis, the anomalous atmospheric conditions in 2020 are similar to the climatological mean state after July 10, so the simulated westerly jet by Exp_South is weak as the Exp_Control and observation (Fig. 11d).
If only the north boundary forcing is included (Exp_North), although the subtropical jet is close to the north boundary and the realistic atmospheric forcing can diffuse from the north boundary through the buffer zone to the interior of the region, Exp_North cannot reproduce the intensity and evolution of the subtropical jet, and the simulated subtropical jet is much weaker than that in Exp_Control (Fig. 11e). These results suggest that there is a systematic interaction between the north cold air, south warm air and Meiyu rainfall. The upper-level jet is in the subtropical frontal region with large north-south temperature gradient. The south boundary forcing in Exp_North is the climatological mean state, leading to a strong monsoon southwesterlies and a weak Meiyu. The weak precipitation releases less latent heat of condensation, causing a weak temperature gradient and weak westerly jet in Exp_North (Fig. 11e).
Without the realistic north boundary forcing, the Exp_East and the Exp_South&East cannot well simulate the westerly jet. Among all sensitivity experiments, only Exp_South&North (Fig. 11h) and Exp_North&East (Fig. 11i) can reproduce the intensity and evolution of the upper-level jet well, suggesting that the north boundary forcing combined with south or east boundary forcing can reproduce the continuous convergence of the north cold air and south warm air, which intensifies the temperature gradient and favors the strong westerly jet and Meiyu. Without the north atmospheric forcing, the westerly jet is weak.