Interdecadal Variations of Persistent Extreme Heat Events in Eastern China Under Global Warming

Naihui Zang (  zangnh18@lzu.edu.cn ) Lanzhou University Junhu Zhao Laboratory for Climate Studies, National Climate Center, China Meteorological Administration Pengcheng Yan Key Laboratory of Arid Climatic Change and Reducing Disaster of Gansu Province / Key Open Laboratory of Arid Climatic Change and Reducing Disaster of China Meteorological Administration, Institute of Arid Meteorology Han Zhang Lanzhou University Shankai Tang Lanzhou University Feng Guolin Lanzhou University


Introduction
According to the special report on climate change from the Intergovernmental Panel on Climate Change, human-induced warming is approximately 1.0°C (± 0.2°C) above pre-industrial levels, increasing at 0.2°C  ). The rate of temperature increase in Europe, Australia, and most parts of Asia is signi cantly higher than the global average, and the increasing trend of EHEs is even higher (Stocker 2014). The recorded warming in China since the early 1960s is almost twice as high as the global mean and approximately one-third higher than the global land average temperature trend ).
Since the beginning of the 21st century, the losses and problems caused to human society by frequent persistent extreme heat events (PEHEs) have become increasingly severe. For instance, the PEHEs that in southwestern China, and the duration of high temperatures at some sites lasted more than 50 days (Peng et al. 2007). In 2013, large-scale PEHEs occurred in South China, and the highest temperature in many places continuously broke historical records. The duration of this event (from June 28 to August 22, lasting 56 days) is also historically rare (Sun et al. 2014;Peng et al. 2016; Yang and Feng 2016). In the summer of 2017, central and eastern China suffered a large-scale PEHE impact ). In the summer of 2018, the number of high-temperature days and the average maximum temperature at most sites in North China and Northeast China exceeded historical records (XU et al. 2019; Ren et al. 2020). Long-duration PEHEs have caused considerable losses to agricultural production and economic development in certain countries. According to statistics, direct nancial losses in China due to meteorological disasters are approximately 200 billion per year, most of which can be attributed to extreme weather (Sun et al. 2011). This number has increased by 60% from 2015 to 2019 to more than 320 billion. At the same time, the impact region of extreme high-temperature events in China has also increased from 12-43% of the country's land area (Qin et al. 2015).
Both the difference in the duration and the scope of occurrence of PEHEs further result in a signi cant impact of differences. The damage caused by large-scale and long-lasting PEHEs is higher than that of de ned EHEs lasting more than 5 days as PEHEs. Ye et al. (2013) de ned EHEs lasting more than 6 days as PEHEs. Jia and Hu (2017) de ned PEHEs with durations longer than 3 d, 5 d, and 7 d as weak, moderate, and strong heatwaves, respectively. The selection of the threshold of the PEHE duration is subjective without taking into account regional differences. P eiderer and Coumou (2018) de ned a persistent event as the duration of a warm or cold day. Their study showed that persistence in summer has increased over the past 60 years in the Northern Hemisphere. Furthermore, the changes are particularly pronounced for prolonged events, suggesting a lengthening in the duration of heat waves.
The above studies show that the threat of PEHEs to the economic development of eastern China will become increasingly serious.
There are considerable differences resulting in the differences in PEHE between regions (Meehl and Tebaldi 2004; P eiderer and Coumou 2018). PEHEs are associated with a persistent anticyclonic anomaly (Zhang et al. 2003;Meehl and Tebaldi 2004;WEI and SUN 2007). Eastern China is broad, and the circulations of different scales work together in this region. The substantial differences in the residence time of anticyclones result in different durations of PEHEs (Yuan et al. 2018). June-August is the main season for PEHEs, but research shows that PEHEs in other months also have a signi cant increasing trend. Our previous study found that EHEs in eastern China show obvious intraseasonal characteristics (Zang et al. 2019). The EHEs mainly occur in the Huanghuai region (the area between the lower Yellow River and the Huai River) in June and primarily occur in the Jiangnan to Jianghuai region from July to August. This is mainly related to the spatial progression of the East Asian summer monsoon. The study also found that since 2010, the frequency and intensity of EHEs decreased in early summer, while the opposite occurred in midsummer.
Based on this, this article conducts a systematic analysis of PEHEs in eastern China and performs a detailed analysis of the regional characteristics, intraseasonal differences, and interdecadal variations of PEHEs of different durations to explore the response of PEHEs to global warming, striving for a deeper understanding of the changing characteristics of PEHEs.

Data
In this study, observed daily surface air temperature data from 1st January 1951 to 31st August 2018 for 2374 stations in China were provided by the National Weather Information Center of the China Meteorological Administration. To ensure the completeness and continuity of the time series, stations with the highest temperature (T max ) missing for more than one day were excluded. Finally, T max data from the May-September period for 759 sites (Fig. 1a) in eastern China from 1961 to 2018 were used.
To analyse the associated circulation anomalies, the data sets of the NCEP-NCAR Reanalysis were used (Kalnay et al. 1996). The variables from the NCEP-NCAR Reanalysis include geopotential height, uwind at multiple pressure levels, with a horizontal resolution of 2.5° × 2.5° during January 1979 to present.

Methods.
An EH day is an effective indicator of EH events, and the Chinese Meteorological Department de nes 35°C as the EH threshold. An EH day is determined to have occurred when the maximum temperature of the day is higher than or equal to 35°C (Shi et al. 2009;Ding et al. 2010;Sun et al. 2011;Hong et al. 2020). In a previous study, we selected 35°C as a xed threshold to analyse the frequency and intensity of summer EHEs in China and found that there are apparent interdecadal variations in EHEs in eastern China. Therefore, this paper still uses this xed threshold to calculate the EHE days in early summer and midsummer rst and then uses the empirical orthogonal decomposition (EOF) method to analyse the main modes and the corresponding time coe cients in eastern China to divide the subregion. Furthermore, according to the temperature series in different regions, the percentile threshold method is used to determine the EHE thresholds. The T max series of climatological summer for sites divided in the same region from 1961 to 2018 is computed, and then the top 10% high-temperature days are de ned as the regional EH day to study the regional and seasonal differences in PEHEs (Huang et al. 1993 occur in the Huanghuai area and south of the Yangtze River. EHEs arise less frequently in the Jianghuai area, so the spatial distribution in the eastern region is inconsistent. In midsummer (July and August), the area from South China to the Jianghuai region is affected by the West Paci c subtropical high, which increases and stretches westward, leading to an increase in the number of EHEs in the Yangtze River Basin (Peng et al. 2016). To further analyse the variations in PEHEs in eastern China, EOF of the anomalies of EHE days in early summer and midsummer from 1961 to 2018 was carried out to study both the spatial distribution and intraseasonal variation characteristics. Figure 1 shows the spatial patterns of the rst two leading EOF modes that analyse the EH day anomalies from 1961 to 2018, both in early summer and midsummer. The EOF1 of EH day anomalies in early summer (Fig. 1b) is characterized by warming in the Yellow-Huai River basin. The warm centre is located in the Huanghuai region. EOF2 features a north-south dipole in EH day anomalies over eastern China (Fig. 1c). The warm centre is located in southern China. The Huanghuai region has negative values.
The spatial characteristics of the rst leading mode of the EH day anomaly in midsummer ( Fig. 1d) are obviously different from those in early summer. Compared with early summer, the positive centre of the rst leading mode moved from the Huanghuai region to the middle and lower reaches of the Yangtze River. In midsummer, the second leading mode shows that the EH day anomaly over South China is opposite to that over Huanghuai basins (Fig. 1e). To explore the differences in PEHEs in eastern China, we divided all the stations in eastern China into four subregions for comparison according to the EOF results of the anomalies of EH days in early summer and midsummer.  There are obvious differences in the frequency and duration of EHEs in eastern China. In NC and HHV, EHEs occur 4.8 times per year on average at each site. However, in the YRV and SC, the average EHE occurrence per site is only 3.5 times per year. That is, the frequency of EHE occurrence in the northern region is higher than that in the southern region (Sun et al. 2011). For PEHEs that lasted more than 3 days, there was no signi cant difference in the average frequency of the 4 regions (0.91 (NC), 1.16 (HHV), 1.06 (YRV), 1.01 (SC), respectively). Although the frequency of EHEs in the north is more frequent than that in the south, the number of days and the overall duration of EHEs in the north are lower than those in the south. This shows that the main manifestation of the difference in EHEs in various regions of eastern China is the difference in the frequency distribution of PEHEs that last less than 3 days. The article will conduct a speci c analysis in the next section.
In addition, most of the EHEs that occurred in the northern region began to increase in May of the 21st century and are EHEs lasting 1-2 days. Due to the lack of long-term PEHE event formation conditions (P eiderer and Coumou 2018), EHEs persisting for more than 3 days are less frequent. For the same reason, in the southern region, in September, except for a small number of EHEs lasting 1-2 days, EHEs lasting more than 3 days are rare. Therefore, the current May and September EHEs have smaller impact on PEHEs in eastern China than June-August, and subsequent studies mainly focus on EHEs in summer (June-August).

The interdecadal variations of PEHEs
To analyze the variations of PEHEs lasting 3 or more days between different regions, the duration of the EHEs at each site is determined separately. Figure 3 shows the annual average of EHE duration-frequency curve of a single station in four regions of eastern China. In the beginning, the frequency value of PEHEs in four regions is at the peak. Then, the frequency decays exponentially as the corresponding duration increases. P eiderer and Coumou (2018) also found similar attenuation characteristics when they studied the changes in persistent temperature events in the Northern Hemisphere land area. The relationship between the duration and frequency of different regions is quite different, which is related to the differences in the formation mechanism of PEHEs in eastern China ( Zhang et  The frequency of PEHEs that last from 3 to 6 days (short-term PEHEs) has the fastest attenuation rate, and the attenuation amplitude is higher than 60%. In this range, the attenuation amplitude of the PEHE frequency in the HHV was the largest, and in the 1960s, the attenuation amplitude of the PEHE frequency in the HHV exceeded 85%.
The frequency of short-term PEHEs in each region generally showed the characteristics of decadal variations that rst decreased and then increased. Since the 1960s, the PEHE frequency began to decline until it reached a minimum in the 1980s and then began to increase. The frequency of PEHEs that occurred in the 2010s reached the maximum. The interdecadal variations in PEHE frequency in eastern China are consistent with those in EHE frequency (Wang et al. 2013;Zang et al. 2019). The decadal trends of short-term PEHE frequency in the YRV and SC are the same as those in NC and HHV. However, the frequency of short-term PEHEs in the YRV and SC since the beginning of the 21st century is signi cantly higher than in the previous stage. In the 2000s especially, the frequency of short-term PEHEs even exceeded that of the 2010s. The frequency of PEHEs lasting more than 3 days in the four regions is roughly the same. The frequency of short-term PEHEs in NC and HHV in each decade is the same as that of YRV and SC, indicating that the difference in the number of EHE days in the eastern region is related to the long-term PEHE frequency.
In the four regions of eastern China, the frequency of PEHEs lasting 7 days or longer per year (long-term PEHEs) for a single station is generally low, so its change with duration also shows a characteristic of decreasing uctuations. As shown in the gure, the frequency of long-term PEHEs in the YRV and SC is signi cantly higher than that of NC and HHV. There are noticeable regional differences in the frequency of PEHEs in eastern China. Short-term PEHEs occur mainly in the north, and PEHEs in the south are more persistent. The peaks of the frequency of the long-term PEHEs in the four regions correspond to different durations (7 days (NC), 9 days (HHV), 9 days (YRV), and 10 days (SC)). The lower the local latitude, the higher the duration of the frequency corresponding to the peak and the stronger the persistence. In the 2010s, especially in typical high-temperature years, the circulation system of the West Paci c subtropical high was signi cantly enhanced (Peng et al. 2016). Due to the feedback of the West Paci c subtropical high, the frequency of long-term PEHEs in the summer of 2010 increased rapidly in the south, which explains the slight decrease in the frequency of short-term PEHE events compared to the 2000s. At the same time, this illustrates the impact of global warming on long-term PEHE events in southern China more obviously. However, the frequency and sustained intensity of long-term PEHEs in the YRV are signi cantly stronger than in SC at lower latitudes, especially the frequency of PEHEs that last for more than 10 days. This may be related to the summer drought resulting from an anomalous geopotential height (Sun et al. 2011).
To further analyse the interdecadal changes and regional differences in summer PEHEs in eastern China, Fig. 4 shows the interannual changes in the frequency of short-term and long-term PEHEs of each station in regions of eastern China and their nine-year moving average.
From the 1960s to the 2000s, except for the long-term PEHE events in NC (Fig. 4e), the frequency of single-station PEHEs in the four regions of eastern China all showed the characteristics of interdecadal changes that rst decreased and then increased, especially in the Huanghuai region. After the 1960s, the frequency of short-term PEHEs decreased signi cantly. Since the late 1970s, the frequency of PEHEs in all regions of eastern China has undergone a signi cant interdecadal transition. The interdecadal transition in NC (Fig. 4a) occurred in the late 1980s, that of the HHV (Fig. 4b) and YRV (Fig. 4c) in the mid-1980s, and that of SC (Fig. 4d) in the early 1970s. The lower the latitude, the earlier the turning point.
In the late 1990s and early 2000s, the frequency of PEHE events in NC increased signi cantly. In the 2010s, the frequency of short-term PEHEs decreased in the YRV and SC, while the frequency of long-term PEHEs increased.

Intraseasonal differences in PEHE persistence in summer
Previous studies have shown that there are obvious intraseasonal changes in the temporal and spatial distributions of EHEs in summer in eastern China and there are also obvious regional differences in interdecadal changes in eastern China (Qiao et al. 2018). To explore the intraseasonal variation and interdecadal variation of PEHEs in eastern China, the average monthly duration-frequency curve of EHEs per station in each decade in the four regions of eastern China is shown in Fig. 5.
There were obvious regional differences in the timing of PEHEs in eastern China. From June to August, short-term PEHEs occurred in NC (Fig. 5a) and HHV (Fig. 5b), but they were mainly concentrated in June and July. The frequency of long-term PEHEs was low, mainly concentrated in June and July. PEHEs in the YRV (Fig. 5c) and SC (Fig. 5d) were concentrated in July and August.
The variations in the frequency of persistent extreme high temperature in the four regions of eastern China with duration are similar to the exponential decay changes in the traditional summer. The exponential decay trend of PEHE frequency with duration is less affected by intraseasonal changes. The frequency of PEHEs in the four regions dropped rapidly within the range of short-term PEHEs. Simultaneously, the frequency of PEHEs (long-term PEHEs) for more than 7 days showed a decrease in uctuations.
Although the frequency changes over time are similar, in northern regions, especially in NC, the frequency and persistence of PEHEs in July and August are signi cantly more robust in response to global warming than in June. Since the 1990s, the frequency of the long-term PEHEs in midsummer in NC is at least three times that of previous decades. Not only has the frequency of PEHEs increased signi cantly, but its sustainability has been considerably enhanced.

Intraseasonal differences in interdecadal changes in PEHEs
To further explore the interannual variation of PEHEs in various regions, the nine-year moving average curve and linear trend of the monthly short-term and long-term PEHE frequency in the four regions of eastern China from 1961 to 2018 are shown in Fig. 6. Since the acceleration of global warming in the 1990s, the intraseasonal distribution of PEHE frequency in NC and HHV in eastern China has changed signi cantly. The occurrence time of PEHEs gradually changed from June and July to the whole summer, and the trend of different months also appeared to be different.
From the 1960s to the 1990s, variations in the frequency of short-term PEHEs from June to July in NC (Fig. 6a & 6g) and HHV (Fig. 6b & 6h) were consistent with the trend of the traditional summer (Fig. 4). In August of the late 1990s, there was a signi cant increase in the frequency of short-term PEHEs in the NC and HHV. In the 2010s especially, the frequency of short-term and long-term PEHEs in some years approached or even exceeded the frequency of PEHE in June. Before the 2010s, the trend of the shortterm PEHE 9-year moving average curve changes in June and July in NC and HHV was the same. However, since the beginning of the 21st century, the trend in the frequency between June and July has also appeared to be different, and the correlation between the trend in July and August is higher than the correlation between June and July. Since global warming, the feedback mechanism within the season has undergone certain changes.
PEHEs in the YRV (Fig. 6c) and SC (Fig. 6d) were concentrated in July and August. During this period, the system affecting the two regions was mainly the western Paci c subtropical high (Yuan et al. 2018). Short-term PEHEs have a higher frequency and shorter duration, and the possibility of process interruption is relatively small. Therefore, the nine-year moving average of the short-term PEHE frequency over time in the two regions is basically consistent with the traditional summer PEHEs (Fig. 4c & 4d), and differences between the trend of frequency in July and August is relatively small. However, the nine-year moving average of the long-term PEHE frequency in July and August in the YRV (Fig. 6g) was quite different in the period from the late 1980s to the early 1990s and the period of 2000s. This is consistent with the statistical analysis of Shi et al. (2009) in the typical years with more high-temperature days in July in East China from 1960 to 2005. In these two periods, the number of typical years with more hightemperature days in July accounted for ten high temperatures, i.e., 70% of a typical year (1988,1990,1992,1994,2001,2003,2004). At the same time, August accounted for only 10% (2003) in this period (Shi et al. 2009), so the long-term PEHEs in the YRV have a large intraseasonal difference. This result may be related to the difference in the number of landing typhoons during the high-temperature period in East China. In July, a typical year with more high-temperature days, the average number of typhoons landing in East China is lower. Therefore, the continuous high-temperature processes in these two periods are not easily interrupted by the impact of precipitation caused by typhoons, and long-term PEHEs are more likely to occur.
In addition, since global warming, there are apparent intraseasonal differences in the regions where the frequency of short-term PEHEs has increased signi cantly in eastern China. The region with the most signi cant increase in short-term PEHEs in June was NC, compared with July in the HHV, and August in YRV (all passed the 95% signi cance test). The region with the most signi cant increase in the long-term PEHE frequency in July and August was the YRV (both values passed the 95% signi cance test). Figures 7 and 8 show the average duration of PEHEs in June, July, and August, which lasted more than 3 days (all-PEHEs) and 7 days (long-term PEHEs), respectively. The interdecadal trends of the average duration of PEHEs in the same month are the same, while the spatial distributions are different.

Intraseasonal spatial distributions of PEHEs
In June (Fig. 7a), the area of the average duration of all PEHEs is distributed in NC and HHV, consistent with the above conclusion. The area of long-term PEHEs is mainly scattered in northern regions. Affected by plum rain, there are relatively fewer EHEs in the YRV in June. It is worth noting that the PEHE centres exceeding 3 days in the HHV are mainly located in inland areas, showing the opposite distribution from July, which is supposed to be affected by the northern region of the plum rain (Tao et al. 1958).
In July (Fig. 7b), the scope and persistence of PEHEs increased signi cantly in NC. The high-value centre of the duration of all PEHEs in HHV also moved from inland to coastal areas. The incidence of PEHEs in the YRV and SC has increased rapidly, and the persistence is far beyond that in northern China. The most persistent PEHEs in eastern China are roughly located in the YRV. In August (Fig. 7c), the area where PEHEs occurred decreased, but since the 2010s, the persistence of PEHEs in the YRV and HHV has increased obviously (Fig. 8c), even exceeding that in July (Fig. 8b).

Comparisons of composite circulation anomalies for different regions
In the northern-hemispheric mid-latitudes, several extreme events have been linked to anomalous atmospheric circulation patterns favoring the persistence of local weather conditions (Qiao et al. 2011, P eiderer and Coumou 2018). The circulation associated with EHEs in eastern China typically shows an anomalous anticyclone that enhances temperature through adiabatic heating (Sun et al. 2011, Chen & Lu, 2015. In order to analyse the likely mechanism which results in the differences in the PEHE in eastern China, the circulation anomalies associated with EH are systematically compared. Figure 9 and Fig. 10 show the average composite of 500hPa and 850hPa geopotential height anomaly in eastern China, and 850hPa (e, f) meridional wind anomaly of strong-persistence years since the 1990s in SC and YRV. There is an anomalous anticyclone over the region in the middle and low troposphere in the four regions, respectively.
In June, there is a signi cant anticyclone over the NC (Fig. 9a & 9b) in both the middle and lower troposphere, and the centre are both located in the north of 40°N. The circulation anomalies in HHV ( Fig. 9c & 9d) are similar to NC, so the nine-year moving average of the frequency of the PEHE in NC and HHV is likely in June. Figure 10a and 10b shows the composite anomalies of the strong-persistence years in YRV in July. There is also a signi cant anticyclone in the middle and lower troposphere, but located in the east of the region. Meanwhile, the centre of composite anticyclone anomalies in SC in July is in the southeast of the region (Fig. 10c & 10d). The differences in the position of the anticyclone anomalies between June and July resulting in the intraseasonal variations of the PEHE in eastern China. In addition, in southern China, there is a positive meridional wind anomaly in the lower troposphere (Fig. 10e &10f), which is also an important reason for the difference between PEHE in southern and northern China

Variations Of The Persistence In Pehes Under Global Warming
The above analysis has shown the regional differences in the frequency of PEHEs in eastern China. Since global warming, the intraseasonal variation in PEHEs in the HHV has differed signi cantly from that in the previous period. The persistence of PEHEs in the YRV and SC has increased. Therefore, we analysed the persistence of PEHEs in the two periods before and after global warming (P1: 1961-1990; P2: 1991-2018). In addition, in NC (Fig. 11a) and HHV (Fig. 11b), the probability distribution of P2 on the right side (larger value) is higher than P1, indicating that the average duration of PEHEs in these regions has increased (Zhan et al. 2020). In other words, in the HHV, since global warming, the persistence of PEHEs has increased, and the average intensity of the PEHEs has increased at the same time.

Conclusion And Discussion
Using the daily maximum temperature data of 759 stations in eastern China from May to September from 1961 to 2018 and the data sets of the NCEP-NCAR Reanalysis, this paper analyses the temporal and spatial characteristics and regional characteristics of PEHEs in NC, HHV, YRV, and SC. We distinguished duration in this study and discussed the persistence of the EHE. And the PEHE was discussed month by month instead of traditional summer to study the intraseansonal variations. The conclusions are as follows: The frequency of PEHEs in eastern China decays exponentially with time. In terms of spatial distribution, there is a regional difference in the duration and intraseasonal differences in the occurrence time. The northern site is dominated by multi-frequency, short-duration EHEs, compared to the opposite in the south. PEHEs in NC and HHV mainly occur from June to July, while in the south, they mainly occur from July to August.
Since the 1990s, the response of PEHEs in northern regions to the global warming has become more apparent with more frequency, stronger persistence and obvious intraseasonal differences. The frequency of PEHEs increases in July and August, shows an opposite linear trend in June. The interdecadal trend in PEHEs also showed apparent differences. Since the 2000s, the frequency and persistence of PEHEs has increased signi cantly in the YRV and SC, especially the long-term PEHEs since 2010s. The differences in the position of the anticyclone anomalies in both the middle and lower troposphere resulting in the intraseasonal variations of the PEHE in eastern China.
This study is mainly based on the PEHEs of a single site. The average annual situation and each decade are represented by the site. The current results show that interdecadal and variation of the PEHE and circulation associated with differences of the PEHE are consistent with previous studies in EHE in eastern China. In future work, it is necessary to conduct a more in-depth exploration of the characteristics and mechanisms of the large-scale variations of PEHEs since global warming on the basis of this work. The statistical result can then be used in combination with a dynamic forecast and can be applied to seasonal prediction.

Declarations Con ict of Interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper.

Availability of data and material
The NCEP-NCAR Reanalysis datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request. The observed daily surface air temperature data for 2,374 stations that are available from the National Weather Information Center of the China Meteorological Administration.

Code availability
The code analysed during the current study are available from the corresponding author on reasonable request.

Figure 1
The spatial distribution of the 759 sites in eastern China (a), and the rst two leading EOF modes of the anomaly of the number of EH days in early summer for the 1961-2018 period in eastern China: (b) EOF1 of early summer, (c) EOF2 of early summer, (d) EOF1 of midsummer, (e) EOF2 of midsummer. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.       The average composite of 500hPa (a, b) and 850hPa (c, d) geopotential height anomaly of strongpersistence years since the 1990s in eastern China in June. (a, c) NC, (b, d) HHV. The area that at the 95% con dence level are dotted. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 8
The same as Figure 7, but lasting 7 days or more. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. The average composite of 500hPa (a, b) and 850hPa (c, d) geopotential height anomaly of strongpersistence years since the 1990s in eastern China in June. (a, c) NC, (b, d) HHV. The area that at the 95% con dence level are dotted. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. The average composite of 500hPa (a, b), 850hPa (c, d) geopotential height anomaly and 850hPa (e, f) meridional wind anomaly of strong-persistence years since the 1990s in eastern China in July. (a, c, e) YRV, (b, d, f) SC. The area that at the 95% con dence level are dotted. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its