3.1 Temporal and spatial characteristics of EHEs
Affected by Jianghuai Meiyu and other factors, the spatial distribution of EHE days in eastern China has visible intraseasonal changes (Yuan et al. 2018; Zang et al. 2019). EHEs in early summer (June) mainly 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 Pacific 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 first 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 first 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 first 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.
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South China (SC) includes stations within 22°-28°N,110°-123°E.
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The Yangtze River valley (YRV) includes stations within 28°-33°N,110°-123°E.
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The Huanghuai region (HHV) includes stations within 33°-38°N,110°-123°E.
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North China (NC) includes stations within 38°-43°N,110°-120°E.
In previous studies, the eastern coastal areas of China were often divided into South China, the Yangtze River valley, and North China based on the overall climate characteristics of the local area (Chen and Lu 2015; Tang et al. 2020). According to the spatial characteristics of the EOF of the anomaly of the number of EH days, this paper divides the area north of the Yangtze River into the Huanghuai region and North China, which can better reflect the regional and seasonal differences of PEHEs.
Previous studies have shown that there are apparent early onset phenomena of EHEs in eastern China, especially in the past 10 years, and the range and intensity of EH days in May have increased significantly (Zang et al. 2019; LU et al. 2020, Wang et al. 2021). Therefore, this paper analyses the average frequency of EHEs per station in four subregions from May to September since 1961 (Fig. 2). Figure 2 shows that the frequency of EHEs in eastern China declined from the end of the 1960s, and the decline in the HHV was relatively large. In the 1980s, the frequency of EHEs in the HHV and YRV reached a minimum. The four regions showed an increasing trend in the late 1980s. In the 1990s, the frequency of EHEs in the eastern region began to increase gradually, and the increase in North China was the most significant.
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 significant 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 specific 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 (Pfleiderer 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).
3.2 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. Pfleiderer 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 al. 2003; Sun et al. 2011; Yuan et al. 2018).
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 first 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 significantly 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 fluctuations. As shown in the figure, the frequency of long-term PEHEs in the YRV and SC is significantly 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.
Pfleiderer and Coumou (2018) pointed out that global warming has increased the EH of summer in the mid-latitudes of the Northern Hemisphere. From the late 1990s to the early 21st century, North China was controlled by a ridge of continental high pressure and the northwestern boundary of the Western Pacific subtropical high (WEI and SUN 2007). Their effect significantly increased the PEHE persistence in North China in July during this period. However, in the 2000s, the frequency of PEHEs in North China was the lowest, which may be related to the stable temperature increase caused by the PDO phase adjustment during this period (DU et al. 2018).
In the 2010s, especially in typical high-temperature years, the circulation system of the West Pacific subtropical high was significantly enhanced (Peng et al. 2016). Due to the feedback of the West Pacific 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 significantly 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 first decreased and then increased, especially in the Huanghuai region. After the 1960s, the frequency of short-term PEHEs decreased significantly. Since the late 1970s, the frequency of PEHEs in all regions of eastern China has undergone a significant 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 significantly. In the 2010s, the frequency of short-term PEHEs decreased in the YRV and SC, while the frequency of long-term PEHEs increased.