Extreme heatwave over Eastern China in summer 2022: the role of three oceans and local soil moisture feedback

Eastern China experienced persistent regional extreme heatwaves in the summer of 2022, with disparate spatial features and formation mechanisms in different months. We quantitatively assessed the relative contributions of three oceans, i.e. tropical Indian Ocean and Pacific and North Atlantic, and the local soil moisture–temperature feedback using linear regression. The results showed that the monthly mean atmospheric circulation anomalies failed to explain the extreme heatwave in June 2022. The combined contribution of the tropical Indo-Pacific and North Atlantic sea surface temperature anomalies (SSTAs), together with the local soil moisture–temperature feedback, explaining approximately 10% of the temperature anomalies. In July, the tropical Indo-Pacific SSTAs promoted anomalous atmospheric circulation and extreme heat via meridional circulation originating in the Maritime Continent, accounting for approximately 10% of the temperature anomalies, with North Atlantic SSTAs contributing the same percentage by a mid-latitude steady Rossby wave. Local soil moisture–temperature feedback accounted for 42% of the anomalies. The tropical Indo-Pacific SSTAs produced a strong western North Pacific anticyclone in August, but their direct contribution to the temperature anomalies was negligible. The North Atlantic SSTAs contributed 9% of the total via the mid-latitude steady Rossby wave. Local soil moisture–temperature feedback contributed 66%, suggesting that the July heatwave and drought exerted a significant impact on the subsequent August extreme heatwave. Global warming has greatly facilitated extreme heatwaves, accounting for about 30%–40% of these events in summer 2022. These results also suggest that the climatic effects of tropical Indo-Pacific and North Atlantic SSTAs on Eastern China are evident in the month-to-month variation in summer. Our results thus contribute to the understanding and prediction of extreme heatwaves in Eastern China.


Introduction
In the summer of 2022, Eastern China experienced the most intense sustained regional heatwave since 1961, lasting 79 d and covering more than 5 million square kilometers (www.chinanews.com.cn/gn/2022/ 09-05/9844482.shtml). The Eastern China region is the most populous in China, and extreme weather and climate events greatly affect people's lives. Therefore, an in-depth understanding of these extreme heatwave events is crucial for future disaster prevention and mitigation in these regions.
The development of extreme heatwaves is associated with local high-pressure systems, which are accompanied by reduced cloud cover, increased shortwave solar radiation reaching the ground, and adiabatic subsidence, leading to an increase in temperature (Deng et al 2019, Hong et al 2022, Thompson et al 2022. The associated low-level temperature advection (Jiménez-Esteve and Domeisen

2022
) and upper-level westerly jets (Sun 2014, Wang et al 2016 also influence the occurrence of high temperatures. Over Eastern China, especially in the southern regions, the westward expansion or intensification of the western Pacific subtropical high (WPSH) has significant effects on the occurrence of extreme heatwaves (Cao et al 2022, Luo and Lau 2018, Deng et al 2019, 2020, Liu et al 2019b. Moreover, these anomalous atmospheric circulations are closely related to local and remote sea surface temperature (SST) anomalies (SSTAs) (Cao et al 2022, Sun 2014, Chen and Li 2023. Sun (2014) reported that the record-breaking SST in the mid-North Atlantic contributed to anomalous East Asian upper-level westerlies and WPSH via the teleconnection wave train, causing extreme heatwaves over the Jianghuai-Jiangnan region of China in July 2013. Li et al (2015) reported the role of the tropical Indo-Pacific SSTAs and the associated anomalous WPSH in causing the extreme heat over South China in the summer of 2013.
In addition to the atmospheric forcing mechanism associated with SSTAs, the role of local soil moisture-temperature feedback in extreme heatwaves has also been highlighted as an important factor (Fischer et al 2007, Zhang and Dong 2009, Meng and Shen 2014. Dry soil may amplify surface warming by considerably reducing the evaporative cooling effect and increasing the upward sensible heat flux (Thompson et al 2022). Fischer et al (2007) showed that soil moisture reduction favors a positive height anomaly in the upper levels, maintaining local high pressure. Soil moisture deficiencies played critical roles in the intense heatwaves over southeastern, southwestern, and eastern China during the summers of 2003, 2006, respectively (Wang et al 2016. During the summer of 2022, extreme heatwave events occurred mainly north of the Yangtze River valley in June, enclosed within 29 • -40 • N and 102 • -122 • E, and along the Yangtze River valley in July and August, within the regions 23 • -34 • N and 102 • -122 • E, and 25 • -36 • N and 102 • -122 • E, respectively (figures 1(a)-(c) and S1(a)-(c)). These regions show large variability in extreme heatwave frequency (figures S1(d)-(f)) and are hereafter referred to as the key regions. The normalized area-averaged 2 m temperature (T2m) anomalies over the key areas all ranked highest for the period 1979-2022 in June, July, and August 2022 (figures 1(d)-(f)). Along with the persistently high temperatures, the Yangtze River valley also experienced severe drought (figures 1(g)-(i)).
Several studies have revealed its triggering mechanism (Chen and Li 2023, He et al 2023, Wang et al 2023. For example, He et al (2023) emphasized the role of the anomalous zonal flow over the subtropical Tibetan Plateau associated with the tropical Indian Ocean low SSTs and Wang et al (2023) highlighted the role of anomalous anticyclonic circulation in the mid-upper troposphere over central and eastern China. During the summer of 2022, consecutive La Niña, negative Indian Ocean dipole (IOD), and North Atlantic tripole (NAT) SSTA patterns developed. However, it is not clear whether all these SSTAs affected the extreme heatwaves over Eastern China in the summer of 2022. If so, then what are the physical mechanisms involved? What are the relative contributions of the different ocean SSTAs? Moreover, the role of low soil moisture over Eastern China in the extreme heatwaves of summer 2022 deserves further investigation.
Therefore, in this study, we applied linear regression to estimate and compare the quantitative contributions of the oceans and local soil moisturetemperature feedback to the extreme heatwave over Eastern China in the summer of 2022. The monthto-month differences in the extreme heatwaves and effects of tropical Indo-Pacific and North Atlantic SSTAs on the climate over Eastern China were also highlighted.

Data
We used the European Center for Medium-Range Weather Forecasts Reanalysis v5 (ERA5; Hersbach et al 2020) dataset with a 1 • × 1 • horizontal resolution, including monthly atmospheric circulation and temperature at 37 pressure levels, outgoing longwave radiation, T2m, and mean surface heat flux, as well as daily T2m and atmospheric circulation. The soil moisture data at 0-28 cm from ERA5-Land monthly average were used in this study (Muñoz-Sabater et al 2021). The gridded daily maximum temperature and soil moisture at a 0.5 • × 0.5 • resolution for global land areas obtained from the Climate Prediction Center (CPC) of the National Oceanic and Atmospheric Administration (NOAA) was used for comparative analysis. The monthly SST dataset (2 • × 2 • ) was obtained from the Extended Reconstructed SST V5 dataset provided by the US NOAA (Huang et al 2017). A 2-9 years bandpass Lanczos filter was employed to extract the interannual variability after the linear trend were removed (Yu et al 2021).

Methods
The IOD index was expressed as the area-averaged SSTA difference between the tropical western (10 • S-10 • N, 50 • -70 • E) and southeastern (10 Pacific SST gradient index was defined as the areaaveraged SSTA difference between the tropical western (5 • S-5 • N, 110 • -155 • E) and central-eastern (5 • S-5 • N, 160 • E-120 • W) Pacific. The NAT SSTA index was defined as follows: The brackets in equation (1) (Jiang et al 2022b). The wave activity flux, which indicates the direction of the Rossby wave energy propagation, was calculated following Takaya and Nakamura (2001). The detailed definition was presented in the supplementary text S1.
Following Kim et al (2022), the linear regression method (text S2) was used to estimate the contributions of the three ocean SSTAs and local soil moisture to the extreme heatwave of summer 2022. A multiregression method was used to reflect the combined contributions of multiple factors. The student's t-test was employed to assess the significance of results, among which the effective degree of freedom N e is denoted as: where N represents the raw sample size, and r 1 and r 2 indicate the lag-one autocorrelation coefficient of two time series, respectively (Bretherton et al 1999). Additionally, we used empirical orthogonal function (EOF) analysis to obtain the dominant intrinsic mode of tropical Indo-Pacific SSTAs .

Atmospheric circulation associated with extreme heatwaves
The atmospheric circulation anomalies associated with extreme heatwaves over Eastern China in the summer of 2022 are shown in figures 2 and S2. The low-tropospheric above-normal temperature was accompanied by local negative geopotential height anomalies in the middle and upper troposphere in June (figures 2(a) and S2(a)-(c)). This suggests that monthly local large-scale circulation anomalies may not represent strong physical links connected to surface weather extremes. This was confirmed by the non-significant statistical correlation (0.01) between the June T2m index and 500 hPa geopotential height index, which was defined as area-averaged 500 hPa geopotential height anomalies over 29 • -40 • N, 102 • -122 • E region. Equivalent-barotropic anticyclonic circulations dominated the extreme heatwave event during July and August, which is in line with previous studies (  Next, we analyzed how local circulation anomalies contribute to the occurrence of extreme heatwaves. The development of highs and associated divergence reduced mid and low cloud cover and allowed more shortwave radiation to reach and warm the ground (figures S3(a)-(f)). The warming ground, in turn, heated the surface atmosphere via long-wave radiation and sensible heat fluxes (figures S3(g)-(i)). The adiabatic heating associated with the sinking motion also directly warmed the atmosphere (figures 2(a)-(c)). Our further analysis showed that the anomalous easterly wind in August 2022, stronger than any since 1979, significantly contributed to the summer heatwave over Eastern China by facilitating the descent (figures S4(c) and (f)), as reported by He et al (2023). There was a relatively weak anomalous easterly wind in June and July, which could have played a minor role in the extreme heatwave (figures S4(a), (b) and (d), (e)). Local atmospheric circulation can explain the extreme heatwave in July and August 2022, but not that in June 2022. The causes of local atmospheric circulation anomalies are explored in the next section.

Formation and maintenance mechanisms of anomalous atmospheric circulation 3.2.1. Tropical SSTAs
Atmospheric circulation anomalies over East Asia during the summer are closely associated with local and remote SSTAs (Weng et al 2011, Yuan and Yang 2012, Chen et al 2013. We examined the role of tropical SSTAs (figures 2(d)-(f)), which presented a negative IOD in the tropical Indian Ocean (Saji et al 1999) and a Central Pacific La Niña in the tropical Pacific (Kao and Yu 2009). Fan et al (2013) reported that cooling over the tropical central Pacific favors the development and maintenance of northwest Pacific anticyclones in the summer according to the classical Gill theory (Gill 1980), and this was observed in June, July, and August 2022 (figures S2(c), (f) and (i)). The Indo-Pacific SSTAs also influence the SSTAs and convection over the Maritime Continent via zonal  To further analyze the quantitative contribution of the tropical Indo-Pacific SSTAs to the extreme heatwaves in the summer of 2022, regression and EOF analyses were performed. The dominant mode of EOF decomposition of tropical Indo-Pacific SSTAs matches well with corresponding SSTAs in June, July, and August 2022 (figures 2(d)-(f) and S5(a)-(c)). Therefore, the first principal component (PC1) was used to represent the variability of tropical Indo-Pacific SSTA patterns. The tropical Indian Ocean and Pacific SSTAs made a small contribution to the extremely high temperatures in June, which together accounted for 2% of the area-averaged T2m anomalies over the key area (figures 3(a), (d) and (g); table 1). In July, the tropical Indian Ocean and Pacific SSTAs contributed to the development of the sinking motion and extremely high temperatures, accounting for 10% of the T2m anomalies (figures 3(b), (e) and (h); table 1). The tropical Indo-Pacific SSTAs did not lead to sinking motion and extreme heat in August, even with a negative contribution for the IOD (figures 3(c), (f) and (i). However, they contributed to the strong anticyclonic circulation in the western North Pacific, as observed in August 2022 (figures S2(i) and S6(c), (f), (i)). This strong anticyclone favored transporting additional water vapor to the west of the key region, resulting in above-average precipitation, which does not favor high temperatures (figures S6(c), (f) and (i). This finding is consistent with the results of Jiang et al (2022b). They suggested that the low-level cyclone over the western North Pacific associated with the positive IOD and El Niño Modoki contributed to the extreme drought in Southeast China in August 2019. In July, the local meridional circulation ascending in the Maritime Continent and descending in South China triggered a weak high-pressure area and below-average precipitation over the key areas (figures S6(b), (e), (h) and S7(b), (e), (h)). The tropical Indo-Pacific SSTAs efficiently contributed to the occurrence of extreme heatwaves in July but not in August. They mainly contributed to the anticyclone over the western North Pacific, further favoring the westward expansion and intensification of the WPSH in August 2022. During 1979-2021, the correlation between the T2m index and PC1 is significant at the 99% confidence level in July (0.45), but not in June (0.13) and August (0.16), which further verifies the above results.
Therefore, the tropical Indo-Pacific SSTAs only partially explain the extreme heatwave in the summer of 2022, and the tropical SSTAs alone could not cause the summertime extreme heatwave, especially in August. This analysis also suggests that the influence of tropical Indo-Pacific SSTAs on East Asia varies greatly from month to month, and that the development of the extreme summer heatwaves over Eastern China is highly sensitive to the intensity and location of the WPSH.

North Atlantic SSTAs
The North Atlantic SSTAs play an important role in the variation of summertime East Asian atmospheric circulation (Sun 2014. The tripole SSTA pattern, characterized by warming of the central pole and cooling of the south and north poles, developed in the North Atlantic in the summer of 2022 and was strongest in August (figures 2(d)-(f) and S8(a)-(c)). How did these SSTAs contribute to the extreme summer heatwave in Eastern China? The tripole SSTA pattern induced an equivalent barotropic anticyclone over the central North Atlantic under the condition of the thermal wind relation and the geostrophic advection limit in the mid and high latitudes (figures 4(a)-(c) and S8; Yu et al 2021, Jiang et al 2022b, Sheng et al 2022. The upper-level anticyclone was located near the mid-latitude westerly jet in summer 2022 (figures 4(a)-(c)). Therefore, the quasi-stationary Rossby wave developed and propagated eastward along the jet waveguide, distributing the disturbance energy downstream and leading to anomalous atmospheric circulation in the upper troposphere over East Asia (figures 4(a)-(c)).
The 200 hPa geopotential height anomalies and wave activity flux associated with the NAT SSTA index show a wave train like that of 2022, contributing to the upper anomalous high pressure and surface warming over Eastern China (figure 4). The contribution of the North Atlantic SSTAs explained 1%, 10%, and 9% of the T2m anomalies in June, July, and August 2022, respectively (table 1). This result also indicates that the anomalies in the Eurasian circulation associated with the North Atlantic SSTAs exhibit significant month-to-month variations during summer. The correlation coefficients between the T2m index and NAT SSTA index are significant at the 90% confidence level in June (0.29) and July (0.35) but not in August (0.16) during the period 1979-2021. The combined contribution of the tropical Indo-Pacific and North Atlantic SSTAs accounted for 4%, 17%, and 9% of the T2m anomalies in June, July, and August, respectively (table 1). Therefore, the contribution of SSTAs to the extreme summer heatwave in Eastern China is limited, with other physical processes being involved.

Local soil moisture-temperature feedback
Local soil moisture-temperature feedback is a very important factor in the development of summer extreme heatwaves over East Asia, as highlighted by Dong (2009) andLi et al (2019b). Belowaverage soil moisture was observed over key areas in the summer of 2022 (figures 5(a)-(c)). The upward latent heat flux over the southern region was positive in July and negative in August (not shown), indicating a gradual drying of the land surface. Normalized area-averaged soil moisture anomalies over the key region for August reached −3.60, making it the driest since 1979, far drier than the next strongest anomaly of −1.76 in 2013 ( figure S9(c)). The unprecedentedly high values of upward surface sensible heat flux indicated strong local moisture-temperature feedback in August (figures S3(i) and S9(f)). Dry soil reduced evaporative cooling and increased upward sensible heat flux, further warming the atmosphere.
Moreover, dry soils favored the development and persistence of mid-to upper-tropospheric anticyclonic circulation (Fischer et al 2007). Due to the persistence of soil moisture anomalies, these results suggest that the July heatwave and drought had a decisive influence on the extreme heatwave in August. The mid-tropospheric circulation and surface temperature anomalies associated with the local soil moisture index also confirm that dry soils favor the development of mid-tropospheric anticyclones and high surface temperatures in July and August (figures 5(e) and (f)). Local soil moisture accounted for 5%, 42%, and 66% of the T2m anomalies in June, July, and August 2022, respectively (table 1). The local soil moisture index and T2m index yield high correlation values of 0.63 and 0.66 in July and August, respectively, during 1979-2021, further indicating the close connection between local soil moisture and temperature anomalies. The results derived from CPC datasets show high agreement (figure S10).
The combined contribution of tropical Indo-Pacific and North Atlantic SSTAs and local soil moisture accounted for 10%, 52%, and 76% of T2m anomalies in June, July, and August 2022, respectively (figures 5(g)-(i); table 1). After subtracting the linear trend, the area-averaged raw temperature anomalies decreased by 0.72 • C, 0.49 • C, and 0.67 • C in June, July, and August 2022, respectively. This indicated that global warming represented by the linear trend, accounts for approximately 40%, 36%, and 28% in June, July, and August, respectively (table 1), agreeing with the result of Wang et al (2023). Excluding the linear trend, the synergistic effect of circulation anomalies associated with SSTAs in the tropical Indo-Pacific and North Atlantic, together with local soil moisture-temperature feedback, dominated the extreme heatwave in July and August 2022. Local soil moisture-temperature feedback was the most significant factor, particularly in August 2022.

Discussion
Monthly large-scale atmospheric circulation anomalies do not provide a complete explanation for the extreme heatwave in June 2022. We further investigate the related mechanism from the perspective of daily time scale. The Hovmöller diagram of T2m and 200 hPa meridional winds averaged between 29 • and 40 • N shows that several persistent high-temperature processes contributed to the monthly average high temperatures, corresponding to the propagation of recurrent synoptic Rossby wave packets in the same phase (figure 6). Similar behavior of Rossby wave packets was observed during the extreme mid-latitude summer heatwaves of 1994, 2010(Fragkoulidis et al 2018, Kornhuber et al 2019, Rothlisberger et al 2019. These waves led to repeatedly strengthened ridges and high-temperature development (figure S11). This pattern spread across the mid-latitudes of the Northern Hemisphere and caused persistent aboveaverage temperatures in several places, for example, Western Europe, North America, and East Asia (figures 6 and S11).
The Tibetan Plateau, an important source of heat in summer, significantly impacts the climate of East Asia (Liu et al 2020). Jiang et al (2022b) reported that the lack of precipitation over the southern Tibetan Plateau and the associated equivalentbarotropic anticyclone contributed to the extreme drought across southeast China in August 2019. During July and August 2022, persistent above-average temperatures, below-average precipitation, and low soil moisture were observed over the Tibetan Plateau. These factors may trigger extreme heatwaves and droughts over Eastern China via promoting the western expansion and intensification of the WPSH (figures 1(h), (i), 5(b), (c) and S2(d), (e), (g), (h)). The Tibetan Plateau may also serve as an important medium in which the steady Rossby wave associated with the North Atlantic SSTAs propagates eastward and influences East Asia (figure 4; Wang et al 2018, Liu et al 2019a. The sea-ice coverage and temperatures in the Arctic (Cohen et al 2014, development of atmospheric blocking (Dole et al 2011), and quasi-resonant amplification of synoptic-scale Rossby waves (Petoukhov et al 2013) have also been suggested as crucial influences on the mid-latitude climate in summer and could be further factors contributing to the extreme heatwaves over Eastern China in the summer of 2022. The synergistic effects of tropical, mid-latitude, and local factors can easily trigger extreme events. For example, the individual tropical SSTAs in August may not have favored the occurrence of high temperatures over Eastern China, but their resulting circulations worked in concert with the circulation associated with the North Atlantic SSTAs and local soil moisture to cause the extreme heatwaves and droughts of August 2022. Following the study of Li et al (2019a), we identified the synergistic/antagonistic effect of tropical Indo-Pacific SSTAs, North Atlantic SSTAs, and local soil moisture index by employing contingency tables (tables S1-S9). Results shows a synergistic effect between a negative local soil moisture index and a positive PC1 in June and between two variables of a positive NAT SSTA index, a positive PC1, and a negative local soil moisture index in July and August.
The absolute value of linear correlation coefficients between the pairwise variables among PC1, NAT SSTA index, and local soil moisture index is less than 0.3, insignificant at the 95% confidence level (tables S10-S12). The linear trend of each variable was removed before establishing the regression equation. Therefore, we treated the linear warming trend, tropical Indo-Pacific and North Atlantic SSTAs, and soil moisture feedback as independent factors in this study. Using linear regression to provide quantitative contributions of different factors inevitably leads to uncertainty, and employing numerical experiments may be an alternative method to provide more precise results. Such numerical experiments deserve further investigation.

Conclusions
In the summer of 2022, Eastern China experienced the strongest sustained regional heatwave in the period from 1979 to 2022, occurring mainly in the northern part of the Yangtze River in June and along the Yangtze River in July and August. The formation mechanisms of the extreme heatwaves in different months were very different.
The extreme heatwave in June could not be well explained by the local monthly mean circulation. The combined contribution of tropical Indo-Pacific and North Atlantic SSTAs and local soil moisture-temperature feedback accounted 10% of the area-averaged T2m anomalies. The development of recurrent transient synoptic Rossby wave patterns may have dominated this persistent heatwave.
In July, the extreme heatwave was closely associated with a local equivalent-barotropic anticyclonic circulation. Tropical Indo-Pacific SSTAs effectively contributed to the subsidence and high temperatures in key regions through meridional circulation originating from the Maritime Continent. These tropical SSTAs were responsible for 10% of the T2m anomalies, and the North Atlantic SSTAs, which influenced the high temperature in Eastern China by the midlatitude steady Rossby wave, accounted for 10% of the anomalies. Local soil moisture-temperature feedback associated with precipitation deficiency contributed to 42% of the T2m anomalies, and the combined contribution was 52%.
The extreme heatwave and the corresponding anomalous atmospheric circulation were strongest in August during the summer of 2022. The direct contribution of tropical Indo-Pacific SSTAs to T2m anomalies was negligible (1%), but they contributed to the strong western North Pacific anticyclone. The North Atlantic SSTAs contributed 9% to the T2m anomalies via the mid-latitude steady Rossby wave. The largest contribution to this extreme heatwave came from the strong local soil moisture-temperature feedback, accounting for 66%. Their combined contributions were responsible for 76% of the anomalies. The high temperatures and drought in July had a major impact on the extreme heatwave in August because of the persistence of low soil moisture. In addition, global warming has contributed about 30%-40% to these extreme heatwaves in 2022 summer.
Our results quantify the contributions of the three oceans and local soil moisture-temperature feedback to the extreme heatwave of summer 2022. The next step in our research will be to conduct numerical experiments to further investigate the formation mechanism of summertime extreme heatwaves in the mid-latitude regions of the Northern Hemisphere and nonlinear interaction between different impact factors.

Data availability statement
All the datasets in this study are publicly available from the following websites: ECMWF Reanalysis V5 dataset from https://cds.climate.copernicus.eu/ cdsapp#!/search?type=dataset; gridded daily maximum temperature and soil moisture of Climate Prediction Center in National Oceanic and Atmospheric Administration from https://psl.noaa.gov/ data/gridded/index.html; and SST from https://psl. noaa.gov/data/gridded/data.noaa.ersst.v5.html.
All data that support the findings of this study are included within the article (and any supplementary files).