Shift of soil moisture-temperature coupling exacerbated 2022 compound hot-dry event in eastern China

Compound hot-dry events (CHDEs) are among the deadliest climate hazards and are occurring with increasing frequency under global warming. The Yangtze River Basin in China experienced a record-breaking CHDE in the summer of 2022, causing severe damage to human societies and ecosystems. Recent studies have emphasized the role of atmospheric circulation anomalies in driving this event. However, the contribution of land–atmosphere feedback to the development of this event remains unclear. Here, we investigated the impacts of soil moisture-temperature coupling on the development of this concurrent heatwave and drought. We showed that large amounts of surface net radiation were partitioned to sensible heat instead of latent heat as the soil moisture-temperature coupling pattern shifted from energy-limited to water-limited under low soil moisture conditions, forming positive land–atmosphere feedback and leading to unprecedented hot extremes in August. The spatial heterogeneity of hot extremes was also largely modulated by the land–atmosphere coupling strength. Furthermore, enhanced land–atmosphere feedback has played an important role in intensifying CHDEs in this traditional humid region. This study improves the understanding of the development of CHDEs from three aspects, including timing, intensity, and spatial distribution, and enables more effective early warning of CHDEs.


Introduction
Concurrent droughts and heatwaves, known as compound hot-dry events (CHDEs), have been increasing globally in recent decades (Zscheischler and Seneviratne 2017, Zscheischler et al 2018, IPCC 2021).The increasing frequency and intensity of CHDEs pose significant risks to crop yield, water resources, and ecosystem services (AghaKouchak et al 2020, Lesk et al 2021, Yin et al 2023).In the summer of 2022, simultaneous drought and heatwave emerged in many regions of China, especially the Yangtze River Basin (YZB).In July and August, the YZB witnessed the strongest heatwave with the highest average temperature and number of high temperature days (maximum temperature >35  2022).Meanwhile, extreme drought occurred in August due to high temperatures and precipitation deficits (Lin et al 2022, Sun et al 2022), resulting in the most severe decline in ecosystem photosynthesis since 2000 (Wang et al 2023a).Moreover, the extremely hot and dry conditions brought serious threats to the national electric system and water security, cascaded into other hazards including wildfires and crop yield losses, and caused numerous heat-related deaths (Zhao et al 2023, Wang et al 2023b).

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To date, numerous studies have attempted to attribute the causes of such CHDEs in terms of atmospheric circulation and sea surface temperature anomalies (SSTAs).Atmospheric circulation anomalies in eastern China, which are typically associated with the anomalous Western Pacific subtropical high (WPSH) (Guan et al 2019), play a significant role in the development of CHDEs (Hao et al 2022).During summer, regions controlled by the WPSH are accompanied by less precipitation, less cloud cover, and higher solar radiation, resulting in much higher air temperatures (Sun et al 2023).Cold air from the north and moisture from the Indian Ocean are kept out of the WPSH-controlled regions (Mallapaty 2022).Furthermore, atmospheric circulation anomalies over eastern China may be caused by SSTAs of the three oceans (the Pacific, Atlantic and Indian Oceans) and snow cover over the Tibetan Plateau (Wang 2019, Chen and Li 2023, He et al 2023).For instance, SSTAs in the tropical Indo-Pacific and the mid-North Atlantic contributed to the extreme heatwave over the YZB in the summer of 2013 (Sun 2014, Li et al 2015).
Meanwhile, the contributions of landatmosphere coupling and feedback in the development of CHDEs are gradually being noted (Zscheischler andSeneviratne 2017, Lesk et al 2022).Recent studies have found that land−atmosphere coupling can accelerate global warming and increase the likelihood of extreme high temperatures (Miralles et al 2014, Qiao et al 2023).Long-lasting anomalous high-pressure systems, accompanied by higher shortwave radiation and air temperature, can alter land surface energy partitioning and trigger stronger land−atmosphere interactions (Orth 2021).On the one hand, high temperature dries the soil by increasing evaporation and plant transpiration (Gentine et al 2019, Miralles et al 2019).On the other hand, drier soil intensifies surface warming through a decrease in latent heat flux and an increase in sensible heat flux, leading to higher surface temperature and lower atmospheric humidity (Dong et al 2022(Dong et al , 2023)).For example, soil moisture-temperature coupling contributed 50.6% to extreme temperatures over western North America in summer 2021 (Zhang et al 2023b).Previous studies have found that dry soil contributed to heatwaves development over eastern China, northern Europe and North American during the summers of 2013, 2018, and 2022, respectively (Wang et al 2016, Dirmeyer et al 2021, Bartusek et al 2022).
Recently, several studies have explored the key atmospheric processes triggering the 2022 CHDE over the YZB, including the anomalous anticyclone in the mid-upper troposphere (Wang et al 2023b), intensification and westward extension of the WPSH (Zhang et al 2023), SSTAs of the tropical Indo-Pacific and North Atlantic (Jiang et al 2023), and anomalous zonal flow over the subtropical Tibetan Plateau (He et al 2023).In addition, the extremely low soil moisture was found to be strongly associated with the temperature extremes in August (Jiang et al 2023).However, how soil moisture deficits and land−atmosphere interactions altered land surface energy partitioning and intensified this CHDE in different regions remains unclear.The frequency and intensity of CHDEs are expected to increase rapidly worldwide in the future (Vogel et al 2020, Bevacqua et al 2022, De Luca and Donat 2023), which is increasing public concern and makes it urgent to understand the mechanism behind CHDEs from the viewpoint of climate system interactions.
Here, we referred to an analysis of the 2021 record-breaking North American heatwave (Bartusek et al 2022) to investigated the characteristics and development process of this unprecedented CHDE in summer 2022.We then assessed the role of soil moisture-temperature coupling in the intensification and spatial distribution of this CHDE over YZB.Furthermore, we evaluated the effects of land−atmosphere interactions on CHDEs over the past 40 years.

Data
The hourly temperature (2 m), dewpoint temperature (2 m), geopotential height (500 hPa), soil moisture (0-7 cm), wind speed (10 m), surface sensible heat flux, surface latent heat flux, surface net solar radiation, surface net thermal radiation, evaporation and potential evaporation (the amount of evaporation under existing atmospheric conditions from a surface of pure water, giving an indication of the maximum possible evaporation) data with a 0.25 • × 0.25 • spatial resolution for the period 1982-2022 were obtained from the ERA5 dataset provided by the European Centre for Medium Range Weather Forecasts (Hersbach et al 2020).The surface net radiation was calculated by subtracting the surface net thermal radiation from the surface net solar radiation (Guo et al 2016, Zhou andWang 2016).All the hourly data were aggregated into averages of daily or longer time scales in this paper.

Definitions of CHDEs and heatwave intensity
Daily CHDEs were defined by daily mean temperature and soil moisture (negative) simultaneously exceeding the corresponding historical 90th percentile thresholds (T90 and SM n 90) (Ridder et al 2020, Weber et al 2020).For the calculation of T90 and SM n 90 for each grid point, 62 d in July and August of each year within the 30 year baseline period  were used.In addition, the daily hot-only events (T > T90 & SM n ⩽ SM n 90) and dry-only events (T ⩽ T90 & SM n > SM n 90) were also defined as control groups.
To quantify the intensity of heatwaves in CHDEs, the daily heatwave magnitude (Russo et al 2015, Ma and Yuan 2023) of a given compound hot-dry day was calculated as: where T d is the daily mean air temperature of a compound hot-dry day, T 25p and T 75p are the 25th and 75th percentile values of the daily temperature of each grid cell within the baseline period (July andAugust 1982-2011), respectively.The annual heatwave intensity of CHDEs is the average of all the M d (T d ) in each year, and is zero if there was no compound hot-dry day at a given grid cell in a given year.

Metric of soil moisture-temperature coupling
The strength of soil moisture-temperature coupling was assessed using the diagnostic π (Miralles et al 2012), which is based on two energy balances of evaporation and potential evaporation.This metric can be used to determine the related heating processes between land and atmosphere.Based on daily data, this method derives the soil moisture-temperature coupling on a daily scale, and the metric is defined as: The primes represent daily anomalies of each variable expressed as the number of standard deviations relative to the climatological (1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001) mean.R n refers to the surface net radiation, T is the daily temperature, E and E p denote the actual and potential evaporation, respectively, and λ is the latent heat of vaporization.
The e ′ is represented by H ′ − H p ′ and denotes the effect of soil moisture deficits on the surface energy balance.When soil moisture is sufficient to meet atmospheric demand (energy-limited region), the actual evaporation equals the potential evaporation, and e ′ will be zero.As a soil moisture deficit gradually appears, the actual evaporation decreases significantly, but the potential evaporation remains constant with the atmospheric evaporation demand and e ′ becomes positive.Hence, the magnitude of e ′ can indicate the contribution of soil moisture deficiency to sensible heat flux.Only if the contribution of soil moisture deficiency to sensible heat (e ′ ) is accompanied by large anomalous temperature values, the local energy balance may be controlling air temperature with large values of π.This diagnostic has been widely used in previous heatwave studies (Miralles et al 2014, Liu et al 2020, Geirinhas et al 2022, Seo  and Ha 2022).

Characteristics of the unprecedented CHDE in summer 2022
Figure 1 shows the spatial distributions of anomalies in temperature, soil moisture, and geopotential height during July and August of 2022.Under the control of the persistent WPSH, anomalously high temperature and exceptionally low soil moisture emerged over eastern China in July and peaked in August over the YZB (103 For instance, temperature anomalies exceeded 8 • C for nearly 10 d (9-18 August) in the Sichuan Basin, accompanied by extremely low soil moisture (figure S1).With the evolution of heat conditions, the anomalies of 3 d mean YZB temperature, soil moisture, and geopotential height in August 2022 exceeded 3, 5, and 2 standard deviations, respectively (figure 1(d)).Simultaneous heatwaves and droughts, which were typical CHDEs, covered nearly 80% of the study area in mid-August (figures S1 and S2).Notably, the geopotential height anomalies were relatively stable after the YZB-mean temperature became extremely high (exceeding 1.5σ) on 4 August, while temperature and soil moisture (negative) continued to increase for approximately 10 d and peaked in mid-August.
High geopotential height, high net radiation, and dry soil are common drivers of extreme temperatures.Compared to temperature and soil moisture anomalies that exceeded their historical  ranges, the anomalies of geopotential height and net surface radiation anomalies during this event were relatively less extreme (figures S3(a) and (d)).Multiple linear regressions, incorporating the simultaneous anomalies of YZB-mean temperature against soil moisture and geopotential height or surface net radiation, show that nonlinear temperature amplifications maximized during the peak of the event (15-20 August) (figures S3(b) and (e)).The maximum temperature amplifications reached 1.6 • C-2 • C (2σ-3σ amplifications), which contributed to the total ∼5 • C temperature anomaly by ∼32%-40%.This indicated the nonlinear interactions between atmospheric circulation anomalies and land surface processes, which played an important role in amplifying this CHDE over the YZB.

Shift of soil moisture-temperature coupling exacerbated the 2022 CHDE
To examine the interactions between atmospheric circulation anomalies and land surface processes, we analyzed the relationships between temperatures, soil moisture, and surface energy fluxes (figure 2).Generally, soil moisture has a negative correlation with temperature in the YZB (figure 2(a)).Although soil moisture continued to decrease from 29 July to 10 August, the temperatures were close to the climatology (figures 2(a)-(g)).However, an abrupt increase in temperature was observed during 10-13 August, and the soil stopped getting drier thereafter (figure 2(g)).In addition, the R 2 between temperature and net radiation was 0.789 in 2002-2021, which was better than that between temperature and soil moisture/geopotential height (figures 2(b) and S4).However, temperature continued to rise after net radiation anomalies peaked on 13 August and significantly deviated from the regression line.This evidence suggests that net radiation could not explain the variations in temperature and that soil moisture might play a dominant role during the CHDE period over YZB.
Soil moisture significantly affects the land surface energy partitioning process.The surface net radiation can be distributed into latent heat flux (evaporative cooling) and sensible heat flux (heating near-surface air).Figure 2(f) shows that latent heat flux increased linearly with net radiation over the past four decades.Additionally, latent heat flux had a negative correlation with soil moisture (figure 2(e)), indicating that the YZB is a typical energy-limited region.
However, in summer 2022, long-lasting temperature extremes continuously consumed soil water, leading to substantial water loss.Then, the regional mean latent heat flux decoupled from net radiation after early August, when soil moisture became extremely low (figure 2(f)).The decoupling between latent heat flux and net radiation suggested that the YZB experienced a shift from an energy-limited pattern to a water-limited pattern during the CHDE.This transition led to an immediate increase in sensible heat flux (figures 2(c) and (d)), which significantly heated the near-surface air and contributed to the unprecedented high temperatures in the following 10 d, coinciding with the severe drought in August over the YZB.
To quantify the local heating effect caused by the shift in soil moisture-temperature coupling, we applied e ′ (see Methods) to describe the extra amount of heat flux transferred from latent heat to sensible heat due to enhanced soil moisture-temperature feedback.The increase in local sensible heat, which significantly contributes to local air temperature, is usually driven by the increase in net radiation or the increase in the proportion of net radiation allocated to sensible heat.The latter term can be reflected by e ′ .The magnitude of e ′ is mainly determined by actual and potential evaporation.Potential evaporation is related to atmospheric evaporation demand, while actual evaporation is also limited by soil moisture availability (figure S5). Figure 3 stratifies different grid points of the YZB during the 2022 event into two soil moisture-temperature coupling modes.In the strong coupling regions (the grid cells with positive sensible heat anomalies and negative latent heat anomalies, orange dots), soil moisture had largely decreased and temperatures started to increase nonlinearly with soil moisture (figure 3(a)).While in the weak coupling regions (all other grid cells, blue dots), soil moisture decreased with slowly rising temperatures.Almost all dots with temperature anomalies higher than 8 • C were found in the strong coupling regions, and the regional mean e ′ increased dramatically during 7-10 August just before the temperature peaked (figure S6).These results suggest that the excessive sensible heat induced by the shift in soil moisture-temperature coupling exacerbated the temperature extremes over the YZB.
Moreover, during the 2022 CHDE, the extreme temperatures (temperature anomalies >8 • C) of different land grid cells had better consistency with e ′ than with net radiation (figures 3(b) and (c)), which indicates that the spatial distribution of hot extremes was linked to the amount of excessive sensible heat produced by enhanced land-atmosphere interactions.Under the background of large-scale atmospheric circulation anomalies (figure 1(c)), almost the entire YZB experienced similar conditions of precipitation deficits and high net radiation (figure S7(e)).However, temperature anomalies in different regions varied greatly from less than 4 • C to over 8 • C in mid-August.This significant spatial heterogeneity of temperature extremes could not be adequately explained by circulation anomalies and net radiation anomalies but fit the spatial distribution of e ′ very well (figures 3(d) and S7(f)-(i)).In addition, the previous local water storage played an important role in this process.Regions with insufficient water storage were more likely to be drained and shifted to a water-limited pattern with stronger land-atmosphere feedback during a prolonged CHDE, ending with more extreme temperatures (figures 3(a) and S7).

Contributions of enhanced land-atmosphere interaction to CHDEs
As a consequence of global warming, the PDFs of regional temperature and soil moisture show that the YZB climate in July and August tended to be hotter and drier in the latter 20 years (2002-2021) (figure 2).Seo and Ha (2022) noted that hotter and drier conditions can largely enhance local landatmosphere interactions, raising the likelihood of CHDEs over northern East Asia, which is a typical water-limited region.While in the humid region YZB, to identify the impacts of background climate change on land-atmosphere interactions and CHDEs, we further examined the trends of temperature, precipitation, soil moisture, e ′ , and the frequency and heatwave intensity of CHDEs (see methods) during the past 40 years .Figures 4(a)-(c) show that temperature had a significant increasing trend and precipitation had a decreasing trend, accompanied by reductions in soil moisture over the YZB.The change in climatology led to an increase in CHDEs (figures S8(a) and (c)).However, there was still a significant increase in the frequency and heatwave intensity of CHDEs even if we have removed the background climate trends of temperature and soil moisture plotted in figures 4(a) and (c), which indicates the nonlinear interactions between temperature and soil moisture under extreme conditions (figures 4(e) and (f)).Moreover, the trends of CHDEs frequency and heatwave intensity calculated by the temperature and soil moisture without the climate trends shared similar spatial distributions with the trend of e ′ (figure 4(c)), Y Ni et al which implies that the enhanced land-atmosphere interaction may be related to the increasing frequency and intensity of CHDEs.Notably, the trends of these variables were significant in the Sichuan Basin, which was also a hotspot in the 2022 CHDE.Therefore, this area (104 • −108 • E, 28 • −32 • N, outlined by black rectangles in figures 4(a)-(f)) was chosen as a typical region to explore the relationship between landatmosphere feedback and compound events.
To further investigate the contribution of landatmosphere feedback to CHDEs, we compared temperature, soil moisture, e ′ , and net radiation during CHDEs, hot-only events, and dry-only events in the typical region during July and August of 1982-2021 (figure S9).Figures 4(e)-(h) show that net radiation anomalies were similar during CHDEs and hot-only events.However, CHDEs had much higher temperatures and lower soil moisture.Almost all temperature anomalies higher than 8 • C were found in CHDEs.The extremely strong land-atmosphere feedback, represented by the highest e ′ in CHDEs, might contribute to the amplification and prolongation of CHDEs.While dry-only events had the lowest net radiation and temperature, although their landatmosphere feedback was relatively strong.Due to relatively sufficient soil water storage, the heatwave in 2022 July was mitigated to some extent by the increasing latent heat (figure 2(g)).However, as soil water continued to be consumed and eventually fell below a certain threshold, positive soil moisturetemperature feedback was established in August, resulting in unprecedented hot extremes.This shift indicates that long-lasting high-pressure systems and high temperatures can substantially consume water resources and alter the local surface energy partitioning process even in humid regions.

Discussion
Previous studies have mainly focused on the atmospheric circulation anomalies (Hao et al 2022) and SSTAs (Chen and Li 2023) that contributed to this long-lasting high-pressure system in summer 2022.However, hot and dry extremes were distributed unevenly under similar atmospheric circulation backgrounds.Our findings indicate that the spatial heterogeneity of hot extremes was largely modulated by the local land-atmosphere coupling strength.The regions with drier soil and stronger land-atmosphere coupling were accompanied by higher temperature extremes.Therefore, this study provides a better understanding of CHDEs from the viewpoint of climate system interactions.Temperature anomalies can be largely explained by soil moisture anomalies during CHDEs (Jiang et al 2023, Zhang et al 2023).Early prediction and warning of the spatial distribution and intensity of exceptional CHDEs can be achieved by monitoring the state of soil moisture and the strength of land-atmosphere coupling.
Background climate change can increase the frequency and intensity of 2022-like CHDEs both directly and indirectly.The warming temperature and drying soil directly increase the likelihood of each variable exceeding its extreme threshold, causing more severe heatwaves and droughts.However, the correlation between temperature and soil moisture, which is strongly linked by land-atmosphere interactions, can also contribute to the occurrence and further amplification of CHDEs.Previous studies have shown that increasing heat-moisture correlations are accompanied by higher likelihood multiplication factors, indicating that hot and dry events are more likely to occur concurrently in the future (Zscheischler and Seneviratne 2017, Lesk et al 2021).Our study also finds that the increased landatmosphere interaction index e ′ can reflect the trend of frequency and heatwave intensity of CHDEs over the past 40 years in YZB.The occurrence of CHDEs is expected to be more frequent in the future under different warming scenarios (Vogel et al 2020, De Luca and Donat 2023) and is largely modulated by precipitation trends rather than warming trends (Bevacqua et al 2022).Considering the significant soil drying trend in many land areas, including humid regions such as the Amazon, the YZB, and Western Europe (Dai 2013, Qiao et al 2023), the enhanced landatmosphere feedback may contribute to more unprecedented disasters like the 2022 CHDE in China and pose great threats to social and ecological systems around the globe.
We also acknowledge some limitations of the ERA5 reanalysis data and the land-atmosphere interaction index e ′ used in our study.First, the surface energy fluxes used in this paper have some uncertainty.Li et al (2017) evaluated the energy flux from four reanalysis datasets and found large differences among them.ERA-Interim has a much better ability to capture the seasonal variations and interannual variability of observations from seven sites in China.Martens et al (2020) confirmed that the surface energy fluxes from ERA5 are of high quality and generally improved upon ERA-Interim, although ERA5 still overestimates surface latent heat flux over land.Second, the value of e ′ has some uncertainty in representing the extra heating effect of land-atmosphere interactions due to the estimation errors of actual evaporation and potential evaporation.In addition, previous studies have demonstrated that advective and adiabatic heating can have a significant contribution to heatwaves in other regions like the North American, Russia and the North China (Schumacher et al 2019, Zhou and Yuan 2022, Röthlisberger and Papritz 2023).The role of advective and adiabatic heating on CHDEs in the YZB requires further investigation in the future.

Conclusions
The YZB experienced an unprecedented CHDE with extremely high temperatures and low soil moisture in summer 2022.By analyzing the surface energy partitioning process, we found that the YZB is a typical energy-limited region where latent heat flux linearly increased with net radiation.However, large amounts of surface net radiation were partitioned to sensible heat instead of latent heat as the soil moisture-temperature coupling pattern shifted from energy-limited to water-limited under low soil moisture conditions during this event, forming positive land-atmosphere feedback and leading to unprecedented hot extremes in August.The hot extremes also showed the feature of spatial heterogeneity, which was closely associated with local land-atmosphere coupling strength.The results uncover an overlooked impact of land-atmosphere interactions that can explain the characteristics of timing, intensity, and spatial distribution of the 2022 record-breaking CHDE in China.
Furthermore, enhanced land-atmosphere feedback has played an important role in intensifying CHDEs in this traditional humid region over the past 40 years.Given that many regions around the globe are expected to experience significant increases in temperature and soil dryness under global warming, the relationship between enhanced land-atmosphere feedback and more frequent CHDEs in the future requires further investigation.

Figure 1 .
Figure 1.Spatial distributions of monthly averaged eastern Asia (a) temperature, (b) soil moisture, and (c) geopotential height anomalies over July and August 2022.Black boxes outline the study area (YZB; 103 • − 123 • E, 25 • -35 • N).(d) The evolution of standard deviations of the three variables throughout July and August averaged over YZB (land only).The dashed gray lines indicate 1.5σ, and the gray region (29 July-22 August) covers the most extreme stage of this event.

Figure 2 .
Figure 2. Three-day-averaged YZB-mean anomalies of soil moisture and net radiation versus (a), (b) temperature, (c), (d) surface sensible heat flux, and (e), (f) surface latent heat flux from 28 July to 20 August in 1982-2022 (8 dots per year).Blue and yellow represent the two different time periods (1982-2001 and 2002-2021).Dot-dashed lines show linear regressions in the two periods.Red diamonds show the 2022 event and its temporal evolution.The probability density function (PDF) of each variable is shown outside the corresponding figure.(g) The evolution of the YZB-mean standard deviations of the five variables throughout July and August.The dashed gray lines indicate ±1.5σ, the gray region (29 July-22 August) covers the most extreme stage of this event, and the red region indicates the shifting period of soil moisture-temperature coupling.

Figure 3 .
Figure 3. Three-day-averaged anomalies of temperature versus (a) soil moisture, (b) surface net radiation, and (c) e ′ stratified by flux partitioning.Each dot represents one of the three-day-averaged anomalies of each land grid cell in the YZB during this CHDE (28 July-20 August ) (8 dots for one grid cell).Orange dots (total: 9942) represent the grid cells with positive sensible heat flux anomalies and negative latent heat flux anomalies.Blue dots (total: 13 506) show all other land grid cells in the YZB region.Dot-dashed lines show linear regressions of the dots.The PDF of each variable is shown outside the corresponding figure.(d) 6 d-mean spatial distribution for the composite of T' and e' during this CHDE.

Figure 4 .
Figure 4.The spatial distribution of linear trends of (a) daily temperature, (b) monthly precipitation, (c) daily soil moisture, (d) daily e ′ , (e) frequency of CHDEs, and (f) heatwave intensity of CHDEs.(e) and (f) are calculated by the temperature and soil moisture without the background climate trends in (a) and (c).Trends in the dotted area pass the 95% significance test.Black boxes outline the typical region (104 • −108 • E, 28 • −32 • N).The distribution of daily (g) temperature anomalies, (h) soil moisture anomalies, (i) net radiation anomalies, and (j) e ′ of CHDEs, hot-only events, and dry-only events in the typical region.The three lines of boxes denote the 25th percentile, median and 75th percentile from bottom to top.All figures are calculated in July and August during 1982-2021.
Soil moisture is a key factor in controlling the energy partitioning process at the land surface (Seneviratne et al 2006, Teuling et al 2010), contributing to the occurrence and development of droughts and heatwaves (Miralles et al 2014, 2019, Bartusek et al 2022, Zhang et al 2023b).There is an increase in the sensitivity of air temperature to drying soil when soil moisture becomes extremely low, leading to positive soil moisture-temperature feedback that intensifies heatwaves and droughts (Benson and Dirmeyer 2021).Such positive feedback typically occurs in semiarid and arid regions, where latent heat flux is largely limited by soil water availability (Seneviratne et al 2010, Hsu and Dirmeyer 2023).The YZB is a typical energylimited region (Dong et al 2023) with annual mean precipitation exceeding 800 mm (Gao et al 2020).
• C) since meteorological observations were available in 1961 (Jiang et al 2023, Ma and Yuan 2023, Zhang et al 2023, Wang et al 2023b).Approximately 360 million people experienced temperatures exceeding 40 • C at some point during this summer in China (Mallapaty