Does soil-atmosphere moisture coupling exacerbate changes in extreme heat in East Asia under rapid summer warming? What is the solution?

Inner East Asia (IEA) is an important component of the global grassland ecosystem and has experienced a more rapid increase in extreme surface air temperatures compared to the summer mean in recent decades (2001–2020, relative to 1971–1990). This excess hot-extreme warming (EHEW) is particularly pronounced in the southern IEA, where extremely hot temperatures have increased twice as quickly as the summer mean warming, which itself already exceeds global-land warming by more than threefold. A quantitative analysis based on a Lagrangian temperature-anomaly equation along air-parcel backward trajectories initiated on hot days across IEA reveals that the observed EHEW is primarily attributable to increased diabatic heating, which predominantly occurs from 2 days before to the hot days. Meanwhile, changes in heat-prone synoptic-scale circulation only exert a limited influence on the excess warming. Soil drying within IEA appears to be a critical factor contributing to increased diabatic heating through soil moisture–atmosphere coupling, as it limits evaporation and enhances sensitive heat flux, thereby triggering positive soil moisture-temperature feedback. Our analysis underscores the significant impact of local soil moisture deficits on the intensified extreme heat. Urgent implementation of grassland and livestock management strategies, coupled with drought mitigation measures, is essential for adaptation and ecosystem conservation .Keywords: extreme heat, climate change, trajectory, land-atmosphere feedback.Inner East Asia (IEA), including Mongolia and its surroundings (figure 1(b)), is a crucial part of the global grassland ecosystem and exerts vital influences on East Asian and global carbon cycles (Gang et al 2014). It has supported pastoralists for thousands of years and currently sustains approximately 70 million livestock, providing livelihoods for 29% of Mongolia ′ s working-age population, contributing about 15% of the national GDP (Nandintsetseg et al 2021). However, these grassland ecosystems are fragile and particularly vulnerable to climate change and human activities due to their presence in arid and semi-arid climate regions (Na et al 2021, Zhao et al 2021). Since the early 2000s, IEA has experienced dramatic warming in the boreal summer, potentially reaching a tipping point and implying irreversible regional climate change (Dong et al 2016, Zhang et al 2020b, Cai et al 2024). Along with this rapid warming, extremely high temperatures have remarkably increased, often occurring concurrently with extreme dryness (Zhang et al 2020b, Seo and Ha 2022). This combination can have severe and complex impacts on biological systems, particularly affecting grasslands including decreased biomass, plant mortality, and potential shifts in ecosystems from carbon sinks to carbon sources (Easterling et al 2000, Chen et al 2023, Zhang et al 2024a). In fact, these climate changes, compounded by the effect of overgrazing, have led to widespread degradation of grasslands, environmental deterioration, and desert expansion on IEA, threatening long-term livestock production and food security (Bouwman et al 2005, Hilker et al 2014, Zhang et al 2020c, Nandintsetseg et al 2021). Therefore, understanding the physical processes behind the abrupt increase in the extremely hot temperatures in this century is critical to mitigating ecological and social risks. IEA summer warming could shift the daily air temperature distribution toward higher values, resulting in increased hot-extreme temperatures (Donat and Alexander 2012, Lewis and King 2017). This summer′ s warming is driven by both external forcing and internal climate variability. Specifically, increased anthropogenic greenhouse gas emissions and decreased anthropogenic aerosols explain more than half of the observed rapid warming (Dong et al 2016, Zhou et al 2020, Hua et al 2021, Zhang et al 2024b). Additionally, both observations and model simulations have demonstrated that the changes in the phase of two interdecadal sea surface temperature variabilities, the interdecadal pacific variability (IPV) and the Atlantic multidecadal variability (AMV), have decisively influenced the recent warming (Cai et al 2019, 2024, Zhang et al 2024b). These changes trigger or contribute to atmospheric Rossby wave trains across the northern mid-latitudes, thereby contributing to an anticyclonic circulation anomaly over IEA, which could intensify surface warming through adiabatic heating and increased surface solar radiation (Hong et al 2017, Piao et al 2017, Zhang et al 2020a, Sun et al 2022). Furthermore, over the past two decades, hot extremes over IEA have exhibited excess warming rates compared to the summer mean temperatures (figure 1(d)), thereby exacerbating the ecosystem vulnerability to abrupt warming. This excess hotextreme warming (EHEW) suggests alterations in the shape of the temperature distribution and underscores the physical mechanisms for the intensified extreme warming. Three physical processes have been identified as contributing to the hot extremes: temperature advection, adiabatic compression, and diabatic heating (Röthlisberger and Papritz 2023). The dynamic or thermodynamic drivers that enhance these physical processes can create conditions that amplify hot extremes. For instance, changes in atmospheric circulation that result in prolonged and intensified heat-prone weather regimes can exacerbate extreme warming (Mann et al 2017, Kornhuber et al 2019, Teng and Branstator 2019). Differential heating rates across latitudes may lead to nonlinear changes in temperature advection under various weather regimes, facilitating the transport of hot air in heat-prone circulation conditions and amplifying extreme warming (Tamarin-Brodsky et al 2022, Patterson 2023). Moreover, soil moisture conditions can exert a nonlinear control over energy partitioning between latent and sensible heats, thereby mitigating or amplifying extreme temperatures (Seneviratne et al 2006, 2010, Perkins et al 2015, Qiao et al 2023, Cai et al 2024, Zuo et al 2024). In wet energy-limited conditions where soil moisture allows for unrestricted evaporation, soil moisture depletion coincides with enhanced latent heat, which effectively restrains the increases in sensible heat flux and air temperature. Conversely, under water-limited conditions with drying soils, reduced soil moisture restricts evaporation and decreases latent heat, which in turn increases sensible heat and amplifies air temperatures (Pendergrass et al 2020, their figure 3). Therefore, soil moisture exerts opposing effects on air temperatures in these two regimes. Recent studies have highlighted significant drying and increased concurrent heatwave-drought events over IEA, potentially shifting toward water-limited conditions during hot extremes and intensifying hot-extreme warming (Zhang et al 2020b, Seo and Ha 2022). Using a soil moisture–temperature coupling strength index, Zhang et al (2020b, their figure 5) shows that positive soil moisture–temperature feedback existed over IEA, which had significantly intensified in recent decades. However, the relative importance of these physical processes in the EHEW over IEA, as well as the related drivers, remains incompletely understood.3. Result 3.1. Excess warming of extremely hot temperature compared to the summer mean in recent decades The summer mean Tmax anomalies averaged over IEA exhibit an abrupt warming in recent decades, featuring a remarkable warming during 2001–2020 compared to 1971–1990 (figure 1(a); 1.6 ◦C), with the most pronounced increase in southern IEA (figure 1(b)). This abrupt warming in IEA is notably larger than that in the surrounding regions, being twice the global land average (figure 1(a)), and more than three times the global land average in southern IEA (figure 1(b)). Moreover, TX90p has warmed more rapidly than the summer mean Tmax, with an excess warming of about 0.5 ◦C over IEA (figure 1(d)). In southern IEA, the warming of TX90p is nearly twice that of summer mean Tmax (figure 1(d)). Consequently, IEA emerges as a hotspot, experiencing faster warming of TX90p than the summer mean Tmax (the EHEW) during 2001–2020 relative to 1971–1990, and aligning with the finding from previous studies (Wang et al 2022).2001–2020. The influence of physical processes on the temperature evolution along these trajectories is assessed using the Lagrangian temperature-anomaly equation (equation (1)). Six clustered trajectories are selected and named: northwest horizontal, northwest vertical, west horizontal, west vertical, southeast, and east corridors (figure 2(a)). Air parcels descending along these corridors approach IEA and lead to TX90p (figures 2(b) and S9). Relative to 1971–1990, the frequency of the east corridor has increased remarkably during 2001– 2020, with a slight increase in both horizontal corridors (black bars in figure 3(b)). To reveal how circulation changes may lead to these frequency changes, we identify the critical patterns that favor each corridor through a composite analysis of time-evolving circulation based on selected days. These days are chosen when both the daily number and percentage of trajectories in the corridor exceed the 90th percentile of all days with at least one grid experiencing a hot day during 1971–1990 and 2001–2020 (more than 28 d for each corridor). In general, the critical patterns for different corridors associated with the hot-extreme temperatures over IEA are characterized by synoptic-scale waves, with a significant local anticyclone anomaly or ridge when hot-extreme occurs (t = 0). However, these waves exhibit varying amplitudes, propagation trajectories, wave numbers, and phase speeds (figures S2–S7). The vertical and horizontal corridors associated with a similar wave train propagates towards IEA (figures S2 vs. S3 and S4 vs. S5). Among these, the horizontal corridors are linked to more intense wave trains, characterized by prolonged and strongly eastward-propagating ridges over IEA, where diabatic heating plays a more important role in warming the parcels (figures S8(a) vs. S8(b) and S8(c) vs. S8(d)). These prolonged and strong ridges benefit the horizontal corridors by persistently heating the low-level air, thereby shaping TX90p with weaker adiabatic heating. In the east corridor, air parcels descend along the westerlies on the south side of a strong and persistent anticyclone anomaly over northeast Asia, primarily heated by diabatic processes (figures S7 and S8(f)). These westerlies, shaped by the strong anticyclone and the anomalous low pressure southeast of the anticyclone, may be part of a persistent wavenumber-4 pattern. The increased frequencies for horizontal and eastern corridors from 1971–1990 to 2001–2020 may be linked to summer circulation changes over Eurasia, characterized by an atmospheric wave train featuring an anticyclonic anomaly over IEA, potentially strengthening and prolonging the synoptic-scale anticyclonic pattern in situ (figure 3(a)). These circulation changes could be induced by the phase changes in both the IPV and the AMV over recent decades, as evidenced by both observational data and model simulations (Piao et al 2017, Zhang et al 2020a, Sun et al 2022, Cai et al 2024, Zhang et al 2024b). Land– atmosphere coupling can further intensify the anticyclonic anomaly over IEA by warming the land surface through increased shortwave radiation related to reduced cloud cover (Zhang et al 2020b). This warming subsequently heats the overlying atmosphere due to increased upward long-wave radiation and sensible heat flux, thereby reinforcing the anticyclone. Moreover, Yang et al (2024) noted a phase-locking behavior of an amplified wavenumber-4 pattern with an anomalous ridge over IEA since the mid-1990s, which has also contributed to the increased frequency of the east corridor. By linearizing equation (3), we quantify how the increase in TX90p anomalies for each corridor (red bars in figure 3(b)) and the changes in corridor frequencies affect the EHEW from 1971–1990 to 2001–2020. The results demonstrate that the EHEW is primarily induced by the consistent increase in TX90p anomalies in the recent decades (red bar in figure 3(c)). The changes in the corridor frequency, associated with the change in heat-prone weather regimes over Eurasia, exert only a limited influence on the EHEW (figure 3(c)). 3.3. Impacts of soil drying on the EHEW in recent decades To better comprehend the possible mechanisms for the EHEW, we use equations (1) and (2) to quantitatively estimate the related physical processes. For both 1971–1990 and 2001–2020, TX90p anomalies are induced by both adiabatic heating and diabatic heating, while the advective process has a weak cooling effect (figure 3(d)). However, the EHEW from 1971–1990 to 2001–2020 is predominantly driven by enhanced diabatic heating (figure 3(e)), while the changes in synoptic-scale circulation regimes appear to have a limited influence (color vs. gray bars in figure 3(e)). Notably, approximately 88.6% of the enhanced diabatic heating occurs from day −2 to day 0 (hatched bar in figure 3(e)), with the vast majority of air partials already situated within IEA at day −2 (figure S9(a)), suggesting that the enhanced diabatic heating is dominated by localized processes over IEA. We further propose a potential driver for this locally enhanced diabatic heating from 1971–1990 to 2001–2020. IEA has experienced a significant reduction in the summer soil moisture (figure 4(a)), consistent with the findings of Zhang et al (2020b) from both observations and reanalysis datasets (their figure 2). This summer soil drying leads to hot days under drier conditions, especially in southern IEA (figure 4(b)). As discussed in section 1 and revealed by the reduced EF along trajectories from day −2 to day 0 in recent decades (figure 4(c)), this drier condition could shift soil moisture–temperature interaction into a water-limited regime. This result is further supported by the positive (negative) relationship between soil moisture and latent heat flux over IEA on dry (wet) days (figure S9), with dry days defined as the days with soil moisture below −2 standard deviations and wet days above 2 standard deviations. The drier conditions on hot days may benefit the soil moisture–temperature feedback under water-limited conditions, where limited soil moisture reduces latent heat flux and enhances sensible heat flux by altering surface energy partitioning, leading to EHEW. According to the linearized equation (5), about 71.2% of the increased sensible heat fluxes averaged within 2 days before the hot days are contributed by the reduced EF, with a much smaller contribution from the increased net surface radiation flux (29.2%, figure 4(d)). Moreover, soil moisture depletion can reduce cloud cover, allowing the land surface to receive more solar shortwave radiation, thus increasing sensible heat flux. Furthermore, the changes in soil moisture during hot days closely align with the spatial patterns of the EHEW across IEA and its surroundings (figures 1(d) and 4(b)), with a significant spatial correlation coefficient of −0.66, underscoring the critical role of soil drying and reduced EF in driving the EHEW during recent decades. Summary and discussion: Relative to 1971–1990, IEA has experienced a remarkable excess warming of TX90p compared to the summer mean Tmax during 2001–2020. This EHEW is more pronounced over southern IEA and is 2 times the summer Tmax change, making ecosystems more vulnerable to climate change and human activities. Utilizing a Lagrangian temperature-anomaly equation along an air-parcel trajectory, we find that the EHEW is predominantly due to increased diabatic heating, most of which occurs within 2 days before the hot days and is likely to be related to enhanced sensible heat flux. About 71.2% of the increased sensible heat flux is induced by the changes in surface energy partitioning along trajectories from 1971–1990 to 2001–2020, which could be driven by soil drying over IEA. Additionally, curve clustering results show that the changes in the frequency of clustered trajectories have a limited impact on the EHEW, indicating a small contribution from the changes in synopticscale circulation regimes. Similar results in this paper can be mainly achieved with 10 d or 14 d backward trajectories. Our quantitative results emphasize the critical role of soil drying in leading to EHEW over IEA. Soil drying may arise from anticyclonic circulation changes over IEA, which may reduce precipitation and enhance the positive soil moisture– temperature feedback (Piao et al 2017, Zhang et al 2020b). These circulation changes may be induced by the phase shifts in the AMV and the IPV (Cai et al 2024). In addition to natural drivers, increased anthropogenic greenhouse gas emissions and decreased anthropogenic aerosols over North America and Europe since the mid-1970s have been reported as the main drivers of warming over IEA (Sprenger and Wernli 2015, Dong et al 2016, Zhou et al 2020, Hua et al 2021, Zhang et al 2024b), which could trigger positive soil moisture–temperature feedback. As temperatures rise, the atmospheric vapor pressure deficit increases, leading to soil drying and further enhancing the initial temperature increase (Seneviratne et al 2010, Seo and Ha 2022). Moreover, overgrazing could exacerbate soil drying by contributing to widespread grassland decline (Nandintsetseg et al 2021, Zhang et al 2024a, Cai et al 2024). The contributions of various drivers of this drying require further quantitative investigations, including the roles of natural internal variability, human-induced greenhouse gases, aerosol emissions, water withdrawals, and overgrazing. Additionally, accurately simulating soil drying and related land-atmosphere feedback is crucial for future projections of extreme heat changes and could help constrain hot-extreme projections in climate models. In the future, the EHEW may be amplified as global warming could continue to dry soils and shift land-atmosphere interactions into water-limited conditions (Qiao et al 2023). Policymakers should take action to sustain soil moisture, such as maintaining ecosystem stability and controlling livestock numbers, to mitigate the risk of severe heatwaves, droughts, serious economic losses, and threats to food security.

References :

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