Summer Drought Events Research Articles

Interannual Variation of Summer Compound Hot and Drought Events in Xinjiang and Its Relationship with the North Atlantic Sea Surface Temperature

Abstract Xinjiang suffers compound hot and drought events under global warming. However, less attention has been paid to physical mechanisms of the variability of compound hot and drought events in this region. This article investigates the interannual variation of summer (June–August) compound hot and drought events in Xinjiang and its relationship with the sea surface temperature (SST) over the North Atlantic. The results show that its first empirical orthogonal function (EOF) mode features a spatially homogenous pattern. This mode is closely connected with the simultaneous meridional negative–positive–negative SST anomalies over the North Atlantic. The summer North Atlantic tripole SST anomalies can trigger a remarkable wave train extending from the North Atlantic to Eurasia, consequently inducing an anomalous high pressure system over the Iran–Pamir Plateau, which is conducive to the increase in air temperature from the surface to the upper troposphere over Xinjiang. The warmer troposphere further strengthens the western Asian subtropical meridional temperature gradient and thereby enhances the westerly wind to the north flank of the West Asian subtropical westerly jet (WASWJ). As a result, the WASWJ is displaced northward, which intensifies the sinking motion and prevents the water vapor transport to Xinjiang, leading to a decrease in precipitation in the target region. The higher temperature and less precipitation contribute to the occurrence of compound hot and drought events over Xinjiang. Numerical simulations based on the Community Atmosphere Model, version 4 (CAM4), further confirm the relationship between the North Atlantic tripole SST anomalies and compound hot and drought events in Xinjiang during summer on the interannual time scale. Significance Statement Xinjiang, located in northwest China, suffers from frequent and intense compound hot and drought events in recent decades. It is thus urgent to understand the physical mechanisms associated with its occurrence in order for disaster prevention and mitigation. In this study, we reveal that the leading mode of summer compound hot and drought events over Xinjiang, featuring a spatially homogenous pattern, is closely correlated with the simultaneous meridional negative–positive–negative SST anomalies over the North Atlantic. The North Atlantic tripole SST anomalies can lead to an anomalous high pressure system over the Iran–Pamir Plateau and a northward shift of the West Asian subtropical westerly jet through exciting a wave train propagating from the North Atlantic to Eurasia, which consequently contributes to the occurrence of compound hot and drought events over Xinjiang.

Modeling the impact of extreme summer drought on conventional and renewable generation capacity: Methods and a case study on the Eastern U.S. power system

Across recent years, there has been a growing prevalence of extreme weather events throughout the United States, posing significant challenges to the reliable and resilient operation of power systems. Specifically, summer droughts threaten to severely reduce available generation capacity to meet regional electricity demand, potentially leading to power outages. This underscores the importance of accurate resource adequacy (RA) assessment to ensure the reliable operation of the nation’s energy infrastructure. Accurately evaluating the usable capacity of regional generation fleets is a challenging undertaking due to the intricate interactions between power systems and hydro-climatic systems. This paper proposes a systematic and analytical framework to evaluate the impacts of extreme summer drought events on the available capacity of various generating technologies, incorporating both meteorological and hydrologic factors. The framework provides detailed plant-level capacity derating models for hydroelectric, thermoelectric, and renewable power plants, facilitating evaluations with high temporal and spatial resolution. The application of the proposed impact assessment framework to the 2025 generation fleet of the real-world power system within the PJM and SERC regions of the United States yields insightful results. By analyzing the daily usable capacity of 6,055 at-risk generators across the study region, it shows that the summer capacity deration is most significant for hydroelectric and once-through thermal power plants, followed by recirculating thermal power plants and combustion turbines. In the event of the recurrence of the 2007 southeastern summer drought event in the near future, the generation fleet could experience a substantial reduction in available capacity, estimated at approximately 8.5 GW, compared to typical summer conditions. The sensitivity analysis reveals that the usable capacity of the generation fleet would suffer an even more significant decrease under conditions of increasingly severe summer droughts. The proposed approach and the findings of this study provide valuable methodologies and insights, empowering stakeholders to bolster the resilience of power systems against the potentially devastating effects of future extreme drought events.

The Important Role of Reduced Moisture Supplies from the Monsoon Region in the Formation of Spring and Summer Droughts over Northeast China

Abstract Anomalies in atmospheric moisture transport are critical for drought formation, with East Asian monsoon anomalies identified as the primary driver of droughts in East Asia. However, the drought mechanism for the border region between monsoon and nonmonsoon land areas remains unclear due to complex interactions between latitudinal circulation patterns. Using a Lagrangian framework, we quantify moisture supply (MS) during spring and summer drought events within the border region, specifically Northeast China (NEC). Our results reveal that decreased MS from monsoon land areas is more crucial than nonmonsoon land areas and local areas for NEC droughts, with the local water recycling playing a more substantial role in summer than in spring. On average, summer droughts are 4 times more intense than spring droughts. Contributions to MS deficits from monsoon land areas, nonmonsoon land areas, and local areas during spring (summer) droughts are 43.2% (43.8%), 25.1% (19.2%), and 22% (33.8%), respectively. The weakened atmospheric circulation from source regions to NEC contributes to over 80% of MS deficits during droughts. Atmospheric wave trains triggered by North Atlantic sea surface temperature anomalies (SSTA) gradients, along with the weakening of the East Asian subtropical westerly jet in spring and a positive phase of the polar–Eurasian teleconnection in summer, contribute to spring and summer droughts, respectively. A sustained, record-breaking positive SSTA along the western European coast from the spring to summer excited a wave train, and resulted in the extreme 2017 spring–summer drought. These findings provide valuable insights into the drought mechanism within the interaction zone of circulation systems from different latitudes. Significance Statement The complex interactions between atmospheric circulation patterns at different latitudes have led to an insufficient understanding of the mechanisms causing extreme drought in the border region between the monsoon and nonmonsoon land areas. This study aims to investigate the drought mechanisms in the border region from the perspective of moisture tracking. Results indicate a moisture supply deficit from the monsoon region contributes the most (approximately half) to droughts, although the majority of air parcels reaching this region originate from the nonmonsoon region. The weakening of circulation contributes to more than 80% of the moisture supply deficit during droughts, far exceeding the impact of reduced evaporation from the moisture source regions. These findings are conducive to understanding the mechanisms of extreme drought.

The Spatiotemporal Variation Characteristics and Impacts of Summer Heatwaves, Droughts, and Compound Drought and Heatwave Events in Jiangsu Province, China

This study used the “Daily meteorological dataset of basic meteorological elements of China National Surface Weather Station (V3.0)” and applied the absolute threshold method and standardized precipitation evapotranspiration index to identify heatwave events and drought events. This study analyzed the spatiotemporal evolution patterns of three types of summer disaster events, namely, heatwave events, drought events, and compound drought and heatwave events, in Jiangsu Province from 1960 to 2018. Additionally, it investigated and verified the concurrent historical data of the identified years with the most severe occurrence of compound drought and heatwave events and calculated the monthly drought centers and summer accumulations of the standardized precipitation evapotranspiration index (SPEI-3). The results indicate that over the 59 years analyzed, the number of days with a threshold of 35 °C, which were considered hot days, was 503.2, accounting for 9.27% of the total summer days in Jiangsu Province. Both the number of hot days and the frequency of heatwave events showed a clear increasing trend from the northeastern coastal areas to the southwestern regions of Jiangsu Province. The total frequency of drought events at different stations in Jiangsu Province from 1960 to 2018 fell within the range of 50–64. The fitted slope of the frequency of compound drought and heatwave events in Jiangsu Province was −0.021 for the period 1960 to 1989, and 0.079 for the period 1990 to 2018, indicating a higher frequency compared with the preceding 30 years. This trend aligned with the rise in heatwave events experienced in Jiangsu Province in recent years. The frequency and duration of compound drought and heatwave events in Jiangsu Province exhibited an increasing spatial pattern from the southwestern parts to the northeastern parts. This study’s verification established that the identification of compound drought and heatwave events was relatively accurate.

Perennial forages in cold‐humid areas: Adaptation and resilience‐building strategies toward climate change

AbstractIn the continental cold and humid areas of northeastern North America, climate change by mid‐century may increase the yield potential of perennial forage species as a result of increased temperature and atmospheric CO2 concentration, and a longer growing season compared to the 1990−2000 period. More winter thaws and less snow cover along with more summer drought events, however, may reduce winter survival and summer regrowth, thus potentially reducing the persistence of perennial forages over time. Based on nearly two decades of research on the effects of climate change on forages in cold and humid areas of northeastern North America, through field experiments and modeling, we summarize in this review the expected effects of climate change on forage production in terms of yield, nutritive value, and winter survival. We also propose a set of 12 adaptation and resilience‐building strategies for forage systems that can be implemented at the field and farm levels. Research priorities for the future, including the improvement of species, forage management practices, and modeling tools, are also identified. Implementing strategies to alleviate or take advantage of the effects of a changing climate may improve productivity and resilience of well‐adapted forage systems in cold and humid areas.

A model framework to investigate the role of anomalous land surface processes in the amplification of summer drought across Ireland during 2018

AbstractDue to its latitude and ample year‐round rainfall, Ireland is typically an energy‐limited regime in the context of soil moisture availability and evapotranspiration. However, during the summer of 2018, regions within the country displayed significant soil moisture deficits, associated with anomalous atmospheric forcing conditions, with consequent impacts on the surface energy balance. Here, we explore the utility of a physically based land surface scheme coupled with observational, global gridded reanalysis and satellite‐derived data products to analyse the spatial and temporal evolution of the 2018 summer drought event in Ireland over grassland, which represents the dominant agricultural land‐cover. While the surface–air energy exchanges were initially dominated by atmospheric anomalies, soil moisture constraints became increasingly important in regulating these exchanges, as the accumulated rainfall deficit increased throughout the summer months. This was particularly evident over the freer draining soils in the east and southeast of the country. From late June 2018, we identify a strong linear coupling between soil moisture and both evapotranspiration and vegetation response, suggesting a shift from an energy‐limited evapotranspiration regime into a dry or soil water‐limited regime. Applying segmented regression models, the study quantifies a critical soil moisture threshold as a key determinant of the transition from wet to dry evaporative regimes. These findings are important to understand the soil moisture context under which land–atmosphere couplings are strongest in water‐limited regimes across the country and should help improve the treatment of soil parameters in weather prediction models, required for subseasonal and seasonal forecasts, consequently enhancing early warning systems of summer climate extremes in the future.

Spatial patterns of atmospheric vapour transport and their connection to drought in New Zealand

AbstractThe impacts of drought can be substantial, particularly for primary producing nations such as New Zealand. Despite the importance of drought, identifying connections to atmospheric drivers remains a challenge. Vertically integrated water vapour transport (IVT) is critical to understanding the connection between ocean, atmosphere and the land surface, and as such the drivers of drought events. Self‐organizing maps (SOMs) are used here to characterize the spatial patterns of IVT over New Zealand. In turn, the occurrence of these patterns during drought conditions (defined using the standardized precipitation and evapotranspiration index) across New Zealand is then identified and discussed. The resulting classification highlights a collection of high and low IVT magnitude nodes (i.e., spatial patterns). Low IVT nodes occur 40.57% of the time during all drought events and 45.86% during the top 99th percentile events. Conversely, high IVT magnitude nodes occur less frequently during drought events, at 23.77 and 20.96% of the time during all drought events and the top 99th percentile events, respectively. The low IVT nodes are also particularly dominant during summer and autumn season drought events. An increase in the frequency of occurrence of drought‐associated nodes is observed over the course of the study period, which may in part be associated with El Niño–Southern Oscillation (ENSO)‐driven interannual variability in New Zealand climate. Overall, the results demonstrate the importance of atmospheric circulation‐driven disruptions in moisture transport for drought development, while also providing a first moisture transport weather classification for New Zealand.

Study of historical groundwater level changes in two Belgian chalk aquifers in the context of climate change impacts

Abstract In southern Belgium, 23% of abstracted groundwater volumes are from chalk aquifers, representing strategic resources for the region. Due to their specific nature, these chalk aquifers often exhibit singular behaviour and require specific analysis. The quantitative evolution of these groundwater resources is analysed for the Mons Basin and Hesbaye chalk aquifers as a function of past evolution, in the short and long term. Groundwater level time series exhibit decreases when analysed over different periods. This is particularly visible for the Hesbaye chalk aquifer when comparing the 1960–90 and 1990–2020 periods. Such decreases are associated with observed temperature increases and precipitation decreases, inducing a decrease of aquifer recharge, and a probable increase of groundwater abstraction in the adjacent catchment. Past evolution is also discussed considering recent winter and summer drought events. The aquifers exhibit long delays in response to recharge events, particularly where the thickness of the partially saturated zone plays a crucial role in observed delays. Regarding future evolution, simulations of the impact of climate changes using medium–high emission scenarios indicate a probable decrease of the groundwater levels over the Hesbaye chalk aquifer.

Drought Monitoring over Nepal for the Last Four Decades and Its Connection with Southern Oscillation Index

This study identified summer and annual drought events using the Standard Precipitation Index (SPI) for 107 stations across Nepal from 1977 to 2018. For this, frequency, duration, and severity of drought events were investigated. The SPI4 and SPI12 time scales were interpolated to illustrate the spatial patterns of major drought episodes and their severity. A total of 13 and 24 percent of stations over the country showed a significant decreasing trend for SPI4 and SPI12. Droughts were recorded during El Niño and non-El Niño years in Nepal. Among them, 1992 was the worst drought year, followed by the drought year, 2015. More than 44 percent of the locations in the country were occupied under drought conditions during these extreme drought events. Droughts have been recorded more frequently in Nepal since 2005. The areas of Nepal affected by extreme, severe, and moderate drought in summer were 8, 9, and 18 percent, while during annual events they were 7, 11, and 17 percent, respectively. Generally, during the drought years, the SPI and Southern Oscillation Index (SOI) have a strong phase relation compared to the average years.

Has the risk of a 1976 north‐west European summer drought and heatwave event increased since the 1970s because of climate change?

AbstractIn the summer of 1976, north‐west Europe experienced an exceptional heatwave and drought, which impacted agriculture and public water supply. This study aims to assess how the likelihood of the event in the present‐day climate has changed since 1976 because of climate change. The analysis focuses on the England and Wales region, which was particularly badly impacted. Three key factors contributing to the extreme summer were identified: the dry preceding winter–spring period, the dry summer and the hot summer. Following the principles of event attribution, three methods are used to evaluate the change in event risk: one using observational data, a second using CMIP5 coupled climate models and a third using HadGEM3‐A atmosphere‐only simulations. This is the first time that this method has been used to evaluate how the risk of a historical extreme event has changed since it originally occurred. The results from the three methods agree qualitatively. The probability of a summer at least as hot as 1976 has increased significantly between the 1970s and the present‐day climate (estimated risk ratios 11 (5–95% confidence interval (CI) [7,14]), 9 (CI [4,28]) and 19 (CI [5,25]) based on the three respective methods). In contrast, no significant change in the probability of an extreme dry winter–spring or an extreme dry summer was found. However, the joint probability of an extreme dry winter–spring followed by an extreme hot summer and the probability of an extreme hot and dry summer have both increased significantly between the 1970s and the present day (estimated risk ratios between 5 and 79, and between 3 and 39, respectively). Water resource systems should therefore be robust enough to cope with more frequent occurrences of summers as extreme as 1976.

Changing pattern of drought in Nepal and associated atmospheric circulation

Understanding the spatial and temporal characteristics of drought is vital for the agricultural, hydrological, ecological, and socio-economic systems. This study investigates the spatial and temporal variation of drought characteristics in Nepal using the Standardized Precipitation Evapotranspiration Index (SPEI) based on observational monthly temperature and precipitation data from 1987 to 2017. The drought variation was observed in the Western, Central, and Eastern regions of Nepal at short-term (SPEI4) and long-term (SPEI12) timescales, revealing the increased drought frequency, duration, and severity in the recent decade. The frequency of short-term drought (SPEI4 ≤ -1) has elevated by 15%, 17%, and 15% in Western, Central, and Eastern regions, respectively, during 2005–2017 compared to 1987–2004. Moreover, the interdecadal increase of drought characteristics was prominent after 2004, revealed by the SPEI12 with aggravated and prolonged drought episodes. The summer drought index (SPEI4-Sep) showed an interannual variation in the Western region at a seasonal timescale, whereas decadal variation in the Central and Eastern regions. Meanwhile, summer drought events increased five times in the Central region, two times in the Eastern Region, and did not observe any change in the Western region after 2004. The large-scale atmospheric circulations revealed the negative summer precipitation anomalies due to weakening wind anomalies with anti-cyclonic circulation and moisture divergence over the Indo-Gangetic plain and Nepal, respectively, leads to enhanced drought after 2004. Furthermore, the moisture transport from the Arabian Sea and the Bay of Bengal to the study region is weaker, resulting in more drought events, especially in the Central and Eastern regions.

Long-term agricultural management impacts arbuscular mycorrhizal fungi more than short-term experimental drought

Agricultural management practices and extreme weather events associated with climate change can influence the diversity and abundance of arbuscular mycorrhizal fungi (AMF) with potential consequences for crop production. However, the importance of the interactive effects of long-term agricultural management and extreme weather events on AMF communities in agricultural soils is not yet fully explored. A short-term drought experiment with rainout-shelters was performed in winter wheat fields in a long-term agricultural trial with organic (biodynamic) and conventional management practices. During four months of the winter wheat growing period (March–June 2017), the rainout-shelters reduced the ambient precipitation by 65% on average. At two sampling dates, the AMF diversity and community composition were assessed using a single-molecule real-time (SMRT) DNA sequencing. A total of 955 amplicon sequence variants (ASVs), belonging to twelve genera were identified. The long-term farming systems and the short-term experimental drought did not affect AMF ASV diversity levels. The AMF community composition at the genus level differed between the organic and the conventional farming systems, but no distinctive communities were found in response to the experimental drought. The three most abundant genera Acaulospora, Paraglomus and Funneliformis were correlated to the two farming practices. Our study demonstrates that AMF communities in agricultural soils are responsive to long-term farming systems, and are resistant to one short-term summer drought event.

Physical storylines of future European drought events like 2018 based on ensemble climate modelling

In the aftermath of observed extreme weather events, questions arise on the role of climate change in such events and what future events might look like. We present a method for the development of physical storylines of future events comparable to a chosen observed event, to answer some of these questions. A storyline approach, focusing on physical processes and plausibility rather than probability, improves risk awareness through its relation with our memory of the observed event and contributes to decision making processes through their user focus. The method is showcased by means of a proof-of-concept for the 2018 drought in western Europe. We create analogues of the observed event based on large ensemble climate model simulations representing 2 °C and 3 °C global warming scenarios, and discuss how event severity, event drivers and physical processes are influenced by climate change. We show that future Rhine basin meteorological summer droughts like 2018 will be more severe. Decreased precipitation and increased potential evapotranspiration, caused by higher temperatures and increased incoming solar radiation, lead to higher precipitation deficits and lower plant available soil moisture. Possibly, changes in atmospheric circulation contribute to increased spring drought, amplifying the most severe summer drought events. The spatial extent of the most severe drought impacts increases substantially. The noted changes can partly be explained by changes in mean climate, but for many variables, changes in the relative event severity on top of these mean changes contribute as well.

Super absorbent polymers mitigate drought stress in corn (Zea mays L.) grown under rainfed conditions

Under climate change, the rainfall is predicted to become more intense and less frequent, causing summer drought events and rainwater loss by deep percolation, resulting in both inefficient use of rainwater and yield reduction. Super absorbent polymers (SAPs) have been studied as soil amendments in arid and semi-arid environments, but little is known about potential use for drought stress mitigation in humid subtropical climates under rainfed conditions. This study investigated the potential of three K-based SAPs (Stockosorb 660, Hydrosource and SuperAB A200) to act as water reservoirs during periods of drought. In the laboratory, SAPs mixed with sandy soil were tested for water absorption capacity (WAC) and re-swelling capacity, saturated hydraulic conductivity (Ks), water holding capacity (WHC) and storage of simulated rainfall. SAPs showed high WAC, re-swelling capacity, reduced Ks and increased soil WHC with performance dependent more on SAPs concentration than SAPs type. The three SAPs effectively reduced rainwater transport and increased soil water storage during simulated rainfall. In North Carolina, USA, an outdoor pot study with corn in loamy sand soil under rainfed conditions tested the effects of the three SAPs at three concentrations (0.15%, 0.30% and 0.45% w/w) on rainwater transport/storage, biomass production, and biomass rainwater productivity (WPbiomass). Under field conditions, 41.76% of rainwater was percolated in the absence of SAPs while in SAPs-amended soil (0.45% w/w) only 4.8%, 5.5% and 6.02% was lost by percolation, for Stockosorb 660, Hydrosource and SuperAB A200, respectively. The retained rainwater resulted in higher leaf water potential leading to enhanced plant growth and WPbiomass. Stockosorb 660 performed best in terms of rainwater saving, while Hydrosource was the most effective in terms of plant growth and WPbiomass. The results suggest that SAPs soil amendment is a potential approach to mitigate drought stress under rainfed conditions in humid subtropical climates.

Recent increasing frequency of compound summer drought and heatwaves in Southeast Brazil

An increase in the frequency of extremely hot and dry events has been experienced over the past few decades in South America, and particularly in Brazil. Regional climate change projections indicate a future aggravation of this trend. However, a comprehensive characterization of drought and heatwave compound events, as well as of the main land–atmosphere mechanisms involved, is still lacking for most of South America. This study aims to fill this gap, assessing for the first time the historical evolution of compound summer drought and heatwave events for the heavily populated region of Southeast Brazil and for the period of 1980–2018. The main goal is to undertake a detailed analysis of the surface and synoptic conditions, as well as of the land–atmosphere coupling processes that led to the occurrence of individual and compound dry and hot extremes. Our results confirm that the São Paulo, Rio de Janeiro and Minas Gerais states have recorded pronounced and statistically significant increases in the number of compound summer drought and heatwave episodes. In particular, the last decade was characterized by two austral summer seasons (2013/14 and 2014/15) with outstanding concurrent drought and heatwave conditions stemmed by severe precipitation deficits and a higher-than-average occurrence of blocking patterns. As result of these land and atmosphere conditions, a high coupling (water-limited) regime was imposed, promoting the re-amplification of hot spells that resulted in mega heatwave episodes. Our findings reveal a substantial contribution of persistent dry conditions to heatwave episodes, highlighting the vulnerability of the region to climate change.

Metabolic responses of date palm (Phoenix dactylifera L.) leaves to drought differ in summer and winter climate.

Drought negatively impacts growth and productivity of plants, particularly in arid and semi-arid regions. Although drought events can take place in summer and winter, differences in the impact of drought on physiological processes between seasons are largely unknown. The aim of this study was to elucidate metabolic strategies of date palms in response to drought in summer and winter season. To identify such differences, we exposed date palm seedlings to a drought-recovery regime, both in simulated summer and winter climate. Leaf hydration, carbon discrimination (${\Delta}$13C), and primary and secondary metabolite composition and contents were analyzed. Depending on season, drought differently affected physiological and biochemical traits of the leaves. In summer, drought induced significantly decreased leaf hydration, concentrations of ascorbate, most sugars, primary and secondary organic acids, as well as phenolic compounds, while thiol, amino acid, raffinose and individual fatty acid contents were increased compared with well-watered plants. In winter, drought had no effect on leaf hydration, ascorbate and fatty acids contents, but resulted in increased foliar thiol and amino acid levels as observed in summer. Compared with winter, foliar traits of plants exposed to drought in summer only partly recovered after re-watering. Memory effects on water relations, and primary and secondary metabolites seem to prepare foliar traits of date palms for repeated drought events in summer. Apparently, a well-orchestrated metabolic network, including the anti-oxidative system, compatible solutes accumulation and osmotic adjustment, and maintenance of cell-membrane stability strongly reduces the susceptibility of date palms to drought. These mechanisms of drought compensation may be more frequently required in summer.

Air pollutants associated with surface meteorological conditions in São Paulo’s ABC region

Air pollution is one the main environmental problems in urban areas like the Metropolitan Area of São Paulo (MASP) in Brazil, where millions of inhabitants are exposed to pollution concentrations above the standards, with potential health impacts. Exposure is unequal throughout MASP, relying on the dynamics of local emission sources interplaying with weather and climate in a regional scale. The ABC region — ABC standing for Santo André, São Bernardo do Campo and São Caetano do Sul, the cities the area originally comprised of — is MASP’s largest industrial center, sitting in its southeast border, and encloses environmental protection areas. That leads to a unique emission profile that differ from the metropolis center. This study aims to characterize the variability of atmospheric pollutants in the ABC region in 2015, investigating possible sources and associations with surface meteorological conditions. Multivariate statistical analyses were applied to data from seven air quality monitoring stations and surface meteorological variables. Results show that São Bernardo do Campo stood out, with O3 concentrations 20% higher (43±19 µg.m-3) than the other sites, while São Caetano do Sul had the highest annual mean PM10 concentrations (39±19 µg.m-3), mostly related to vehicular emissions. Relative humidity was negatively correlated with primary pollutants, while temperature and radiation correlated with O3. Unusually high O3 concentrations were observed in January of 2015, concomitant with negative anomalies of precipitation and relative humidity, likely associated with the 2014/2015 summer drought event in Southeast Brazil. Overall, results show that local emission sources significantly impact air pollution loading and its diurnal variability, particularly in the case of primary pollutants. Climate modulates the seasonal concentration variability, and regional scale weather phenomena may impact air quality conditions. To reach concentration standards everywhere, policy makers must be aware of processes occurring in different spatial scales that determine air quality.

Changes in compound drought and hot extreme events in summer over populated eastern China

Compound extreme events always cause severe impacts on human society and the natural system, especially in the populated areas. However, studies on their changes depend on definitions of the related extreme events. In this study, a newly defined compound drought and hot extreme events (CDHEEs) index based on the day-night concurrent hot extreme index and daily drought monitoring index is defined to explore possible changing features in summer-time CDHEEs from 1961 to 2018 in the densely populated areas of eastern China. Results reveal that the occurrence of CDHEEs shows a significant increase in the densely populated areas with the strengthening interaction of drought and hot extreme events, especially since the late 1990s. Further analyses indicate that urbanization and regional climate also moderate changes in CDHEEs, as we found that CDHEEs increased more prominently in the cities, especially in the drier regions. As a result, the population exposed to CDHEEs has doubled since the late 1990s in the densely populated areas of eastern China.

A synoptic view on intra-annual density fluctuations in Abies alba

Tree growth, as an archive of environmental conditions, has proven to provide valuable proxies for climate reconstructions. Density measurements were successfully used in the field of dendroclimatology in the past, with recent efforts to obtain sub-annual predictors from density profiles. Particular attention is laid on anomalies in the density profile named intra-annual density fluctuations (IADFs). Density fluctuations in earlywood with higher than ordinary density are assumed to be triggered by drought events in spring or early summer. How they are formed, linked to chemical and anatomical characteristics and their functionality is not well understood, although necessary to interpret growth-climate relationships of this trait. An explorative assessment of radial profiles from silver fir, showing an IADF in earlywood of 2005, included cell wall and lumen area, cellulose to lignin ratio, microfibril angle, tracheid length as well as size and shape of pits, beside density. The resulting synoptic view of these traits depicted deviations at the IADF position, indicating that multiple phases of wood formation are affected by a triggering event. These anatomical and chemical adaptions result in a hydraulically safer tissue with improved mechanical strength. The presented approach provides first insights on multiple involved wood traits and leads towards a more comprehensive understanding of IADF formation.

Divergent responses of ecosystem water-use efficiency to extreme seasonal droughts in Southwest China

The recurrent drought extremes have resulted in deleterious impacts on ecological security. Despite that many attempts have been made to explore ecosystem responses to different severities of droughts, a deep understanding of how ecosystem water-use efficiency (WUE) responds to extreme seasonal droughts is critical for predicting the trends under future climate change, especially in the ecologically-fragile karst ecosystem across Southwest China. This study systematically examined the spatio-temporal variations of ecosystem WUE over the karst and non-karst areas, as well as their divergent responses to different seasonal droughts. Our findings revealed the apparent increase in drought frequency, duration, and severity in Southwest China during the past four decades. Meanwhile, spring and summer drought events were the prevailing drought types. Compared with the non-karst area, multi-year mean WUE in the karst area was relatively lower, whereas the area exhibiting significant increase in WUE (p < 0.01) accounted for 39.3% and 22.3%, respectively. However, the effects of drought on ecosystem WUE varied in different seasons with more severe consequence in the karst ecosystem. During the early stage of autumn-spring drought in 2009/2010, ecosystem WUE was apparently larger than the baseline condition with the difference turning to be negative anomalies during the peak period, whereas the effect of summer drought in 2011 led to negative anomalies nearly throughout the duration. Further analysis revealed that the anomalies in evapotranspiration acted a prominent role in altering WUE at the onset of both droughts, while ecosystem WUE was mainly determined by the sensitivity of gross primary production during the later stage. All analyses are beneficial for expecting the coupling relationship between global carbon and water cycles to future climate change, particularly as droughts are projected to increase in terms of frequency and severity.