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Hydrometeorological droughts are complex hazards expressed through the relative deviation in water availability relative to long‐term average conditions. The development and propagation of drought is governed by hydrological processes at different spatial scales including precipitation, evapotranspiration, overland flow, soil moisture, groundwater storage and discharge. Detailed investigation is thus required to evaluate associated linkages among various types of hydrometeorological drought to understand the likely impacts of climate change on drought characteristics (magnitude, frequency, duration and propagation). This study explores the impact of climate change on hydrometeorological drought for 10 Irish catchments, using standardized drought indices representing different components of the hydrological system. We employ 12 Global Climate Models from the Coupled Model Inter‐comparison Project phase 6 (CMIP6), forced with SSP370 and bias corrected to catchment conditions to force a conceptual hydrological model to generate hydrological projections for each catchment. The results indicate substantial drying during summer with associated increases in summer drought magnitude and frequency. However, simulations show a wide range of change, especially for hydrological drought (discharge and baseflow). Only modest changes in the magnitude and frequency of hydrological drought events were found, with increases in winter and spring precipitation offsetting summer dryness. Similarly, the probability of meteorological drought propagating to hydrological events (Standardized Streamflow Index/Standardized Baseflow Index), shows modest increases under the climate change projections considered. Findings highlight that drought, especially during summer are a critical climate change risk for adaptation. Plain Language Summary: Climate change aggravates the impact of hydrometeorological droughts on water availability and distribution in Ireland. Rainfall, evaporation, overland flow, soil moisture, groundwater storage, and the size and strength of discharges are the main factors that cause drought extent and duration. Therefore, for the sustainable development of ecosystems and water resources in a country, it is a requirement to evaluate associated linkages among various types of hydrometeorological drought to understand the likely impacts of climate change on drought characteristics (magnitude) and propagation. This paper explores the impact of climate change on hydrometeorological drought for 10 Irish catchments. The findings indicate significant summer drying, as well as increases in the magnitude and frequency of summer droughts. The probability of meteorological drought propagating to hydrological events, shows modest increases under the climate change projections considered. Drought, particularly during the summer, is a critical climate change risk for adaptation, according to the findings. Key Points: Changes in hydrometeorological drought characteristics under changing climate conditions were estimated; and the results indicate substantial drying during summer with associated increases in summer drought magnitude and frequencyIncreases in winter and spring precipitation result in a decrease in the frequency and magnitude of drought events outside summer and are critical for maintaining baseflow during summer. This also raises the risk of extreme droughts for a dry winter/springThe probability of drought propagation from meteorological to hydrological events increases for the 2050s but decreases for the end of century, highlighting the importance of increases in winter and spring precipitation to drought risk [ABSTRACT FROM AUTHOR]

This paper focuses on the interdecadal variation in drought in Northeast China (NEC) in recent decades. Two dry and wet periods have occurred in the past 61 years (1950–2010). The mechanism underlying the interdecadal variation in drought in NEC is further analyzed; the results indicate that the direct cause is the interdecadal change of the Okhotsk High (OH). A strong (weak) OH is conducive (detrimental) to the convergence of cold air transported by northeast cold vortex and water vapor from the east of NEC, eventually leading to an increase (decrease) in precipitation. Furthermore, we seek to understand why OH displays such an interdecadal variation. The interdecadal increases and decreases in sea ice in the Barents Sea and the Kara Sea explain most of the interdecadal change of the OH. When more sea ice is present in this area, more energy is transferred eastward from the sea ice area to Northeast Asia, weakening the OH; in contrast, less sea ice strengthens the OH. Sensitivity simulations by Community Atmosphere Model, version 4 (CAM4) using sea ice forcing in the Barents Sea and the Kara Sea yield similar results to the observed data, further confirming our conclusion. The results of this paper provide a new understanding of the interdecadal variation in drought in NEC, and it may help to improve our ability to predict drought and provide early warning in the future. Key Points: The drought over Northeast China has significant interdecadal variability during the past decadesThe Okhotsk High acts as the direct factor causing interdecadal changes in NEC drought by affecting atmospheric circulationThe interdecadal increases and decreases in sea ice in the Barents Sea and the Kara Sea can modulate the Okhotsk High changes [ABSTRACT FROM AUTHOR]

The tropical Asian regions are particularly prone to catastrophic droughts due to high temperature and evaporation and their sensitivity to variable oceanic‐atmospheric circulation. The extent to which future global warming may intensify droughts in this populous region is a matter of utmost concern. Palaeoclimatic data can help inform scientists learn more about the temporal patterns and drivers of monsoon change over geological timescales. In this paper, we present a new 8,200 year biomarker record preserved in the annually laminated sediments of Maar Lake Twintaung, Myanmar. The δ13C27‐35 sedimentary record is mainly derived from leaf wax lipids in the lake catchment, which is presently dominated by C3 vegetation in the modern lake catchment. The δ13C27‐35 is mainly regulated by plant physiological and biochemical responses to drought stress and, as such, can be used as a proxy of past changes in moisture. The δ13C27–35 record reveals distinct decadal‐to‐centennial‐scale droughts superimposed on a trend of gradually decreasing summer monsoon intensity from the mid‐Holocene to late Holocene. Within the limits of the dating uncertainties, these decadal‐to‐centennial‐scale droughts are found to be well correlated with the southward shift of the Intertropical Convergence Zone. It highlights thepimportance of the Intertropical Convergence Zone shift (tropical monsoon trough) in regulating monsoon rainfall in the region. In addition, most of the droughts are likely to be linked with active volcanism and solar minima and suggest a coupled process between external drivers and internal climate dynamics. Key Points: Leaf wax n‐alkanes record in varved sediment of Maar lake Twintaung, MyanmarThe result indicates distinct decadal‐to‐centennial‐scale droughts over the past 8.2 kaThe drought cycles link with the ITCZ shift, solar and volcanic eruption [ABSTRACT FROM AUTHOR]

The Tarim River, the largest inland river in China, sits in the Tarim River Basin (TRB), which is an arid area with the ecosystem primarily sustained by water from melting snow and glaciers in the headstream area. To evaluate the pressures of natural disasters in this climate‐change‐sensitive basin, this study projected flash droughts in the headstream area of the TRB. We used the variable infiltration capacity (VIC) model to describe the hydrological processes of the study area, Markov chain Monte Carlo to quantify the parameter uncertainty of the VIC model. Ten downscaled general circulation models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) were used to drive the VIC model, and the standardized evaporative stress ratio was applied to identify flash droughts. The results demonstrated that the VIC model after Bayesian parameters uncertainty analysis can efficiently describe the hydrological processes of the study area. In the future (2021–2100), compared with the plain region, the alpine region has higher flash drought frequency and intensity. Compared with the historical period (1961–2014), the frequency, duration, and intensity of flash droughts tend to increase throughout the study area, especially for the alpine area. Moreover, based on variance decomposition, CMIP6 model is the most important uncertainty source for flash drought projection, followed by the shared socioeconomic pathway of climate change scenario and VIC model parameters. Plain Language Summary: The Tarim River is the largest inland river in China, which is an arid area with the ecosystem primarily sustained by water from melting snow and glaciers in the headstream area. Compared with long‐duration drought, flash drought is more difficult to forecast and identify. We used the variable infiltration capacity model to build a hydrological model of the headstream area of the Tarim River Basin, and the model output (ET data) was used to identify flash droughts. Based on 10 representative Coupled Model Intercomparison Project Phase 6 (CMIP6) models, this study projected the future flash droughts in the study area under climate change. The findings indicated that the alpine region is likely to experience a greater frequency and intensity of flash droughts in the future (2021–2100) compared to the plain region. Compared with the historical period (1961–2014), the frequency, duration, and intensity of flash droughts tend to increase throughout the study area, especially for the alpine area. In addition, the result of uncertainty decomposition revealed that the CMIP6 model is the most important uncertainty source for flash drought projection. Key Points: The hydrological processes of study area are well described by the variable infiltration capacity model and Bayesian uncertainty analysisCompared with the plain region, the alpine region has higher flash drought frequency and intensity in the futureThe Coupled Model Intercomparison Project Phase 6 model is the most important uncertainty source for flash drought projection [ABSTRACT FROM AUTHOR]

Drought is a recurring challenge, especially in the arid and semi‐arid regions. Therefore, understanding its spatial and temporal variation is important for pre‐disaster planning and sustainable water resource management during drought. However, the sparse and short length of the instrumental records in midlatitude East Asia hampered the full understanding of drought variation, especially at multidecadal and longer timescales. Here, we compiled a tree‐ring network of 364 chronologies in midlatitude East Asia and reconstructed the variation of the averaged May–August Standardized Precipitation Evapotranspiration Index at a spatial resolution of 0.5° × 0.5°, for the period 1501–2000 CE. The reconstruction captured the spatial and temporal variation of actual drought well and has a strong association with a drought atlas that was derived from historical documents. The study area has experienced an expansion of dry areas during the past decades; however, it is still within the natural variability of the past half millennium. According to drought variation during the instrumental period, the study area was divided into four subregions; however, running correlations showed that drought variation in some subregions was similar in past centuries, except in recent decades. This study provides new data for understanding the spatiotemporal variability of drought in midlatitude East Asia, and the vanished correlation between drought variation in some regions during the past decades indicates the complex forcing mechanism of regional drought variation. Plain Language Summary: We present a 500‐year drought reconstruction for midlatitude East Asia with a spatial resolution of 0.5° × 0.5°, by using a tree‐ring network of 364 chronologies. Based on this reconstruction, we found that the recent dry area expansion in midlatitude East Asia is still within the range of natural variability of the past 500 years. Our results also showed that, except for the past several decades, the drought variation in some subregions was significantly correlated. Key Points: We reconstructed drought variation of midlatitude East Asia over the past half millennium at a spatial resolution of 0.5°The recent drying and dry area expansion are within the range of natural variability of the reconstructionIn some regions, drought variations are currently uncorrelated, but have been similar in past centuries [ABSTRACT FROM AUTHOR]

Based on the standardized precipitation evaporation index and reanalysis data, this article reveals that the relationship between the North Pacific Oscillation (NPO) and the drought over southern China in summer shifts from an insignificant correlation to a significant negative correlation after 2000. During the significant period (2001–2018), the positive NPO phase corresponds to an anomalous anticyclonic circulation dominating the mid‐low latitude North Pacific, which suppresses the moisture transport into southern China. Meanwhile, the easterly anomalies south of the anomalous anticyclonic circulation prevailing in the equatorial Pacific are coupled with a cooling of sea surface temperature in the central tropical Pacific, which enhances the Walker circulation and further induces an anomalous meridional circulation with the descending anomalies over southern China. Such moisture and dynamical conditions favor the occurrence of drought in southern China. During the former period (1983–2000), however, the NPO‐related atmospheric circulations over the North Pacific shift northward. In this case, the NPO shows insignificant relations with the moisture and dynamical conditions over southern China, thereby diluting the linkage of the NPO to the drought in southern China. The interdecadal strengthening in the relationship between the NPO and above atmospheric conditions is attributed to the westward (southward) extension of the NPO northern (southern) center. The larger interannual variability of spring sea ice in the Bering Sea and the southward movement of summer storm track over the North Pacific are hypothesized to account for the expansion of the NPO structure. Plain Language Summary: Southern China is an economically developed region with a large population. In recent decades, droughts frequently occurred in this region and severely affected agriculture production, forest‐fire prevention, and human lives. Therefore, it is very necessary to understand the physical mechanisms underlying the variability of droughts in southern China. In this study, we addressed the relationship of summer drought in southern China with the North Pacific Oscillation (NPO), which is characterized by the north–south seesaw pattern in sea level pressure over the North Pacific. We revealed that their relationship exhibits an interdecadal change around 2000, from an insignificant correlation to a significant negative correlation. Such an interdecadal strengthening is due to the expansion of the NPO structure, which may result from the changes of the spring sea ice in the Bering Sea and the summer storm track over the North Pacific. These findings are expected to deepen our understanding for the variation of drought in southern China and are also important for disaster prevention and mitigation. Key Points: Relationship between the summer drought in southern China and the North Pacific Oscillation (NPO) has strengthened after 2000Such a strengthening is attributed to the westward (southward) extension of the NPO northern (southern) centerThe expansion of the NPO structure is related to changes of spring sea ice in the Bering Sea and summer storm track over the North Pacific [ABSTRACT FROM AUTHOR]

Isoprene is a hydrocarbon emitted in large quantities by terrestrial vegetation. It is a precursor to several air quality and climate pollutants including ozone. Emission rates vary with plant species and environmental conditions. This variability can be modeled using the Model of Emissions of Gases and Aerosols from Nature (MEGAN). MEGAN parameterizes isoprene emission rates as a vegetation‐specific standard rate which is modulated by scaling factors that depend on meteorological and environmental driving variables. Recent experiments have identified large uncertainties in the MEGAN temperature response parameterization, while the emission rates under standard conditions are poorly constrained in some regions due to a lack of representative measurements and uncertainties in landcover. In this study, we use Bayesian model‐data fusion to optimize the MEGAN temperature response and standard emission rates using satellite‐ and ground‐based observational constraints. Optimization of the standard emission rate with satellite constraints reduced model biases but was highly sensitive to model input errors and drought stress and was found to be inconsistent with ground‐based constraints at an Amazonian field site, reflecting large uncertainties in the satellite‐based emissions. Optimization of the temperature response with ground‐based constraints increased the temperature sensitivity of the model by a factor of five at an Amazonian field site but had no impact at a UK field site, demonstrating significant ecosystem‐dependent variability of the isoprene emission temperature sensitivity. Ground‐based measurements of isoprene across a wide range of ecosystems will be key for obtaining an accurate representation of isoprene emission temperature sensitivity in global biogeochemical models. Plain Language Summary: Plants emit a reactive gas called isoprene which has a large impact on air quality and climate throughout the world. This impact can be studied and quantified using computer models, but there are large uncertainties in modeled isoprene emission rates. There are few measurements to constrain the emission rates of many vegetation species. There are also uncertainties in the relationship between isoprene emissions and temperature, which makes it difficult to predict how air quality may change in a warming climate. Our goal in this study was to reduce uncertainties in a widely‐used isoprene emission model by constraining the model with observations. We used satellite observations to constrain the emission rates in a diverse range of ecosystems, but these results were sensitive to many different sources of error including drought stress. Using ground‐based observations, we found that isoprene emissions were five times more sensitive to temperature at a measurement site in the Amazon rainforest than they were at a UK measurement site. Updating the temperature sensitivity of isoprene emission models has the potential to improve models of air quality during extreme heat events and in a warming climate. Key Points: Satellite and ground based observations were used to optimize an isoprene emission model in a Bayesian model‐data fusion frameworkOptimization with satellite‐based emissions was highly uncertain due to observation biases and a high sensitivity to model input errorsGround‐based observations showed that Amazonian isoprene emissions were 5× more sensitive to temperature than UK isoprene emissions [ABSTRACT FROM AUTHOR]

Understanding the runoff response to climate change in the Tibetan Plateau (TP) is critical to water resources management since TP is the "Water Tower of Asia" and is sensitive to climate change. However, previous studies have paid more attention to the changes in annual and monthly runoff. Therefore, the changes in the characteristics of floods and droughts are still unknown, particularly in the Yarlung Zangbo River basin (YRB) in the southern TP. Therefore, this study simulates the runoff in four subbasins in the YRB using the GR4J_SNOW model to depict the characteristics (including frequency, severity, peak, and duration) of floods on a daily scale and those of droughts on a monthly scale. It is done in historical (1979–2014) and future (2049–2084) periods under different climate scenarios (RCP4.5 and RCP8.5), and then the changes in floods and droughts between the two periods are comprehensively compared. The results indicate that the YRB has experienced marginal streamflow change during the historical period. Under future varying climate change scenarios, however, the YRB might become wetter with increasing frequency, volume, and peak of floods, along with decreasing frequency, severity, and duration of droughts. In the upstream YRB, covered by more snow with a drier climate, the flood and drought characteristics appear to be more sensitive to climate change than in the downstream YRB. This study is helpful for a better understanding of the hydrological process in the TP under climate change and for forming sustainable water resource strategies for the surrounding regions of the TP. Plain Language Summary: The Tibetan Plateau (TP) is the Asian water tower, providing water resources to hundreds of millions of people living downstream. The Yarlung Zangbo River basin (YRB), located in the southern part of the TP, has attracted increasing attention because most of the Tibetan people live there and it is the upper stream of the Brahmaputra River. It is not clear how floods and droughts, the two most serious natural disasters, could change under climate change in the YRB. This study simulates the changes in flood and drought characteristics in the YRB in the historical (1979–2014) and future (2049–2084) periods under different climate scenarios (RCP4.5 and RCP8.5). Our results show that the YRB could become wetter with increasing floods and mitigated droughts in the future, despite minor changes in flood and drought characteristics in the historical period. The change could be stronger upstream than downstream. This study can be helpful for long‐term water resource management and planning on the TP. Key Points: The characteristics of floods and droughts in the Yarlung Zangbo River basin (YRB) are comprehensively analyzed in the past and futureThe YRB is expected to become wetter in the future, reflected by increased floods and alleviated droughtsThe flood and drought characteristics in its drier upstream are more sensitive to climate change than those in the humid downstream [ABSTRACT FROM AUTHOR]

Drought can develop rapidly over a short period, the so‐called "flash drought". The widely used standardized precipitation evapotranspiration index (SPEI), with traditional time scales longer than 1 month, cannot easily capture flash drought signals. Here, the SPEI with a 5‐day (pentad) time scale was proposed to investigate flash droughts in China from 1980 to 2017. New criteria for flash droughts based on the high‐resolution index were also developed. Flash droughts were stronger and longer in western and northern China. The intensity and duration of mild, moderate, and severe flash droughts increased during the study period, while severe flash droughts showed the largest increase. The high‐resolution SPEI permits us to examine the process of a drought event, that is, how a mild flash drought develops into a severe flash drought. The contribution of precipitation deficit to the duration of mild flash droughts was 50%. As the intensity of flash droughts increased, the contribution of high temperature and increased net radiation to flash drought duration increased from 50% (mild flash droughts) to 74% (severe flash droughts). When flash droughts occurred, Northwestern China and Southwestern China were dominated by unusually weak water vapor transport, while Northeastern China, Northern China, Southern China ,and Middle‐Lower Yangtze were mainly dominated by abnormal downward motion. The downward motion was generally accompanied by clear sky conditions with fewer clouds, higher air temperature, and higher surface solar radiation, which accelerated and amplified flash droughts triggered by initial precipitation deficits. Plain Language Summary: In recent years, a rapidly developing drought has attracted widespread attention, and it is known as the "flash drought". However, existing drought indices do not capture the signals of flash drought event life cycles. In this study, the existing drought index (SPEI) was upgraded to a higher temporal resolution to explore the spatial distribution and temporal changes in flash droughts in China. The results showed that the flash drought was intense and long‐lasting in western and northern China. In the past 38 yr, flash droughts have become more intense and more difficult to recover from, especially severe flash droughts. As the intensity of flash droughts increased, the impact of precipitation decreased, while temperature and net radiation gradually played leading roles. Flash droughts in Northwestern China and Southwestern China were mainly related to weak water vapor transport, while they were affected by strong downward motion in Northeastern China, Northern China, Southern China, and Middle‐Lower Yangtze. The downward motion was generally accompanied by clear sky conditions with fewer clouds, higher air temperature, and higher surface solar radiation, which accelerated and amplified flash droughts triggered by initial precipitation deficits. Key Points: A high temporal resolution drought index and new criteria were developed to identify flash droughts over ChinaThe intensity and duration of the flash drought increased significantly from 1980 to 2017 over China, especially the severe flash droughtThe precipitation deficit initiated a mild flash drought, and interactions with air temperature and net radiation amplified the drought intensity [ABSTRACT FROM AUTHOR]

A severe persistent drought occurred in South China during autumn–winter 2020/2021, causing enormous economic losses and severe water shortage in South China. Focusing on this event, this study analyzed the climate anomalies associated with this event, and investigated the underlying mechanisms as well as their relationship with the leading modes of the autumn and winter precipitation in eastern China. The results suggest that the drought in South China during autumn 2020 is mainly attributed to the water vapor divergence anomalies induced by an anomalous anticyclone over eastern China and an anomalous cyclone over the South China Sea, which are caused by an anomalous Eurasian atmospheric wave‐train triggered by the negative sea ice anomalies in the Barents Sea‐Kara Sea region and a La Niña‐like condition in the tropical Pacific sea surface temperatures, respectively. The continued drought in South China during winter 2020/2021 is mainly attributed to the water vapor divergence anomalies induced by an anomalous cyclone over the western North Pacific, which is caused by the continued La Niña‐like condition. The mechanisms of the first two leading modes of autumn precipitation in eastern China both contributed to the drought in South China during autumn 2020, whereas the mechanisms of the first leading mode of winter precipitation in eastern China played an important role in causing the drought in South China during winter 2020/2021. Numerical experiments demonstrate important impact of the ENSO and Arctic sea ice anomalies on the East Asian climate anomalies during autumn–winter 2020/2021. Key Points: The drought in South China during autumn 2020 is attributed to combined effect of El Niño‐Southern Oscillation (ENSO) and Arctic sea ice anomaliesThe drought in South China during winter 2020/2021 is mainly attributed to the La Niña‐like conditionNumerical experiments demonstrate important impact of ENSO and Arctic sea ice on East Asian climate anomalies during autumn–winter 2020/2021 [ABSTRACT FROM AUTHOR]

Strengthened land‐atmosphere coupling in the northeastern United States (NEUS), accompanied by a positive soil moisture‐rainfall feedback, may lead to more drought. Coupling between the land and atmosphere emerges when low soil moisture values limit surface latent heat flux, or evapotranspiration, so that a majority of absorbed solar radiation is emitted from the surface as sensible heat. In this study, the Weather Research and Forecasting model was run with four prescribed soil moisture levels across 7 years to elucidate the strength of land‐atmosphere coupling under potential, future soil moisture states in the NEUS. Under drier conditions, land‐atmosphere coupling strengthens, and a positive soil moisture‐precipitation feedback develops in all years despite differences in the amount of moisture advected into the study domain. As snowpack decreases and evaporative demand increases, soil conditions may become drier in future summers over the NEUS, resulting in the more frequent development of drought. Key Points: The strength of land‐atmosphere coupling in the northeastern United States (NEUS) may increase under drier soil moisture conditionsA positive soil moisture‐rainfall feedback may develop under land‐atmosphere coupling, increasing the risk for droughtWith decreased snowpack and increased evapotranspiration in the NEUS expected, drought may become more common under climate change [ABSTRACT FROM AUTHOR]

This study examines August droughts in northern China (north of 35°N) at interannual and interdecadal timescales. During the period from 1968 to 1992, the precipitation in the western and eastern regions of northern China showed an out‐of‐phase pattern. During the period from 1994 to 2018, the climate in the Tarim Basin became wetter, but there was no significant connection with drought in North China. Further studies demonstrated that the Atlantic sea surface temperature (SST) and regional soil moisture anomalies were related to drought changes in northern China. Before the early 1990s, the east‐west standardized precipitation index dipole mode in northern China was physically linked with the wet soil in central Asia in the preceding June; moreover, the wet land surface could persist until August and cause anticyclonic and cyclonic systems over Mongolia and the subtropical northwestern Pacific, thus contributing to precipitation surplus in northwest China and a deficit in North China. After the early 1990s, however, wet conditions over the Tarim Basin were related to wet soil in the East European Plain in the preceding July: the wet soil could persist into August and generate anomalous circulation over the Tarim Basin, leading to abnormal humidity. Additionally, the SST conditions in the North (South) Atlantic are key sources of climate predictability for northern China drought at the interseasonal scale after (before) the early 1990s. Key Points: The spatial distribution of the standardized precipitation index in northern China shows a marked zonal dipole patternThe drought in northern China underwent a remarkable interdecadal change in approximately 1992The Atlantic sea surface temperature and regional soil moisture anomalies were responsible for the drought changes in northern China [ABSTRACT FROM AUTHOR]

Hot droughts (i.e., concurrent droughts and hot anomalies) have drawn much attention recently, but current knowledge about early propagation and concurrence remains limited. The 2019 autumn severe hot drought (2019AHD) over the middle‐lower reaches of the Yangtze River (MLYR) is employed for process‐based analysis, while water vapor transport, heating budgets, and atmospheric dynamics are comprehensively investigated. The main achievements are as follows: (a) The 2019AHD over MLYR is not isolated and is related to early propagation from adjacent North China (NC) two months earlier. (b) Direct physical causes are explored. Drought evolution is related to anomalous water vapor transport dominated by low‐level cyclonic anomalies over the East China Sea. Concurrently, local low‐level hot anomalies over MLYR are related to amplified horizontal advection heating, while near‐surface warming is favored by decreased low cloud cover and enhanced downward shortwave radiation. (c) Regional‐scale concurrence of droughts and hot extremes might be understood by the combined effects of amplified subsidence (suppressing precipitation formation) and intensified low‐level divergence (reducing low cloud cover). (d) From a large‐scale dynamic perspective, early propagation and development of the 2019AHD are related to the southward shift and enhancement of the subsidence branch of meridional circulations. Meanwhile, intensity variation in 200 hPa cyclonic anomalies over MLYR corresponds well with local drought development, and the enhanced intensity at the time of drought occurrence is probably related to Silk Road Pattern–like wave train propagation over Eurasia. These insights can help us understand the mechanisms and detect early signals of hot droughts. Plain Language Summary: Hot droughts (i.e., concurrent droughts and hot anomalies) are compound climate extreme events with amplified disastrous impacts compared to individual drought or heatwave. They have drawn much attention recently, but current knowledge about concurrences remains limited. To understand it, the present study employs the 2019 autumn severe hot drought over the middle‐lower reaches of the Yangtze River for process‐based analysis. Water vapor transport, heating budgets, and atmospheric dynamics are comprehensively investigated. Further, the concurrence of drought and hot extremes during the autumns over the middle‐low reach of the Yangtze River are examined and investigated from the dynamical and statistical perspectives. It is found that intensified vertical subsidence and enhanced low‐level divergence tend to be responsible for the concurrence. Physically, the amplified vertical subsidence suppresses precipitation formation, and intensified low‐level divergence might lead to near‐surface warming through less‐than‐normal low cloud fraction and greater‐than‐normal net shortwave radiation. Achievements herein can also help understand the mechanisms behind compound climate extreme events. Key Points: The 2019 hot drought over the middle‐lower reaches of the Yangtze River is related to early propagation in North China two months earlierIntensified vertical subsidence and enhanced low‐level divergence tend to be responsible for concurrence of droughts and hot extremesLocal 200‐hPa cyclonic anomalies at the time of drought occurrence are related to Silk Road Pattern‐like wave trains over Eurasia [ABSTRACT FROM AUTHOR]

Among the various environmental factors that affect isoprene emissions, drought has only been given limited attention. Four different drought response (DR) schemes were implemented in the Model of Emissions of Gases and Aerosols from Nature (MEGAN, version 2.1), and the Community Multiscale Air Quality (CMAQ) model was applied to investigate the drought impacts on air quality during both drought and normal years in China. Generally, all DR schemes decrease isoprene emissions except for mild drought conditions. The significant decrease and even termination of isoprene emissions are predicted in South China under severe drought conditions. During the drought period, the DR scheme considering both mild and severe drought (SMD) improves the model performance especially in severe drought‐hit regions when compared with the Ozone Monitoring Instrument (OMI) averaged formaldehyde vertical column density (HCHO VCD). The results show that most of the DR schemes decrease simulated ozone (O3) and secondary organic aerosols (SOA) levels. For both O3 and SOA, noticeable changes are predicted in the Sichuan Basin (5 ppb and 4 µg m−3 for O3 and SOA, respectively). This investigation is the first modeling study to investigate the impacts of isoprene drought response on air quality in China. Plain Language Summary: In the last 50 years, drought has become more severe, prolonged, and frequent in China, which is expected to affect air quality by changing Biogenic volatile organic compound (BVOC) emissions. The BVOCs are regarded as the significant precursor of secondary pollutants such as O3 and secondary organic aerosols (SOA), and thus play an important role in the air quality. This is the first modeling study to evaluate drought impacts on atmospheric chemistry and air quality in China using different isoprene drought response schemes. The drought scheme considering both mild and severe drought has improved the model performance at severe drought‐hit regions when compared with the satellite formaldehyde vertical column density. Most of the drought schemes predict lower isoprene emissions and thus leading to lower O3 and SOA levels. During the drought year, O3 and SOA decreases of up to −8% and −30% are simulated for southern China under severe drought conditions. Key Points: First implementation of the Model of Emissions of Gases and Aerosols from Nature (MEGAN) model drought response for investigating China air qualityThe drought scheme considering both mild and severe drought improves the model performance in severe drought‐hit regionsProminent changes of O3 (−8%) and secondary organic aerosols (SOA) (−30%) are predicted in the drought year in southern China [ABSTRACT FROM AUTHOR]

Risks associated to extreme hydrological conditions, such as floods and droughts, are expected to increase in future climate because of projected changes in precipitation and temperature. Assessing how wet and dry persisting conditions (or spells) will evolve in future climate is a crucial step in the study of extreme hydrological events. Projected changes over the north‐eastern part of North America in the annual number of wet days, wet, and dry spells characteristics (number, duration), and wet spells intensities are analyzed. Two regional climate model ensembles are considered: the multimember ensemble from the Canadian RCM v5 (CRCM5‐LE), and 16 simulations from the NA‐CORDEX multimodel ensemble both using the Representative Concentration Pathway 8.5 (RCP8.5) scenario. Comparison of observed and simulated wet and dry spells characteristics is first performed in present climate (1971–2000). Regional Climate Model (RCM)s tend to generate too many wet days and wet or dry spells resulting in shorter dry spells and slightly longer wet spells. Modeling uncertainties are accounted for a bigger contribution to the bias than internal variability since the multimodel ensemble dispersion is the largest. Throughout the 21st century, both ensembles project significant trends in winter at higher latitudes resulting in increasing wet day frequency, increasing number of wet spell, longer wet spells and shorter dry spells. For other seasons, internal variability of the CRCM5‐LE and differences among the various NA‐CORDEX simulations seems to overwhelm the climate change signal. Wet spell intensities increases are projected for all seasons over almost the entire domain. Globally, wetter climate with potential significant hydrological impacts are expected in many regions. Key Points: Wet and dry spells future evolution are assessed using two RCMs ensemble: CRCM5‐LE and NA‐CORDEXModeling uncertainties are accounted for a bigger contribution to the bias than internal variabilityBoth internal variability and modeling uncertainties seems to overwhelm the climate change signal [ABSTRACT FROM AUTHOR]

Drought is among the costliest natural disasters that affect the economy, food and water security, and socioeconomic well‐being of about 1.4 billion people in India. Despite the profound implications of droughts, the propagation of meteorological to hydrological droughts in India is not examined. Here, we use observations and simulations from a well‐calibrated and evaluated Variable Infiltration Capacity (VIC) model to estimate drought propagation in India. Standardized Precipitation Index (SPI), Standardized Soil Moisture Index (SSMI), and Standardized Streamflow Index (SSI) were estimated for 223 catchments in India to represent meteorological, agricultural, and hydrological droughts, respectively. We estimated drought propagation time for these catchments located in 18 major Indian subcontinental river basins. Internal propagation of hydrological drought was estimated using optimal hydrological Instantaneous Development Speed (IDS) and Instantaneous Recovery Speed (IRS) from onset to the termination. Indus, Sabarmati, and Godavari river basins have higher propagation time of meteorological to hydrological droughts. The high (low) development rate of hydrological drought is followed by the high (low) recovery rate for most of the locations. We find significant influence of Seasonality Index (SI) and Base Flow Index (BFI) on propagation time of meteorological to hydrological droughts in the Indian subcontinental river basins. Overall, understanding of drought propagation, development/recovery speed, and their deriving factors can assist in the management and planning of water resources in India. Key Points: Drought propagation was estimated using SPI and the VIC model‐simulated SSI at 223 locations in the Indian subcontinentIndus, Sabarmati, and Godavari river basins have longer drought propagation timesA majority of catchment characteristics do not show a strong relationship with drought parameters [ABSTRACT FROM AUTHOR]

This study compares various drought indices, the Palmer Drought Severity Index (PDSI), the Standardized Precipitation Evapotranspiration Index (SPEI), and the Standardized Precipitation Index (SPI), and the uncertainties in each to the input data from which they are derived. The abilities of the PDSI, SPEI, and SPI to capture drought periods are assessed through a comparison with soil moisture estimates from two generations of the Global Land Data Assimilation System (GLDAS). This comparison shows that the skill with which a drought index represents variations in soil moisture does not necessarily improve when evapotranspiration is included (i.e., PDSI/SPEI rather than SPI), though this depends on location and the time scale of the drought. The differences in the abilities of the drought indices to represent soil moisture are also compared to the magnitude of the uncertainty in each index arising from the choice of input data. In many cases, the uncertainties in the variations of the PDSI, SPEI, and SPI to the choice of input data are larger in magnitude than the differences between the indices themselves, particularly when considering the dry tails of the distribution. The results show that no one drought index outperforms the others during drought conditions. Key Points: Skill of PDSI, SPEI, and SPI is based on the region and length of drought examined and the depth at which soil moisture deficits are computedChoice of input data changes the skill of the drought index such that the differences between them for drought identification become smallThe uncertainty in the choice of LSMs also played a significant role in uncertainties in the skill of the drought indices [ABSTRACT FROM AUTHOR]

Long‐lasting and severe droughts seriously threaten agriculture, ecosystems, and society. Summer 2018 in central Europe was characterized by unusually persistent heat and drought, causing substantial economic losses, and became a part of a several years long dry period observed across this region. This study assesses the magnitude of the recent drought within a long‐term context and links the increased drought severity to changes in atmospheric circulation. Temporal variability of drought conditions since the late 19th century was analyzed at seven long‐term stations distributed across the Czech Republic using the Palmer Drought Severity Index and the Standardized Precipitation Evaporation Index. The Palmer Z Index and a variation of the Standardized Precipitation Evaporation Index were used to study rapidly emerging short‐term droughts and to link these episodes to atmospheric circulation. Changes in circulation were analyzed through circulation types calculated from flow strength, direction and vorticity in mean sea level pressure data from the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis for 1948–2018. Increasing drought severity across the Czech Republic with record‐low values of the drought indices during 2015–2018 was found. The trend was distinctive in both vegetation (April–September) and cold (October–March) periods. The tendency toward more severe droughts in recent decades was linked to changes in frequency of dry and wet circulation types, highlighting the important role of atmospheric circulation in regional climate. It remains an open question whether the significantly increasing frequency of dry circulation types in the vegetation period is related to climate change, or rather represents multidecadal climate variability. Key Points: The 2015–2018 drought was record‐breaking in the context of observational record in central EuropeStatistically significant trends toward drier conditions were found for long‐term drought indicesIncreased drought severity in the recent decades was linked to higher frequency of dry circulation types at the expense of wet types [ABSTRACT FROM AUTHOR]

The standardized precipitation evapotranspiration index (SPEI) is one of the well‐established drought metrics worldwide. It is simply computed using precipitation and atmospheric evaporative demand (AED) data. Although AED is considered a key driver of drought variability worldwide, it could have less impact on drought in specific regions and for particular times as a function of the magnitude of precipitation. Specifically, the influence of the AED might overestimate drought severity during both normal and humid periods, resulting in "false alarms" about drought impacts on physical and human environments. Here, we provided a global characterization of the sensitivity of the SPEI to changes of the AED. Results demonstrate that the contribution of AED to drought severity is largely impacted by the spatial and temporal variability of precipitation. Specifically, the impact of AED on drought severity was more pronounced during periods of low precipitation, compared to wet periods. Interestingly, drought severity in humid regions (as revealed by SPEI) also showed low sensitivity to AED under drier conditions. These results highlight the skill of SPEI in identifying the role of AED in drought evolution, especially in arid and semiarid regions whose climate is characterized typically by low precipitation. This advantage was also evident for humid environments, where SPEI did not overestimate drought severity due to the increased AED. These findings highlight the broader applicability of SPEI to accurately characterize drought severity worldwide. Key Points: Atmospheric Evaporative Deamdn mostly affect SPEI in dry periodsThere are spatial differences in the influence of the AED on drought assessment by means of the SPEIAED is not overestimating drought severity since it shows a comprehensive influence on dryness [ABSTRACT FROM AUTHOR]

The frequency of extreme drought events in northern China has increased since the mid‐1990s, and such a decadal anomalous trend may have a vital influence on national economics and food supply. Based on the analysis of observational datasets and experiments conducted by a linear baroclinic model, this study claims that the intraseasonal circulation anomalies related to extreme drought are linked to the eastward shift in the negative phase of the summer North Atlantic Oscillation (SNAO). When the SNAO is in a negative phase, the cyclonic eddy‐driven jet can enhance the eastward extension of the North Atlantic jet stream (NAJS), contributing to a closer connection between the NAJS and the Asian‐African jet stream and enhancing the waveguide. The cyclonic eddy in the exit of the jet stream excites anomalous Rossby wave energy that originates from the NAJS exit and spreads to the Caspian Sea, Mongolia, and northern China; this associated wave train, which is similar to the Silk Road Pattern (SRP), enhances the anticyclonic anomaly over northern China, establishing a pronounced link with extreme drought in northern China. Thus, the negative phase of the SNAO may be a contributing driver of extreme drought in northern China. Key Points: The extreme drought events in northern China are linked to the eastward shift of the summer North Atlantic Oscillation (SNAO)Cyclone eddy‐driven jet can enhance east extension of North Atlantic jet stream (NAJS) when the SNAO is in the negative phaseThe cyclonic eddy excites anomalous Rossby wave energy originating from the NAJS exit, which enhances the anticyclonic anomaly over northern China [ABSTRACT FROM AUTHOR]

The reliable projection of future changes in hydrological drought characteristics plays a crucial role in providing meaningful insights into agricultural development and water resources planning under climate change. In this study, we develop probabilistic projections of hydrological drought characteristics through a convection‐permitting climate simulation and a multimodel hydrological prediction for two major river basins in South Texas of the United States. The probabilistic hydrological drought projection depicts the future evolution of spatiotemporal characteristics of droughts under best‐ and worst‐case scenarios. Our findings reveal that there is a considerable variation in hydrological drought regimes near the urban area in South Texas. And the prolonged severe drought events are expected to be punctuated by the increasing extreme precipitation in a changing climate. This could lead to an increasing number of the dry‐wet abrupt alternation events. Moreover, hydrological droughts are projected to occur more frequently for the fall season in the Guadalupe River Basin and for the winter season in the Blanco River Basin, advancing our understanding of future changes in seasonal characteristics of hydrological droughts at a river basin scale. Key Points: Probabilistic projections of hydrological drought characteristics are developed at a river basin scaleHydrological droughts are expected to occur more frequently by the end of the 21st centuryThe number of dry‐wet abrupt alternation event is projected to increase in a changing climate [ABSTRACT FROM AUTHOR]

Compound drought and heatwave (CDHW) events have garnered much attention in recent studies. However, thus far, the identification of such events is oversimplified, and their association with natural climate variability is not fully explored. Here, we derive anomalies in the weekly self‐calibrated Palmer Drought Severity Index (sc_PDSI) and daily maximum temperatures to identify CDHW events from 1982 to 2016 over 26 climate regions across the globe. Using a Poisson Generalized Linear Model (GLM), we analyze yearly occurrences of seasonal CDHW events and their association with the warm and cold phases of El Nino Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and North Atlantic Oscillation (NAO). ENSO exhibits robust association with CDHW events over the Southern Hemisphere during the austral summer and fall, while PDO influences their occurrences over the Western North America in the Northern Hemisphere during the boreal summer, which is supported by the composites of anomalies in the atmospheric circulations and surface energy budget. However, NAO association with CDHW events is relatively weak. The CDHW occurrence over other regions is driven by a combination of these large‐scale natural forcing. Our analyses also highlight that the cooccurrence of weekly to submonthly scale anomalies in the observed temperature and precipitation may not be always aligned between the observations and the reanalysis. Therefore, caution must be exercised while explaining such observed anomalies on the basis of reanalysis‐based circulations and surface energy budget. Overall, our analyses provide a new insight towards concurrent extremes and should help foster research efforts in this area. Plain Language Summary: The cooccurrence of drought and heatwave events has increased considerably during the last few decades over many regions of the globe. Therefore, it is imperative to develop an understanding of the factors that influence their regional occurrences. In this study, using a Poisson Generalized Linear Model framework, we identify major regional drivers that are associated with the occurrences of compound drought and heatwave events. Our results show that seasonal occurrences of these compound extremes over many regions in the Southern Hemisphere is associated with the positive phase of ENSO while negative phase of PDO influences their occurrences in the Western North America. Such occurrences in other regions exhibit influence of more than one mode of natural variability. The associations of ENSO and PDO with compound drought and heatwave events can be explained in terms of atmospheric circulations and surface energy budget anomalies. Key Points: Associations between seasonal occurrences of compound drought and heatwave (CDHW) events and the major natural modes of climate variability are investigatedENSO is the most influential natural mode of variability associated with the seasonal occurrences of CDHW events over tropical regions in the Southern HemisphereAnomalies in seasonal composites of pertinent variables explain most of these associations except for few regions [ABSTRACT FROM AUTHOR]

Exploring the dynamic evolution of the abrupt alternation between wet and dry spells in adjacent months plays a crucial role in water resources planning and agricultural development in a changing climate. The dry‐wet abrupt alternation (DWAA) has been studied based on hydrometeorological observations over the past several years. However, little effort has been made to explore DWAA from a climate projection standpoint. Furthermore, few studies have investigated potential interrelationships between DWAA and heavy rainfall. In this study, the interrelationships between DWAA events and heavy rainfall with various intensities as well as potential evapotranspiration are revealed explicitly through the convection‐permitting climate simulations for 10 climate divisions over Texas in the United States. Our findings disclose that the increasing heavy rainfall and potential evapotranspiration lead to more frequent occurrence of DWAA events over a larger spatial extent. Heavy rainfall with daily precipitation greater than 20 mm contributes most to the occurrence of DWAA. In addition, a severe phenomenon of dry‐wet‐dry alternation is projected to appear due to the increasing number of heavy rainfall and drought events as well as the deteriorated soil water holding capacity under global warming. Plain Language Summary: The dry‐wet abrupt alternation (DWAA) has dramatic negative impacts on water security and agricultural production. DWAA is a devastating natural disaster which is characterized by an abrupt alternation between wet and dry spells in adjacent months. Exploring the underlying mechanism and complex evolution of DWAA is thus crucial to helping policymakers and stakeholders develop sound adaptation and mitigation plans for reducing potential risks of natural extreme events. We find that the DWAA event is projected to occur simultaneously in multiple climate divisions, especially for the most populous region of South Texas. Heavy rainfall with daily precipitation greater than 20 mm contributes most to the occurrence of DWAA. Due to the increasing number of drought and heavy rainfall events, a severe super extreme event of the dry‐wet‐dry alternation is projected to appear by the end of this century, whereas there is no historical record on the occurrence of such an extreme event over Texas in the period of 1981–1995. Key Points: Heavy rainfall with daily precipitation greater than 20 mm contributes most to the occurrence of dry‐wet abrupt alternation eventsAbrupt alternation between wet and dry spells is projected to occur more frequently over a larger spatial extent by the end of the 21st centuryAbrupt alternation (dry‐wet‐dry) is expected to occur due to the increasing number of heavy rainfall and drought events in a changing climate [ABSTRACT FROM AUTHOR]

Compared with drought onset, the quantification and attribution of drought demise are not well studied. Meteorological droughts usually terminate more rapidly than they initiate, making it hard to define a transition period leading to drought demise with monthly data. In this study, methods of quantifying and attributing drought demise are applied to the Modern‐Era Retrospective Analysis for Research and Applications version 2 (MERRA‐2) using modified Standardized Precipitation Index representing meteorological drought calculated at pentad intervals to resolve subseasonal demise events. Methodologies to attribute three specific causes of drought demise in nine climate regional divisions over conterminous U.S. (CONUS) are developed. The three phenomenological causes are tropical cyclones, atmospheric rivers, and changes in land atmospheric feedback. Atmospheric river is the most common of the factors for drought demise, over most of the regions in the eastern, and central U.S. tropical cyclones are important causes over the Southwest, the South in fall, and the Southeast in summer. Evolving land atmospheric feedback is a factor mainly over the central and southwestern United States. These attributions estimated may not be representative of the long‐term climatologies of drought demise due to the short duration of MERRA‐2. A representativeness test is conducted for estimating the three impacts on drought demise using subsamples from a large ensemble of climate model simulation including several centuries of data. Thirty to forty years is not long enough to be representative of local long‐term statistics for the attribution of the causes of demise of extreme events like drought but may be adequate at regional scales. Key Points: The transition period leading to drought demise is definedA framework for the attribution of several causes of drought demise is devisedSeasonal variability of drought demise causes is demonstrated [ABSTRACT FROM AUTHOR]

Heat waves lead to major impacts on human health, food production, and ecosystems. To assess their predictability and how they are projected to change under global warming, it is crucial to improve our understanding of the underlying processes affecting their occurrence and intensity under present‐day climate conditions. Beside greenhouse gas forcing, processes in the different components of the climate system—in particular the land surface, atmospheric circulation, and the oceans—may play a key role in changing the odds for a particular event. This study aims to identify the role of the individual drivers for five heat waves (and, in some cases, of concurrent droughts) in the recent decade. Simulations are performed with the Community Earth System Model using nudging of horizontal atmospheric circulation and prescription of soil moisture. The fully constrained model accurately reproduces how anomalous an event was. Factorial experiments, which force the model toward observations for one or several key components at a time, allow us to identify how much of the observed temperature anomaly of each event can be attributed to each driver. Considering all analyzed events, atmospheric circulation and soil moisture play similarly important roles, each contributing between 20% and 70% to the events' anomalies. This highlights that the role of thermodynamics can be just as important as that of the dynamics for temperature extremes, a possibly underestimated feature. In addition, recent climate change amplified the events and contributed between 10% and 40% of the events' anomalies. Key Points: Observed temperature anomalies are well reproduced in a climate model when constrained by observed atmospheric circulation and soil moistureIn these constrained runs soil moisture and atmospheric circulation are equally important for temperature anomalies of five recent heat wavesDisentangling the physical drivers for the five investigated heat waves reveals a warming signal over the recent decades [ABSTRACT FROM AUTHOR]

Probabilistic projections of future drought characteristics play a crucial role in climate change adaptation and disaster risk reduction. This study presents a copula‐based probabilistic framework for projecting future changes in multivariable drought characteristics through convection‐permitting Weather Research and Forecasting simulations with 4‐km horizontal grid spacing. A probabilistic multivariate drought index is introduced to examine the joint effects of drought indicators with uncertainty intervals for four major river basins located in South Central Texas of the United States. Markov chain Monte Carlo is used to address uncertainties in assessing copula parameters and in predicting climate‐induced changes in hydrological regimes. Our findings reveal that the severity and intensity of drought episodes can be amplified when considering the compound effects of soil moisture and runoff regimes by using the probabilistic multivariate drought index. The South Central Texas region is projected to experience more drought events with shorter duration and higher intensity in a changing climate. The drought severity will not necessarily increase due to the decreasing drought duration. In addition, our findings indicate that the intensity of future droughts is expected to increase as a result of the deficiency of soil moisture even though precipitation extremes are projected to become more frequent. Moreover, climate change impacts on multivariate drought characteristics will intensify with the increasing temporal scales (i.e., short‐, medium‐, and long‐term droughts) although the number of future drought events may decrease by the end of this century. Plain Language Summary: Projecting future changes in drought characteristics plays a crucial role in climate change adaptation and disaster risk reduction. Here we present a copula‐based probabilistic framework for conducting high‐resolution projections of multivariable drought characteristics through convection‐permitting Weather Research and Forecasting simulations with the 4‐km horizontal grid spacing. A probabilistic multivariate drought index is also introduced to examine the joint effects of drought indicators for four major river basins located in South Central Texas of the United States. Our findings reveal that the severity and intensity of drought episodes can be amplified when considering the compound effects of soil moisture and runoff regimes. Furthermore, the long‐lasting droughts are expected to evolve into a series of short‐duration droughts due to the projected increase in the frequency and intensity of extreme precipitation in a changing climate. This study provides meaningful insights into the evolution of future drought characteristics in view of the increasing trend in extreme precipitation, which plays an important role in facilitating sustainable agricultural development and water resources planning under climate change. Key Points: A copula‐based probabilistic framework was proposed for conducting high‐resolution projections of multivariate drought characteristicsHigh‐resolution climate projections were developed using the convection‐permitting WRF model with the 4‐km horizontal grid spacingA probabilistic multivariate drought index was introduced to examine joint effects of soil moisture and runoff deficits [ABSTRACT FROM AUTHOR]

A severe drought hit the Greater Horn of Africa (GHA) in 2014, but it remains unclear whether this extreme event was attributable to anthropogenic climate change or part of longer‐term natural cycles. Precipitation patterns are known to be changing across the GHA, but trajectories in land surface variables are much less well known. We simulated the GHA land surface environment to assess the balance between natural cycles and human‐induced climate change. Using a new form of event attribution study where we focused on both climate variables and also directly simulated land surface variables, we combined publicly volunteered distributed computing with land surface simulations to quantify land surface responses. Uncertainty was quantified both for climate model and land surface model outputs. We identified two distinct "drought trajectories" in the GHA bimodal seasonality area during the March–May (Long Rains season) of 2014. Human‐induced climate change may have resulted in regions from Lake Nalubaale (Lake Victoria) to Northern Kenya receiving less precipitation in this season and having up to 20% higher probability of drought‐level evapotranspiration rates (increasing drought). In contrast, the simulated anthropogenic climate change signal for this season induced somewhat wetter conditions and up to 20% lower probability of drought‐level evapotranspiration in Eastern Ethiopia, Southern Somalia, and coastal Kenya (decreasing drought). Uncertainties in our modeling system varied by region and variable of focus, but broadly we found that land surface simulation uncertainty neither added significantly to climate model uncertainty nor significantly reduced it. Key Points: Precipitation patterns are known to be changing across the Horn of Africa, but trajectories in land surface variables are less well knownWe identify two distinct "drought trajectories" in the Horn of Africa: one of increasing drought and one of decreasing droughtChanges in water cycle impacts may best be assessed through attribution studies that include changes in land surface variables [ABSTRACT FROM AUTHOR]

Reservoirs provide water for irrigation and hydropower and protect downstream regions from flooding. Reservoir storage is affected by drought, which hampers the ability to provide water for irrigation and hydropower. Despite the need for reservoir storage prediction for planning and decision‐making, a reservoir storage forecast system at 1‐ to 3‐month lead has been lacking for major reservoirs in India. Here we evaluate the potential of observed accumulated precipitation, standardized precipitation index (SPI), standardized precipitation evapotranspiration index (SPEI), standardized streamflow index (SSI), and observed reservoir storage to provide reservoir storage anomaly forecast at 1‐ to 3‐month lead for the dry season (February to May) using a statistical approach. We find that accumulated precipitation for 3–11 months is strongly associated with the monthly reservoir storage anomalies in India. Moreover, accumulated precipitation and observed reservoir storage provide a reasonable (R2 = 0.7) forecast skill for reservoir storage anomalies at 1‐ to 3‐month lead. Similarly, SPI and SPEI can be used to predict reservoir storage anomalies at 1‐ to 3‐month lead in India. Since the prediction skill from SPI and SPEI is similar, reservoir storage at monthly timescale is largely affected by accumulated precipitation instead of variability in air temperature. We find that the forecast skill for the 1‐month lead from SPI, SPEI, and SSI is similar for Indira Sagar (Narmada) and Minimata (Mahanadi) reservoirs; however, prediction skill for reservoir storage anomalies improves substantially using SSI for 2‐ to 3‐month lead. Reservoir storage anomalies forecast at 1‐to 3‐month lead can be valuable for water management‐related decision‐making and planning during the dry season in India. Key Points: We developed reservoir storage index (RSI) to show reservoir storage condition in IndiaWe evaluated the potential of observed accumulated precipitation, SPI, SPEI, and SSI for reservoir storage forecast at 1‐ to 3‐month leadReservoir storage anomaly prediction has a similar skill for SPI, SPEI, and SSI for 1‐month lead where at higher lead (2‐3 months), SSI shows better forecast skill than SPI and SPEI [ABSTRACT FROM AUTHOR]

Partitioning of the available energy at the Earth's surface into the latent heat (LE) and sensible heat (H) fluxes has important climatological, hydrological, and physiological implications. With the prediction of more frequent droughts in central Europe in the near future, there is a particular need to understand variability in available energy partitioning under drought stress conditions at forest ecosystems that are common in the region, such as Norway spruce and European beech. Using eddy covariance measurements from two Norway spruce sites with contrasting wet and dry climates and one European beech site in the Czech Republic, it was found that the proportion of energy partitioned into H was greater at the spruce sites than at the beech site in all conditions during the growing season. The difference between the mean midday (09:00–15:00) β values for low stress conditions and drought stress conditions was much smaller at the European beech site (β = 1.04 vs. 1.11) than at the wet (β = 1.52 vs. 2.50) and dry Norway spruce (β = 1.80 vs. 2.70) sites, indicating that β was not as sensitive to drought stress at the European beech site as at the Norway spruce sites. The high β values and enhancement of drought conditions through positive feedback processes at Norway spruce stands mean that the potential substitution of Norway spruce monocultures with mixed broadleaf‐coniferous stands in central Europe will likely lessen the severity of droughts and heat waves in the region. Plain Language Summary: Partitioning of the available energy at the Earth's surface into the latent heat and sensible heat fluxes plays an important role in processes such as variability in the height of the convective boundary layer and associated weather processes. With the prediction of more frequent droughts in central Europe in the near future, there is a particular need to understand how available energy partitioning varies under different environmental conditions at forest ecosystems that are common in the region, such as Norway spruce and European beech. Using eddy covariance measurements from two Norway spruce sites with contrasting wet and dry climates and one European beech site, it was found that the proportion of energy partitioned into sensible heat was greater at the spruce sites than at the beech site in all conditions during the growing season. The partitioning of available energy was more sensitive to drought stress at the Norway spruce sites than at the European beech site. The high sensible heat fluxes at Norway spruce plantations during drought conditions means that the potential large‐scale substitution of Norway spruce monocultures with mixed broadleaf‐coniferous plantations in central Europe could lessen the severity of droughts in the region. Key Points: The proportion of energy partitioned into sensible heat was greater at the spruce sites than at the beech site in all conditionsThe partitioning of available energy was more sensitive to drought stress at the Norway spruce sites than at the European beech siteThe results suggest that widespread conversion of spruce monocultures to mixed forests could reduce the severity of droughts [ABSTRACT FROM AUTHOR]

The arid/semiarid region of Northwest China (NWC, north to 35°N, and west to 100°E in China) experienced significant wetting after the late 1980s. We used both the summer precipitation and standardized precipitation indices (precipitation index [PI] and standardized precipitation index) to reexamine the regime shift. Consistent with the rainfall, the drought condition represented by the standardized precipitation index was significantly alleviated over most areas of NWC during P2 (1989–2016) than that during P1 (1961–1988). Along with the increased rainfall over the NWC, significant changes also occur in the interannual variation‐related circulation. The Ural High (UH) was found to be closely related with the interdecadal change during the late 1980s. The NWC PI‐related wave propagation from the UH weakened after the late 1980s. The persistent tropical Indian Ocean sea surface temperature (SST) from the preceding winter to the summer were also found to influence the NWC PI through modulating the South Asian High during P1. However, during P2, the ENSO signals in the preceding winter, which can be memorized by the Indian Ocean SST in the spring and propagate to the Indo‐Pacific SST in the summer, can influence the NWC PI. The wave train pattern in the convection from the Maritime Continent to NWC and northwestern Europe seems to be a key bridge. The UH seems to be independent of the above SST‐forced atmospheric circulations. The SST signals of the preceding winter can provide useful information for the summer drought prediction in NWC. Key Points: The summer drought over Northwest China (NWC)‐related atmospheric circulation show significant changes at the late 1980sThe Ural High's impact on the atmospheric circulation over NWC becomes much weaker during P2 (1989‐2016) than that during P1 (1961‐1988)The key SST regions related with the NWC drought shifted from the tropical Indian Ocean during P1 to the Indo‐Pacific region during P2 [ABSTRACT FROM AUTHOR]

Investigating the spatiotemporal link among different drought types is of great significance for understanding the causative mechanism of droughts. In this study, we propose a new approach for connecting meteorological and hydrological drought events by considering their overlaps in drought persistence time and spatial coverage in three dimensions (i.e., latitude, longitude, and time). The variable infiltration capacity model with daily hydrometeorological observations in and around the Yellow River basin for the period of 1961–2012 is used to derive the standardized precipitation evapotranspiration index and standardized runoff index. Results show that minor meteorological drought events with short durations and small areas generally are not prone to cause a hydrological drought, especially in winter for the source region, leading to failure matches between the two drought types. By contrast, meteorological droughts of large magnitudes are more likely to influence a hydrological drought in different ways. In three dimensions, the transformation from meteorological to hydrological droughts is characterized by lengthened duration, lagged response time, and attenuated drought severity, which is in accordance with previous studies. Moreover, the drought area also decreases during this process. The increase in potential evapotranspiration from February to April after 1998 is the reason for the attenuation of drought area. The enlarged evaporative demand from potential evapotranspiration aggravates the atmospheric dry condition and promotes the development of meteorological drought but does not necessarily cause a hydrological drought. Key Points: Drought characteristics were identified from a three‐dimensional perspectiveA new approach for linking meteorological and hydrological events was proposedDrought propagation features were supplemented from a three‐dimensional perspective [ABSTRACT FROM AUTHOR]

The contributions of oceanic and atmospheric variability to spatially widespread summer droughts in the contiguous United States (hereafter, pan‐CONUS droughts) are investigated using 16‐member ensembles of the Community Climate Model version 3 (CCM3) forced with observed sea surface temperatures (SSTs) from 1856–2012. The employed SST forcing fields are either (i) global or restricted to the (ii) tropical Pacific or (iii) tropical Atlantic to isolate the impacts of these two ocean regions on pan‐CONUS droughts. Model results show that SST forcing of pan‐CONUS droughts originates almost entirely from the tropical Pacific because of atmospheric highs from the northern Pacific to eastern North America established by La Niña conditions, with little contribution from the tropical Atlantic. Notably, in all three model configurations, internal atmospheric variability influences pan‐CONUS drought occurrence by as much or more than the ocean forcing and can alone cause pan‐CONUS droughts by establishing a dominant high centered over the U.S. montane west. Similar results are found for the Community Atmosphere Model version 5 (CAM5). Model results are compared to the observational record, which supports model‐inferred contributions to pan‐CONUS droughts from La Niñas and internal atmospheric variability. While there may be an additional association with warm Atlantic SSTs in the observational record, this association is ambiguous due to the limited number of observed pan‐CONUS droughts. The ambiguity thus opens the possibility that the observational results are limited by sampling over the twentieth century and not at odds with the suggested dominance of Pacific Ocean forcing in the model ensembles. Key Points: Models forced with observed sea surface temperatures replicate spatially widespread summer droughts over the contiguous United StatesVariability in the tropical Pacific explains almost all the ocean forcing of these droughts in the models; the Atlantic plays an ambiguous roleContributions to these droughts from internal atmospheric variability are roughly equal to or greater than the collective ocean forcing [ABSTRACT FROM AUTHOR]

High‐resolution paleoclimate records are essential for improving our understanding of internal variability and the detection and attribution of forced climate system responses. The densely populated northeastern United States is at risk from increasing temperatures, severe droughts, and extreme precipitation, but the region has limited annual and seasonal‐resolution paleoclimate records beyond the instrumental record. Chamaecyparis thyoides, L. (B.S.P.), Atlantic white cedar, a wetland conifer found within 200 km of the Atlantic coastline of the United States, is a promising tree‐ring proxy that can fill in these data gaps. Here, we develop and analyze a new network of Atlantic white cedar tree‐ring chronologies across the northeastern United States and demonstrate that site selection is important for regional paleoclimate reconstructions. Ring width variability reflects winter through summer temperatures at inland and hydrologically stable sites in the northernmost section of the species' range. Ombrotrophic sites along the coast record hydrological signals and correlate with growing season precipitation. We demonstrate skillful regional climate field reconstructions for the last several centuries and show the increased skill from incorporating our moisture sensitive sites into broad‐scale products like the North American Drought Atlas. This comprehensive understanding of the species' climate responses leads to a tree‐ring network that provides the long‐term multivariate climate context at multidecadal and centennial time scales for the large‐scale ocean‐atmospheric processes that influence the climate of the region. We use this network to examine the covariance of temperature and drought across the New England area over the past two centuries. Key Points: Atlantic white cedar tree‐ring chronologies are used for field reconstruction of northeastern U.S. temperaturesAtlantic white cedar growing in ombrotrophic environments are significantly correlated with precipitation, not temperatureCurrent temperature and precipitation trends are unlike those seen in the multicentury climate reconstructions [ABSTRACT FROM AUTHOR]

We perform the first uncertainty quantification analysis of the turn‐of‐the‐century drought in the western United States using a large perturbed‐parameter ensemble of the Community Atmosphere Model version 4.0 (CAM4). We develop several metrics to characterize the aridity bias, spatial extent, and tropical forcing of the drought and use statistical models to ascertain that the modeled drought was mainly sensitive to CAM4 parameters related to deep convection and clouds. Deep convection parameters account for over half the variance across the drought metrics. We employ observed estimates of these drought metrics to infer probability distributions of the model parameters that lead to a better agreement with observations, thereby providing guidance on how to improve the simulation of drought in CAM4. We find that in some cases, the suggested parameter values that improve the simulation of one drought characteristic would degrade the simulation of another, suggesting that there is a complex relationship between the model parameters and drought in the western United States. We also demonstrate reductions in the uncertainty of the drought metrics of up to 30% by constraining with observations of all metrics. Furthermore, we demonstrate for the first time that improvements to the simulation of the spatial extent of the turn‐of‐the‐century drought also lead to improvements in the spatial extent of the Australian Millennium drought, suggesting that physics of drought as encoded by the model parameters may be generalizable. Key Points: Deep convection parameters account for more than half the CAM4 ensemble variance in metrics used to quantify droughtWe found optimal parameter values that may lead to a better reproduction of drought in CAM4We reduce the uncertainty of the CAM4 simulation of a metric that quantifies the spatial extent of drought by 40% [ABSTRACT FROM AUTHOR]

Studying the tendency of soil temperature and moisture can provide scientific support for setting soil parameters for land surface hydrological model. Previous research has shown that there is long‐range correlation of soil temperature or moisture; however, driving factors of long‐range correlation variations and the cross correlation between temperature and moisture under multiple timescales have rarely been discussed. To explore the driving factors, we compared long‐range correlation between shady and sunny slopes. Adaptive fractal analysis was utilized to analyze long‐range cross correlation (LRCC) and long‐range correlation (LRC). Experimental data were collected between 9 August 2013 and 3 December 2014 at A'rou observation stations, Babao River Basin. Results show that two slopes have some common characteristics. (1) Soil temperature and moisture display the consistent change. (2) The persistence intensity of moisture, temperature, and their interaction intensity change at 9, 6, and 6‐day timescales, respectively. (3) LRC and LRCC indices demonstrate the stratification heterogeneity. Results also show that LRC and LRCC indices on two slopes have differences. The soil system was stratified into three layers to explain these differences over large timescales. (1) At 4‐ to 20‐cm depth, with significant influence of solar radiation and evapotranspiration on sunny slope, moisture shows weaker persistence. There is stronger antipersistence of temperature and stronger hydrothermal interaction on sunny slope. (2) At 20‐ to 80‐cm depth, there is weaker persistence of soil moisture on shady slope because of unstable groundwater and side runoff supply. (3) At 80‐ to 160‐cm depth, with seasonal melt‐water effect on sunny slope, persistence of moisture was considerably weaker. Key Points: Comparison of long‐range correlations of soil moisture between the shady slope and sunny slope. Comparison of long‐range correlations of soil temperature between the shady slope and sunny slope. Comparison of long‐range cross correlation between the soil moisture and soil temperature between the shady slope and sunny slopeLong‐range correlation and long‐range cross correlation display vertical depth variation and timescale variationWe seek out driving factors of stratification heterogeneity of LRC and LRCC in different soil depths [ABSTRACT FROM AUTHOR]

Drought is the most recurrent and destructive hazard in arid and semiarid regions and will only become more complex under climate change. It is vital to characterize the various types of drought, to investigate the potential factors affecting different types of drought, and to assess the relationship between drought types. In this study, the Standardized Precipitation Index and the Standardized Runoff Index were used to characterize meteorological and hydrological drought, respectively, to investigate drought characteristics and mechanisms in 17 catchments on the Loess Plateau from 1961 to 2013. Furthermore, the propagation time from meteorological to hydrological drought was explored and the potential factors influencing drought propagation time were investigated. The results indicate that the Loess Plateau has experienced an increased tendency toward both meteorological and hydrological droughts over the period 1961–2013, with hydrological drought more serious than meteorological drought at various drought assessment time scales. Moreover, average drought duration and severity were greater for hydrological drought than meteorological drought. Maximum 5‐day precipitation (Rx5day) was the dominant extreme climate index for explaining variance in meteorological drought at the annual time scale. Owing to the greater complexity underlying hydrological drought, Rx5day, the number of warm days (Tx90p), and the number of warm nights (Tn90p) all contribute to the variance in hydrological drought. Furthermore, the percentage of forested land had a significant positive association (p < 0.001) with propagation time, whereas the percentage of land given over to pasture had a significant negative association (p < 0.001) with propagation time. Key Points: Loess Plateau has experienced an increased tendency toward both meteorological and hydrological droughts over the period 1961–2013Average drought duration and severity were greater for hydrological drought than meteorological droughtChanges in land use and land cover can alter the propagation time from meteorological to hydrological drought [ABSTRACT FROM AUTHOR]