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The Yangtze River basin (YRB) experienced a record-breaking mei-yu season in June–July 2020. This unique long-lasting extreme event and its origin have attracted considerable attention. Previous studies have suggested that the Indian Ocean (IO) SST forcing and soil moisture anomaly over the Indochina Peninsula (ICP) were responsible for this unexpected event. However, the relative contributions of IO SST and ICP soil moisture to the 2020 mei-yu rainfall event, especially their linkage with atmospheric circulation changes, remain unclear. By using observations and numerical simulations, this study examines the synergistic impacts of IO SST and ICP soil moisture on the extreme mei-yu in 2020. Results show that the prolonged dry soil moisture led to a warmer surface over the ICP in May under strong IO SST backgrounds. The intensification of the warm condition further magnified the land thermal effects, which in turn facilitated the westward extension of the western North Pacific subtropical high (WNPSH) in June–July. The intensified WNPSH amplified the water vapor convergence and ascending motion over the YRB, thereby contributing to the 2020 mei-yu. In contrast, the land thermal anomalies diminish during normal IO SST backgrounds due to the limited persistence of soil moisture. The roles of IO SST and ICP soil moisture are verified and quantified using the Community Earth System Model. Their synergistic impacts yield a notable 32% increase in YRB precipitation. Our findings provide evidence for the combined influences of IO SST forcing and ICP soil moisture variability on the occurrence of the 2020 super mei-yu. [ABSTRACT FROM AUTHOR]

Heat extremes have severe implications for human health, ecosystems, and the initiation of wildfires. While they are mostly introduced by atmospheric circulation patterns, the intensity of heat extremes is modulated by terrestrial evaporation associated with soil moisture availability. Thereby, ecosystems provide evaporative cooling through plant transpiration and soil evaporation, which can be reduced under water stress. While it has been shown that regional ecosystem water limitation is projected to increase in the future, the respective repercussions on heat extremes remain unclear. In this study, we use projections from 12 Earth system models to show that projected changes in heat extremes are amplified by increasing ecosystem water limitation in regions across the globe. We represent the ecosystem water limitation with the ecosystem limitation index (ELI) and quantify temperature extremes through the differences between the warm-season mean and maximum temperatures. We identify hotspot regions in tropical South America and across Canada and northern Eurasia where relatively strong trends towards increased ecosystem water limitation jointly occur with amplifying heat extremes. This correlation is governed by the magnitude of the ELI trends and the present-day ELI which denotes the land–atmosphere coupling strength determining the temperature sensitivity to evaporative cooling. Many regions where ecosystem functioning is predominantly energy-limited or transitional in the present climate exhibit strong trends towards increasing the water limitation and simultaneously experience the largest increases in heat extremes. Sensitivity of temperature excess trends to ELI trends is highest in water-limited regions, such that in these regions relatively small ELI trends can amount to drastic temperature excess trends. Therefore, considering the ecosystem's water limitation is key for assessing the intensity of future heat extremes and their corresponding impacts. [ABSTRACT FROM AUTHOR]

Southeastern South America (SESA) is a highly productive agricultural region and a hot spot for land‐atmosphere interactions. To evaluate the impact of dry soil moisture anomalies (SMAs) on SESA climate and the sensitivity of the regional climate response to the location of SMAs, we perform three experimental simulations using the Community Earth System Model (CESM) with prescribed dry SMAs over (a) SESA, (b) western SESA, and (c) eastern SESA. The dry SESA and eastern SESA simulations show widespread negative precipitation anomalies. In contrast, the dry western SESA simulation shows positive precipitation anomalies over northeastern Argentina, which are associated with the enhanced southward moisture flux co‐located with the South American low‐level jet exit region. A composite analysis of extremely dry cases over western SESA using reanalysis data agrees with the findings from our CESM experiment. These findings have potential implications for subseasonal forecasting in this region. Plain Language Summary: Large‐scale soil moisture anomalies evolve slowly and can provide an opportunity for better weather forecasting at timescales longer than 2 weeks. Therefore, it is critical to understand the causal physical mechanism and evaluate whether the regional climate response is sensitive to the location of soil moisture anomalies, especially in a productive agricultural region like southeastern South America (SESA). Using a numerical climate model, we simulate the impacts of dry soil over (a) SESA, (b) western SESA, and (c) eastern SESA. The simulations show that dry soil over western SESA can alter regional atmospheric circulation in the proximity of the existing corridor of poleward moisture transport, hence enhancing rainfall over northeastern Argentina. Conversely, dry soil over eastern SESA or the entire SESA region results in less precipitation because enhanced northerly transport is not co‐located with the low‐level wind corridor. Analysis of a dataset that incorporates observations supports our findings from numerical simulations. Key Points: The impact of dry soil moisture anomalies (SMAs) on southeastern South America (SESA) regional climate is sensitive to the location of SMAsThis study provides a causal mechanism linking soil moisture to precipitation via atmospheric circulationWhen western SESA has dry soil, it generates anomalous geostrophic wind, which is co‐located with the low‐level jet exit region [ABSTRACT FROM AUTHOR]

Land surface‐atmosphere coupling and soil moisture memory are shown to combine into a distinct temporal pattern for wildfire incidents across the western United States. We investigate the dynamic interplay of observed soil moisture, vegetation water content, and atmospheric dryness in relation to fuel loading, fire ignition and post‐fire recovery. We find that positive soil moisture anomalies around 5 months before fire ignition increase biomass growth in the subsequent months, thereby shaping fire‐prone vegetation conditions. Then, concurrent decrease in soil moisture, vegetation dehydration, and atmospheric dryness collectively contribute to the occurrence of fire ignition events. This is followed by a rapid recovery in both soil and atmospheric moisture within several weeks after the fire incidents. Our findings provide insights into understanding of wildfire ignition dynamics, supporting fire modeling and enabling improved fire predictions, early warning systems, and mitigation strategies. Plain Language Summary: We find that wildfire are part of a distinct temporal pattern of soil moisture, vegetation water content and atmospheric dryness dynamics that begin about 5 months before the incidents. We analyze anomalies in soil moisture, vegetation water content, vapor pressure deficit and precipitation before and after large wildfires during the warm season across various US climate regions. We document distinct patterns in soil aridity and atmospheric dryness, fuel load, and vegetation dehydration that contribute to the occurrence of wildfires. Our observation‐based analyses show that soil moisture anomalies significantly control the growth of biomass several months before the fire occurrence, which in turn affects the availability of fire fuel. Notable dry anomalies with fuel accumulation in the dry western climate regions are observed approximately 4 to 5 months before the fires, followed by a quick recovery trend several weeks after the fire in wet regions. Our results indicate that in the majority of the climate regions, there are significant dry anomalies preceding the occurrence of wildfires. Understanding the dynamics of these environmental variables can be the basis for improved wildfire hazard predictions and mitigation strategies. Key Points: Wetter‐than‐average soil moisture 4–5 months before wildfires results in increased aboveground biomass accumulationSoil moisture decrease and atmospheric drying after peak biomass lead to dry fuel availability at the time of wildfire ignitionThe temporal pattern is evident in mapped data on soil moisture, vegetation water content and wildfire incidents across western United States [ABSTRACT FROM AUTHOR]

We demonstrate an indirect, rather than direct, role of quasi-resonant amplification of planetary waves in a summer weather extreme. We find that there was an interplay between a persistent, amplified large-scale atmospheric circulation state and soil moisture feedbacks as a precursor for the June 2021 Pacific Northwest "Heat Dome" event. An extended resonant planetary wave configuration prior to the event created an antecedent soil moisture deficit that amplified lower atmospheric warming through strong nonlinear soil moisture feedbacks, favoring this unprecedented heat event. [ABSTRACT FROM AUTHOR]

The Central American Dry Corridor experienced five consecutive years of drought from 2015 to 2019. Here, we find that the severity of this drought was driven primarily by rainfall deficits in July–August. To determine if the magnitude of this event was outside the range of natural variability, we apply a statistical resampling method to observations that emulates internal climate variability. Our analyses show that droughts similar to the 2015–2019 event are possible, although extremely rare, even without anthropogenic influences. Persistent droughts in our ensemble are consistently linked to stronger easterly winds associated with the Caribbean Low‐Level Jet. We also examine the effects of temperature on soil moisture during this drought using the Palmer Drought Severity Index and show that anthropogenic warming increases the likelihood of severe deficits. Multi‐year droughts are likely to worsen by the end of the 21st century due to the compound effects of anthropogenic climate change. Plain Language Summary: Climate models project that Central America is one of the global hotspots for future decreases in precipitation as a result of human‐caused climate change. This is particularly concerning for the Dry Corridor region, which is already prone to frequent droughts and high levels of food insecurity among households. Much of this region experienced severe rainfall deficits between 2015 and 2019, provoking the question of whether or not this drought was caused by climate change or if it could have occurred because of natural climate variability alone. Using a statistical model, we show that while 2015–2019 was the driest period in the observational record, droughts as bad as this one are possible even without the influence of human‐caused climate change. We examine the additional role of temperature since it can modulate drought severity through its influence on soil moisture. We find warming temperatures increase the occurrences of greater soil moisture deficits. We also determine that the strength of the Caribbean Low‐Level Jet, which transports moisture from the Caribbean Sea into Central America, is strongly associated with persistent dry conditions in the region. Key Points: The 2015–2019 drought was severe, but it falls within the range of natural climate variabilityJuly–August deficits were the most significant drivers of overall droughtPositive easterly zonal wind anomalies associated with the Caribbean Low‐Level Jet are strongly linked to regional precipitation deficits [ABSTRACT FROM AUTHOR]

Heat extremes have severe implications for human health, ecosystems and the initiation of wildfires. Whereas they are mostly introduced by atmospheric circulation patterns, the intensity of heat extremes is modulated by vegetation functioning associated with soil moisture availability. Thereby, vegetation provides evaporative cooling through transpiration, which can be reduced under water stress. While it has been shown that regional ecosystem water limitation is projected to increase in the future, the respective repercussions on heat extremes remain unclear. In this study we use projections from eight Earth system models to show that projected changes in heat extremes are amplified by increasing ecosystem water limitation in regions across the globe. We represent ecosystem water limitation with the Ecosystem Limitation Index (ELI) and quantify temperature extremes through the differences between warm-season mean and maximum temperatures. We identify hotspot regions in tropical South America and across Northern Eurasia where relatively strong trends towards increased ecosystem water limitation jointly occur with amplifying heat extremes. This correlation is governed by the magnitude of the ELI trends and the present-day ELI which denotes the land-atmosphere coupling strength determining the temperature sensitivity to evaporative cooling. Many regions where vegetation functions are predominantly energy-limited or transitional in present climate exhibit strong trends towards increasing water limitation and simultaneously experience the largest increases in heat extremes. Therefore, considering the ecosystem's water limitation is key for assessing the intensity of future heat extremes and their corresponding impacts. [ABSTRACT FROM AUTHOR]

Soil moisture (SM) affects weather through its impact on surface flux partitioning, influencing vertical atmospheric profiles and circulations driven by differential surface heating. In West Africa, observational studies point to a dominant negative SM‐precipitation feedback, where dry soils help to initiate and maintain convection. In this context, serious concerns exist about the ability of models with parameterised convection to simulate this observed sensitivity of daytime convection to SM. Here, we evaluate the effect of initial SM perturbations in a short‐range ensemble forecast over West Africa, comparing the UK Met Office Global and Regional Ensemble Prediction System (MOGREPS) with parameterised convection (GLOB‐ENS) to its regional convection‐permitting counterpart (CP‐ENS). Results from both models suggest SM perturbations introduce considerable spread into daytime evaporative fraction (EF) and near‐surface temperatures. This spread is still evident on Day 3 of the forecast. Both models also show a tendency to increased afternoon rainfall frequency over negative EF anomalies, reproducing the observed feedback. However, this effect is more pronounced in CP‐ENS than GLOB‐ENS, which illustrates the potential for process‐based forecast improvements at convection‐permitting scales. Finally, we identify persistent biases in rainfall caused by land cover mapping issues in the operational GLOB‐ENS setup, emphasising the need for careful evaluation of different mapping strategies for land cover. [ABSTRACT FROM AUTHOR]

The potential effects of climate change on plant reproductive phenology include asynchronies with pollinators and reductions in plant fitness, leading to extinction and loss of ecosystem function. In particular, plant phenology is sensitive to extreme weather events, which are occurring with increasing severity and frequency in recent decades and are linked to anthropogenic climate change and shifts in atmospheric circulation. For 15 plant species in a Venezuelan cloud forest, we documented dramatic changes in monthly flower and fruit community composition over a 35‐year time series, from 1983 to 2017, and these changes were linked directly to higher temperatures, lower precipitation, and decreased soil water availability. The patterns documented here do not mirror trends in temperate zones but corroborate results from the Asian tropics. More intense droughts are predicted to occur in the region, which will cause dramatic changes in flower and fruit availability. [ABSTRACT FROM AUTHOR]

Projections of changes in extreme droughts under future climate conditions are associated with large uncertainties, owing to the complex genesis of droughts and large model uncertainty in the atmospheric dynamics. In this study we investigate the impact of global warming on soil moisture drought severity in west-central Europe by employing pseudo global warming (PGW) experiments, which project the 1980–2020 period in a globally warmer world. The future analogues of present-day drought episodes allow for investigation of changes in drought severity conditional on the historic day-to-day evolution of the atmospheric circulation. The 2018 west-central European drought is the most severe drought in the 1980–2020 reference period in this region. Under 1.5, 2 and 3 ∘ C global warming, this drought episode experiences strongly enhanced summer temperatures but a fairly modest soil moisture drying response compared to the change in climatology. This is primarily because evaporation is already strongly moisture-constrained during present-day conditions, limiting the increase in evaporation and thus the modulation of the temperature response under PGW. Increasing precipitation in winter, spring and autumn limits or prevents an earlier drought onset and duration. Nevertheless, the drought severity, defined as the cumulative soil moisture deficit volume, increases considerably, with 20 % to 39 % under 2 ∘ C warming. The extreme drought frequency in the 1980–2020 period strongly increases under 2 ∘ C warming. Several years without noticeable droughts under present-day conditions show very strong drying and warming. This results in an increase in 2003-like drought occurrences, compounding with local summer temperature increases considerably above 2 ∘ C. Even without taking into account a (potentially large) dynamical response to climate change, drought risk in west-central Europe is strongly enhanced under global warming. Owing to increases in drought frequency, severity and compounding heat, a reduction in recovery times between drought episodes is expected to occur. Our physical climate storyline provides evidence complementing conventional large-ensemble approaches and is intended to contribute to the formulation of effective adaptation strategies. [ABSTRACT FROM AUTHOR]

Drought is a widespread and destructive natural hazard and is projected to occur more frequently and intensely, with more severe impacts in a changing environment. In this study, we used the standardized precipitation index (SPI) at various time scales (i.e., 3, 6, and 12 months) to provide an overall view of drought conditions across Sichuan Province, southwestern China, from 1961 to 2016. Then, the relationship between the SPI and the soil moisture anomalies was analyzed. Furthermore, the causes of SPI drought from the perspective of large-scale atmospheric circulation were assessed in the study area. The results showed that most stations with decreasing trends were located in the eastern part of Sichuan Province, while most stations with increasing trends were located in the northwestern part, indicating that the eastern region presented a drying trend, while the northwestern part exhibited a wetting trend. The specific analysis focused on extreme drought indicated an increasing occurrence the probability of extreme drought events, which could induce a high potential drought risk in the study area. The SPI values had a strong relationship with the soil moisture anomalies, and the linear correlation coefficients decreased as the time scale increased. This result indicated that SPI3 (3-month SPI) could be regarded as a good predictor of soil moisture drought. The cross wavelet analysis revealed that the Southern Oscillation Index (SOI) had statistically significant correlations with the SPIs in Sichuan Province. The results of this study are useful for assessing the change in local drought events, which will help reduce the losses caused by drought disasters in Sichuan Province. [ABSTRACT FROM AUTHOR]

During summer 2021, Western North America (WNA) experienced an unprecedented heatwave with record-breaking high temperatures associated with a strong anomalous high-pressure system, i.e., a heat dome. Here, we use a flow analog method and find that the heat dome over the WNA can explain half of the magnitude of the anomalous temperature. The intensities of hot extremes associated with similar heat dome-like atmospheric circulations increase faster than background global warming in both historical change and future projection. Such relationship between hot extremes and mean temperature can be partly explained by soil moisture-atmosphere feedback. The probability of 2021-like heat extremes is projected to increase due to the background warming, the enhanced soil moisture-atmosphere feedback and the weak but still significantly increased probability of the heat dome-like circulation. The population exposure to such heat extremes will also increase. Limiting global warming to 1.5 °C instead of 2 °C (3 °C) would lead to an avoided impact of 53% (89%) of the increase in population exposure to 2021-like heat extremes under the RCP8.5-SSP5 scenario. The heat dome explains about 55% of the 2021 Western North American high temperatures. The intensity of heat extremes associated with such circulations are increasing faster than background global warming due to soil moisture feedbacks. [ABSTRACT FROM AUTHOR]

Snow on the Tibetan Plateau (TP) is an important signal for the prediction of East Asian climate. In this study, the relationship between the TP spring snowmelt and spring and summer precipitation in Northwest China (NWC) was investigated, along with the possible mechanisms linked to the impacts of snowmelt on precipitation. The results showed that the TP spring snowmelt had significant impacts on spring and summer precipitation in NWC. For example, when there was a large spring snowmelt in the central- eastern TP, the spring and summer precipitation in the Hexi Corridor and southeast NWC was excessive, especially in summer; when there was a large spring snowmelt in the northern TP, the spring and summer precipitation was deficient across the whole of NWC, while a large spring snowmelt in the western TP led to deficient spring and summer precipitation in eastern NWC but excessive precipitation in western NWC. The possible mechanisms for this included the fact that more spring snowmelt over the TP led to higher soil moisture contents, which further resulted in weakened subtropical westerly and enhanced ridge over Xinjiang. By changing the TP thermal forcing, these anomalous atmospheric circulation conditions transported water vapor into NWC, thus creating excessive summer precipitation in that region. [ABSTRACT FROM AUTHOR]

Abstract Terrestrial ecosystems are fully coupled with the climate. The planet has been greening owing to the increased vegetation growth in response to the changing atmosphere, which in turn has feedback on the climate. Greening has slowed down the rise in global land-surface air temperature mainly through a coincident increase of evapotranspiration and precipitation in wet regions. In dry regions, greening intensifies the decrease in soil moisture induced by greening-enhanced transpiration. Uncertain, however, is how the climate effects of greening in semi-arid lands might differ for variable wet and dry conditions. Here, we focus on the biosphere–atmosphere interactions in Australia by modeling the perturbation of vegetation changes under various states of sea surface temperature (SST), including the climatology mean, El Niño, and La Niña conditions. For the dry conditions of El Niño, greening exacerbates water stress and largely depletes the soil moisture, while for the wet conditions of La Niña, greening-enhanced evapotranspiration and precipitation resupply the soil moisture. For the normal conditions using the climatology mean SST, a small decrease in soil moisture occurs but with large spatial contrast because of heterogeneous changes of evapotranspiration and precipitation induced by greening. We emphasize that the alternating dry and wet conditions modulated by the large-scale climate variability are vital to understanding the response of climate to greening. Furthermore, vegetation-based warming mitigation policies need to be cautious when inferring distinct climate effects associated with greening.

Soil moisture is one of the essential climate variables with a potential impact on local climate variability. Despite the importance of soil moisture, studies on soil moisture characteristics in Ethiopia are less documented. In this study, the spatiotemporal variability of Ethiopian soil moisture (SM) has been characterized, and its local and remote influential driving factors are investigated. An empirical orthogonal function (EOF) and KMeans clustering algorithm have been employed to classify the large domain into homogeneous zones. Complex maximum covariance analysis (CMCA) is applied to evaluate the covariability between SM and selected local and remote variables such as rainfall (RF), evapotranspiration (ET), and sea surface temperature (SST). Inter-comparison among SM datasets highlight that the FLDAS dataset better depicts the country's SM spatial and temporal distribution (i.e., a correlation coefficient r = 0.95 , r m s d = 0.04 m 3 m - 3 with observations). Results also indicate that regions located in northeastern Ethiopia are drier irrespective of the season (JJAS, MAM, and OND) considered. In contrast, the western part of the country consistently depicted a wetter condition in all seasons. During summer (JJAS), the soil moisture variability is characterized by a strong east–west spatial contrast. The highest and lowest soil moisture values were observed across the country's central western and eastern parts, respectively. Furthermore, analyses indicate that interannual variability of SM is dictated substantially by RF, though the impact on some regions is weaker. It is also found that ET likely drives the SM in the eastern part of Ethiopia due to a higher atmospheric moisture demand that ultimately invokes changes in surface humidity and rainfall. A composite analysis based on the extreme five wettest and driest SM years revealed a similar spatial distribution of wet SM with positive anomalies of RF across the country and ET over the southern regions. Remote SSTs are also found to have a significant influence on SM distribution. In particular, equatorial central Pacific and western Indian oceans SST anomalies are predominant factors for spatiotemporal SM variations over the country. Major global oceanic indices: Oceanic Nino Index (ONI), Indian Ocean Dipole (IOD), Pacific warm pool (PACWARMPOOL), and Pacific Decadal Oscillations (PDO) are found to be closely associated with the SM anomalies in various parts of the country. The associationship between these remote SST anomalies and local soil moisture is via large-scale atmospheric circulations that are linked to regional factors such as precipitation and temperature anomalies. [ABSTRACT FROM AUTHOR]

Drought is complex and is also one of the main disasters affecting China. Exploring the response of agricultural drought and meteorological drought to climatic factors helps us to understand the causes of drought. In this paper, we evaluated the temporal and spatial characteristics of soil drought and meteorological drought (SMD) and explored their responses to climatic factors and latent heat fluxes (LHF), and then explained their variation from the perspective of atmospheric circulation. The following results were obtained. (1) Meteorological drought has gradually increased in the Liaohe River Basin, on the North China Plain, and on the Loess Plateau while average soil moisture has been maintained at only approximately 25%. The impacts of drought are very serious in these regions. (2) LHF response to short time-scale (3-month scale) drought performance is high in the dry season, and the regions with high correlation coefficients are spatially distributed and concentrated in the monsoon climate zone. The regions with high correlation coefficients between drought and LHFs on long time scales (12-month scale) are concentrated in the coastal basin of southeast China. (3) Short- and long-term SMDs showed highly responsive and significant relationships with PDO, showing variations in the southeast coastal basin, the Pearl River basin, the northwest inland basin and the eastern part of the Heilongjiang basin, with a maximum correlation coefficient of 0.21 (p < 0.01). The short-term SMD in the northwestern inland region was significantly negatively correlated with AMO (correlation coefficient of −0.19, p < 0.01). the Nino3.4 index is significantly positively correlated with the SMD in the southeast coastal region of China, with a maximum correlation coefficient of 0.23 (p < 0.01). The decrease in convective precipitation led to a stronger association between soil and meteorological drought and climatic factors. This study helps to reveal the changing patterns of SMDs and can also be used globally to identify the local development patterns of drought under climate change. [ABSTRACT FROM AUTHOR]

Terrestrial ecosystems are fully coupled with the climate. The planet has been greening owing to the increased vegetation growth in response to the changing atmosphere, which in turn has feedback on the climate. Greening has slowed down the rise in global land-surface air temperature mainly through a coincident increase of evapotranspiration and precipitation in wet regions. In dry regions, greening intensifies the decrease in soil moisture induced by greening-enhanced transpiration. Uncertain, however, is how the climate effects of greening in semi-arid lands might differ for variable wet and dry conditions. Here, we focus on the biosphere–atmosphere interactions in Australia by modeling the perturbation of vegetation changes under various states of sea surface temperature (SST), including the climatology mean, El Niño, and La Niña conditions. For the dry conditions of El Niño, greening exacerbates water stress and largely depletes the soil moisture, while for the wet conditions of La Niña, greening-enhanced evapotranspiration and precipitation resupply the soil moisture. For the normal conditions using the climatology mean SST, a small decrease in soil moisture occurs but with large spatial contrast because of heterogeneous changes of evapotranspiration and precipitation induced by greening. We emphasize that the alternating dry and wet conditions modulated by the large-scale climate variability are vital to understanding the response of climate to greening. Furthermore, vegetation-based warming mitigation policies need to be cautious when inferring distinct climate effects associated with greening. [ABSTRACT FROM AUTHOR]

The middle reaches of the Yellow River (MRYR) are an important base for agricultural and husbandry production and coal and coal-based power and chemical industries. Understanding the variability of autumn (especially September) precipitation over the MRYR region and the associated atmospheric circulation anomalies and precursory signals is of great importance for the prevention and mitigation of meteorological disasters during autumn rainy season. This study primarily explored precursory signals for September precipitation over the MRYR from the perspectives of sea surface temperature (SST) and soil moisture (SM) anomalies. The results reveal that the northward-shifted East Asian westerly jet (EAWJ) and the strengthened and westward-extended western Pacific subtropical high (WPSH) are responsible for more precipitation over the MRYR region. Further analyses show that the September MRYR precipitation is significantly related to the preceding July–August southern Pacific SST pattern (SPSP) and Yunnan-Guizhou Plateau (YGP) SM. The preceding SPSP anomaly, which reflects the La Niña/El Niño-like SST anomalies, can be maintained until September and plays an important role in modulating the September MRYR precipitation. Moreover, the above SST anomalies may adjust the SM anomalies in the YGP during July–August. The SM anomalies in The YGP persist from July–August to September and eventually affect the MRYR precipitation through exciting an anomalous vertical motion during September. The effect of the preceding SPSP anomaly on the September MRYR precipitation decreases when the SM effect is absent, which suggests that the YGP SM anomalies act as a bridge linking the preceding Pacific SST anomalies and the ensuing September MRYR precipitation. This study discloses the joint contribution of the preceding Pacific SST and YGP SM anomalies to the September MRYR precipitation and may shed new light on the short-term prediction of autumn precipitation over the MRYR. [ABSTRACT FROM AUTHOR]

In the springs of 2021 and 2023, northern East Asia experienced intense dust activities resulting in significant environmental and ecological impacts. This study explores the climate factors responsible for these strong dust activities in both springs. Results show that the anomalous surface warming during the late winter and early spring and the anomalous dry surface soil moisture in the spring are the underlying surface factors contributing to the occurrence of intense dust activities in these two years. Furthermore, we find that the anomalous wave patterns over the Eurasian continent are dynamic factors driving the initiation of dust activities in these two springs. The intensified north-south pressure gradient in Gobi Desert (GD) associated with the wave patterns significantly increases near-surface wind speeds, favoring the occurrence of dust activities. The sea surface temperature (SST) anomalies in the North Atlantic and tropical eastern Pacific can significantly influence the wind speed anomalies in northern East Asia, thereby impacting dust activities. In 2021, the increase in wind speed in the GD was influenced by both the warm SST anomalies in the North Atlantic and the cold SST anomalies in the tropical eastern Pacific. Nevertheless, in 2023, the predominant influence came from the exceptionally strong warming in the North Atlantic, offsetting the opposing effect of warm SST anomalies in the tropical eastern Pacific. Sensitivity experiments confirm the important role of the SST anomalies in the North Atlantic and tropical eastern Pacific on the dust activities in GD. These findings play an insight into understanding of the mechanisms behind the recent variations in dust activities in northern East Asia. • Surface warming and drier soils are the surface factors responsible for the intense dust activities in 2021 and 2023. • SST anomalies in the North Atlantic and tropical Pacific can influence the wind speed in Gobi Desert (GD), thereby impacting dust activities. • Sensitivity experiments confirm the important role of the SST anomalies in the key regions on the dust activities in GD. [ABSTRACT FROM AUTHOR]

This study investigated the relationship between extreme precipitation and near‐surface temperature (precipitation–temperature relation) from two different perspectives, the rate of change of precipitation with temperature and dynamic (i.e., effect of the change in atmospheric motion) and thermodynamic (i.e., effect of the change in atmospheric moisture content) aspects, using a 5‐km dynamical downscaled hundreds‐year data set for past climate condition (PAST; from 1951 to 2010) and future climate condition (FUTURE; 4°C warmer than the preindustrial condition). Initially, using the observation and the PAST and FUTURE data sets, it was found that the 99th and 99.9th percentile hourly precipitation for each temperature bin (P99 and P99.9, respectively) paralleled the slope of the Clausius–Clapeyron (C–C) relation for a certain temperature range over the Tokachi River basin in Hokkaido, the northern island of Japan; however, both P99 and P99.9 decreased in the high‐temperature range. Next, we examined the cause of the P99 and P99.9 differences between PAST and FUTURE for each temperature bin by classifying dynamic and thermodynamic factors. The result showed that the thermodynamic effect dominates the differences in P99 and P99.9 between PAST and FUTURE, which means that the thermodynamic effect is the main component of the precipitation–temperature relation. Similar analyses were applied to the whole river basin, including the mountainous area. The results showed that the differences in P99 and P99.9 between PAST and FUTURE are mainly due to the thermodynamic contribution, regardless of plain or mountain area. Using such large model data sets, we could make a robust assessment of the precipitation–temperature relation and the dynamic and thermodynamic contributions to precipitation changes. Moreover, using the 5‐km resolution hundreds‐year data set enabled us to quantify the spatial distribution of such precipitation characteristics over a thousands of square kilometer catchment. [ABSTRACT FROM AUTHOR]

Heat waves such as the one in Europe 2003 have severe consequences for the economy, society, and ecosystems. It is unclear whether temperatures could have exceeded these anomalies even without further climate change. Developing storylines and quantifying the highest possible temperature levels is challenging given the lack of a long homogeneous time series and methodological framework to assess them. Here, we address this challenge by analyzing summer temperatures in a nearly 5000-yr preindustrial climate model simulation, performed with the Community Earth System Model CESM1. To assess how anomalous temperatures could get, we compare storylines generated by three different methods: 1) a return-level estimate, deduced from a generalized extreme value distribution; 2) a regression model, based on dynamic and thermodynamic heat wave drivers; and 3) a novel ensemble boosting method, generating large samples of reinitialized extreme heat waves in the long climate simulation. All methods provide consistent temperature estimates, suggesting that historical exceptional heat waves such as those in Chicago in 1995, Europe in 2003, and Russia in 2010 could have been substantially exceeded even in the absence of further global warming. These estimated unseen heat waves are caused by the same drivers as moderate observed events, but with more anomalous patterns. Moreover, altered contributions of circulation and soil moisture to temperature anomalies include amplified feedbacks in the surface energy budget. The methodological framework of combining different storyline approaches of heat waves with magnitudes beyond the observational record may ultimately contribute to adaptation and to the stress testing of ecosystems or socioeconomic systems to increase resilience to extreme climate stressors. [ABSTRACT FROM AUTHOR]

El Niño is a dominant source of interannual climate variability around the world. Based on the observed and reanalyzed datasets for the period of 1958–2019, this study explores the influence of El Niño on the spring precipitation over the Indochina Peninsula (ICP). The results show that El Niño has a significant negative correlation with the following spring precipitation over the ICP. However, this climatic teleconnection of El Niño was unstable, with an obvious interdecadal strengthening since the early 1990s. During the decaying spring, the El Niño–related sea surface temperature (SST) anomalies would induce an abnormal downward motion along with an anomalous low-level anticyclone over the western North Pacific. Before the early 1990s, such El Niño–induced atmospheric circulation anomalies were located to the east of the ICP, exerting little influence on the spring ICP precipitation. In contrast, since the early 1990s, the abnormal downward motion and anomalous low-level anticyclone extended westward covering the whole ICP, hampering local spring precipitation. This interdecadal change is owing to a relatively stronger intensity and longer duration of the El Niño–related warm SST anomalies over the tropical central Pacific in the epoch after the early 1990s (1992–2019) than in the previous decades (1958–91). Our findings highlight a strengthening effect of El Niño on the following spring climate over the ICP since the early 1990s, which has great implications for the regional climate prediction. Significance Statement: Spring precipitation variation has profound economic and societal influences for the Indochina Peninsula (ICP) that consists of several agriculture-based countries. Moreover, the local soil moisture anomaly induced by the ICP spring precipitation is an important seasonal predictor for the East Asian summer monsoon and extreme climate. Here this study identifies a strengthening influence of El Niño on the following spring precipitation over the ICP, due to a relatively stronger intensity and longer duration of the El Niño–related warm SST anomalies over the tropical central Pacific since the early 1990s. This may provide an important implication for the seasonal predictions of summer monsoon and extreme climate over East Asia and thus potentially enhance the predictability of regional climate. [ABSTRACT FROM AUTHOR]

• The vegetation drought was alleviating during 1982–2020 across China. • Spring drought and summer drought were more likely to occur in South China. • NPI-NAO-AO had the most significant impact on vegetation drought. • Soil moisture was the primary driving factor of vegetation drought. Vegetation drought can reduce the net ecosystem productivity, resulting in disturbances to the sustainable development of humans and nature. In this study, Vegetation Health Index (VHI) was used to identify vegetation drought. The spatial and temporal evolution of vegetation drought was characterized using the Pixel-based Trend Identification Method (PTIM) in China from 1982 to 2020. Moreover, multiple cross-wavelet techniques were used for revealing the correlations between atmospheric circulation factors and their combinations with vegetation drought. The results indicates that: (1) there is a decreasing trend of vegetation drought in China during the study period, while it will show an increasing trend in the future; (2) the South China is prone to spring drought and summer drought, and the Sichuan Basin is prone to autumn drought and winter drought; (3) in spring, the most obvious trend of vegetation drought mitigation appears; (4) a combination of atmospheric circulation factors, North Pacific Index (NPI)-North Atlantic Oscillation (NAO)-Arctic Oscillation (AO), has the most significant impact on vegetation drought across China. [ABSTRACT FROM AUTHOR]

Rapid drought intensification, or flash droughts, is often driven by anomalous atmospheric ridging and can cause severe and complex impacts on water availability and agriculture, but the full range of variability of such events in terms of intensity and frequency is unknown. New tree‐ring reconstructions of May–July mid‐tropospheric ridging and soil moisture anomalies back to 1500 CE in the central United States—a hotspot for flash drought—suggest that over the last five centuries, anomalies in these two variables combined to indicate flash‐drought conditions in ∼17% of years and exceptionally severe flash drought in ∼4% of years, similar to frequencies in recent decades. However, over one‐third of all inferred exceptional flash droughts occurred since 1900, suggesting the 20th century was highly flash‐drought prone. These results may guide future work to diagnose the roles of external, oceanic, and land‐surface forcing of warm‐season atmospheric circulation and hydroclimate over North America. Plain Language Summary: In 2012, the central United States experienced a "flash drought," when rapid soil drying due to persistently low precipitation totals and high temperatures in late spring and summer caused billions of dollars in damages and agricultural losses. Such events are difficult to forecast because flash droughts can develop in a matter of weeks, and only a handful of flash droughts have been observed in recent decades, giving high uncertainty as to their likelihood. Here, we use tree rings to create two independent annual records of central United States soil moisture and the principal atmospheric circulation pattern known to cause flash droughts that extend back to the year 1500. Taken together, these records provide a new five‐century perspective on these crucial components of flash drought and reveal for the first time the long‐term behavior of central United States flash droughts, including frequency and cyclicity of exceptional events. We find that the instrumental record is a good representation of the long‐term likelihood of flash droughts. This apparent agreement with long‐term average conditions is largely by chance; however, as the reconstructions indicate large century‐to‐century variations in flash‐drought frequency and magnitude over the past 500 years. Key Points: We developed tree‐ring reconstructions of central U.S. soil moisture and the atmospheric ridge that causes flash droughts back to 1500 CEObservations capture flash drought's long‐term probability distribution, but reconstructions reveal centennial fluctuations in variabilityCycles in reconstructed atmospheric ridging may aid diagnosis and prediction of flash drought on interannual to decadal time scales [ABSTRACT FROM AUTHOR]

Land-atmosphere interactions are often interpreted as local effects, whereby the soil state drives local atmospheric conditions and feedbacks originate. However, nonlocal mechanisms can significantly modulate land-atmosphere exchanges and coupling. We make use of GCMs at different resolutions (low ~1° and high ~0.25°) to separate the two contributions to coupling: better represented local processes versus the influence of improved large-scale circulation. We use a two-legged metric, complemented by a process-based assessment of four CMIP6 GCMs. Our results show that weakening, strengthening, and relocation of coupling hot spots occur at high resolution globally. The northward expansion of the Sahel hot spot, driven by nonlocal mechanisms, is the most notable change. The African easterly jet's horizontal wind shear is enhanced in JJA due to better resolved orography at high resolution. This effect, combined with enhanced easterly moisture flux, favors the development of African easterly waves over the Sahel. More precipitation and soil moisture recharge produce strengthening of the coupling, where evapotranspiration remains controlled by soil moisture, and weakening where evapotranspiration depends on atmospheric demand. In SON, the atmospheric influence is weaker, but soil memory helps to maintain the coupling between soil moisture and evapotranspiration and the relocation of the hot spot at high resolution. The multimodel agreement provides robust evidence that atmospheric dynamics determines the onset of land-atmosphere interactions, while the soil state modulates their duration. Comparison of precipitation, soil moisture, and evapotranspiration against satellite data reveals that the enhanced moistening at high resolution significantly reduces model biases, supporting the realism of the hot-spot relocation. [ABSTRACT FROM AUTHOR]

Understanding the Tibetan Plateau (TP) thermal processes is of utmost significance in changing climate. This study investigates the effect of soil moisture in changing the TP thermal profile and consequently summer precipitation in South Asia (SA). Soil moisture from Special Sensor Microwave Imager (SSM/I) developed from the F-08, F-11, and F-13 fundamental climate data record and atmospheric reanalysis from ERA-Interim, MERRA-2, and NCEP/CFSR during 1988–2008 are used. A generalized linear method that assesses the reciprocal forcing between two connected fields, named the coupled manifold technique (CMT), is applied to TP soil moisture and SA summer precipitation. It is revealed that interannual variations of SA precipitation are significantly (confidence level = 99%) impacted by TP soil moisture and the explained ratio of variance in SA is 0.3–0.4. Composite analysis indicates that SA summer precipitation has positive anomalies in response to dry TP soil moisture in the previous spring and vice versa. For understanding the possible mechanism, thermal processes, relative humidity, wind components, and moisture flux anomalies were calculated for dry and wet TP soil moisture and summer precipitation. The results suggested that TP soil moisture is likely to regulate near-surface energy balance and diabatic heating profile over TP. As a result, the surrounding lower-level westerlies (easterlies) (at 850 hPa) converge (diverge), associated with divergence (convergence) at the upper troposphere (200 hPa). The westerlies (easterlies) are usually moisture-rich (moisture-deficient) and thus cause more (less) precipitation in western (eastern) SA. It is thus suggested that the spring soil moisture may affect the thermal profile of TP, affecting the summer precipitation in SA as a consequence. [ABSTRACT FROM AUTHOR]

The seasonal prediction of precipitation extremes over the Yangtze River basin (YRB) has always been a great challenge. This study investigated the effects of spring soil moisture over the Indo-China Peninsula (ICP) on the following summer mei-yu front and YRB precipitation extremes during 1961-2010. The results indicated that the frequency of summer YRB precipitation extremes was closely associated with the mei-yu front intensity, which exhibited a strong negative correlation with the preceding spring ICP soil moisture. However, the lingering climate influence of the ICP soil moisture was unstable, with an obvious weakening since the early 1990s. Due to its strong memory, an abnormally lower spring soil moisture over the ICP would increase local temperature until the summer by inducing less evapotranspiration. Before the early 1990s, the geopotential height elevation associated with the ICP heating affected the western Pacific subtropical high (WPSH), strengthening the southwesterly summer monsoon. Consequently, the mei-yu front was intensified as more warm, wet air was transported to the YRB, and local precipitation extremes also occurred more frequently associated with abnormal ascending motion mainly maintained by the warm temperature advection. In the early 1990s, the Asian summer monsoon underwent an abrupt shift, with the changing climatological states of the large-scale circulations. Therefore, the similar ICP heating induced by the anomalous soil moisture had different effects on the monsoonal circulation, resulting in weakened responses of the mei-yu front and YRB precipitation extremes since the early 1990s. [ABSTRACT FROM AUTHOR]

We analyze the observed relationship between sea surface temperatures (SSTs) over the Atlantic Ocean and the normalized difference vegetation index (NDVI) in the Orinoco and Amazon basins. Monthly correlations between anomalies of NDVI and SSTs are computed for different regions of the Atlantic Ocean.We also use a mixture of observations and reanalysis products to analyze lagged correlations. Our results show that during August-September (i.e., the dry-to-wet transition season), changes in NDVI in the central Amazon and the so-called Arc of Deforestation are associated with precedent changes in the SSTs of the tropical North Atlantic (TNA) and the Caribbean (CABN) during March-June. Anomalous warming of the CABN and TNA generates changes in surface winds and atmospheric moisture transport in the region, decreasing precipitation, with consequent decreases of soil moisture, moisture recycling, and NDVI. An increase in TNA and CABN SSTs during March-June is also associated with an increase of NDVI over the northern Orinoco during June (i.e., the wet season). Unlike in the southern Amazon, precipitation and soil moisture in the Orinoco basin do not exhibit significant changes associated with SSTs. By contrast, atmospheric moisture recycling and transport increase with warmer SSTs in the TNA. Therefore, for the Orinoco, the link between SSTs and NDVI appears to be related not to changes in precipitation but to changes in moisture recycling. However, the causality between these changes needs to be further explored. These findings highlight the contrasting responses of the Amazon and Orinoco basins to Atlantic temperatures and the dominant role of atmospheric moisture transport linking these responses. [ABSTRACT FROM AUTHOR]

Climate change presents risks both in terms of warming and increased variability that are heightened when compounded. It is thus notable that the simulations in the Coupled Model Intercomparison Project Phase 5 (CMIP5) showing greater Northern midlatitude continental warming also show a greater increase in monthly average temperature variance, particularly in Europe. European variability increases with warming at a rate of 0.40°C2/°C (95% C.I. [0.28, 0.50]), with local warming rates explaining 71% of the intermodel difference in variability changes. Coupling between warming and variance increases the probability of high temperatures compared to a scenario where variance is stable. If warming were to reach 6°C, the risk of monthly average temperature exceeding a 30°C threshold is 4 times greater in the increased‐variance scenario. Despite the simple scaling across models suggesting some common origin, changes in model temperature and variance potentially involve a range of mechanisms whose contributions remain unclear. Plain Language Summary: Extreme and persistent summertime temperatures present risks to human health, property, and natural systems. The frequency of hot extremes increases in response to both growing mean temperature and increasing variability. Here we show that models that predict higher summertime warming also predict greater increases in temperature variability in midlatitude continents, especially in Europe. The combination of warming and increased variability makes the chances of hot extremes much greater than if temperature variance stayed constant. The reasons for increased variability may involve changes in soil moisture, variability in local radiation, and changes in atmospheric circulation, but exactly why a simple scaling between warming and increased variance emerges in Europe and elsewhere is unclear. Key Points: Summertime temperature variability and warming in NH midlatitudes covary across CMIP5 models, especially in EuropeSuch covariance between warming and increased variability accentuates risks of future high‐temperature eventsAlbeit a weaker relationship than with warming, increases in variability also scale with biases in simulated historical variability [ABSTRACT FROM AUTHOR]

Rapid-onset droughts, known as flash droughts, can have devastating impacts on agriculture, water resources, and ecosystems. The ability to predict flash droughts in advance would greatly enhance our preparation for them and potentially mitigate their impacts. Here, we investigate the prediction skill of the extreme 2012 flash drought over the U.S. Great Plains at subseasonal lead times (3 weeks or more in advance) in global forecast systems participating in the Subseasonal Experiment (SubX). An additional comprehensive set of subseasonal hindcasts with NASA’s GEOS model, a SubX model with relatively high prediction skill, was performed to investigate the separate contributions of atmospheric and land initial conditions to flash drought prediction skill. The results show that the prediction skill of the SubX models is quite variable.While skillful predictions are restricted to within the first two forecast weeks in most models, skill is considerably better (3–4 weeks or more) for certain models and initialization dates. The enhanced prediction skill is found to originate from two robust sources: 1) accurate soil moisture initialization once dry soil conditions are established, and 2) the satisfactory representation of quasi-stationary cross-Pacific Rossby wave trains that lead to the rapid intensification of flash droughts. Evidence is provided that the importance of soil moisture initialization applies more generally to central U.S. summer flash droughts. Our results corroborate earlier findings that accurate soil moisture initialization is important for skillful subseasonal forecasts and highlight the need for additional research on the sources and predictability of drought-inducing quasi-stationary atmospheric circulation anomalies. [ABSTRACT FROM AUTHOR]

The mechanisms causing European heatwaves in recent years, particularly their differences among several heatwaves, are poorly understood. Here atmospheric circulation anomalies and soil moisture‐temperature coupling during the summer 2018 heatwave are comprehensively examined and compared with the 2003 and 2010 heatwaves using ERA5 reanalysis, model simulations, and eddy covariance flux measurements. We show that the 2018 heatwave successively affected northern and central Europe, and the peak temperature in Finland and northwest Russia broke historical records of the past 40 years. Although three heatwaves were all initially triggered by atmospheric circulation anomalies, the strong moisture‐temperature coupling were found to further strengthen the 2003 and 2010 heatwaves. This coupling was also strong in central Europe during 2018 heatwave, but was weak in the northern European center of the heatwaves. The high temperature in 2018 was mainly due to increases in the amount of net surface radiation caused by the clear skies associated with reduced precipitation. Furthermore, we also find that land cover plays a critical role in determining the occurrence and strength of soil moisture‐temperature coupling. Cropland/grassland depletes soil moisture more readily than forests, thereby triggering a more rapid release of sensible fluxes as observed during 2018 heatwave. Plain Language Summary: In summer 2018, Europe was attacked by strong heatwave. This heatwave successively affected northern and central Europe, and temperature in Finland and northwest Russia broke historical records. Similar with heatwaves happened in 2003 and 2010, the 2018 heatwave was triggered by atmospheric circulation anomalies. However, different from the strong soil moisture‐temperature coupling during 2003 and 2010 heatwaves, this coupling was relative weak in northern Europe during 2018 heatwave because of the relatively small soil moisture deficits. Key Points: The 2018 heatwave successively affected northern and central Europe, and temperature in Finland and northwest Russia broke historical recordThe moisture‐temperature coupling was strong in central Europe, which was relatively weak in northern Europe during 2018 heatwaveLand cover plays a critical role in determining the occurrence and strength of soil moisture‐temperature coupling [ABSTRACT FROM AUTHOR]

The multidecadal hydroclimate variations of the Seine basin since the 1850s are investigated. Given the scarcity of long-term hydrological observations, a hydrometeorological reconstruction is developed based on hydrological modeling and a method that combines the results of a downscaled long-term atmospheric reanalysis and local observations of precipitation and temperature. This method improves previous attempts and provides a realistic representation of daily and monthly river flows. This new hydrometeorological reconstruction, available over more than 150 years while maintaining fine spatial and temporal resolutions, provides a tool to improve our understanding of the multidecadal hydrological variability in the Seine basin, as well as its influence on high and low flows. This long-term reconstruction allows analysis of the strong multidecadal variations of the Seine river flows. The main hydrological mechanisms at the origin of these variations are highlighted. Spring precipitation plays a central role by directly influencing not only the multidecadal variability in spring flows but also soil moisture and groundwater recharge, which then regulate summer river flows. These multidecadal hydroclimate variations in the Seine basin are driven by anomalies in large-scale atmospheric circulation, which themselves appear to be influenced by sea surface temperature anomalies over the North Atlantic and the North Pacific. The multidecadal hydroclimate variations also influence high and low flows over the last 150 years. The analysis of two particularly severe historical droughts, the 1921 and the 1949 events, illustrates how long-term hydroclimate variations may impact short-term drought events, particularly through groundwater–river exchanges. The multidecadal hydroclimate variations described in this study, probably of internal origin, could play an important role in the evolution of water resources in the Seine basin in the coming decades. It is therefore essential to take the associated uncertainties into account in future projections. [ABSTRACT FROM AUTHOR]

The process of soil drying following a single rainfall input offers an integrated perspective on soil-vegetation water dynamics in responses to atmospheric conditions during periods without rainfall. In this work, the soil moisture dry-down time scale events (τ) was calculated using surface soil moisture data from the SMOS mission, with the objective to explore if the spatio-temporal variability of τ could be used as a proxy for regional flooding and waterlogging characterization. Our working hypothesis is that soil moisture dries up more slowly under flooded conditions as a result of slower surface water elimination by infiltration and capillary rise of water from the saturated zone close to the surface. A clear difference precipitation-moisture coupling was detected between two regions with different flooding dynamics. In a region where flooding is triggered by precipitation excesses on weekly-to-monthly time scales and where the coupling between precipitation and evapotranspiration is strong, a positive correlation between dry-down and 6-month accumulated precipitation anomaly was found for all seasons except winter. By contrast, in the other region where flooding is largely de-coupled from precipitation and evapotranspiration, but rather coupled to ground water table dynamics on time scales from several months to years, no significant correlations were found. These results are based on a short period of data: March 2010 – November 2014. [ABSTRACT FROM AUTHOR]

The ERA-Interim (1979–2016) and National Center for Environmental Prediction (NCEP) (1958–2016) reanalysis data are applied in an empirical orthogonal function (EOF) decomposition of the summer zonal wind anomaly in the upper troposphere (at 200 hPa) over Asia. Consistent spatio-temporal modes are derived from the two datasets on different time scales, with the first three dominant modes representing the principal features of the summer Asian subtropical westerly jet (ASWJ). The first and third modes respectively describe the meridional position deviation and the overall strength variations of ASWJ, and both exhibit zonally symmetric distributions, which have been extensively investigated. The second mode describes a zonally asymmetric distribution of the jet over Asia, which reflects coordinated variations in the upstream and downstream systems and has not yet been systematically studied. In this study, we focus on the analysis of the second mode of ASWJ. The observation results show that the second mode is closely related to external forcing factors as well as the large-scale circulation at mid-high latitudes. Through the interaction of the preceding North Atlantic sea surface temperature (SST), the simultaneous soil moisture, and the atmospheric circulation that propagates zonally via the waveguide effect of the jet, the ocean–land–atmosphere coupled system can produce coordinating impacts on the variability of ASWJ. These results aid in a more complete and comprehensive understanding of the physical characteristics of the summer westerly jet and provide a new perspective for the study of the variability of ASWJ. [ABSTRACT FROM AUTHOR]

Dry land surface conditions have been shown to amplify extreme heat events in Europe, but the extent to which this influence involves modification of the overlying atmospheric circulation has yet to be fully established. Here, this issue is addressed using two Community Earth System Model ensembles, with the same heatwave‐inducing atmospheric circulation pattern imposed over different land surface states. These two ensembles differ in the vertical level above which the circulation is constrained (surface vs. upper troposphere). Soil moisture anomalies are found to play an important role in dictating heatwave intensity among ensemble members. The heatwave is approximately 0.1°C hotter per standard deviation soil moisture reduction when the troposphere is free to respond to surface conditions than when it is constrained, implying that a portion of the land surface influence involves feedbacks through the atmospheric circulation. The additional atmospheric response also allows for nonlocal heatwave amplification in subsequent months. Plain Language Summary: Seasonally persistent European heatwaves are socioeconomically costly. Though established by atmospheric circulation patterns, heatwaves can be intensified by dry land surface conditions, increasing societal risks. Here, we show that the manner in which dry land surface conditions affect European heatwave intensity is twofold: through local heating and through changes to the structure of the atmosphere. The additional influence of the land surface on the atmosphere can make the same European heatwave up to 0.4°C hotter overall and can cause surface drying in other regions. This nonlocal mechanism leads to hotter conditions in those regions in subsequent months. Key Points: By constraining atmospheric circulation, a seasonally persistent European heatwave is imposed over different initial land surface statesThe prescribed heatwave is amplified by local dryness directly and indirectly through the modification of the near‐surface atmosphereThe atmospheric response to surface dryness allows for an additional 0.1°C of warming per standard deviation of soil moisture drying [ABSTRACT FROM AUTHOR]

This article presents the "screen-level and surface analysis only" (SSA) system at the European Centre for Medium-Range Weather Forecasts (ECMWF). SSA is a simplification of the operational land–atmosphere weakly coupled data assimilation (WCDA). The goal of SSA is to provide 1) efficient research into land surface developments in NWP and 2) land reanalyses with land–atmosphere coupling. SSA maintains a coupled forecast model between assimilation cycles, but the atmospheric analysis is not performed; rather, it is forced from an archived analysis. Hence, SSA is much faster than WCDA, although it lacks feedback between the land and atmospheric analyses. A global sensitivity analysis was performed over one year to compare the WCDA and SSA systems. Prescribed proxy 2-m temperature/humidity screen-level observation errors were approximately doubled in the soil moisture data assimilation, thereby reducing the average size of the root-zone soil moisture analysis increments by about 60%. The systematic impact of these changes on the WCDA surface and near-surface atmospheric dynamics was effectively captured by SSA, although the short-term impact was underestimated. Importantly, the SSA forecast verification scores accurately reflected those of WCDA: atmospheric 1–10-day temperature/humidity forecasts were degraded in the tropics and lower midlatitudes up to about 700 hPa. The soil moisture analysis performance was not significantly impacted. These results endorse SSA as an NWP research tool and confirm the role of assimilating proxy screen-level observations in the soil moisture analysis to improve weather forecasts. Appropriate use and limitations of SSA are considered. [ABSTRACT FROM AUTHOR]

Because of the interactive margin between the East Asian summer monsoon and westerly circulation, summer rainfall in northern China (NC) exhibits high variability. By employing reanalysis data and geostationary satellite data from the Fengyun-2G (FY-2G) satellite and using the linear baroclinic model (LBM) and Hybrid Single-Particle Lagrangian Integrated Trajectory model, this study suggests a tripole pattern in summer rainfall over NC and the Indian subcontinent (IS) that is related to the Indian summer monsoon. The distributions of atmospheric circulation indicate three teleconnections: one is from the IS via the Indo-China Peninsula (ICP) and NC, enhancing the Pacific–Japan (PJ) pattern; another is from the IS via west-central Asia and NC, arousing a Eurasian wave pattern; and the third is an IS–TP–NC pattern via the Tibetan Plateau (TP). Those teleconnections modulate vorticity and atmospheric stability over NC. In addition, along with the circulation distribution related to those teleconnections, two pathways of moisture transport related to the IS rainfall are suggested, except for moisture transport via the Bay of Bengal: one is from the Indo-Pacific to NC due to enhancing cyclones over the Indo-Pacific and a PJ-like pattern; and another is from the IS to NC via the TP within the midtroposphere, which modulates midtroposphere moisture fluxes and atmospheric stability over NC. Both teleconnections and moisture transport result in anomalous rainfall over NC. This study reveals a new mechanism and pathway of the Indian summer monsoon impacting NC rainfall, possibly explaining the reason behind the high variability in NC rainfall. [ABSTRACT FROM AUTHOR]

The differences in planetary boundary layer characteristics, in particular atmospheric boundary layer height (ABLH), humidity, and local circulations in pre-monsoon and monsoon period over the Erhai Lake, were simulated by the lake-atmosphere coupled model WRF v3.7.1. No lake simulations were also conducted to investigate lake effects over complex topography. During pre-monsoon period, local circulation was fully developed under weak synoptic system. The ABLH ran up to 2300 m or so. During monsoon period, temperature difference between land and lake became smaller, resulting in weaker local circulations. The height of circulation reduced by 500 m, and ABLH ran up to 1100 m during the day. Enhanced soil moisture and low surface temperature due to monsoon rainfalls in July could be the main reason for the slightly lower ABLH over the Erhai Lake area. Specific humidity of the boundary layer increased 8.8 g kg−1 or so during monsoon period. The Erhai Lake enlarged thermal contrast between valley and mountain slope in the Dali Basin. The lake reduced air temperature by 2~3 °C during daytime and increased air temperature by nearly 2 °C in the evening. Due to its small roughness length and large thermal capacity, the Erhai Lake enlarged lake-land temperature difference and local wind speed. A cyclonic circulation was maintained by the combination of mountain breeze and land breeze in the south of the lake. The lake decreased air temperature, increased specific humidity, and reduced ABLH during daytime, whereas the opposite effect is presented at night. [ABSTRACT FROM AUTHOR]

In this study, the influence of the large-scale circulation patterns on temperature in Europe and Serbia is examined. Among the large-scale circulation patterns in the Northern Hemisphere, less attention has been paid to the impact of the East Atlantic/West Russia pattern (EA/WR). The relationship between the EA/WR pattern and geopotential height at 500 hPa is investigated using reanalysis data from National Centres for Environmental Prediction – National Centre for Atmospheric Research. Temperature anomalies have been explored in relation to strong positive and negative phases of the EA/WR pattern for all months. Analyzing the correlation between the EA/WR index (EA/WRI) and geopotential height at 500 hPa, a centre with negative correlation has been found throughout the year over Russia, north of the Caspian Sea. Positive (negative) temperature anomalies prevailed over Eastern (Western) Europe for the strong negative EA/WR phase (EA/WRI < −1). The temperature anomalies associated with the strong positive phase of the EA/WR pattern (EA/WRI > 1) reflect below-average temperatures over Eastern Europe. In addition, we explored the combined effects of positive and negative phases of the North Atlantic Oscillation (NAO) or East Atlantic (EA) pattern with the EA/WR pattern on temperature variations in Europe and Serbia. We find that the effect of the EA/WR pattern on temperature changes is dependent of the EA phase but not of the NAO phase over Serbia. When the EA/WRI is negative and EA pattern or NAO are in positive phase, the positive temperature anomalies prevailed over most of Europe, including Serbia. The highest values of temperature anomalies exist over Serbia for EA/WRI < −1 and EA index > 1. It is found that this case appears more frequently in the last 20 years, contributing to the warming in Europe and Serbia. [ABSTRACT FROM AUTHOR]

Time of emergence of anthropogenic climate change is a crucial metric in risk assessments surrounding future climate predictions. However, internal climate variability impairs the ability to make accurate statements about when climate change emerges from a background reference state. None of the existing efforts to explore uncertainties in time of emergence has explicitly explored the role of internal atmospheric circulation variability. Here a dynamical adjustment method based on constructed circulation analogs is used to provide new estimates of time of emergence of anthropogenic warming over North America and Europe from both a local and spatially aggregated perspective. After removing the effects of internal atmospheric circulation variability, the emergence of anthropogenic warming occurs on average two decades earlier in winter and one decade earlier in summer over North America and Europe. Dynamical adjustment increases the percentage of land area over which warming has emerged by about 30% and 15% in winter (10% and 5% in summer) over North America and Europe, respectively. Using a large ensemble of simulations with a climate model, evidence is provided that thermodynamic factors related to variations in snow cover, sea ice, and soil moisture are important drivers of the remaining uncertainty in time of emergence. Model biases in variability lead to an underestimation (13%-22% over North America and <5% over Europe) of the land fraction emerged by 2010 in summer, indicating that the forced warming signal emerges earlier in observations than suggested by models. The results herein illustrate opportunities for future detection and attribution studies to improve physical understanding by explicitly accounting for internal atmospheric circulation variability. [ABSTRACT FROM AUTHOR]

This paper examines the local and non-local climatic effects of wetter arid areas of Northwestern China (ANWC) in summer using a fully coupled climate model. In response to soil moisture increase, the local air temperature significantly decreases, and precipitation increases in summer. The wetter ANWC results in a warmer and rainless climate over the Central part of North China Plain (CNCP). While the influence of increasing soil moisture on the Tibetan Plateau (TP) precipitation is negligible due to the high altitude. A cooler climate over ANWC leads to an abnormal descending motion occurring. Meanwhile, an anomalous high pressure and an abnormal anticyclonic circulation occurs in 500 hPa over the west of ANWC. Correspondingly, a huge anomalous cyclone circulation occurs over the northeastern part of China and Mongolia, which can hinder the northward movement of the East Asian summer Monsoon (EASM) and reduce the water vapor transport into Central China Plain. As a result, the weakened EASM thus leads to rainless over CNCP. As soil moisture increases over ANWC is stronger, the magnitudes of the influence on the atmospheric field become larger. This study highlights the wetter ANWC effect on local climate and provides more information about the mechanism of wetter ANWC remote effect on non-local climate. • This paper examines the local and non-local climatic effects of wetter arid areas of Northwestern China (ANWC) in summer using a fully coupled climate model. • It finds that the wetter ANWC can result in a warmer and rainless climate over the Central part of North China Plain (CNCP) through a complex atmospheric circulation response. • A cooler climate leads to an abnormal descending motion occurring over ANWC. • A huge anomalous cyclone circulation over the northeastern part of China and Mongolia can block the East Asian summer Monsoon northward and decrease the water vapor transport into Central China Plain. • This study highlights the wetter ANWC effect on local climate and provides more information about the mechanism of wetter ANWC remote effect on non-local climate. [ABSTRACT FROM AUTHOR]

A series of stationary wave model (SWM) experiments are performed in which the boreal summer atmosphere is forced, over a number of locations in the continental United States, with an idealized diabatic heating anomaly that mimics the atmospheric heating associated with a dry land surface. For localized heating within a large portion of the continental interior, regardless of the specific location of this heating, the spatial pattern of the forced atmospheric circulation anomaly (in terms of 250-hPa eddy streamfunction) is largely the same: a high anomaly forms over west-central North America and a low anomaly forms to the east. In supplemental atmospheric general circulation model (AGCM) experiments, similar results are found; imposing soil moisture dryness in the AGCM in different locations within the U.S. interior tends to produce the aforementioned pattern, along with an associated near-surface warming and precipitation deficit in the center of the continent. The SWM-based and AGCM-based patterns generally agree with composites generated using reanalysis and precipitation gauge data. The AGCM experiments also suggest that dry anomalies imposed in the lower Mississippi River valley have remote surface impacts of particularly large spatial extent, and a region along the eastern half of the U.S.-Canadian border is particularly sensitive to dry anomalies in a number of remote areas. Overall, the SWM and AGCM experiments support the idea of a positive feedback loop operating over the continent: dry surface conditions in many interior locations lead to changes in atmospheric circulation that act to enhance further the overall dryness of the continental interior. [ABSTRACT FROM AUTHOR]

Atmospheric variability is driven not only by internal dynamics, but also by external forcing, such as soil states, SST, snow, sea-ice cover, and so on. To investigate the forecast uncertainties and effects of land surface processes on numerical weather prediction, we added modules to perturb soil moisture and soil temperature into NCEP's Global Ensemble Forecast System (GEFS), and compared the results of a set of experiments involving different configurations of land surface and atmospheric perturbation. It was found that uncertainties in different soil layers varied due to the multiple timescales of interactions between land surface and atmospheric processes. Perturbations of the soil moisture and soil temperature at the land surface changed sensible and latent heat flux obviously, as compared to the less or indirect land surface perturbation experiment from the day-to-day forecasts. Soil state perturbations led to greater variation in surface heat fluxes that transferred to the upper troposphere, thus reflecting interactions and the response to atmospheric external forcing. Various verification scores were calculated in this study. The results indicated that taking the uncertainties of land surface processes into account in GEFS could contribute a slight improvement in forecast skill in terms of resolution and reliability, a noticeable reduction in forecast error, as well as an increase in ensemble spread in an under-dispersive system. This paper provides a preliminary evaluation of the effects of land surface processes on predictability. Further research using more complex and suitable methods is needed to fully explore our understanding in this area. [ABSTRACT FROM AUTHOR]

In this study, the robustness of a previously developed classification system that categorizes convective thunderstorm events initiated during various synoptic and dynamic conditions is analyzed. This classification system was used to distinguish between organized and unorganized convection and then used to determine whether unorganized convection occurs preferentially over wet or dry soils. The focus is on 12 events that occurred in synoptically benign (SB) environments where the Great Plains low-level jet was not present (noLLJ), and whether these events were accurately classified as unorganized convection is evaluated. Although there is a small sample size, the results show that the classification system fails to differentiate between local unorganized convection and large-scale organized convection under SB–noLLJ conditions. The authors conclude that past studies that have used this classification to study how soil moisture influences unorganized convection should be revisited. Additional variables and/or alternative precipitation datasets should be employed to enhance the robustness of the classification system. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau plays a significant role in atmospheric circulation and the Asian monsoon system. Turbulent surface fluxes and the evolution of boundary-layer clouds to deep and moist convection provide a feedback system that modifies the plateau's surface energy balance on scales that are currently unresolved in mesoscale models. This work analyses the land surface's role and specifically the influence of soil moisture on the triggering of convection at a cross section of the Nam Co Lake basin, 150 km north of Lhasa using a cloud-resolving atmospheric model with a fully coupled surface. The modelled turbulent fluxes and development of convection compare reasonably well with the observed weather. The simulations span Bowen ratios of 0.5 to 2.5. It is found that convective development is the strongest at intermediate soil moisture. Dry cases with soils close to the permanent wilting point are moisture limited in convective development, while convection in wet soil moisture cases is limited by cloud cover reducing incoming solar radiation and sensible heat fluxes, which has a strong impact on the surface energy balance. This study also shows that local development of convection is an important mechanism for the upward transport of water vapour, which originates from the lake basin that can then be transported to dryer regions of the plateau. Both processes demonstrate the importance of soil moisture and surface-atmosphere interactions on the energy and hydrological cycles of the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Potential biases in tree-ring reconstructed Palmer drought severity index (PDSI) are evaluated using Thornthwaite (TH), Penman-Monteith (PM), and self-calibrating Penman-Monteith (SC) PDSI in three diverse regions of the United States and tree-ring chronologies from the North American drought atlas (NADA). Minimal differences are found between the three PDSI reconstructions and all compare favorably to independently reconstructed Thornthwaite-based PDSI from the NADA. Reconstructions are bridged with model-derived PDSI_TH and PDSI_PM, which both closely track modeled soil moisture (near surface and full column) during the twentieth century. Differences between modeled moisture-balance metrics only emerge in twenty-first-century projections. These differences confirm the tendency of PDSI_TH to overestimate drying when temperatures exceed the range of the normalization interval; the more physical accounting of PDSI_PM compares well with modeled soil moisture in the projection interval. Remaining regional differences in the secular behavior of projected soil moisture and PDSI_PM are interpreted in terms of underlying physical processes and temporal sampling. Results demonstrate the continued utility of PDSI as a metric of surface moisture balance while additionally providing two recommendations for future work: 1) PDSI_PM (or similar moisture-balance metrics) compare well to modeled soil moisture and are an appropriate means of representing soil-moisture balance in model simulations and 2) although PDSI_PM is more physically appropriate than PDSI_TH, the latter metric does not bias tree-ring reconstructions of past hydroclimate variability and, as such, reconstructions targeting PDSI_TH can be used with confidence in data-model comparisons. These recommendations and the collective results of this study thus provide a framework for comparing hydroclimate variability within paleoclimatic, observational, and modeled data. [ABSTRACT FROM AUTHOR]

Understanding how different physical processes can shape the probability distribution function (PDF) of surface temperature, in particular the tails of the distribution, is essential for the attribution and projection of future extreme temperature events. In this study, the contribution of soil moisture-atmosphere interactions to surface temperature PDFs is investigated. Soil moisture represents a key variable in the coupling of the land and atmosphere, since it controls the partitioning of available energy between sensible and latent heat flux at the surface. Consequently, soil moisture variability driven by the atmosphere may feed back onto the near-surface climate-in particular, temperature. In this study, two simulations of the current-generation Geophysical Fluid Dynamics Laboratory (GFDL) Earth System Model, with and without interactive soil moisture, are analyzed in order to assess how soil moisture dynamics impact the simulated climate. Comparison of these simulations shows that soil moisture dynamics enhance both temperature mean and variance over regional 'hotspots' of land-atmosphere coupling. Moreover, higher-order distribution moments, such as skewness and kurtosis, are also significantly impacted, suggesting an asymmetric impact on the positive and negative extremes of the temperature PDF. Such changes are interpreted in the context of altered distributions of the surface turbulent and radiative fluxes. That the moments of the temperature distribution may respond differentially to soil moisture dynamics underscores the importance of analyzing moments beyond the mean and variance to characterize fully the interplay of soil moisture and near-surface temperature. In addition, it is shown that soil moisture dynamics impacts daily temperature variability at different time scales over different regions in the model. [ABSTRACT FROM AUTHOR]

The present study investigates the characteristics and factors of the variability of summer precipitation over the Tibetan Plateau (TP). It is found that interannual variations of summer precipitation tend to be opposite over the eastern and western TP, manifesting an east-west TP precipitation dipole pattern (TPPD). The summer TPPD is significantly correlated with the spring sea surface temperature (SST) in the tropical Indian Ocean, and the latter can be considered an important precursor for the former. The impacts of spring SST in the tropical Indian Ocean on summer TPPD are through oceanic and land relay effects. For the oceanic relay effect, the positive SST anomaly (SSTA) in the tropical Indian Ocean persists from spring to summer and induces the upper-tropospheric positive geopotential height anomaly and anomalous anticyclone to the south of the TP by stimulating a Rossby wave response. Winter El Niño and Indian Ocean Dipole (IOD) contribute to the persistence of the SSTA in the Indian Ocean from spring to summer through modulating a spring asymmetric mode in the Indian Ocean. The SSTAs in the Indian Ocean affect the western North Pacific anticyclone at lower troposphere through modulating the Indo-Western Pacific Ocean capacitor (IPOC) mode. These circulation anomalies adjust the variability of the summer TPPD. For the land relay effect, the SSTA in the tropical Indian Ocean induces the soil moisture (SM) anomaly in the Turan Plain through modulating atmospheric circulation around the Arabian Peninsula and the Mediterranean Sea during spring. The SM anomaly in the Turan Plain is maintained from spring to summer and induces the upper-tropospheric negative geopotential height anomaly and anomalous cyclone north of the TP during summer by stimulating an anomalous vertical motion. Through the above oceanic and land processes, a north-south opposite pattern of geopotential height anomalies around the TP appears and eventually modulates water vapor transport into the TP and hence the formation of the TPPD during summer. Further analyses reveal that the impact of the winter El Niño and IOD and spring tropical Indian Ocean SST on the summer TPPD becomes weaker when one of the abovementioned relay effects is absent. • An east-west TP precipitation dipole pattern (TPPD) during summer was revealed. • Spring tropical Indian Ocean SST can affect summer TPPD through two relay effects. • Physical process linking spring SSTA in the Indian Ocean and summer TPPD was found. [ABSTRACT FROM AUTHOR]

In order to predict the response of alpine ecosystems to global warming and to provide the experimental data needed for Atmosphere Global Circulation Models (AGCMs) coupled with Land Surface Models (LSMs), a warming experiment using infrared heaters was conducted in the alpine meadow ecosystem of the Qinghai-Tibet Plateau. Five replicate blocks with three 2 × 2 m treatment plots in each block were randomly installed. The treatment plots were control plots (C) at ambient temperatures, moderately warmed plots (W1), and intensely warmed plots (W2), manipulated by using 130 W m-2 and 150 W m-2 infrared heaters, respectively. The results showed that when significant warming increased the daily mean soil surface temperature by 1-3 °C compared to temperatures in the control plots during the warm season (seasonal frozen soil thaw), the soil temperature gradients from depths of 0 to 100 cm significantly increased by 0.02-0.04 °C cm-1 during the day and 0.01-0.03 °C cm-1 at night. Volumetric soil liquid water content significantly decreased by 2.6-3.4% in shallow soil (5-15 cm) and significantly increased by 0.7-5.1% in deep soil (100 cm) compared to the control plots. Likewise, soil liquid water content gradients at depths between 10 and 20 cm significantly decreased by 0.2-0.3% cm-1 and significantly increased by 0.01-0.08% cm-1 at depths between 20 and 100 cm. Warming did not cause significant vapor water change in the atmosphere near the soil surface. Based on these results, it can be concluded that increasing soil temperatures accelerated the processes of ground heat flux, sensible heat, and latent heat, which caused significant change in soil water content and in its gradients. [ABSTRACT FROM AUTHOR]

The influence of the Pacific-North American (PNA) pattern, the northern annular mode (NAM), and the El Niño-Southern Oscillation (ENSO) on extreme temperature days and months over North America is examined. Associations between extreme temperature days and months are strongest with the PNA and NAM and weaker for ENSO. In general, the association with extremes tends to be stronger on monthly than daily time scales and for winter as compared to summer. Extreme temperatures are associated with the PNA and NAM in the vicinity of the centers of action of these circulation patterns; however, many extremes also occur on days when the amplitude and polarity of these patterns do not favor their occurrence. In winter, synoptic-scale, transient weather disturbances are important drivers of extreme temperature days; however, many of these smaller-scale events are concurrent with amplified PNA or NAM patterns. Associations are weaker in summer when other physical mechanisms affecting the surface energy balance, such as anomalous soil moisture content, also influence the occurrence of extreme temperatures. [ABSTRACT FROM AUTHOR]