Dry Events Research Articles

How Have Hydrological Extremes Changed over the Past 20 Years?

Abstract Severe floods and droughts, including their back-to-back occurrences (weather whiplash), have been increasing in frequency and severity around the world. Improved understanding of systematic changes in hydrological extremes is essential for preparation and adaptation. In this study, we identified and quantified extreme wet and dry events globally by applying a clustering algorithm to terrestrial water storage (TWS) data from the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (FO). The most intense events, ranked using an intensity metric, often reflect impacts of large-scale oceanic oscillations such as El Niño–Southern Oscillation and consequences of climate change. The severity of both wet and dry events, represented by standardized TWS anomalies, increased significantly in most cases, likely associated with intensification of wet and dry weather regimes in a warmer world, and consequently, exhibited strongest correlation with global temperature. In the Dry climate, the number of wet events decreased while the number of dry events increased significantly, suggesting a drying trend that may be attributed to climate variability and possible increases in irrigation and reliance on groundwater. In the Continental climate where temperature has risen faster than global average, dry events increased significantly. Characteristics of extreme events often showed strong correlations with global temperature, especially when averaged over all climates. These results suggest changes in hydrological extremes and underscore the importance of quantifying total water storage changes when studying hydrological extremes. Extending the GRACE/FO record, which spans 2002 to the present, is essential to continuously tracking changes in TWS and hydrological extremes.

Anthropogenic Influence on Decadal Changes in Concurrent Hot and Dry Events over China around the Mid-1990s

The frequency and duration of observed concurrent hot and dry events (HDEs) over China during the growing season (April–September) exhibit significant decadal changes across the mid-1990s. These changes are characterized by increases in HDE frequency and duration over most of China, with relatively large increases over southeastern China (SEC), northern China (NC), and northeastern China (NEC). The frequency of HDEs averaged over China in the present day (PD, 1994–2011) is double that in the early period (EP, 1964–81); the duration of HDEs increases by 60%. Climate experiments with the Met Office Unified Model (MetUM-GOML2) are used to estimate the contributions of anthropogenic forcing to HDE decadal changes over China. Anthropogenic forcing changes can explain 60%–70% of the observed decadal changes, suggesting an important anthropogenic influence on HDE changes over China across the mid-1990s. Single-forcing experiments indicate that the increase in greenhouse gas (GHG) concentrations dominates the simulated decadal changes, increasing the frequency and duration of HDEs throughout China. The change in anthropogenic aerosol (AA) emissions significantly decreases the frequency and duration of HDEs over SEC and NC, but the magnitude of the decrease is much smaller than the increase induced by GHGs. The changes in HDEs in response to anthropogenic forcing are mainly due to the response of climatological mean surface air temperatures. The contributions from changes in variability and changes in climatological mean soil moisture and evapotranspiration are relatively small. The physical processes associated with the response of HDEs to GHG and AA changes are also revealed.

Reconstruction of fluvio‐lacustrine landscapes and settlement history in the Texcoco region, Mexico, using a modern geomorphic analog

AbstractLocated in the Basin of Mexico, the eastern shore of former Lake Texcoco sustained a variety of human occupations throughout the Holocene, including preceramic hunter‐gatherers, incipient agriculturalists, and a variety of settlements in the ceramic periods. Nonetheless, the environmental dynamics of occupations on the lakeshore have not been fully addressed. The Archaic preagricultural Texcoco Man site (>5000 B.C.E.) and the Late Formative TX‐LF‐14 site (c. 550‐200 B.C.E.), among others, occupy this fluvio‐lacustrine transitional environment. Few stratigraphic works in and around the sites have been performed. Consequently, it is difficult to understand the dynamics of the sedimentary system in space and time. This work highlights and describes the fluvio‐lacustrine sedimentary dynamics and the resulting landscape that past societies inhabited on the eastern shore of Texcoco Lake. Because the study area has been altered by historic and modern draining, our work employs Lake Santiaguillo and its main tributary, the Tejamen River in the Durango state, as a modern analog to study their sedimentary dynamics. The analyses of surface geomorphology in the Texcoco study area were employed to corroborate the modern analog interpretation. To achieve these goals, we conducted a GIS‐based morphometric analysis and LANDSAT‐8 imagery to study the variations in landforms through wet and dry events. The results indicate an increase in the lake volume, low bifurcation in the active fluvial channels, few inundated surfaces, and the presence of bird‐foot deltaic channels during high precipitation events. In contrast, low precipitation events are characterized by reduced lake volume, increased fluvial channel bifurcation, and expanded floodplains. This heterogeneous landscape thus provided a rich source of diverse natural resources of saline and freshwater aquatic habitats. Simultaneously, constant or recurring flooding events generated a challenging landscape for prehistoric settlers who implemented diverse technologies, such as the construction of tlateles, on the levees of deltaic channels to reduce the risk and impact of flooding events.

The influence of tropical Atlantic sea-surface temperatures and the North Atlantic Subtropical High during the Maya Droughts

The frequency and duration of Late-Holocene hydrologic extremes in northern Guatemala were investigated using multiple sedimentological and geochemical proxies preserved in a sediment core collected from Lake Petén Itzá. A general trend of increasing aridity in the Maya Lowlands during the past 2000 years was punctuated by several multidecadal- to centennial-scale drought events recorded in the Petén Itzá sediments. In particular, the period spanning the Maya Terminal Classic Period and the Medieval Climate Anomaly (MCA), between 800 and 1300 CE, was marked by several extreme droughts and included the driest conditions of the past 2000 years between 950 and 1100 CE. Similarities between our data and other existing regional paleoclimate records suggest regional drying events during this time may have been driven by a common mechanism. Specifically, comparisons between these records and tropical Atlantic sea surface temperatures (SSTs) suggest that the dry intervals may have been driven by a westward expansion of the North Atlantic Subtropical High pressure system. This period was unique in the general agreement between regional proxy records, which are otherwise notably heterogeneous during the Late-Holocene. During the Little Ice Age (LIA; 1400–1800 CE) mean precipitation at Petén Itzá was further reduced, and multidecadal drying events were recorded between 1500–1530, 1600–1640, and 1770–1800 CE. However, regional hydroclimatic coherency was weaker during the LIA, suggesting that additional climatic mechanisms played a more important role in local-scale hydrology during that time.

Influence of climate variability on fire generation

This study evaluates the occurrence of dry and wet events and their relationship with fires in southern Pampas, Argentina. The intensity and magnitude of dry and wet events were determined based on the regional series of the Standardized Precipitation and Evapotranspiration Index (SPEI) for the 2000-2021 period. The data obtained were related to the El Niño Oceanic Index (ONI) to analyze the incidence of El Niño and La Niña events in generating them. Fires in the region were detected using remote sensing techniques, considering the number of events, their intensity, extent, and duration. The southern Pampas experiences marked rainfall variability, with 15 dry events, 11 wet years, and 2 standard years recorded for the period analyzed. Extreme dry years were, on average, more intense (SPEI = -2.14) and occurred mainly during the negative ONI phase. In contrast, extreme wet years exhibited lower intensity (SPEI = 1.98), and only the most intense ones were related to neutral ONI phases. We analyzed a representative extremely dry (ED) and an extremely wet events (EW) to interpret the relationship between climate variability and the spatiotemporal variability of fires in the region. It was observed that during the EW event (2014-2015, SPEI = 1.52, and El Niño event until 2015) the number of fires was higher compared to an ED event (2008-2009, SPEI = -2.22, and La Niña event during 2008), with 460 and 205 fires, respectively. The intensity was higher in the EW (302.6 and 31.5 MW), while the area presented considerable differences (1722 and 815.5 km2, respectively). Finally, the duration of the fires was shorter in ED than in EW (6 and 8 months, respectively). These results were related to vegetation health (NDVI = 0.29 and 0.41 and EVI = 0.15 and 0.21 in ED and EW, respectively) and changes in land covers. This study provides a solid database for future research efforts and sustainable land management plans.

Responses of Soil Microbial Survival Strategies and Functional Changes to Wet-Dry Cycle Events.

Soil microbial taxa have different functional ecological characteristics that influence the direction and intensity of plant-soil feedback responses to changes in the soil environment. However, the responses of soil microbial survival strategies to wet and dry events are poorly understood. In this study, soil physicochemical properties, enzyme activity, and high-throughput sequencing results were comprehensively anal0079zed in the irrigated cropland ecological zone of the northern plains of the Yellow River floodplain of China, where Oryza sativa was grown for a long period of time, converted to Zea mays after a year, and then Glycine max was planted. The results showed that different plant cultivations in a paddy-dryland rotation system affected soil physicochemical properties and enzyme activity, and G. max field cultivation resulted in higher total carbon, total nitrogen, soil total organic carbon, and available nitrogen content while significantly increasing α-glucosidase, β-glucosidase, and alkaline phosphatase activities in the soil. In addition, crop rotation altered the r/K-strategist bacteria, and the soil environment was the main factor affecting the community structure of r/K-strategist bacteria. The co-occurrence network revealed the inter-relationship between r/K-strategist bacteria and fungi, and with the succession of land rotation, the G. max sample plot exhibited more stable network relationships. Random forest analysis further indicated the importance of soil electrical conductivity, total carbon, total nitrogen, soil total organic carbon, available nitrogen, and α-glucosidase in the composition of soil microbial communities under wet-dry events and revealed significant correlations with r/K-strategist bacteria. Based on the functional predictions of microorganisms, wet-dry conversion altered the functions of bacteria and fungi and led to a more significant correlation between soil nutrient cycling taxa and environmental changes. This study contributes to a deeper understanding of microbial functional groups while helping to further our understanding of the potential functions of soil microbial functional groups in soil ecosystems.

Atmospheric deposition of microplastics in Shiraz, Iran

The atmosphere plays a critical role in the regional and global transportation and redistribution of microplastics (MPs). However, the significance of rainfall and its means of scavenging MPs are not well understood. In this study, MP deposition was determined during successive dry and rainy events over eight consecutive days in the Shiraz region of Iran. Flux magnitudes and temporal distributions at six sites within and outside the city (including a remote, non-urbanised location) were similar and revealed a progressive increase in MP abundance and deposition during dry periods (up to about 50 MP m−2 h−1) and subsequent relatively rapid wet deposition (washout) by incipient rainfall (peaking at about 130 MP m−2 h−1). Wet deposition of MPs progressively decreased throughout the rainfall event, but with evidence of secondary peaks, before atmospheric accumulation and dry deposition increased during the next dry event and the cycle continued. Dry and wet deposition were dominated by fibres (that included polyester-polyethylene terephthalate, polystyrene, polyvinyl chloride and polyethylene) but the evolution of deposition did not appear to be associated with changes in MP size. These observations indicate that, as with other airborne pollutants, initial rainfall is an efficient scavenger of atmospheric MPs. Along with variations in methodology, this effect may contribute to the wide variation in MP fluxes reported in the literature.

Anthropogenic Influence on the Record-Breaking Compound Hot and Dry Event in Summer 2022 in the Yangtze River Basin in China

Abstract CMIP6 simulations suggest that the probability of the compound hot and dry event occurring in summer 2022 in the Yangtze River basin in China is enhanced by anthropogenic effect by 7 times.

Quantitative pollen-based paleoclimate reconstructions for the past 18.5 ka in southwestern Yunnan Province, China

Quantitative reconstruction of past climate plays an important role in understanding global and regional climate changes and validating climate models. Although some important progress has been made in quantitative paleoclimate reconstructions based on terrestrial records in the Indian summer monsoon region, high-resolution quantitative studies spanning the last ∼20 ka are still relatively sparse with differing results. This study presents high-resolution quantitative variations in mean temperature of the coldest month (MTCM, the first controlling factor of the regional vegetation), mean annual temperature (MAT, the second controlling factor), and mean annual precipitation (MAP) from Lake Tengchongqinghai in southwestern China, based on an updated modern pollen dataset and the fossil pollen record spanning the last 18.5 ka. The results show that temperature and precipitation increased gradually from 18.5 ka, and peaked from 7.2 to 4.5 ka when MAT was on average 1.0 °C higher than the modern observational value (14.5 °C), corresponding to the mid-Holocene thermal maximum (HTM), and then decreased gradually. The total reconstructed ranges are between −2.2 and 9.2 °C for MTCM, 7.7 and 17.2 °C for MAT, and 840 and 1300 mm for MAP. On top of this overall climate trend, seven abrupt cold and dry events were detected during the periods of 16.2–14.8 ka, 12.8–11.5 ka, ∼11.1 ka, 9.1–8.4 ka, ∼7.7 ka, 4.3–3.7 ka, and 0.68–0.009 ka (1270–1950 CE). The results of this quantitative reconstruction were validated by both statistical and ecological evaluations. We conclude that the trend of climatic change since 18.5 ka in this study area was primarily driven by June, July, August, and September solar insolation and changes in radiative forcing and greenhouse gas concentrations. The abrupt changes may be caused by changes in the Atlantic meridional overturning circulation, solar activity, the position of the Intertropical Convergence Zone, and volcanic activity.

Serengeti–Masai Mara ecosystem dynamics inferred from rainfall extremes

The Serengeti–Masai Mara Ecosystem (SMME) is an iconic ecological and biodiversity resource in East Africa with a spectacular great animal migration feature. Environmental shocks like droughts, floods, and land degradation threaten the SMME’s ecological functioning. However, the spatiotemporal ecosystem dynamics during climate extremes are inadequately examined. Here we quantified water availability and vegetation changes during extreme wet (EWE) and dry (EDE) events over the SMME for wet and dry seasons in 1982–2020. We derived extreme events from seasonal mean precipitation anomaly exceeding standard deviation and applied composite and correlation techniques to assess their dynamics with animal populations and migration prospects. Wet season EWE increases vegetative and moist conditions over southern SMME, suggesting elongating migrants’ occupancy compared to normal periods and delayed northward migration. Wet season EDE significantly suppresses these features, heightening ecosystem competition and survival threats, proposing an earlier northward migration. Dry season EWE increases vegetation and water availability over northern SMME, prompting the late southward migration. Dry season EDE significantly reduces vegetation and water availability over northern SMME, suggesting an early southward migration towards regions with more vegetation and increased water availability. The SMME also experiences multiple EDE occurring in consecutive seasons, prolonging dry conditions that aggregate wildlife survival threats. Notably, two EDE prevailed consecutively from the 1993 dry season to the 1994 wet season, coupled with a population decline of wildebeest (1.2–0.9 million), buffalo (40–20 thousand), and lion (1.3–0.9 thousand). We also note a reversal from more EDE to EWE during the study period. Prevalent EWE can lessen wet and vegetative conditions distribution gradient, which is imperative for the functioning of the SMME migratory ecosystem. Our study unveiled hotspot areas of extremes-driven ecosystem changes for the sustainable SMME migratory functioning essential for framing meaningful conservation management policies under climate change.

A 225-year pine (Pinus latteri) tree-ring record of pre-monsoon relative humidity variation in Nan province of northern Thailand and the linkage with large-scale ocean-atmospheric circulations

Thailand is bounded by the Gulf of Thailand in the Pacific Ocean and the Andaman Sea in the Indian Ocean. Variations in the Thailand monsoon (TM) are strongly influenced by the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) phenomenon. The climatology of Thailand is of great importance for understanding seasonal responses to large-scale climatic variations and associated ocean-atmospheric forcing. This study presents the first longest reconstruction (1792–2016 CE) of March–May relative humidity (RH) based on the tree-ring width chronology of Pinus latteri trees from Nan province, northern Thailand. The growth of P. latteri was mainly limited by pre-monsoon RH variability (r = 0.773, p < 0.001, n = 66). The field spatial correlations with the gridded climate datasets demonstrated that the climatic signals retained in our reconstruction represented regional climate variations. Extremely wet years were observed in 1879, 1887, 1908, 1929, 1942, 1974, and 2013 CE, whereas extremely dry years were found in 1795–1796, 1905, 1960, 1992, and 2010 CE. Spectral analysis and spatial correlation analysis revealed pre-monsoon RH variability in the northern Thailand is linked to large-scale ocean-atmospheric circulations, mainly ENSO and IOD activities due to sea surface temperatures variation in the Pacific and Indian Oceans. ENSO-related climate anomalies during the past 225 years were strongly associated in our reconstruction, of which a total of 63% extremely and abnormally wet years occurred during the La Niña events, whereas 59% of extremely and abnormally dry years corresponded to El Niño events. Our reconstruction indicated increasing extreme wet and dry events since 1900 CE attributed to ENSO activities, and thus provide useful insights to understand occurrence of extreme weather events in Thailand climatology.

Model Validation and Sensitivity Analysis of Coupled Non‐Equilibrium Heat and Mass Transfer in Porous Media With Application to Evaporation From Bare Soils

AbstractMathematical models in engineering play an important role in understanding and predicting the behavior of a system. A mechanistic coupled liquid water, water vapor and heat transfer model incorporating kinetic phase change accounting for real‐time interfacial area between water and gas phases was developed to predict coupled subsurface processes and evaporation (drying) rates from bare soils. To enhance the model capability to predict evaporation rates, the air resistance associated with the viscous sublayer was implemented in energy and mass exchange across the soil‐air interface (the land‐atmosphere boundary condition [BC]). The atmospheric stability condition was also considered in the calculation of sensible heat and vapor fluxes at the ground surface. This comprehensive model was validated against measured field data from bare soil test plots from a green roof study, during temperate summer conditions in Canada, demonstrating that the model captured the main coupled processes in the subsurface of bare soil during drying periods. A sensitivity analysis was performed to determine the importance of various components of the comprehensive model. Removal of viscous sublayer resistance in the vapor transfer BC resulted in poorer predictions of evaporation (drying) rates. Incorporating the atmospheric stability function accounting for real‐time atmospheric conditions did not improve the predictive capability for the simulated drying events compared to the case when only a neutral atmospheric condition was implemented. Neglecting heat transfer associated with hydrodynamic dispersion of water vapor in the subsurface had limited impact on subsurface temperature predictions.

Operational prediction of extreme bushfires

Fire behaviour is changing as new modes of fire spread dominate in south-eastern Australia, making extreme bushfires significant risk drivers during bad fire seasons. This is linked to changes in climate and landscape hydrology. A Hierarchical Predictive Framework has been developed specifically to predict the risk of extreme bushfires in south-east Australia. It uses temperature anomalies and the accumulation of river drying events to predict seasonal risk. If that risk is raised, a Blow-Up Fire Outlook model looks at fire weather forecasts and fireground context for specific risk prediction. This guides operational intelligence gathering to support decision-making by incident management teams. The Hierarchical Predictive Framework is based on over 20 years of archival data on extreme bushfires and their context. It is intended to work alongside, but not replace, existing operational systems. Tools like the framework can help keep fire crews safe in the face of risks like branched troughs and the rapid rise in prominence of foehn-driven fires in Australia, as well as the rapidly growing threat from fires that spawn pyrocumulonimbus clouds (pyroCbs).

Record-breaking and unprecedented compound hot and dry summers in Europe under different emission scenarios

After analysing observed summer compound hot and dry (CHD) events over Europe from 1950 to 2022, we employ a large ensemble of high-resolution regional climate model simulations to investigate CHD events under different emission scenarios. By the end of the century, even under a low-emission scenario, model results show a likely increase in the frequency and extension of CHD events over most (60%) of Europe. In particular, the fraction of land projected to be hit once every two years nearly doubles (at least 15%, likely range 6–21) compared to the historical period (8%, 6.5–10), and at least 5,3% (1–7) of land will be hit every year. Under a high-emission scenario, 50% of the Iberian Peninsula is projected to be hit at least twice every three years (20.3 times in 30 years, likely range 17.2–24.2), compared to 1 in ten years in the historical period, whereas 50% of the British Islands, France, and the Mediterranean will be hit more than once every two years. Moreover, 10% of European land will be hit nearly once every 7 years (4.2 times, 3.2–5.6) by CHD events whose intensity equalled or even surpassed the maximum recorded during 1950–2022, and 20% of the Iberian Peninsula once every 5 years. The increase in record-breaking or unprecedented CHD events is mostly related to the increase in record-breaking heatwaves, which is likely over most regions even for the low-emission scenario. In contrast, the increase in record-breaking drought events is limited to southern Europe under the medium- and high-emission scenarios.

Invasive plant species interact with drought to shift key functions and families in the native rhizosphere

Background and aimsInteractions between species invasions and climate change have the potential to drive changes in plant communities more than either factor alone. One pathway through which these effects can occur is via changes to the rhizosphere microbial community. Invasive plants can alter these microbial communities affecting natives’ abilities to compete with invaders. At the same time, climate change is leading to more frequent extreme wet and dry events. Understanding the response of plant communities to these combined global change drivers requires a comprehensive approach that assesses the relationship between plant competition and belowground rhizosphere microbial community responses.MethodsHere we use a field experiment in a California grassland with a set of six native annual forbs (i.e., wildflowers) and three invasive annual grasses to test how competition with invasive plants alters both identity and function in the native rhizosphere microbiome, and whether competition between these groups interacts with rainfall to amplify or ameliorate microbial shifts.ResultsMetagenomics of rhizosphere communities revealed that drought combined with competition from invaders altered a higher number of functions and families in the native rhizosphere compared to invasive competition alone or drought alone. Watering combined with invasion led to fewer shifts.ConclusionThis suggests invasion-driven shifts in the microbial community may be involved in weakening natives’ ability to cope with climate change, especially drought. Understanding the role of the microbial community under invasion and climate change may be critical to mitigating the negative effects of these interacting global change drivers on native communities.Graphical abstractUnderstanding plant community response to global change drivers requires a comprehensive approach that assesses the relationship between plant competition and belowground rhizosphere microbial community responses. (a) In this work, we use a field experiment in a California grassland with a set of native forbs (purple) and invasive grasses (teal) to assess the combined effects of competition and water availability (drought, control, watered) on the rhizosphere microbiome. (b) Drought combined with competition from invaders altered the relative abundance of 36 functions (white) and 22 microbial families (blue) in the native rhizosphere compared to the effects of competition (3 functions, 16 families) or drought alone on natives (not shown: 5 functions, 0 families). (c) Additionally, regardless of watering treatment, invasive grasses sourced more of the taxonomic community in native-invasive mixes and this was exacerbated during drought. Overall, these results suggest invasion-driven shifts in the microbiome may be involved in weakening natives’ ability to cope with climate change, especially drought.

Extraction and identification of spectrum characteristics of coal and rock hydraulic fracturing and uniaxial compression signals

Microseismic (MS) events generated during coal and rock hydraulic fracturing (HF) include wet events caused by fracturing fluid injection, in addition to dry events caused by stress perturbations. The mixture of these two events makes effective fracturing MS events pickup difficult. This study is based on physical experiments of different coal and rock HF and uniaxial compression. The differences of waveform characteristic parameters of various coal and rock ruptures were analyzed using the Hilbert–Huang transform, leading to some useful conclusions. The phase characteristics of the acoustic emission (AE) energy differed significantly and responded well to the pumping pressure curve. The AE waveforms of HF exhibit similar energy and frequency distribution characteristics after Empirical mode decomposition. The main frequency bands for coal, sandstone, and shale samples are 100–300 kHz, while the mudstone sample is in the range of 50–150 kHz. The decay ratios for coal, sandstone, shale and mudstone samples are 0.78, 0.83, 0.67 and 0.85, respectively. When compared to the uniaxial compression test, the main frequency bands of HF were reduced for coal, sandstone and mudstone samples, whereas shale remained essentially unchanged. The duration, instantaneous energy, and total energy of the HF waveform are smaller than those of uniaxial compression, while the decay ratio is greater, especially for the mudstone samples. The waveform characteristic parameters, trained using the multilayer perceptron neural network, can effectively identify HF and uniaxial compression events with an accuracy of 96%.

Compound events of wet and dry extremes: Identification, variations, and risky patterns

Compound hydrometeorological extremes have been widely examined under climate change, they have significant impacts on ecological and societal well-being. This study sheds light on a new category compound of contrasting extremes, namely compounding wet and dry extremes (CWDEs). The CWDEs are characterized as devastating dry events (EDs) accompanied by wet extremes (EWs) in a given time window. Notably, we first adopt a separate system to identify coinciding events considering the different evolving processes and impacting patterns of EDs and EWs. The peak-over-threshold and standardized index methods are used in a daily and monthly window to identify EWs and EDs respectively. Furthermore, the spatial-temporal changes and risky patterns of CWDEs are revealed by using the Mann-Kendall test, the Ordinary Least Squares, and the Global and Local Moran indices. Germany is the study case. As one major finding, the results indicate a pronounced seasonal effect and spatial clustering pattern of CWDEs. The summer is the most vulnerable period for CWDEs, and the spatial hotspots are mainly located in the southern tip of Germany, as well as in the vicinity of the capital city Berlin. Besides, robust uptrends in CWDEs across all evaluation metrics have been discovered in historical periods, and the moist climate and complex geography collectively contribute to severe CWDEs. Unexpectedly, the study finds that compounding events in dry regions are mainly driven by wet extremes, whereas they show a higher dependency on dry anomalies in wet regions. The research provides new insights into compound extremes which are composed of individual hazards with distinct features. Related findings will aid decision-makers in producing effective risk mitigation plans for prioritizing vulnerable regions. Lastly, the robust framework and open access data allow for extensive exploration of various compounding hazards in different regions.

Spatial-temporal changes of compound temperature-humidity extremes in humid subtropical high-density cities: An observational study in Hong Kong from 1961 to 2020

In the humid subtropics, rising temperatures can cause higher humidity via enhanced evaporation which exacerbates heat-related health problems. This study uses multi-station observational data to reveal the spatial-temporal changes of compound temperature-humidity extreme events in Hong Kong during 1961–2020. Based on the 90th and 10th percentiles of temperature and specific humidity, four types of compound events were identified, i.e., Compound Hot and Wet (CHW), Hot and Dry (CHD), Cold and Wet (CCW), and Cold and Dry (CCD) events. Over the past six decades, there has been a significant increase of CHW (+3.45 events/decade) and decrease of CCD (−3.00 events/decade). The greatest increase of CHW was observed during the warm period of the 2010s (+4 events/year/month). Meanwhile, the trends of CHD and CCW were less evident. Spatially, more frequent compound events (especially those with high humidity (CHW and CCW)) were observed in built-up areas compared to rural areas, while the intensity of these events remained similar. The results imply that both regional climate and urban factors contribute to the increase of extreme hot and humid weather. The study generalizes mechanisms for these spatial-temporal changes, and discussed implications for compound extremes management in Hong Kong and other similar cities.

Assessment of blue water-green water interchange under extreme warm and dry events across different ecohydrological regions of western Canada

As key drivers of terrestrial ecosystems and food production, green water (GW) and blue water (BW) are interlinked through natural and anthropogenic processes. Their relationship has reportedly been non-stationary, with an anticipated shift from BW to GW in the future. However, these potential shifts are poorly characterized, particularly in response to future extreme warm-dry events. To address this gap, we used a process-based agro-hydrological model to project GW-BW interlinkages and their potential shifts across three different ecohydrological regions – mountainous, crop, and natural lands – in the Nelson River Basin, the largest agricultural basin in Canada and a major breadbasket for the world. Our results indicate dissimilarity in the physical processes that link GW and BW across different ecohydrological regions, and therefore, disproportional projected changes in BW and GW. In mountainous and natural lands, with projected decreases in precipitation in extreme warm-dry years of up to 56% and 46% under SSP126 and SSP585 scenarios, BW decreases by 64% and 42% while GW decreases only by 37% and 29%. The decline in GW is expected to be less because of the availability of evaporative water supplied by legacy soil moisture and groundwater contributions from earlier seasons. This available water to supply evaporative demand represents a delayed overall shift of BW to GW. In crop lands, however, results show a significant decrease of precipitation (45%-55%), BW (up to 42%), and GW (up to 42%), with no remarkable shift of BW to GW in extreme warm-dry years in the future. The projected BW and GW declines in crop lands may impose severe water shortages on local and regional food production. Projected declines in BW from the mountains that serve as source water for downstream irrigation may exacerbate such possible shortages.

Hot and dry compound events in South America: present climate and future projections, and their association with the Pacific Ocean

Compound hot and dry events can cause greater impacts than those generated by individual extreme events. Understanding the physical mechanisms that lead to their development is particularly important for an early warning. The aim of this study is to assess the ability of global climate models (GCMs) to simulate hot/dry compound events in South America (SA) during the historical period 1979–2014, in comparison with observational and reanalysis datasets. Additionally, this work seeks to investigate the potential changes in these events under two future climate scenarios for the period 2065–2100. Furthermore, we analyze the spatial patterns of sea surface temperature anomalies (SSTA) in the Pacific Ocean associated with these events in tropical and extratropical SA. In the historical period, reanalysis tends to overestimate the number of hot/dry events, while the ensemble median of GCMs performs better than the individual ones. The future projections under the high emissions scenario show longer heat waves, but a low model agreement about the number of compound events in tropical SA. For southern SA, an increase in the annual frequency of compound events is projected, and more than two hot/dry events per year are expected to occur relative to the 1979–2014 baseline. Finally, we find that compound events in tropical SA are favored during the El Niño phase, even though two other SSTA patterns have gained prominence in recent years. In southern SA, hot/dry events are associated with the negative phase of the Pacific Decadal Oscillation and the La Niña phase.