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  1. Abstract Climate change amplifies dry and hot extremes, yet the mechanism, extent, scope, and temporal scale of causal linkages between dry and hot extremes remain underexplored. Here using the concept of system dynamics, we investigate cross-scale interactions within dry-to-hot and hot-to-dry extreme event networks and quantify the magnitude, temporal-scale, and physical drivers of cascading effects (CEs) of drying-on-heating and vice-versa, across the globe. We find that locations exhibiting exceptionally strong CE (hotspots) for dry-to-hot and hot-to-dry extremes generally coincide. However, the CEs differ strongly in their timescale of interaction, hydroclimatic drivers, and sensitivity to changes in the soil-plant-atmosphere continuum and background aridity. The CE of drying-on-heating in the hotspot locations reaches its peak immediately driven by the compounding influence of vapor pressure deficit, potential evapotranspiration, and precipitation. In contrast, the CE of heating-on-drying peaks gradually dominated by concurrent changes in potential evapotranspiration, precipitation, and net-radiation with the effect of vapor pressure deficit being strongly controlled by ecosystem isohydricity and background aridity. Our results help improve our understanding of the causal linkages and the predictability of compound extremes and related impacts. 
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    Free, publicly-accessible full text available December 1, 2024
  2. Abstract

    The 2022 Compound Drought and Heatwave (CDHW) caused widespread crop damage, water shortages, and wildfires across Europe. Our study analyzed this event’s severity and return period (RP) and compared it with past mega CDHWs in Europe. The hardest‐hit areas were Iberian Peninsula, France, and Italy, where temperatures exceeded 2.5°C above normal, and severe droughts persisted from May to August. Using a Bayesian approach, we estimated the RP for the 2022 CDHW event, which was unprecedented in Northern Italy, Iberian Peninsula, and western parts of France, with RPs of 354, 420, and 280 years, respectively. The reduced soil moisture due to precipitation deficits and high temperatures contributed to the persistence and severity of drought, creating a positive feedback loop where dry soils led to even drier conditions. In light of our findings, it is evident that global warming poses increased risks of severe CDHW events, which are likely to increase.

     
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  3. Abstract

    Flash Drought (FD) has garnered much attention in recent years, with significant advancements in the indicators applied for identifying these rapidly intensifying events. However, the difference in existing FD definitions and methodologies among research communities and the choice of different data sources underscores the importance of addressing the uncertainties associated with the global FD characteristics and their drivers. This study compares two key FD indicators derived based on evaporative stress ratio (ESR) and root‐zone soil‐moisture (RZSM) using three different data sources to investigate the uncertainties in global FD frequency and intensity (speed), and the influencing drivers. The results suggest that such disparities are significant in the two FD indicators across different climate regions of the globe. The results highlight varying spatial drivers of FD frequency, intensity, and their evolution, potentially linked to background aridity. Changes in precipitation, temperature, vapor pressure deficit, and soil‐temperature coupling play an important role with a cascading (concurrent) impact on the evolution of FD based on RZSM (ESR). The relationship between ESR and RZSM fails to explain most of the variance in each of these indicators specific to the FD episodes, especially in the transitional and humid climate regimes. Overall, the results highlight the necessity of more nuanced methodologies for deriving FD indicators that can efficiently couple the rapid soil‐moisture depletion rates in deeper layers with changes in atmospheric evaporative demand which has direct implications on vegetation health.

     
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  4. Abstract

    The significant impact of flash droughts (FDs) on society can vary based on a combination of FD characteristics (event counts, mean severity, and rate of intensification), which is largely unexplored. We employed root‐zone soil‐moisture for 1980–2018 to calculate the FD characteristics and integrated them to formulate a novel multivariate FD indicator for mapping the global FD hotspot regions. The potential influence of climate characteristics (i.e., anomalies, aridity, and evaporative fractions) and land‐climate feedbacks on the evolution of multivariate FD indicator is investigated. Our results indicate that precipitation is the primary driver of FD evolution, while the effect of temperature, vapor pressure deficit, and land‐climate interaction varies across the climate divisions after the onset of the events. The magnitude of multivariate FD indicator decreases with increased climate aridity, and it is significant in the global humid regimes, underscoring the importance of water and energy supply as limiting factors regulating FD‐risk.

     
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  6. Abstract

    Compound drought and heatwaves can cause significant damage to the environment, economy, and society. In this study, we quantify the spatio‐temporal changes in compound drought and heatwave (CDHW) events by integrating weekly self‐calibrated Palmer Drought Severity Index (sc_PDSI) and daily maximum temperatures during the period 1983 to 2016. Multiple data products are used to examine the robustness of sc_PDSI in the compound event analysis. The results consistently suggest significant increases in drought‐related heatwaves and affected global land area in recent (warmer) periods. Several regions across the globe witnessed rise in CDHW frequency (one to three events/year), duration (2–10 days/year), and severity. This increasing pattern is spatially asymmetric, and greater amplification is observed across the Northern hemisphere due to recent warming. Furthermore, the background aridity influences the spatiotemporal evolution of CDHW events. The results can be applied to minimize the impacts of extreme CDHWs in critical geographical regions.

     
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  7. Abstract

    Globally, heat stress (HS) is nearly certain to increase rapidly over the coming decades, characterized by increased frequency, severity, and spatiotemporal extent of extreme temperature and humidity. While these characteristics have been investigated independently, a holistic analysis integrating them is potentially more informative. Using observations, climate projections from the CMIP5 model ensemble, and historical and future population estimates, we apply the IPCC risk framework to examine present and projected future potential impact (PI) of summer heat stress for the contiguous United States (CONUS) as a function of non‐stationary HS characteristics and population exposure. We find that the PI of short‐to‐medium duration (1–7 days) HS events is likely to increase more than three‐fold across densely populated regions of the U.S. including the Northeast, Southeast Piedmont, Midwest, and parts of the Desert Southwest by late this century (2060–2099) under the highest emissions scenario. The contribution from climate change alone more than doubles the impact in the coastal Pacific Northwest, central California, and the Great Lakes region, implying a substantial increase in HS risk without aggressive mitigation efforts.

     
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  8. Abstract

    Compound drought and heatwave (CDHW) events have garnered much attention in recent studies. However, thus far, the identification of such events is oversimplified, and their association with natural climate variability is not fully explored. Here, we derive anomalies in the weekly self‐calibrated Palmer Drought Severity Index (sc_PDSI) and daily maximum temperatures to identify CDHW events from 1982 to 2016 over 26 climate regions across the globe. Using a Poisson Generalized Linear Model (GLM), we analyze yearly occurrences of seasonal CDHW events and their association with the warm and cold phases of El Nino Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and North Atlantic Oscillation (NAO). ENSO exhibits robust association with CDHW events over the Southern Hemisphere during the austral summer and fall, while PDO influences their occurrences over the Western North America in the Northern Hemisphere during the boreal summer, which is supported by the composites of anomalies in the atmospheric circulations and surface energy budget. However, NAO association with CDHW events is relatively weak. The CDHW occurrence over other regions is driven by a combination of these large‐scale natural forcing. Our analyses also highlight that the cooccurrence of weekly to submonthly scale anomalies in the observed temperature and precipitation may not be always aligned between the observations and the reanalysis. Therefore, caution must be exercised while explaining such observed anomalies on the basis of reanalysis‐based circulations and surface energy budget. Overall, our analyses provide a new insight towards concurrent extremes and should help foster research efforts in this area.

     
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