Vegetation plays a crucial role in soil moisture regulation and the development of rapid-onset droughts known as flash droughts. We use climate model experiments with the Community Earth System Model (CESM2) to examine how the vegetation response to rising CO2 impacts projections of future flash drought in the Northern Hemisphere mid-latitudes. By isolating the influences of CO2 fertilization and CO2 stomatal conductance effects from CO2 radiative forcing, we find that: (a) CO2-induced changes to plant characteristics are of sufficient magnitude to modify flash drought characteristics, (b) CO2 fertilization effects counteract the CO2 stomatal conductance effects on projected flash drought occurrence, and (c) the combined influence of the vegetation response to rising CO2 can either amplify or counteract CO2 radiative-driven flash drought changes depending on location. In water-limited regions such as the western United States, the Mediterranean Basin, the Middle East, and west/central Asia where CO2 fertilization dominates and surface vegetation strongly controls water availability, elevated leaf area offsets reductions in stomatal conductance and transpiration, increasing the likelihood of future flash droughts. Vegetation-driven increases in flash drought in these areas are generally aligned in sign with projected increases due to radiative forcing. Conversely, in more energy limited regions such as western Canada, East Asia, and parts of Europe, preserved soil moisture from reduced stomatal conductance and transpiration suppresses flash droughts despite increased leaf area from CO2 fertilization. These reductions in flash drought from vegetation counteract radiative-driven increases. This study elucidates physical processes underlying projected flash drought development, improving predictive capabilities and mitigation strategies.

Plant physiological responses to rising CO2 have been shown to contribute to increasing extreme heat; but their impacts on co-occurrences of high heat and humidity have not been assessed previously. Since heat stress depends on both, reductions in evapotranspiration and increases in sensible heat can incite competing influences on co-occurrence metrics (e.g., heat index). Here we analyze plant physiological forcing in idealized simulations that isolate plant physiological from radiative impacts of rising CO2. Our results demonstrate that increasing temperature has a larger influence than declining moisture, leading to overall CMIP6 multi-model mean heat index increases. Model differences are driven by varying levels of transpiration decline, which can be partially offset by leaf-area-driven increases in canopy evaporation in some models/regions; as highlighted by differences between two versions of one model (CESM) with high and low levels of leaf-area change. This analysis helps clarify the role of plants in future climate and human health.

Analogs of present-day rapid ice melt can be found in episodic discharges of icebergs that occurred during glacial periods called Heinrich events. This introduces excess meltwater into the North Atlantic and weakens the Atlantic thermohaline circulation (AMOC), triggering a hydrologic cycle–AMOC collapse feedback as the atmospheric energy transport compensates for reduced northward heat transport. Here we employ a novel series of 100-year North Atlantic “hosing” simulations to investigate atmospheric and oceanic energy transport response from freshwater forcing, focusing in particular on the role of atmospheric rivers (ARs) within atmospheric energy transport. Importantly, we use an “overwriting” methodology that allow us to attribute AMOC weakening to added North Atlantic meltwater and subsequent hydrologic cycle responses, respectively. In contrast to far-reaching response of transient eddies, our results show a substantial increase in moisture convergence from ARs that is geographically constrained to the North Atlantic midlatitudes. Such AR changes nevertheless comprise an important component of net precipitation changes over the Euro-Atlantic sector, with the amount being comparable to that from transient eddies over the subpolar Atlantic. Over the course of the century-long simulations, we demonstrate that hydrologic cycle responses to North Atlantic freshening and subsequent feedbacks, including those from ARs, account for approximately half of the simulated AMOC collapse. Our work highlights the dynamics of atmospheric moisture transport response to North Atlantic freshening events and elucidates how intensifying moisture transport may accelerate AMOC collapse in the future.

The spatial extent of an extreme heat event influences the total exposure of people and natural systems to heat-related stresses, straining water, energy, and emergency management resources. Here, we quantify how the contiguous area of individual heat wave events varies across heat wave types, time of year, and in response to observed climate change within the Berkeley Earth Surface Temperature Dataset. Across the mid-high latitudes, cold season heat waves cover areas that are 1.25 to 3 times larger than warm season events, and daytime heat waves impact 1.25 to 2 times the area of nighttime heat waves. The reverse relationship is found throughout tropical regions. Average heat wave size, regardless of type or season, has increased across most land in recent years, often by 1.5 to 2 times in the mid-latitudes. The contiguous spatial extent of dry soil anomalies and lower tropospheric subsidence events have also increased in some locations, potentially contributing to the increases in heat wave size.

Hydroclimate volatility refers to sudden, large and/or frequent transitions between very dry and very wet conditions. In this Review, we examine how hydroclimate volatility is anticipated to evolve with anthropogenic warming. Using a metric of ‘hydroclimate whiplash’ based on the Standardized Precipitation Evapotranspiration Index, global-averaged subseasonal (3-month) and interannual (12-month) whiplash have increased by 31–66

This study examines changes to daily weather patterns associated with the occurrence of the precipitation deficits that contribute to seasonal drought events, with durations from 3–8 months in the Northeast US, for the period 1980–2018. Our over-arching question is the extent to which seasonal droughts are due to a shift in frequency to drier patterns as compared to a suppression of precipitation during wetter patterns. Seasonal drought events are defined using the 3-month Standardized Precipitation Evapotranspiration Index (SPEI). The changes to daily weather are considered with respect to twenty characteristic weather patterns for the region, identified by applying Self-Organizing Map (SOM) analysis to daily reanalysis 500-hPa geopotential height and 900-hPa horizontal wind anomalies. This allows an analysis of the drought-related changes to the structure and frequency of the weather patterns that determine the production of precipitation. Twenty-two seasonal Northeast US drought events are identified. The precipitation deficits associated with these droughts is, on average, ∼70% due to suppression of precipitation in wet patterns and ∼30% due to a change in frequency from wetter to drier patterns. Nearly 80% of the precipitation deficits associated with seasonal drought are associated with changes to only eight of the twenty daily weather patterns, with the relative importance of those patterns changing seasonally. Key features of seasonal drought weather in the cool season are flattening of ridges or shifting in trough location eastward; and, in the warm season, weakening of troughs along with ridge-building, or westward shifts in trough location.

This study examines changes to daily weather patterns associated with the occurrence of the precipitation deficits that contribute to seasonal drought events, with durations from 3 to 8 months in the Northeast United States, for the period 1980-2018. Our overarching question is the extent to which seasonal droughts are due to a shift in frequency to drier patterns as compared to a suppression of precipitation during wetter patterns. Seasonal drought events are defined using the 3-month standardized precipitation evapotranspiration index (SPEI). The changes to daily weather are considered with respect to 20 characteristic weather patterns for the region, identified by applying self-organizing map (SOM) analysis to daily reanalysis 500-hPa geopotential height and 900-hPa horizontal wind anomalies. This allows an analysis of the drought-related changes to the structure and frequency of the weather patterns that determine the production of precipitation. Twenty-two seasonal Northeast U.S. drought events are identified. The precipitation deficits associated with these droughts are, on average,-70% due to suppression of precipitation in wet patterns and-30% due to a change in frequency from wetter to drier patterns. Nearly 80% of the precipitation deficits associated with seasonal drought are associated with changes to only eight of the 20 daily weather patterns, with the relative importance of those patterns changing seasonally. Key features of seasonal drought weather in the cool season are flattening of ridges or shifting in trough location eastward; and, in the warm season, weakening of troughs along with ridge building or westward shifts in trough location.

Predicting and managing the impacts of fl ash droughts is difficult fi cult owing to their rapid onset and intensifica- fi ca- tion. Flash drought monitoring often relies on assessing changes in root-zone soil moisture. However, the lack of widespread soil moisture measurements means that fl ash drought assessments often use process-based model data like that from the North American Land Data Assimilation System (NLDAS). Such reliance opens fl ash drought assessment to model biases, particularly from vegetation processes. Here, we examine the influence fl uence of vegetation on NLDAS-simulated fl ash drought characteristics by comparing two experiments covering 1981-2017: open loop (OL), which uses NLDAS surface meteorological forcing to drive a land surface model using prognostic vegetation, and data assimilation (DA), which instead assimilates near-real-time satellite-derived leaf area index (LAI) into the land surface model. The OL simulation consistently underestimates LAI across the United States, causing relatively high soil moisture values. Both experiments produce similar geographic patterns of fl ash droughts, but OL produces shorter duration events and regional trends in fl ash drought occurrence that are sometimes opposite to those in DA. Across the Midwest and Southern United States, fl ash droughts are 4 weeks (about 70%) longer on average in DA than OL. Moreover, across much of the Great Plains, fl ash drought occurrence has trended upward according to the DA experiment, opposite to the trend in OL. This sensitivity of fl ash drought to the representation of vegetation suggests that representing plants with greater fi delity could aid in monitoring fl ash droughts and improve the prediction of fl ash drought transitions to more persistent and damaging long-term droughts.

We present one year of delta D, delta 18 O, d-excess, and Delta ' O-17 data from monthly precipitation at a Caribbean coastal site in Panama and from tap waters across the country to constrain geographic, climate, and moisture source controls on isotopic variability and better understand the sources and mechanisms of precipitation in Central America, a region facing significant modifications to the annual rainfall cycle due to climate change. Monthly precipitation delta D ranged from - 52.2 to +14.3 %o, delta 18 O from - 7.6 to +0.4 %o, d-excess from +7.1 to +11.6 %o, and Delta ' 17 O from +11 to +29 per meg. Rainy season precipitation samples were found to have lower delta D, delta 18 O, and d-excess due to Rayleigh distillation during the condensation and rainout of Pacific moisture over the central cordilleras, which results in decoupling between d-excess and Delta ' 17 O. Outlier Delta ' 17 O values during peak dry and rainy months may reflect seasonal changes in water vapor sourcing, from Caribbean to Pacific and/or locally recycled moisture, or may be a result of organic contamination. Tap water delta D ranged from - 82.3 to - 14.3 %o, delta 18 O from - 11.6 to - 2.4 %o, d-excess from +4.3 to +12.2 %o and Delta ' 17 O from - 2 to +84 per meg. Tap water delta D and delta 18 O values increase eastward due to lower orographic effects and Pacific and locally recycled moisture contributions to rainfall and greater secondary evaporation. Tap water d-excess and Delta ' 17 O values are also de-coupled but lack clear spatial trends and controls. The results of this study indicate the promise of adding Delta ' 17 O to the isotopic toolkit in tropical mountainous regions with complicated water cycling dynamics and provide a baseline for future triple oxygen isotope investigations.

Land surface evapotranspiration (ET) is a major source of moisture for the global hydrologic cycle. Though the influence of the land surface is well documented, moisture tracking analyses aimed at quantifying the contribution of the land surface to precipitation have often relied on offline tracking approaches that require simplifying assumptions and can bias results. Additionally, the contribution of the ET components (transpiration ( T ), canopy evaporation ( C ), and ground evaporation ( E )) individually to precipitation is not well understood, inhibiting our understanding of moisture teleconnections in both the current and future climate. Here, we use the Community Earth System Model version 1.2 with online numerical water tracers to examine the contribution of local and remote land surface ET, including the contribution from each individual ET component, to precipitation across North America. Much of northern and northeastern North America receives up to 80% of summertime precipitation from land surface ET, and over 50% of that moisture originates from transpiration alone. Local moisture recycling constitutes an essential source of precipitation across much of the southern and western regions of North America, suggesting precipitation across the region is sensitive to local land surface conditions, including soil moisture and vegetation state. The reliance on locally recycled moisture is far less pronounced across northern and eastern North America, where remotely sourced moisture, particularly from transpiration, dominates precipitation contributions. The results highlight regions that are especially sensitive to land cover and hydrologic changes in local and upwind areas, providing key insights for drought prediction and water resource management.

Over the course of a season, a location’s precipitation is comprised of moisture sourced from a diverse set of geographic regions. Seasonal extremes in precipitation may arise from changes in the contribution of one or several of these sources. Here, we use the Community Earth System Model with numerical water tracers to quantify the contribution of locally sourced, known as precipitation recycling, versus remotely sourced precipitation to seasonal wet and dry extremes across North America. The greatest impact of recycling on both wet and dry extremes is found in the Interior West of the United States where changes to recycling contribute as much as 25%–30% of drought deficit and pluvial surplus. Recycling contributions are smaller across the eastern U.S., generally less than 8%, highlighting the greater role of imported moisture for explaining hydroclimate extremes in these regions. Robust contributions of precipitation recycling to drought and pluvials across the Interior West are driven by consistent changes to local evaporation and the conversion of local evaporation to local precipitation during extreme hydroclimate conditions. The results are consistent with an energy-limited and water-limited evaporation framework and provide a new estimate of the role of local processes in shaping hydroclimate extremes.

Spatially compounding extremes pose substantial threats to globally interconnected socio-economic systems. Here we use multiple large ensemble simulations of the high-emissions scenario to show increased risk of compound droughts during the boreal summer over ten global regions. Relative to the late twentieth century, the probability of compound droughts increases by ~40% and ~60% by the middle and late twenty-first century, respectively, with a disproportionate increase in risk across North America and the Amazon. These changes contribute to an approximately ninefold increase in agricultural area and population exposure to severe compound droughts with continued fossil-fuel dependence. ENSO is the predominant large-scale driver of compound droughts with 68% of historical events occurring during El Niño or La Niña conditions. With ENSO teleconnections remaining largely stationary in the future, a ~22% increase in frequency of ENSO events combined with projected warming drives the elevated risk of compound droughts. The co-occurrence of drought across different regions will have far-reaching effects on global agriculture and food supply. Model projections show an increased likelihood of these compound droughts under a high-emissions scenario, with a ninefold increase of farm land and population exposure.

The Holocene thermal maximum, a period of global warmth evident in early to mid-Holocene proxy reconstructions, is controversial. Most model simulations of the Holocene have not reproduced this warming, leading to a disagreement known as the Holocene Temperature Conundrum. Pollen records document the expansion of vegetation in the early and mid-Holocene African Sahara and Northern Hemisphere mid- and high latitudes, which has been overlooked in previous modeling studies. Here, we use time slice simulations of the Community Earth System Model to assess the impact of Northern Hemisphere vegetation change on Holocene annual mean temperatures. Our simulations indicate that expansion of Northern Hemisphere vegetation 9000 and 6000 years ago warms Earth's surface by ~0.8° and 0.7°C, respectively, producing a better match with proxy-based reconstructions. Our results suggest that vegetation change is critical for modeling Holocene temperature evolution and highlight its role in driving a mid-Holocene temperature maximum.

In this paper, we analyzed the association among trends in COVID-19 cases, climate, air quality, and mobility changes during the first and second waves of the pandemic in five major metropolitan counties in the United States: Maricopa in Arizona, Cook in Illinois, Los Angeles in California, Suffolk in Massachusetts, and New York County in New York. These areas represent a range of climate conditions, geographies, economies, and state-mandated social distancing restrictions. In the first wave of the pandemic, cases were correlated with humidity in Maricopa, and temperature in Maricopa and Los Angeles. In Suffolk and New York, cases were correlated with mobility changes in recreation, grocery, parks, and transit stations. Neither cases nor death counts were strongly correlated with air quality. Periodic fluctuations in mobility were observed for residential areas during weekends, resulting in stronger correlation coefficients when only weekday datasets were included in the analysis. We also analyzed case-mobility correlations when mobility days were lagged, and found that the strongest correlation in the first wave occurred between 12 and 14 lag days (optimal at 13 days). There was stronger but greater variability in correlation coefficients across metropolitan areas in the first pandemic wave than in the second wave, notably in recreation areas and parks. In the second wave, there was less variability in correlations over lagged time and geographic locations. Overall, we did not find conclusive evidence to support associations between lower cases and climate in all areas. Furthermore, the differences in cases-mobility correlation trends during the two pandemic waves are indicative of the effects of travel restrictions in the early phase of the pandemic and gradual return to travel routines in the later phase. This study highlights the utility of mobility data in understanding the dynamics of disease transmission. It also emphasizes the criticality of timeline and local context in interpreting transmission trends. Mobility data can capture community response to local travel restrictions at different phases of their implementation and provide insights on how these responses evolve over time alongside disease trends.

Sea ice melt and ocean heat accumulation in the Arctic are strongly influenced by the presence of atmospheric water vapor during summer. While the relationships between water vapor concentration, radiation, and surface energy fluxes in the Arctic are well understood, the sources of summer Arctic water vapor are not, inhibiting understanding and prediction of Arctic climate. Here we use the Community Earth System Model version 1.3 with online numerical water tracers to determine the geographic sources of summer Arctic water vapor. We find that on average the land surface contributes 56% of total summer Arctic vapor with 47% of that vapor coming from central and eastern Eurasia. Given the proximity to Siberia, near-surface temperatures in the Arctic between 90°E-150°E, including the Laptev Sea, are strongly influenced by concentrations of land surface-based vapor. Years with anomalously large concentrations of land surface-based vapor in the Arctic, and especially in the Laptev Sea region, often exhibit anomalous near-surface poleward flow from the high latitudes of Siberia, with links to internal variability such as the Arctic Dipole anomaly.

During the early to mid-Holocene, changes in orbital precession led to considerable increases in West African monsoon (WAM) rainfall compared to today and shifted its reach further north. However, climate proxies and paleoclimate model simulations disagree over fundamental aspects of the mid-Holocene (MH; 6 ka BP) enhancement of the WAM. Here, we use a water isotope-enabled Earth system model (iCESM1) to, for the first time, directly compare simulated northern African hydroclimatic change between the mid-Holocene and pre-industrial era (PI) with the hydroclimate signal inferred from leaf wax n-alkanes in order to study the WAM's past spatial change. iCESM1 simulates a northernmost WAM extent of ∼24°N, which broadly agrees with the extent inferred from pollen and dust records (23–28°N) but falls short of that from leaf wax n-alkanes (27–31°N). While the isotopic composition of rainfall (δDP) inferred from leaf wax n-alkanes is lower during the MH than the PI, simulated MH δDP is higher in northwestern Africa, especially in boreal fall. This discrepancy can be reconciled by interpreting the inferred signal of leaf wax n-alkanes as being reflective of the isotopic composition of soil water (δDS) and its subsequent influence by soil evaporation. We postulate that leaf wax n-alkanes may overestimate inferred mean annual precipitation rates in the MH by not incorporating the enrichment of precipitation shown by iCESM1. Our results have broad implications for reconstruction of past hydrologic change in northern Africa and lend further support to the northernmost WAM extents inferred from pollen and dust records.

Spatially compounding droughts over multiple regions pose amplifying pressures on the global food system, the reinsurance industry, and the global economy. Using observations and climate model simulations, we analyze the influence of various natural Ocean variability modes on the likelihood, extent, and severity of compound droughts across ten regions that have similar precipitation seasonality and cover important breadbaskets and vulnerable populations. Although a majority of compound droughts are associated with El Niños, a positive Indian Ocean Dipole, and cold phases of the Atlantic Niño and Tropical North Atlantic (TNA) can substantially modulate their characteristics. Cold TNA conditions have the largest amplifying effect on El Niño-related compound droughts. While the probability of compound droughts is ~3 times higher during El Niño conditions relative to neutral conditions, it is ~7 times higher when cold TNA and El Niño conditions co-occur. The probability of widespread and severe compound droughts is also amplified by a factor of ~3 and ~2.5 during these co-occurring modes relative to El Niño conditions alone. Our analysis demonstrates that co-occurrences of these modes result in widespread precipitation deficits across the tropics by inducing anomalous subsidence, and reducing lower-level moisture convergence over the study regions. Our results emphasize the need for considering interactions within the larger climate system in characterizing compound drought risks rather than focusing on teleconnections from individual modes. Understanding the physical drivers and characteristics of compound droughts has important implications for predicting their occurrence and characterizing their impacts on interconnected societal systems.

Paleoclimate proxies indicate that changes in insolation since the mid-Holocene have driven widespread hydrologic changes across the midlatitudes. It is unclear how atmospheric rivers (ARs), which are fundamental to global moisture transport today, may have contributed to these Holocene hydroclimate changes. Here, we use a set of climate model simulations with the Community Earth System Model (CESM), and introduce an AR algorithm optimized to identify ARs within different climate states, to show that changes to the location and intensity of landfalling ARs explain the majority of the precipitation difference between the mid-Holocene and the preindustrial period in several midlatitude regions. During the mid-Holocene, enhanced seasonality increased summer season AR vapor content and displaced ARs poleward of their preindustrial period trajectories, especially in the Northern Hemisphere. Consequently, in high midlatitude coastal areas of western North America and East Asia, ARs account for greater than 10% more of total precipitation during the mid-Holocene, and nearly 100% of the simulated change in precipitation between the two climates. The simulated AR changes are consistent with moisture-sensitive proxy records and with present-day relationships between ARs and regional circulation, enhancing confidence that ARs served as the underlying synoptic mechanism responsible for mid-Holocene hydroclimate anomalies in several coastal mid-latitude areas. The results indicate that ARs are sensitive to background climate state, and suggest that changes in ARs may have contributed to hydroclimate changes throughout Earth's past. (C) 2020 The Author(s). Published by Elsevier B.V.