Temperature and precipitation source variability and glacial dynamics in the southwestern United States at Fish Lake, Utah, since late MIS 4

Abstract Small‐scale turbulence above 10 km is investigated using observations from two recent airborne field campaigns: the Asian Summer Monsoon Chemical & Climate Impact Project (ACCLIP) and the Dynamics and Chemistry of the Summer Stratosphere (DCOTSS). Turbulence is enhanced by factors of 2–24 inside clouds, within 100 km of active deep convection, above atmospheric jets, and above mountains coincident with strong low‐level winds. In DCOTSS, which sampled outflow from overshooting convection over the continental United States, turbulence occurred most frequently near the tropopause, indicating enhanced troposphere‐stratosphere exchange. In ACCLIP, conducted over the northwestern Pacific, turbulence was most common in the upper troposphere. We evaluate the performance of several turbulence diagnostics computed from the European Centre for Medium‐Range Weather Forecasts' fifth reanalysis (ERA5) and operational forecasts, along with the clear‐air turbulence flag recently introduced in the forecasts. Among the diagnostics tested, TI3 computed from operational forecast data is most skillful.

Abstract. The isotopic composition of water vapor in the upper troposphere and lower stratosphere (UTLS) can be used to understand and constrain the budget and pathways of water transport into that region of the atmosphere. Measurements of the water isotopic composition help further understanding of the region's chemistry, radiative budget, and the sublimation and growth of polar stratospheric clouds and high-altitude cirrus, both of which are also important to stratospheric chemistry and Earth's radiation budget. Here we present the first intercomparison of water isotopic composition (δD) using in situ measurements from the ChiWIS, Harvard ICOS, and Hoxotope instruments and satellite retrievals from ACE-FTS. The in situ data come from the AVE-WIIF, TC4, CR-AVE, StratoClim, and ACCLIP field campaigns, and satellite retrievals of isotopic composition are derived from the ACE-FTS v5.2 data set. We find that in all campaign intervals, satellite retrievals above about 14 km altitude are depleted by up to 150 ‰ with respect to in situ measurements. This difference persists even in transit flights through stratospheric air in high-latitude regions, which should be relatively free of observational biases present in other regions. We also use in situ measurements from the ChiWIS instrument, which has flown in both the Asian Summer Monsoon (AM) and the North American Monsoon (NAM), to confirm the isotopic enhancement in δD observed in satellite retrievals above the NAM.

Abstract Ephemerally flooded playas are common in the southwestern United States and globally in drylands. Often formed in closed basins, playas are depressions which inundate infrequently from local precipitation and streamflow produced near the playa or from upland areas. Few studies have quantified the hydrologic connectivity between upland catchments and playas using observations. Here, we used rain gauge‐corrected precipitation from weather radar and water level measurements in 18 playas of the Chihuahuan Desert to identify precipitation thresholds leading to playa inundation over a 6.4‐year period. Geospatial data sets on topography, soil properties, and vegetation cover were employed to determine the controls on inundation. Only 9.4% of all precipitation events above 1 mm led to inundation, with 69.8% of all inundations occurring during the North American monsoon (NAM, July‐September). Mean and standard deviations (Std) of runoff ratios at all playas were 2.74 ± 4.08% and 3.29 ± 5.19% for annual and NAM periods. At the annual scale, playa inundation occurred when mean precipitation thresholds of 18.3 ± 7.5 mm (event total) and 12.0 ± 4.5 mm/hr (60‐min intensity) were exceeded. Across all playas, inundation occurrence and volume were related most strongly to precipitation metrics and catchment area, with secondary controls of soil and terrain properties. The explanatory power of the derived regressions describing the inundation response across the playas were significantly improved when considering their geological origin. As a result, the inundation response classification system could be applied to ephemeral playas in other arid and semiarid landscapes.

Abstract Here, we present a first assessment of the US Department of Agriculture’s (USDA) “Grass-Cast Southwest,” which is a forecasting tool for rangeland aboveground net primary productivity (ANPP) for the southwest region of the United States. Our results show that ANPP forecasts in early April were relatively close to the observation-based ANPP estimates in late May for all years evaluated ( R = 0.6–0.9). The relatively high predictability of spring rangeland productivity in this region is likely because it is strongly driven by antecedent winter/early spring precipitation. Conversely, the first summer forecasts produced in June did not consistently predict the final observation-based ANPP estimates in late August ( R = −0.5–0.7), likely because summer rangeland productivity in this region is highly dependent on variable, less predictable precipitation from the North American Monsoon (NAM). Antecedent El Niño Southern Oscillation (ENSO) indices could be used to improve Grass-Cast Southwest performance in both the spring and summer. The ENSO JFM (January–March) index was significantly positively correlated with rangeland productivity during the spring season, whereas ENSO MAM (March–May) was significantly negatively correlated with rangeland productivity during the summer season.

Climate is a key factor shaping plant communities. Given its high temporal variability, understanding the mechanisms behind community dynamics is essential. This review examines the role of climatic drivers such as the Intertropical Convergence Zone, North American Monsoon, Cold Fronts, El Niño-Southern Oscillation, and Atlantic Meridional Overturning Circulation, in both the present and the late Pleistocene and Holocene, using evidence from sedimentary proxies. We conducted a meta-analysis of palynological records from Mexico based on sedimentary sequences covering at least one glacial and one interglacial period. We focus on Marine Isotope Stages (MIS) 1 (14-7.2 kyr), 2 (29-14 kyr), 3 (57-29 kyr), 5e (126-116 kyr), and 6 (191-126 kyr) to assess how climate influenced plant communities. Results show clear contrasts between glacial and interglacial periods, except for MIS 2 and 3, highlighting region-specific responses to climatic forcing. Northern Mexico was strongly affected by the North American Monsoon, producing unexpectedly wetter conditions during MIS 2 than during the Holocene. In the Trans-Mexican Volcanic Belt, elevation plays a major role: the eastern mountains generate orographic rainfall and block moisture from reaching western areas, especially during glacial periods. In southern Mexico, vegetation shifts are linked to movements of the Intertropical Convergence Zone. These dynamics drive latitudinal and elevational vegetation migrations, and sometimes the emergence of non-analog communities. Understanding how long-term climate processes have historically shaped vegetation is crucial for informing conservation strategies in the context of ongoing global warming.

Abstract. Water vapor in the upper troposphere and lower stratosphere plays a crucial role for climate, affecting radiation, chemistry, and atmospheric dynamics. This study applies a simplified Lagrangian method to reconstruct stratospheric water vapor based on satellite observations from the Stratospheric Aerosol and Gas Experiment III on the International Space Station (SAGE III/ISS) and the Aura Microwave Limb Sounder (MLS). The objective is to improve understanding of moisture enhancements in the Asian and North American monsoons and to identify the key factors contributing to reconstruction biases. The performance of Lagrangian reconstructions significantly improves with the size of trajectory ensembles but exhibits a general dry bias. The reconstruction represents the summertime local water vapor maximum well in the Asian monsoon, particularly above the tropopause, but not in the North American monsoon. The main dehydration region diagnosed from trajectories indicates that water vapor in the Asian monsoon is predominantly controlled by local tropopause temperatures. The dry bias in reconstructions below the tropopause over the Asian monsoon shows a positive correlation with convection intensity, particularly in the western part of the monsoon region, suggesting that underestimated moistening from convection may contribute to this bias. Water vapor mixing ratios in the North American monsoon are largely influenced by long-range transport from dehydrated regions over southern Asia and additional local moistening. The limited performance of reconstructions in the North American monsoon is therefore likely linked to underestimation of local convection or uncertainties in long-range transport.

Background: Post-wildfire debris flows (PFDFs) frequently threaten life, property, and infrastructure in California. To date, there is no comprehensive assessment of the spatial distribution, seasonality, atmospheric drivers, and interannual variability across the state. Aims: We develop a database of PFDF events in California for the period 2000-2024 and analyze the database to describe spatial and temporal variability and impacts of PFDFs. Methods: We use peer reviewed literature, media, and agency reports to compile the PFDF event database and various meteorological sources to classify events by storm type. Key Results and Conclusions: We identify 97 PFDF events producing 595 individual PFDFs; events occur predominantly in the Transverse Ranges and the Sierra Nevada. There is high interannual variability in PFDF events. Event frequency tends to be greatest following years with well above average area burned. PFDF events occur predominantly in the cool season (October-May) and 55% are associated with atmospheric rivers. Approximately 31% of PFDF events occur in the warm season (June-September) associated with the North American Monsoon, tropical cyclones, and other thunderstorms. Implications: Improved understanding of where, when, and why PFDFs occur supports hazard planning and mitigation efforts and allows us to track changes in a warming climate.

Climate projections suggest ecosystems could face drastic changes due to global climate change, including more severe and frequent droughts than those recorded in the last century. Paleoclimatic series provide more extensive information than that available with instrumental records, allowing for the analysis of trends and recurrence of extreme events over a longer time periods. The objective of this research was to reconstruct the precipitation variability for southwestern Chihuahua, based on the tree-ring records of Pinus arizonica Engelm. and to assess the influence of ocean atmospheric circulations like El Niño Southern Oscillation (ENSO) and the North American Monsoon (NAM) on both low- and high-frequency climate variability. We developed three dendrochronological series covering 214 years (1802–2015), 265 years (1750–2014) and 369 years (1647–2015), for the Talayotes (TAL), Predio Particular Las Chinas (PPC) and El Cuervo (CUE) sites, respectively. The 369-year regional chronology was significantly related to cumulative precipitation variability between January and September. Recurring droughts were observed at approximately 50-year intervals. This regional climate variability was significantly related (p < 0.05) to Niño 3 SST and PDSI (JJA) indices. Maximum and minimum extreme events reconstructed in the last 369 years were synchronized with ENSO events, both in the El Niño warm phase and the La Niña cold phase. These results suggest that P. arizonica tree rings record shared a common response to the regional climate that was significantly modulated by ENSO and the NAM. This is the first dendroclimatic study to reconstruct summer precipitation patterns in northern Mexico, which is valuable given the importance of this seasonal precipitation on the regional economy.

Abstract This study investigated four top‐decile precipitation regimes in Colorado (CO) using a k‐means clustering approach. Two cool‐season regimes featured northwesterly and southwesterly integrated vapor transport (IVT) impacting western CO, characterized by a high frequency of anomalous landfalling West Coast Atmospheric Rivers (ARs) that penetrated inland facilitated by an upstream trough over the Intermountain West. Another cool‐season regime with southeasterly IVT impacting eastern CO was characterized by anomalous Gulf of America ARs facilitated by a trough and cut‐off low over the Great Plains. In contrast, a warm‐season regime is influenced by IVT from Gulf of America ARs facilitated by the North Atlantic subtropical high and the North American Monsoon circulation, impacting southwestern and southeastern CO. Representative storm examples from each regime are highlighted to examine the processes and impacts of top‐decile precipitation. These regimes provide further opportunity to assess predictability associated with AR and non‐AR related precipitation extremes in CO.

Abstract Hot droughts, or compound drought and heatwaves, have a significant impact on arid regions in southwestern North America (SWNA). In the summer of 2023, SWNA experienced unusually intense hot droughts during both the daytime and nighttime, with their severity amplified three to five times above values during the prior four decades. Over this period, a significant increase was found in the coupling between nighttime and daytime conditions at local scales and across different regions of the SWNA. The hot drought was due to a suppressed North American monsoon (NAM) in response to synoptic‐scale subsidence and moisture divergence. Nevertheless, we identified a remote connection between soil moisture in upwind areas and hot drought conditions in downwind regions across the US‐Mexico border. Our findings indicate that hot droughts need to be analyzed over complete diurnal cycles and over regions that are connected through atmospheric pathways to improve their prediction, response, and mitigation.

Abstract Lightning is a critical climate variable, due to both its significance as a metric of atmospheric thresholding and its significance as a natural hazard. While lightning density is often studied as a marker of local convective dynamics, it is also a player in the larger coupled systems linking the local atmospheric column, the land surface, and dynamic moisture advection. Aiming to bridge the land‐atmosphere gap in lightning studies, the research investigates the interplay between soil moisture (SM), convective available potential energy (CAPE), precipitation, wind shear, atmospheric moisture, and lighting density. Employing spatial correlations (r) and year‐over‐year change analyses, satellite (SMAP) and reanalysis (ERA5 and NARR) data from 2016 to 2021 show the seasonal and interannual co‐evolution of lightning and its land‐atmosphere covariates. Across the continental United States (CONUS), CAPE was identified as the most effective proxy for lightning density, particularly in summer (r = 0.80). Notably, the southeastern U.S. displayed a significant connection between SM and lightning (r = 0.60), representing the role of thunderstorms in seasonal land surface moisture as well as feedbacks from the land surface to convective processes upstream of lightning. In contrast, the arid southwestern U.S., another region of high thunderstorm occurrence, exhibited reduced correlations with SM (r = 0.12), likely due to both the reduced persistence of moisture anomalies in arid regions and the relatively weaker land surface feedbacks compared to the influence of advection by the North American monsoon. The coupling of SM was most pronounced in the southeastern U.S. during the summer months (JJA), while no clear pattern was identifiable elsewhere within CONUS. Wavelet analyses suggest seasonal changes in the lead‐lag behavior of SM and lightning density, with SM commonly leading in the Southeast in JJA. Year‐to‐year change analysis during JJA revealed aligning trends, reinforcing the relationship between summertime SM and lightning. This study provides a baseline reference for coupled land and atmosphere feedbacks between terrestrial lightning, its precursors, and its effects.

Abstract This study explores the relationship between spring soil moisture and snowpack in the Colorado Plateau (CP) and the onset of the North American Monsoon (NAM) using reanalysis data. We find that increased spring soil moisture in the CP corresponds to later NAM onset dates throughout much of the southwestern United States (US) and northwestern Mexico. Anomalously moist spring soils in the CP increase the seasonal‐average surface latent heat flux and decrease sensible heat flux, resulting in lower late spring and early summer surface temperatures and a weakened thermal gradient between the land and ocean. Decreased spring sensible heat flux also corresponds to decreased early summer geopotential heights over the southwestern US, a weaker NAM circulation, and reduced moisture flux into the NAM region. NAM onset dates in years with the highest spring soil moisture or snow depth content in the CP are delayed 1 week on average in the NAM region. Conversely, NAM onset dates in years with the lowest spring soil moisture or snow depth content in the CP occur 1 week earlier on average. While increased spring snow in the CP is correlated with a delayed NAM onset, the relationship between spring albedo and NAM onset lacks statistical significance, suggesting that CP snow anomalies affect the NAM by contributing soil moisture via snowmelt, rather than directly affecting the surface energy budget. Results indicate a mechanistic pathway that links spring soil moisture and snowpack anomalies in the CP to NAM development, which will be tested in future numerical simulations.

Abstract The western United States (WUS) experienced a record-breaking drought from September 2020 to August 2021. Here, we examine how anthropogenic climate change impacted its severity by comparing its present-day meteorological and hydrological conditions with an identical drought had it played out in a preindustrial climate. These events are simulated using the Weather Research and Forecasting (WRF) Model as well as an offline calibrated land surface model. To simulate preindustrial hydrologic conditions, we modify the forcing data used to drive our offline land surface model using two different techniques: (i) implementing the pseudo–global warming method with global climate model (GCM) deltas (13 model mean) to produce dynamically downscaled changes and (ii) directly applying GCM deltas. Our dynamically downscaled simulation exhibits significant anthropogenic drought-period precipitation increases leading to enhanced streamflow in all three major WUS riverine basins. This is distinct from the hydrologic changes derived directly from GCM deltas and found in previous literature, especially across the Colorado River basin where GCMs show precipitation decreases in response to warming. These disparities may be the result of several interconnected factors, such as (i) large uncertainty in GCM precipitation changes, (ii) variation in GCM connective parameterizations, (iii) inaccurate portrayal of the North American monsoon in GCMs and WRF, and (iv) WRF grid boundary placement. Due to these shortcomings, we cannot make any conclusive statements about the drought’s climate response, but instead, we use this work as a laboratory to explore methodological and physical complexities in WUS drought attribution to support other modelers undertaking future studies. Significance Statement This study aims to better understand how human-caused climate change impacted the record-breaking western United States (WUS) drought that occurred between 2020 and 2021. We employ two widely used techniques and found considerably different results, with one showing that climate change decreased drought severity and the other showing the opposite. This disparity is particularly large across the Colorado River basin. While it is difficult to draw any sweeping conclusions about the impact of climate change from these results, we use this as an opportunity to discuss the strengths and weaknesses of these modeling techniques with the goal of helping the regional modeling community understand and confront model limitations as they continue to investigate the impact of climate change on WUS drought.

Abstract While large-scale climate drivers such as El Niño–Southern Oscillation (ENSO) have been widely studied in relation to western U.S. droughts, the influence of synoptic-scale atmospheric patterns remains less well understood. This study examines the 2021–22 drought in the western United States by analyzing synoptic-scale conditions using the fifth major global reanalysis produced by ECMWF (ERA5) and National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis data, comparing them to the 1991–2020 climatology and the nondrought year of 2019. Results show that both winters during the drought period were dominated by anomalously strong upper-level ridging and expansive surface high pressure systems over the Pacific, producing persistent blocking patterns that limited precipitation across much of the West. In summer, while surface pressure anomalies were near climatological norms, a persistent upper-level ridge continued to reinforce regional dryness. However, the North American monsoon brought substantial moisture to the interior Southwest in both 2021 and 2022, partially alleviating drought conditions in that region. These findings underscore the critical role of synoptic-scale circulation features—particularly atmospheric blocking—in determining seasonal drought severity. The study highlights the importance of incorporating synoptic-scale analysis into drought forecasting and water resource management to improve preparedness and long-term resilience. Significance Statement This study highlights how persistent high pressure blocking systems during the wet winter season can prolong drought conditions into the dry summer months in the western United States, exacerbating water scarcity and impacting ecosystems and agriculture. However, the North American monsoon plays a crucial role in delivering seasonal moisture to the Southwest, partially alleviating drought in that region. Understanding these synoptic patterns is essential for improving drought forecasting and water resource management, helping policymakers and stakeholders develop more effective mitigation strategies in an increasingly variable climate.

Influence of ENSO on moisture provenance and stable isotope dynamics during the North American Monsoon

ABSTRACT Objective The expansion of nonnative Smallmouth Bass Micropterus dolomieu into the Grand Canyon ecosystem downstream of Glen Canyon Dam is widely seen as a potential threat to native fishes, particularly the Humpback Chub Gila cypha, which is listed as threatened under the Endangered Species Act. This concern stems from observations in the Green and upper Colorado rivers, where dam-modified habitats and reservoir introductions have allowed Smallmouth Bass to become established and impact native fish species. Our objective was to compare habitat conditions between a Smallmouth Bass population center in the Green River—where Humpback Chub are rare—and two major Humpback Chub population centers in the Colorado River in Grand Canyon, where Smallmouth Bass invasion is a conservation concern, to help assess invasion risk and inform management actions downstream of Glen Canyon Dam. Methods We developed a conceptual model of Smallmouth Bass recruitment potential emphasizing the roles of temperature, turbidity, and the timing of these conditions, based on literature for Smallmouth Bass and Largemouth Bass Micropterus nigricans. Using U.S. Geological Survey gauge station data, we summarized daily temperature and turbidity patterns near a known Smallmouth Bass population center in the middle Green River (near Jensen, Utah) and compared those with conditions in the Colorado River in Glen Canyon and Grand Canyon downstream from Glen Canyon Dam. We then used our conceptual model of Smallmouth Bass early life history, along with observed turbidity and temperature patterns, to explore how irregular recruitment could affect Smallmouth Bass population dynamics and individual growth potential using an age-structured population model and a bioenergetics model. Results Although warm reservoir releases due to low water storage in Lake Powell has made temperature regimes in Glen and Grand canyons more similar to those of the middle Green River, the Colorado River ecosystem in Grand Canyon has a very different turbidity regime from that of the middle Green River. The Colorado River in Grand Canyon becomes very turbid during late summer and fall, when tributaries experience flash floods during the North American monsoon season. These periods of high turbidity coincide with critical early life stages of Smallmouth Bass, likely limiting foraging efficiency, growth, and overwinter survival. Our population model indicates that rapidly growing and sustained Smallmouth Bass populations require successful recruitment at least once every 2 years, but empirical data suggest that these conditions occur only every 5–7 years in Grand Canyon. This would make long-term establishment of self-sustaining Smallmouth Bass populations in Grand Canyon unlikely or at least highly uncertain. Conclusions Despite the presence of nonnative predators, Humpback Chub populations in Grand Canyon have expanded during the past 10–15 years, very likely due to favorably warm river temperature. While Smallmouth Bass in Glen or Marble canyons (between Glen Canyon Dam and the Little Colorado River) may pose a downstream dispersal risk (and potentially a threat to Humpback Chub populations), high turbidity conditions downstream of the Little Colorado River reduce the likelihood of persistent local Smallmouth Bass recruitment. These results suggest that Smallmouth Bass invasion risk to Humpback Chub is not uniform across the Colorado River basin and should be evaluated in the context of the different site-specific water quality and flow regime conditions, other known risks to Humpback Chub, and Humpback Chub population size in different parts of the basin.

Abstract Farmworkers in California’s Imperial Valley face a disproportionate risk of heat stress from prolonged exposure to extreme temperatures. However, heat stress is more than just extreme temperature and depends on humidity, winds, and sun exposure. These conditions can be aggregated into the wet-bulb globe temperature (WBGT), a metric endorsed by the U.S. Occupational Safety and Health Administration (OSHA) for monitoring workplace heat. To better inform heat mitigation strategies, we construct a climatology of summertime WBGT in Imperial Valley for 30 years during 1991–2020. The climatology is constructed from a dynamical downscaling of ERA5 reanalysis to 1.5 km using the Weather Research and Forecasting (WRF) Model, using quantile mapping to bias correct WBGT inputs against regional weather station observations. We find that WBGT is the highest in urban areas, where summertime daily maxima average 34°C and in rural croplands below mean sea level. National Weather Service (NWS) thresholds of WBGT that portend elevated heat stress are frequently exceeded, with 60%–70% of summer days exceeding the highest threshold of 32.2°C. WBGT also shows statistically significant warming trends over populated areas, and the onset of consecutive extreme WBGT days is occurring earlier in summer. Furthermore, breaks in the North American monsoon result in lower WBGTs, suggesting a source of intraseasonal WBGT predictability. These results underscore the need to downscale climate projections for the near future while bolstering heat mitigation and adaptation plans for vulnerable communities. Significance Statement Farmworkers in California’s Imperial Valley are at heightened risk of heat stress due to their prolonged heat exposure and unique social vulnerabilities. This heat stress can be proxied by the wet-bulb globe temperature (WBGT), which aggregates temperature, humidity, winds, and sun exposure. Using high-resolution climate modeling, we characterize summertime WBGTs in the Imperial Valley during 1991–2020. We find that the WBGT is the highest in urban areas and where croplands are below mean sea level. In fact, the average summertime WBGT is frequently above National Weather Service thresholds that indicate when rest periods should be taken. Furthermore, long-term warming trends and an earlier onset of extreme WBGT days put farmworkers increasingly at risk to heat stress. Additionally, we note that periods of relief from heat stress tend to occur during breaks in the North American monsoon, offering a route to improving forecasts of heat comfort. These results underscore the need for localized WBGT climate projections while bolstering heat mitigation and adaptation plans for vulnerable farmworkers.

Leaching dynamics of Zn2+, Pb2+ and Cu2+ in urban soils of a semi-arid region: A quantitative analysis.

Abstract Structural overshoots, where biomass is overallocated to tree leaf area compared to sapwood area, could result in lethal stress during droughts. Climate change may alter climatic cues that drive leaf area production, such as temperature and precipitation, as well as seasonal dynamics that underlie summer rainfall due to the North American Monsoon (NAM). Combined, this could lead to temporal mismatches between leaf area‐driven water demand and availability, and increased drought‐induced mortality events. We used leaf area to sapwood area ratios to investigate the prevalence of overshoots and whether overshoots increase drought‐induced mortality. We measured populations of aspen spanning the northern transition zone of the NAM during and following severe droughts. We observed increased overshoots and drought‐induced mortality in southern latitude populations that rely more on summer monsoon rainfall. Changes in convective activity from low snowpack the preceding winter may be a climatic driver of heightened summer monsoon rainfall in the region and therefore may also trigger increased production of leaf area during wetter summers. Our results suggest that an overshoot of leaf area to sapwood area (AL:AS) ratios is associated with drought‐induced tree mortality and highlight that climate‐change driven alterations to the NAM could have major consequences for tree species' acclimation to environmental change. Read the free Plain Language Summary for this article on the Journal blog.