Search

The timing and geographic extent of a potential “4.2 ky event” remain highly contested. Here we present records of ENSO variability at 3.8 kyBP and 4.1 kyBP derived from a Borneo stalagmite, which suggest a significant change in ENSO properties between these time intervals. The Borneo records show evidence of significantly reduced ENSO activity at 4.1 kyBP, relative to other measured windows within the Holocene. This reduced ENSO activity coincides with a period of drier conditions and enhanced dust events in the Middle East that took place ∼4.0–4.3 kyBP. The Borneo records show evidence of enhanced ENSO activity at 3.8 kyBP. Various hydroclimate changes attributed to the “4.2 ky event” in many regions may thus be reflecting a shift from reduced to enhanced El Niño activity that occurred between 3.8 kyBP to 4.0 kyBP. Research suggests that a significant climate shift may have occurred around 4,000 years ago, though the exact timing and global extent of the event remain uncertain. A change in the frequency or magnitude of the El Niño‐Southern Oscillation (ENSO) is one hypothesized cause, however there are currently few high‐resolution records which resolve ENSO variability 4,000 years ago. We present a new western tropical Pacific stalagmite‐based record of ENSO activity, which shows a significant minimum in ENSO variability around 4,100 years ago. This reduced ENSO activity coincides with a centennial‐scale climate event in the Middle East characterized by drier conditions and enhanced dust events. The Borneo record also shows a significant increase in El Niño/La Niña activity that took place by 3,800 years ago. Hydroclimate changes around 4,000 years ago found in records from multiple regions around the world may therefore reflect a shift from reduced to enhanced El Niño activity. Borneo stalagmite record shows anomalously low El Niño‐Southern Oscillation variability in a 135‐year‐long window, centered at 4.1 kyBPBorneo stalagmite record suggests an abrupt shift from low to high El Niño‐Southern Oscillation variability between 3.8 and 4.0 kyBPRecords of tropical hydroclimate change ∼4,000 years ago may reflect a significant increase in El Niño variability Borneo stalagmite record shows anomalously low El Niño‐Southern Oscillation variability in a 135‐year‐long window, centered at 4.1 kyBP Borneo stalagmite record suggests an abrupt shift from low to high El Niño‐Southern Oscillation variability between 3.8 and 4.0 kyBP Records of tropical hydroclimate change ∼4,000 years ago may reflect a significant increase in El Niño variability

The Asian Summer Monsoon (ASM) affects ecosystems, biodiversity, and food security of billions of people. In recent decades, ASM strength (as represented by precipitation) has been decreasing, but instrumental measurements span only a short period of time. The initiation and the dynamics of the recent trend are unclear. Here for the first time, we use an ensemble of 10 tree ring‐width chronologies from the west‐central margin of ASM to reconstruct detail of ASM variability back to 1566 CE. The reconstruction captures weak/strong ASM events and also reflects major locust plagues. Notably, we found an unprecedented 80‐year trend of decreasing ASM strength within the context of the 448‐year reconstruction, which is contrary to what is expected from greenhouse warming. Our coupled climate model shows that increasing anthropogenic sulfate aerosol emissions over the Northern Hemisphere could be the dominant factor contributing to the ASM decrease. Monsoonal rainfall has a certain influence on agriculture and industry in the regions of Asian Summer Monsoon (ASM). An understanding of the spatial‐temporal variability of the ASM and the associated dynamics is vital for terrestrial ecosystems, water resources, forests, and landscapes. We have developed a 448‐year ASM reconstruction back to 1566 CE using 10 tree ring chronologies from the margin region of ASM. We find that historical severe droughts and locust plague disasters during weak ASM events. The recent decreasing ASM trend persisting for over 80 years is unprecedented over the past 448 years. Coupled climate models show that increasing anthropogenic aerosol emissions are the dominant underlying factor. Our aim is that the time series will find a wide range of utility for understanding past climate variability and for predicting future climate change. A 448‐year Asian Summer Monsoon reconstruction that extends back to 1566 CE was developed using an ensemble of 10 tree ring chronologies from northwest ChinaThe recent 80‐year decreasing trend of the Asian Summer Monsoon was unprecedented over the past 448 yearsCoupled climate models showed that the unprecedented decreasing trend was likely due to increasing anthropogenic aerosols

This observational study uses high‐resolution data to investigate mesoscale signatures of the North Atlantic Oscillation (NAO) in the atmosphere and ocean. The production of spatially high‐pass (“mesoscale”) filtered kinetic energy by buoyancy and shear effects in the atmosphere, along with sea surface temperature and surface currents in the ocean is described, and a difference with NAO phase is found. In positive NAO winters, an extension of the Gulf Stream warm core is observed, along with a displacement and acceleration of surface currents. Mesoscale activity in the atmosphere appears to follow this extension of the warm core of the Gulf Stream. This, we suggest, is a new feature of NAO air‐sea interactions which is embedded within the basin‐scale NAO patterns discussed previously in the literature. Further downstream, mesoscale activity seems to reflect more passively the general poleward migration of synoptic activity in positive NAO conditions. Robust atmospheric mesoscale signals are embedded within basin‐scale NAO variability in the western North AtlanticThis mesoscale variability might be a response to changes in the temperature and velocity structure of the Gulf Stream driven by the NAOStudy examines the new high‐resolution ERA5 atmospheric reanalysis

In tropical paleoclimate studies, paleo‐precipitation is often reconstructed from proxies via the “amount effect,” that is, the empirical inverse relationship between local precipitation amount (P) and the oxygen isotopic composition of precipitation (δ18OP). However, recent research has illustrated numerous microphysical and dynamical controls on δ18OPthat do not necessarily covary with P, complicating the reconstruction of circulation features like the Intertropical Convergence Zone. Here we introduce a new conceptual and statistical model for δ18OPthat better captures the physical foundations for δ18OPas a tracer of hydrological balance. We find that bulk precipitation microphysics and cloud type exert comparable influences on δ18OP. Moisture transport plays an important secondary role in regions of deep atmospheric convection such as the Intertropical Convergence Zone and Indo‐Pacific Warm Pool. Our findings help reconcile conflicting interpretations of Intertropical Convergence Zone excursions, and provide a firm physical grounding for more nuanced, accurate interpretations of past hydroclimate using water isotope proxies. The oxygen isotopic composition of tropical precipitation is a powerful tool for “fingerprinting” the history of evaporation, condensation, and transport that water was subjected to in the atmosphere before it reached the ground as precipitation. For this reason, the oxygen isotopic composition of precipitation is commonly employed as a water cycle tracer, both in modern‐day contexts and in geologic archives. Translating oxygen isotope ratios into metrics of circulation and climate is not always straightforward, however, due to the range of processes that can affect precipitation. Here we introduce a novel conceptual and statistical framework for interpreting oxygen isotope ratios in tropical precipitation by deconvolving its multiple competing influences. We find that the relative importance of each factor varies geographically. Moisture source is particularly important around the Indo‐Pacific Warm Pool, while cloud type exerts strong influence in regions where stratiform clouds are abundant. These results help to reconcile conflicting interpretations of how the Intertropical Convergence Zone and other key features of tropical circulation respond to climate forcings, which are critical questions for past climate reconstructions as well as future climate projections. We present a new conceptual framework and statistical model for the oxygen isotopic composition of precipitation at tropical stationsCloud type is leading influence where stratiform rain is abundant; moisture transport plays key role along tropical rain belt perimetersδ18OPand Precip are both water cycle integrators, but δ18OPis a more reliable proxy for large‐scale hydrological processes than Precip

Paleoclimate reconstructions of El Niño/Southern Oscillation (ENSO) behavior often rely on oxygen isotopic records from tropical corals (δ18O). However, few reef‐based observations of physical conditions during El Niño events exist, limiting our ability to interpret coral δ18O. Here we present physical and geochemical measurements from Palmyra Atoll (5.9°N, 162.1°W) from 2014–2017, along with a data assimilation product using the isotope‐enabled Regional Ocean Modeling System (isoROMS). Coral δ18O signals are comparably strong in 2014–2015 and 2015–2016; notably, over 50% of the signal is driven by seawater δ18O, not temperature. If a constant seawater δ18O:salinity relationship were present, this would imply a comparable salinity anomaly during both events. However, salinity changes are much larger during 2014–2015, indicating a highly nonstationary relationship. isoROMS then shows that advection strongly influences δ18O during both the 2014–2015 and 2015–2016 El Niño, driving differences in the salinity/seawater δ18O relationship. This demonstrates the need for considering ocean dynamics when interpreting coral δ18O. Coral reefs record past El Niño events through changes to their oxygen isotopic composition, but since very few observations exist of local reef conditions during El Niño, the exact effects of El Niño on local δ18O are not well known. We measured oceanographic conditions at Palmyra Atoll during two El Niño events, along with the isotopic compositions of seawater, rainwater, and corals themselves. Salinity changes were much stronger during the smaller 2014–2015 El Niño, but the coral and seawater δ18O changes were comparable—similar coral δ18O changes came about for different reasons during these two events. Ocean dynamics appears to be the dominant factor in coral and seawater δ18O anomalies. This indicates that properly interpreting results from coral δ18O records requires considering how ocean circulation has varied in the past. A new set of physical oceanographic and oxygen isotopic observations was collected at Palmyra Atoll, spanning two El Niño eventsPalmyra experienced stronger seawater δ18O changes during the weak 2014–2015 El Niño than during the strong 2015–2016 El NiñoOceanic dynamics strongly affect coral and seawater δ18O, and must be considered when interpreting coral‐based reconstructions A new set of physical oceanographic and oxygen isotopic observations was collected at Palmyra Atoll, spanning two El Niño events Palmyra experienced stronger seawater δ18O changes during the weak 2014–2015 El Niño than during the strong 2015–2016 El Niño Oceanic dynamics strongly affect coral and seawater δ18O, and must be considered when interpreting coral‐based reconstructions

Thank you to those who reviewed in 2017 for Geophysical Research Letters.

Reconstructions of ocean temperatures prior to the industrial era serve to constrain natural climate variability on decadal to centennial timescales, yet relatively few such observations are available from the west Pacific Warm Pool. Here we present multiple coral‐based sea surface temperature reconstructions from Yongle Atoll, in the South China Sea over the last ~1,250 years (762–2013 Common Era [CE]). Reconstructed coral Sr/Ca‐sea surface temperatures indicate that the “Little Ice Age (1711–1817 CE)” period was ~0.7°C cooler than the “Medieval Climate Anomaly (913‐1132 CE)” and that late 20th century warming of the western Pacific is likely unprecedented over the past millennium. Our findings suggest that the Western Pacific Warm Pool may have expanded (contracted) during the Medieval Climate Anomaly (Little Ice Age), leading to a strengthening (weakening) of the Asian summer monsoon, as recorded in Chinese stalagmites. Tropical Pacific climate variability influences global climate system on interannual and longer timescales. In contrast to the eastern and central tropical Pacific, where paleo–sea surface temperature data are relatively abundant, such data are fairly sparse in the western tropical Pacific, which limits our understanding of Pacific Warm Pool and its role in low‐frequency climate variability. Here we present a multiple coral‐based estimates of monthly resolved sea surface temperature from the Yongle Atoll, South China Sea, that span segments of the last 1,250 years. We demonstrate significant variations in mean sea surface temperature over this period—with warmer conditions during the so‐called “Medieval Climate Anomaly” and cooler conditions during the so‐called “Little Ice Age,” in keeping with compilations of Northern Hemisphere temperature over this period. Our data also reveal unprecedented warming of sea surface temperatures in the late 20th century and early 21st century that are unprecedented in the past 1,250 years. We believe that our manuscript will be of broad interest to geologists, climatologists, and coral reef scientists, given the paucity of climate records from the region. Coral Sr/Ca reconstructions indicate interannual to centennial‐scale SST variations in the tropical western Pacific over the past 1,250 yearsWestern tropical Pacific SST was relatively cool during the “Little Ice Age” and warm during the “Medieval Climate Anomaly”The late 20th century is the warmest interval of the last millennium in the western tropical Pacific Ocean

Appreciation of Peer Reviewers for 2015

Major volcanic eruptions generate widespread ocean cooling, which reduces upper ocean stratification. This effect has the potential to increase nutrient delivery into the euphotic zone and boost biological productivity. Using externally forced last millennium simulations of three climate/Earth System models (Model for Interdisciplinary Research On Climate (MIROC), Community Earth System Model (CESM), and LOch‐Vecode‐Ecbilt‐CLio‐agIsm Model (LOVECLIM)), we test the hypothesis that large volcanic eruptions intensify nutrient‐driven export production. It is found that strong volcanic radiative forcing enhances the likelihood of eastern Pacific El Niño‐like warming in CESM and LOVECLIM. This leads to an initial reduction of nutrients and export production in the eastern equatorial Pacific. However, this initial response reverses after about 3 years in association with La Niña cooling. The resulting delayed enhancement of biological production resembles the multiyear response in MIROC. The model simulations show that volcanic impacts on tropical Pacific dynamics and biogeochemistry persist for several years, thus providing a new source for potential multiyear ecosystem predictability. Major volcanic eruptions can impact multiyear biological production in the tropical PacificLong‐term ocean response to volcanic forcing is obtained in three Earth System models

Speleothem oxygen isotopes (δ18O) are often used to reconstruct past rainfall δ18O variability, and thereby hydroclimate changes, in many regions of the world. However, poor constraints on the karst hydrological processes that transform rainfall signals into cave dripwater add significant uncertainty to interpretations of speleothem‐based reconstructions. Here we present several 6.5 year, biweekly dripwater δ18O time series from northern Borneo and compare them to local rainfall δ18O variability. We demonstrate that vadose water mixing is the primary rainfall‐to‐dripwater transformation process at our site, where dripwater δ18O reflects amount‐weighted rainfall δ18O integrated over the previous 3–10 months. We document large interannual dripwater δ18O variability related to the El Niño–Southern Oscillation (ENSO), with amplitudes inversely correlated to dripwater residence times. According to a simple stalagmite forward model, asymmetrical ENSO extremes produce significant offsets in stalagmite δ18O time series given different dripwater residence times. Our study highlights the utility of generating multiyear, paired time series of rainfall and dripwater δ18O to aid interpretations of stalagmite δ18O reconstructions. Six year northern Borneo dripwater isotope time series dominated by ENSO variabilityMixing of vadose waters governs rainfall‐to‐dripwater isotope transformationDifferent flow pathways produce different dripwater time series from same input

Recent paleoclimate reconstructions have suggested millennial‐scale variability in the Indo‐Pacific Warm Pool region coincident with events of the last deglaciation. Here, we present a new stalagmite oxygen isotope record from northern Borneo, which today is located near the center of the region's mean annual intertropical convergence zone. The record spans the full deglaciation, and reveals for the first time distinct oxygen isotope variations at this location connected with the Bølling‐Allerød onset and the Younger Dryas event. The full deglaciation in the Borneo stalagmite proxy reconstruction appears remarkably similar to a 20–11 ka transient simulation of rainfall over the area produced using the isotope‐enabled Community Earth System Model. In this model, periods of weakened Atlantic Ocean meridional overturning circulation are associated with an anomalous Western North Pacific anticyclone, which is produced in boreal autumn and shifts south over Borneo during boreal winter, causing dry conditions. Here, we aim to resolve conflicting evidence of how tropical convection in Borneo in the Indo‐Pacific oceanic region, a critical region associated with El Niño events and the Asian and Australian‐Indonesian monsoon systems, was affected by past changes in the strength of Atlantic Ocean circulation. Analyzing a 20,000+ year old Borneo cave stalagmite along its growth axis, we find large oxygen isotope shifts coincident with two prominent millennial‐scale periods of reduced Atlantic Ocean overturning circulation strength. The isotope record is similar to a neighboring southwestern Philippine stalagmite record, and we interpret the signal as drier mean annual conditions over the region during these events compared to the background state; an interpretation supported by simulations in three independent state‐of‐the‐art climate models. One model suggests that these conditions in Borneo were driven by the southward shift of an anomalous anticyclone in the boreal winter season. By combining geochemical and model evidence, this study has shown that rainfall decreased for millennial periods during the end of the ice age, when the Atlantic Ocean overturning circulation weakened. In doing so, we have elucidated further how this region responds to major changes in global climate and ocean conditions. A new stalagmite record reveals for the first time distinct Bølling‐Allerød and Younger Dryas oxygen isotope variations in northern BorneoThe stalagmite oxygen isotope pattern strongly resembles the recent iCESM transient annual rainfall simulation from 20 to 11 kaBorneo drying during Heinrich 1 and the Younger Dryas may be attributed to an anomalous boreal winter Western North Pacific anticyclone A new stalagmite record reveals for the first time distinct Bølling‐Allerød and Younger Dryas oxygen isotope variations in northern Borneo The stalagmite oxygen isotope pattern strongly resembles the recent iCESM transient annual rainfall simulation from 20 to 11 ka Borneo drying during Heinrich 1 and the Younger Dryas may be attributed to an anomalous boreal winter Western North Pacific anticyclone

Boreal winter near‐surface atmospheric circulations over the Hawaiian region are known to influence the state of the tropical Pacific and initiate the development of El Niño–Southern Oscillation (ENSO) events. Here we show that these same preceding near‐surface circulations have an additional influence on the longitudinal position of the resultant ENSO‐related sea surface temperatures (SSTs) as well, with warm (cold) events systematically shifted to the east (west) of the typical SST anomalies. In influencing this positioning, these atmospheric circulations in turn modify the near‐ and far‐field climate responses to these SSTs such that during warm events, the typical ENSO‐related responses east (west) of the dateline are generally enhanced (reduced); conversely, during cold events, the typical ENSO‐related responses are generally reduced (enhanced). The fact that the extratropical atmospheric circulations in question influence the asymmetry of ENSO extremes with a 12 month lead time carries important implications for predicting the socioeconomic impacts of these events. Extratropical SLP changes impact the longitudinal structure of ENSO eventsThese impacts are found in observations and coupled‐climate model simulationsSLP‐induced ENSO asymmetries result in appreciable teleconnection differences

The tropical Pacific influences climate patterns across the globe, yet robust constraints on decadal to centennial‐scale climate variations are difficult to extract from sparse instrumental observations in this region. Oxygen isotope (δ18O) records from long‐lived corals enable the quantitative reconstruction of tropical Pacific climate variability and trends over the twentieth century and beyond, but such corals are exceedingly rare. Here, we use multiple short coral δ18O records to create a coral ‘ensemble’ reconstruction of twentieth century climate in the central tropical Pacific. Ten U/Th‐dated fossil coral δ18O records from Kiritimati Island (2°N, 157°W) span 1891 CE to 2006 CE, with the younger samples enabling quantitative comparison to a large ensemble of modern coral records and instrumental sea surface temperature. A composite record constructed of modern and fossil Kiritimati coral δ18O records shows a shift toward warmer and fresher conditions from 1970 CE onward, consistent with previously published records in this region. The tropical Pacific plays a major role in shaping global temperature and rainfall patterns, but sparse instrumental observations prior to ∼1950 CE hamper the detection of potential anthropogenic climate signals against the rich background of natural variability in this region. This study demonstrates the utility of reconstructing twentieth century climate trends using an ensemble of multiple, short coral records in data‐sparse regions of the tropics. When applied to the central equatorial Pacific, coral data from Kiritimati Atoll resolve a twentieth century shift toward warmer and wetter conditions in this region, in line with evidence from continuous, century‐long coral records previously published from this region. Ensembles of short fossil coral δ18O records yield robust estimates of nineteenth‐twentieth century climate variability and trendsSeawater δ18O contributions to Kiritimati coral δ18O are larger on multi‐decadal timescales than on interannual timescalesCoral δ18O records reveal a trend toward warmer and wetter conditions in the central equatorial Pacific over the twentieth century Ensembles of short fossil coral δ18O records yield robust estimates of nineteenth‐twentieth century climate variability and trends Seawater δ18O contributions to Kiritimati coral δ18O are larger on multi‐decadal timescales than on interannual timescales Coral δ18O records reveal a trend toward warmer and wetter conditions in the central equatorial Pacific over the twentieth century

Coral oxygen isotopes (δ18O) from the central equatorial Pacific provide monthly resolved records of El Niño‐Southern Oscillation activity over past centuries to millennia. However, calibration studies using in situdata to assess the relative contributions of warming and freshening to coral δ18O records are exceedingly rare. Furthermore, the fidelity of coral δ18O records under the most severe thermal stress events is difficult to assess. Here, we present six coral δ18O records and in situtemperature, salinity, and seawater δ18O data from Kiritimati Island (2°N, 157°W) spanning the very strong 2015/16 El Niño event. Local sea surface temperature (SST) anomalies of +2.4 ± 0.4°C and seawater δ18O anomalies of −0.19 ± 0.02‰ contribute to the observed coral δ18O anomalies of −0.58 ± 0.05‰, consistent with a ∼70% contribution from SST and ∼30% from seawater δ18O. Our results demonstrate that Kiritimati coral δ18O records can provide reliable reconstructions even during the largest class of El Niño events. Oxygen isotope anomalies in coral skeletons are a well‐established proxy for changes in tropical Pacific Ocean temperature variations, which have a profound impact on weather extremes around the planet. However, only a handful of calibrations exist that quantify the relationship between ocean temperature and coral oxygen isotopic composition at a given site, especially across extreme events where this relationship may vary most strongly. Here we compare ocean temperature data from loggers installed on the reef at Kiritimati Island (2°N, 157°W) to coral oxygen isotopic records spanning the record‐breaking 2015/16 El Niño event. We find that oxygen isotopes in corals provide accurate reconstructions of ocean temperature extremes during this very strong El Niño event, with ∼70% of the signal originating from ocean temperature and the remainder from increased rainfall. An ensemble of coral oxygen isotope timeseries from the central equatorial Pacific tracks the 2015/16 El Niño eventCoral oxygen isotope records reflect ∼70% contribution from warming and ∼30% from freshening during the 2015/16 El Niño eventIn situseawater oxygen isotope data provide quantitative constraints on temperature versus hydrological contributions to coral records An ensemble of coral oxygen isotope timeseries from the central equatorial Pacific tracks the 2015/16 El Niño event Coral oxygen isotope records reflect ∼70% contribution from warming and ∼30% from freshening during the 2015/16 El Niño event In situseawater oxygen isotope data provide quantitative constraints on temperature versus hydrological contributions to coral records

Atmospheric rivers (ARs) are responsible for the vast majority (∼88%) of flood damage in the Western U.S., an annual average of USD$1.1 billion. Here, using historical flood insurance data, we investigate the genesis characteristics of damaging ARs in the Western U.S.. ARs exceeding USD$20 million in damage (90th percentile), are shown to travel further across the Pacific Ocean, with median genesis locations 8°–27° further westward compared to typical ARs. Identifying regions of preferential genesis of damaging ARs elicit important implications for AR observation campaigns, highlighting distant regions not currently considered for AR reconnaissance. The genesis of damaging ARs is associated with elevated upper‐level zonal wind speeds along with deeper cyclonic anomalies, most pronounced for Washington and Oregon ARs. Linking AR dynamics and lifecycle characteristics to economic damage provides an opportunity for impact‐based forecasting of ARs prior to landfall, supporting efforts to mitigate extreme flood damages. We examine the economic impact of atmospheric rivers (ARs), corridors of elevated atmospheric moisture vapor transport, on the Western U.S.. Atmospheric rivers are responsible for the vast majority of flood damage which is experienced in the Western U.S. causing mean annual damages of USD$1.1 billion, 88% of the total annual flood damage. We use 40 years of historic flood insurance claims paired with a database of all ARs that make landfall on the U.S. West Coast to show, for the first time, the relationship between atmospheric river dynamics and the economic impact for the Western. U.S.. Our results show that damaging ARs tend to originate further from the U.S. coastline than all other, less damaging ARs. We also demonstrate the atmospheric conditions favorable for the initiation of damaging ARs with a particular focus on ARs that travel across the entire Pacific Ocean prior to landfall. Clear implications arise for atmospheric river data collection, identifying important regions which are not current examined where damaging ARs generate. The results presented aid in understanding the cause of damaging floods in the Western U.S. with the aim of supporting atmospheric river forecasts by linking atmospheric conditions to historic damage. Historical flood damages from the Western U.S. are paired with an atmospheric river lifecycle tracking algorithmDamaging atmospheric rivers (ARs) tend to have genesis locations further from the coastline, travel further and have a higher moisture fluxThe genesis of damaging ARs is associated with distinct dipole pressure anomalies paired with an elevated zonal jet stream Historical flood damages from the Western U.S. are paired with an atmospheric river lifecycle tracking algorithm Damaging atmospheric rivers (ARs) tend to have genesis locations further from the coastline, travel further and have a higher moisture flux The genesis of damaging ARs is associated with distinct dipole pressure anomalies paired with an elevated zonal jet stream

Atmospheric rivers (ARs) are the primary mechanism for mid‐latitude water vapor transport, and are identified by a key variable, integrated water vapor transport (IVT). The ability of atmospheric reanalyses in providing a ground‐truth dataset for the IVT field is assessed by comparing ERA5, MERRA‐2, and JRA‐55 data against a large sample (>1,700) of dropsonde profiles deployed in and around ARs. Bias and error increase with IVT magnitude, although asymmetrically around the AR core. A partitioning of the source of error reveals that humidity contributes more to the difference in IVT above 800 hPa, while wind is the dominant source in the lowest levels (to 950 hPa). This quantification of reanalysis error and bias identifies ERA5 as the dataset with the lowest IVT errors and demonstrates remaining challenges in representing the observed state in ARs. Atmospheric rivers (ARs) are long, narrow features in the atmosphere that transport significant amounts of water and energy from lower to higher latitudes, and are identified by integrated water vapor transport (IVT). In this study we compare IVT from >1,700 dropsonde observations to atmospheric reanalyses, a gridded dataset of past atmospheric states constrained by observations. We identify ERA5, the most recent reanalysis with the highest spatial resolution, as the dataset with the lowest IVT errors. IVT is a product of wind and humidity, and we find that the reanalysis wind contributes more to the difference from dropsonde IVT in the lowest layers than reanalysis moisture. Quantify reanalysis error and bias compared to dropsonde integrated vapor transport (IVT)ERA5 simulates integrated vapor transport (IVT) at these point locations better in terms of correlation and standardized errorReanalysis bias and error increase with integrated vapor transport (IVT) magnitude, although asymmetrically around the atmospheric river (AR) core Quantify reanalysis error and bias compared to dropsonde integrated vapor transport (IVT) ERA5 simulates integrated vapor transport (IVT) at these point locations better in terms of correlation and standardized error Reanalysis bias and error increase with integrated vapor transport (IVT) magnitude, although asymmetrically around the atmospheric river (AR) core

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

Time series of cave drip water oxygen isotopes (δ18O) provide site‐specific assessments of the contributions of climate and karst processes to stalagmite δ18O records employed for hydroclimate reconstructions. We present ~12‐year‐long time series of biweekly cave drip water δ18O variations from three sites as well as a daily resolved local rainfall δ18O record from Gunung Mulu National Park in northern Borneo. Drip water δ18O variations closely match rainfall δ18O variations averaged over the preceding 3–18 months. We observe coherent interannual drip water δ18O variability of ~3‰ to 5‰ related to the El Niño–Southern Oscillation (ENSO), with sustained positive rainfall and drip water δ18O anomalies observed during the 2015/2016 El Niño. Evidence of nonlinear behavior at one of three drip water monitoring sites implies a time‐varying contribution from a longer‐term reservoir. Our results suggest that well‐replicated, high‐resolution stalagmite δ18O reconstructions from Mulu could characterize past ENSO‐related variability in regional hydroclimate. Cave stalagmites allow for the reconstruction of past regional rainfall variability over the last hundreds of thousands of years with robust age control. Such reconstructions rely on the fact that differences in the isotopic composition of rainwater set by regional rainfall patterns is preserved as the rainwater travels through cave bedrock to feed the cave drip waters forming stalagmites. Long‐term monitoring of rainwater and cave drip water isotopes ground truth the climate to stalagmite relationship across modern‐day changes in regional rainfall. Twelve years of monitoring data presented in this study identify individual El Niño–Southern Oscillation events in rainfall and cave drip water isotopic composition, providing a strong foundation for stalagmite‐based climate reconstructions from this site. Three 12‐year‐long cave drip water δ18O time series capture El Niño and La Niña events in northern BorneoEstimates of karst residence times range from 3 to 18 months, with a secondary contribution from a longer‐term reservoir at one drip siteDrip water nonstationarity implies multiple stalagmites are required to reconstruct El Niño–Southern Oscillation variability over time