In this study, we investigate the changes in water isotope ratios in the Indian summer monsoon precipitation (delta 18Oprecip) during the Last Glacial Maximum (LGM, similar to 21 ka Before Present) compared to the pre-industrial (PI) period, and the mechanisms driving these changes, using a general circulation model with water isotope and novel water vapor source-tagging capabilities.During the LGM, the model simulates a substantial reduction (15 %) in monsoon precipitation over the Indian subcontinent, consistent with proxy records. This drying in LGM is associated with reduced atmospheric water vapor, a thermodynamic response to cooling, while the westerly circulation, a dynamics response, is strengthened over parts of the subcontinent. Additionally, zonal temperature gradients between a relatively less-cooled tropical Western Pacific Ocean and Indian subcontinent lead to anomalous subsidence over the Indian region, enhancing the drying. Water vapor source tagging shows that while the four dominant moisture sources for the monsoon (South Indian Ocean, Arabian Sea, Indian land recycling, and Central Indian Ocean) remained the same, their contributions were reduced during the LGM. The delta 18Oprecip values over the Indian monsoon region are enriched by approximately 1 parts per thousand in the LGM simulation, and we find that this enrichment was not driven by the local amount effect. A decomposition analysis shows that the enrichment was primarily caused by reduced contributions from distant, isotopically depleted water vapor sources and secondarily by reduced rainout during moisture transport from the Indian Ocean.These findings have important implications for paleoclimate reconstructions, suggesting that delta 18O records from the Indian region could be indicators of broad-scale atmospheric circulation rather than being direct proxies for local precipitation amount.

We present the first results of the Water Isotope Model Intercomparison Project (WisoMIP), with Phase 1 focused on modern simulations (1979-2023) from a suite of isotope-enabled atmospheric general circulation models nudged to ERA5 reanalyzes. Water sources, mixing, and rainout history influence the isotopic composition of vapor and precipitation, making these simulations powerful tools for tracing the global water cycle. By prescribing identical winds, sea surface temperatures, and sea ice conditions, we isolate differences in water isotope behavior across models, controlling for variability in atmospheric dynamics and mean climate. Our analyses show that the ensemble mean best matches observations, as individual model errors cancel out to yield a more accurate representation of Earth's isotope distributions. We also evaluate trends and responses to major climate modes during the recent warming period, highlighting regional and temporal sensitivities in the isotope signals. These diagnostics extend beyond traditional model evaluation metrics (e.g., temperature, precipitation) to reveal uncertainties in physical processes and guide improvements in model parameterizations. The resulting modern nudged ensemble data set serves as a benchmark for isotope-enabled model development, satellite product comparison, and understanding of water cycle changes in a warming climate. Given its standardized design and broad participation, WisoMIP provides a valuable "isotope reanalysis" product for applications ranging from paleoclimate reconstruction to model tuning. Our work demonstrates the importance of coordinated isotope model evaluation in advancing the use of water isotopes as a diagnostic tool in climate science.

Although precipitation isotopes are widely used as hydroclimate proxies, limited understanding of the physical processes controlling their variability, particularly in the tropics, continues to constrain their reliable application. To better characterize these processes, we examined spatial-temporal variations of precipitation isotopes across Southeast Asia by comparing observational data with simulations from isotope-enabled GISS ModelE2.1. Observed and simulated precipitation δ¹⁸O show strong agreement across spatial and temporal scales, from seasonal to decadal variability, demonstrating the model’s ability to capture key hydrological processes. We further quantified the relative influences of local and regional processes on precipitation isotopes. A progressive decline in precipitation δ¹⁸O along monsoon tracks during the Northeast and Southwest monsoons reflects the dominant influence of large-scale monsoonal circulation. Quantitative decomposition of the controlling mechanisms shows that upstream rainout is the primary driver of seasonal precipitation δ¹⁸O variations, explaining ∼39% – 48% of the seasonal δ¹⁸O range, followed by the isotopic composition of source vapor. These findings suggest that regional atmospheric processes, including large-scale convection in both source regions and along transport trajectories, govern precipitation isotopes in Southeast Asia. Moreover, the spatially heterogeneous responses of precipitation δ¹⁸O to major climate modes, such as the El Niño–Southern Oscillation and the Indian Ocean Dipole, indicate that different regions are predominantly influenced by distinct climate drivers. By quantifying the relative impacts of local and regional processes, this study enhances the interpretation of isotope-based paleoclimate records and provides new insights into the regional climate dynamics in Southeast Asia.

Numerical water tracers are implemented in the Energy Exascale Earth System Model version 2. Simulations performed with the water-tag-enabled model for both pre-industrial and future greenhouse gas concentrations reveal a marked increase in the role of mid-latitude and southern subtropical regions as exporters of atmospheric moisture-to the extratropical upper troposphere and the tropical free troposphere. For the latter, the northward shift of the Intertropical Convergence Zone increases cross-hemispheric transport of subtropical water vapor to the Northern Hemisphere. In the polar regions, most of the lower tropospheric moistening instead arises from increases in local evaporation. These findings illustrate the utility of the water tags, underscore critical changes in global hydrologic cycle, and provide insight into atmospheric dynamics under future climate scenarios. For applications when a global grid is desired, we additionally propose a novel statistical reconstruction, based on copula modeling, of the joint distribution of origin of water vapor, which reduces the number of tracers from order to order , substantially ameliorating the considerable computational cost of water tracers. This statistical reconstruction is particularly beneficial to the interpretation of the relationship between latitude and longitude of origin of moisture over the tropical oceans and in the lower troposphere over land.

Precipitation isotopes are valuable tracers for understanding the hydrologic cycle and climate variations. Distinct from d‐excess, 17 O‐excess has recently emerged as a promising new tracer of precipitation processes because of its insensitivity to moisture source temperature. However, the control mechanisms on precipitation 17 O‐excess remain poorly understood. In this study, we evaluated the performance of the GISS‐E2.1 climate model in simulating the precipitation isotopes, focusing on 17 O‐excess. Through comprehensive analysis, we explored how variations in seawater isotopes, rain evaporation, kinetic isotope fractionation parameters, and supersaturation factors influence the simulated precipitation d‐excess and 17 O‐excess. Our findings reveal that GISS‐E2.1 accurately captures the spatial distribution and temporal variations of precipitation δ 18 O. Moreover, it reasonably reproduces the spatial patterns of precipitation d‐excess, though slightly underestimating the mean value in the low latitudes. Although most simulated 17 O‐excess values fall within the observed range, evaluating the accuracy of 17 O‐excess simulations is challenging due to the limited availability of observational data. Notably, in tropical regions, the spatiotemporal distributions of d‐excess and 17 O‐excess are sensitive to convective processes, such as rain evaporation. The model's limitations in 17 O‐excess simulation suggest that current formulations are inadequate to fully capture the variability of 17 O‐excess. This underscores the complexity of the processes influencing 17 O‐excess and highlights the need for additional data and further research to comprehensively understand its controlling factors. Our findings contribute to our understanding of the mechanisms driving the observed variation in precipitation triple oxygen isotopes and to the validation and improvement of climate models.

Describing the processes that regulate the flows and exchanges of water within the atmosphere and between the atmosphere and Earth’s surface is critical for understanding environmental change and predicting Earth’s future accurately. The heavy-to-light hydrogen and oxygen isotope ratios of water provide a useful lens through which to evaluate these processes due to their innate sensitivity to evaporation, condensation, and mixing. In this review, we examine how isotopic information advances our understanding about the origin and transport history of moisture in the atmosphere and about convective processes—including cloud mixing and detrainment, precipitation formation, and rain evaporation. Moreover, we discuss how isotopic data can be used to benchmark numerical simulations across a range of scales and improve predictive skill through data assimilation techniques. This synthesis of work illustrates that, when paired with air mass thermodynamic properties that are commonly measured and modeled (such as specific humidity and temperature), water’s isotope ratios help shed light on moist processes that help set the climate state.

Here we present a research framework for the first atmosphere-connected supermodel using state-of-the-art atmospheric models. The Community Atmosphere Model (CAM) versions 5 and 6 exchange information interactively while running, a process known as supermodeling. The primary goal of this approach is to synchronize the models, allowing them to create a new dynamical system which can theoretically benefit from each component model, in part by increasing the dimensionality of the system. In this study, we examine a single untrained supermodel where each model version is equally weighted in creating pseudo-observations. We demonstrate that the models synchronize well without decreased variability, particularly in storm track regions, across multiple timescales, and for variables where no information has been exchanged. Synchronization is less pronounced in the tropics, and in regions of lesser synchronization we observe a decrease in high-frequency variability. Additionally, the low-frequency modes of variability (North Atlantic Oscillation and Pacific North American Pattern) are not degraded compared to the base models. For some variables, the mean bias, as well as the non-interactive ensemble mean, is reduced compared to control simulations of each model version.

Modeling experiments and field campaigns have evaluated shallow convective mixing as a potential constraint on the low-cloud climate feedback, which is critical for establishing climate sensitivity. Yet the apparent relationship between low-cloud fraction and shallow convective mixing differs substantially among general circulation models (GCMs), large eddy simulations, and both remote sensing and in situ observations. Here, we consider how changes in GCMs' representations of subgrid-scale vertical moist fluxes can alter the cloud-mixing relationship. Using vertical profiles of water vapor isotope ratios (delta D) to characterize the strength of shallow convective mixing in a manner that can be compared directly to satellite observations, we evaluate the cloud-mixing relationship produced in tiered experiments with the Community Atmosphere Model (CAM). From versions 5 to 6 of CAM, the most notable physics change is CLUBB, a scheme that unifies the representation of shallow convection and boundary layer turbulence through a joint probability density function (PDF) for subgrid velocity and moisture. CLUBB reduces the covariance between low-cloud fraction and shallow convective mixing, producing a bivariate distribution that is more similar in character to monthly averaged satellite observations. Using parameter sensitivity experiments, we argue that CLUBB's ability to simulate skewness in the distribution of vertical velocity produces more isolated but stronger moist updrafts, which reduce the grid-mean low-cloud fraction while maintaining efficient hydrological connectivity between the boundary layer and the free troposphere. These results suggest that mixing is not an effective predictor of low-cloud feedback in GCMs with PDF closure schemes.

O-17-excess is a relatively new water isotope parameter that could potentially provide useful information about the hydrological cycle. Previous works focusing on O-17-excess in polar regions suggest that it primarily tracks moisture source relative humidity, but little is known about how to interpret O-17-excess data in lower latitudes. Here we present quasi-hourly triple oxygen isotope data of precipitation collected from two tropical cyclones in Texas and Louisiana in 2020 to understand the impacts of environmental and meteorological processes on the(17)O-excess of low-to mid-latitude precipitation. We find that at both hourly time scales and the event scale,O-17-excess is strongly correlated to changes in on-site rainfall intensity and relative humidity, which is consistent with the theory that the isotopic fraction at ion associated with rain re-evaporation lowers the(17)O-excess of the remaining droplet. In addition, although evaporative conditions at the moisture source region may also influence( 17)O-excess of water vapor transported to the precipitation site, their impacts are likely over printed by the post-condensation rain re-evaporation processes. Our results thus suggest that O-17-excess can be used as a proxy for local rather than source region evaporative conditions during tropical cyclones.

Abstract 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.

The quantity and characteristics of atmospheric rivers over Antarctica, which import heat and moisture towards the continent, are a major source of uncertainty in future sea level rise estimates. We employ a new variable-resolution grid over Antarctica, using CESM2 (VR-CESM2), which balances the extensibility of a GCM with the high computational costs of a high-resolution climate model. This setup uses observed sea surface temperature and sea ice concentration, implements moisture-tagging (linking precipitation to a moisture source region on the globe), and produces high spatial and temporal resolution atmosphere and ice sheet surface outputs, which can be used to detect atmospheric rivers and to estimate their impact.As a baseline for experiments testing the relative importance of large-scale drivers, we first quantify, over an idealized 10-year period, the global sources of moisture and the portion of total precipitation that reaches the ice sheet during large-scale vs atmospheric river events (and their associated synoptic characteristics). Beyond this baseline, we will use this setup to perform initial test scenarios assessing the relative impact of reduced sea ice combined with enhanced ocean heat at lower latitudes.

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.

The hydrologic cycle is a fundamental component of the climate system with critical societal and ecological relevance. Yet gaps persist in our understanding of water fluxes and their response to increased greenhouse gas forcing. The stable isotope ratios of oxygen and hydrogen in water provide a unique opportunity to evaluate hydrological processes and investigate their role in the variability of the climate system and its sensitivity to change. Water isotopes also form the basis of many paleoclimate proxies in a variety of archives, including ice cores, lake and marine sediments, corals, and speleothems. These records hold most of the available information about past hydrologic variability prior to instrumental observations. Water isotopes thus provide a ‘common currency’ that links paleoclimate archives to modern observations, allowing us to evaluate hydrologic processes and their effects on climate variability on a wide range of time and length scales. Building on previous literature summarizing advancements in water isotopic measurements and modeling and describe water isotopic applications for understanding hydrological processes, this topical review reflects on new insights about climate variability from isotopic studies. We highlight new work and opportunities to enhance our understanding and predictive skill and offer a set of recommendations to advance observational and model-based tools for climate research. Finally, we highlight opportunities to better constrain climate sensitivity and identify anthropogenically-driven hydrologic changes within the inherently noisy background of natural climate variability.

<p class="p1">Isotope ratios in water vapor record evaporation (E) and precipitation (P) along moisture transport paths. At low latitudes, the path-integrated E-P signal is dominated by local E and P, providing an indicator of tropical water balance. In contrast, at high latitudes, E and P patterns upstream overwhelm local signals, reflecting the dependence on remote moisture sources. This dependence defines the length scales of moisture transport.</p> <p class="p2"> </p> <p class="p1">In the zonal mean, moisture transport length scales can be represented visually in two dimensions by moist isentropic surfaces, along which poleward moisture transport occurs. These surfaces explain why Rayleigh distillation reasonably approximates meridional variations in high-latitude isotope ratios while also providing a physical basis for why polar isotope-temperature relationships are distinct in space and time. </p> <p class="p2"> </p> <p class="p1">Isotopically enabled GCM simulations and short-duration Antarctic ground-based observations both lend support for the isentropic view of moisture transport. They also suggest that this framework provides a simple means to predict changes in length scale in a warmer climate, assuming zonal-mean humidity changes follow Clausius-Clapeyron scaling. However, isotopic observations with the vertical resolution and temporal coverage necessary to easily evaluate recent and expected future variations in moist isentropic transport are lacking.</p> <p class="p2"> </p> <p class="p1">Here, we consider two possible alternative methods for testing predictions about long-term moisture length-scale changes with isotopic observations. Using the two-decade-long AIRS satellite record, we consider the extent to which mid-free tropospheric hydrogen isotope ratios, normalized by humidity, can provide a measure of length scale in a total-column sense. Second, we ask to what extent moist isentropic transport is set by episodic events, such as warm conveyor belts, that can be observed by infrequent but high-vertical-resolution airborne isotopic measurements. We discuss the implications of enhanced transport efficiency, expected in a warmer future, for increasing length scales and strengthening hydrological dependencies between remote locations.</p>

Climate records of ratios of stable water isotopes of oxygen (δ18O) are used to reconstruct the past Indian monsoon precipitation. Identifying the sources of water vapor is important in understanding the role of monsoonal circulation in the δ18O values, to aid in monsoon reconstructions. Here, using an isotope-enabled Earth system model, we estimate the contributions of oceanic and terrestrial water vapor sources to two major precipitation seasons in India-the Southwest monsoon and the Northeast monsoon, and their effects on the δ18O in precipitation (δ18Op). We find that the two monsoon seasons have different dominant sources of water vapor because of the reversal in atmospheric circulation. While Indian Ocean regions, Arabian Sea, and recycling are the major sources of the Southwest monsoon precipitation, North Pacific Ocean and recycling are two crucial sources of Northeast monsoon precipitation. The δ18Op of the Southwest monsoon precipitation is determined by contributions from the Indian Ocean sources and recycling. Despite reduced precipitation, more negative δ18Op values are simulated in the Northeast monsoon season due to larger negative δ18Op contributions from the North Pacific. Our results imply that changes in atmospheric circulation and water vapor sources in past climates can influence climate reconstructions using δ18O.

Climate records suggest a weaker Indian monsoon circulation and drier conditions in the continent during the Last Glacial Maximum (LGM, ~19-23 ka BP). This is mainly due to circulation changes caused by high-latitude ice sheets, tropical and high-latitude SST changes, and lower atmospheric CO2 concentrations compared to pre-industrial (PI). Such changes in boundary conditions and circulation are likely to cause changes in the water vapor sources of monsoon precipitation, with implications for precipitation reconstructions using water isotope proxies. We use the water isotope/water tagging-enabled Community Earth System Model (iCESM) to study the effects of glacial conditions on the sources of water vapor and isotope ratios of precipitation for the Indian monsoon precipitation. We conduct time slice experiments for the PI and the LGM periods following the PMIP4 guidelines. iCESM was successful in identifying the water vapor sources of present-day Indian summer monsoon precipitation, namely the Indian Ocean sources and precipitation recycling. The detailed results of this study will be presented at the meeting.

In this poster, I will present plans to complete a PlioMIP2 simulation at high resolution (HR) in the fully-coupled Community Earth System Model (CESM). This simulation is part of the NCAR Accelerated Scientific Discovery PaleoWeather project and also includes HR simulations of the Early Eocene Climatic Optimum, with CO2 reconstructed to be much higher than today’s, and the Last Glacial Maximum with much lower CO2. CESM is being configured with a resolution of 0.25° for the atmosphere and land, and 0.1° for the ocean and sea ice. This set of fully-coupled paleo-HR climate simulations for past greenhouse and icehouse climates will study the drivers that govern the characteristics of extreme weather events in both atmosphere and ocean under altered climate states. Potential applications include diagnosing changes in atmospheric synoptic features like mesoscale convective systems, tropical cyclones, monsoons, and atmospheric rivers, of coastal upwelling zones, and for their impacts on past regional precipitation patterns. In addition, the paleo-HR simulations will better resolve the landscape of topographies and coasts. Both are critical for accurate comparison with the paleoclimate record since most of the terrestrial and marine paleo-observations come from basins surrounded by mountains and near-shore locations. The paleo-HR simulations will complement the preindustrial, historical and RCP8.5 future simulations available from the iHESP project (https://ihesp.tamu.edu/), resulting in HR simulations to investigate the dynamics that connect past and future climate changes.