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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.

The latitudinal position of the subtropical jet over the Himalayas (Himalayan jet latitude or HJL) controls the region's climate during winter and spring by guiding moisture‐delivering storms. Here we use the Community Earth System Model‐Last Millennium Ensemble to diagnose forced trends in HJL during the past millennium. During 850–1849, there is a weak equatorward trend in winter HJL. In contrast, the spring HJL has a relatively larger poleward trend, and increases in both variance and frequency of poleward/equatorward excursions. We demonstrate changes in orbital precession reduced the thermal gradient between tropical and subtropical Asia, shifting the spring HJL poleward. During 1850–2005, the spring HJL exhibits no trend due to compensating influences from orbital and anthropogenic greenhouse gas forcings. These findings suggest it is essential climate models properly simulate the effects of and potential interactions between orbital forcing and anthropogenic factors to accurately project Himalayan jet variability and associated storm tracks.

Stable oxygen isotopic ratios in corals (δ¹⁸O_coral) are commonly utilized to reconstruct climate variability beyond the limit of instrumental observations. These measurements provide constraints on past seawater temperature, due to the thermodynamics of isotopic fractionation, but also past salinity, as both salinity and seawater δ¹⁸O (δ¹⁸O_sw) are similarly affected by precipitation/evaporation, advection, and other processes. We use historical observations, isotope‐enabled model simulations, and the PAGES Iso2k database to assess the potential of δ¹⁸O_coralto provide information on past salinity. Using ‘‘pseudocorals’’ to represent δ¹⁸O_coralas a function of observed or simulated temperature and salinity/δ¹⁸O_sw, we find that δ¹⁸O_swcontributes up to 89% of δ¹⁸O_coralvariability in the Western Pacific Warm Pool. Although uncertainty in the δ¹⁸O_sw‐salinity relationship influences the inferred salinity variability, corals from these sites could provide valuable δ¹⁸O_swreconstructions. Coordinated in situ monitoring of salinity and δ¹⁸O_swis vital for improving estimates of hydroclimatic change.

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 usingin 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 andin 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.

Natural and social systems worldwide are impacted by climate modes such as the El Niño/Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), making it imperative to understand their sensitivity to climate change. Paleoclimate studies extend the observational climate baseline, and speleothem records (δ¹⁸O_spel) are a common data source. However, relationships between δ¹⁸O_speland climate modes are uncertain; climate models provide a way to test the strength and stability of these relationships. Here, we use the isotope‐enabled Community Earth System Model's Last Millennium Ensemble combined with a forward proxy model to delineate the global expression of modal variability in “pseudo‐stalagmite” (δ¹⁸O_spel) records worldwide. The modeled δ¹⁸O_spelspatially correlates with modal signatures. However, substantial changes in modal variance only modestly affect individual δ¹⁸O_spelvariance. A network of δ¹⁸O_spelrecords, particularly one that straddles the Pacific, significantly improves the reconstructability of ENSO variance.

Teleconnection rainfall over North America may be systematically altered by tropical Pacific mean state changes. Characterizing teleconnection changes to improve prediction requires many realizations of ENSO events, but twentieth century data are temporally limited. To extend twentieth century records, we evaluate ENSO events in a new last‐millennium paleoclimate data assimilation reconstruction to deduce how mean state changes affect the magnitude/extent of ENSO‐driven rainfall in the United States. Despite global cooling during the Little Ice Age, the central‐eastern tropical Pacific warms relative to the Medieval Climate Anomaly, shifting teleconnections eastward and increasing rainfall anomalies in the southwestern United States. Teleconnections strengthen independently of ENSO amplitude; we thus suggest caution in using paleoclimate reconstructions of teleconnection rainfall as a proxy for ENSO amplitude. We demonstrate teleconnection rainfall is sensitive to the pattern of tropical Pacific mean SST changes, underscoring the importance of reducing uncertainties in future warming patterns in the tropical Pacific.

Monsoon responses to eruptions over the last millennium (LM) are examined in an ensemble of climate simulations as a function of eruption hemisphere. A composite analysis reveals a particularly strong sensitivity of monsoon rainfall in the year following Northern Hemisphere (NH) extratropical eruptions. Additional analysis focusing on the 18th century eruption of Mt. Laki and idealized simulations representing an analogue Southern Hemisphere eruption (SH‐Laki) reveal monsoon responses that are approximately symmetric across hemispheres, despite exhibiting asymmetries in other aspects of the climate response. We conclude that 1) latitudinally mirrored eruptions result in approximately symmetric monsoon responses, 2) disproportionate weakening (strengthening) of NH (SH) monsoons by NH eruptions over the LM resulted in part from their relatively high latitudes, and 3) uncertainty in eruption latitude fundamentally limits our ability to accurately simulate associated monsoon and tropical precipitation responses in nature.

Explosive volcanic eruptions are one of the largest natural climate perturbations, but few observational constraints exist on either the climate responses to eruptions or the properties (size, hemispheric aerosol distribution, etc.) of the eruptions themselves. Paleoclimate records are thus important sources of information on past eruptions, often through the measurement of oxygen isotopic ratios (δ¹⁸O) in natural archives. However, since many processes affectδ¹⁸O, the dynamical interpretation of these records can be quite complex. Here we present results from new, isotope‐enabled members of the Community Earth System Model Last Millennium Ensemble, documenting eruption‐inducedδ¹⁸O variations throughout the climate system. Eruptions create significant perturbations in theδ¹⁸O of precipitation and soil moisture in central/eastern North America, via excitation of the Atlantic Multidecadal Oscillation. Monsoon Asia and Australia also exhibit strong precipitation and soilδ¹⁸O anomalies; in these cases,δ¹⁸O may reflect changes to El Niño‐Southern Oscillation phase following eruptions. Salinity and seawaterδ¹⁸O patterns demonstrate the importance of both local hydrologic shifts and the phasing of the El Niño‐Southern Oscillation response, both along the equator and in the subtropics. In all cases, the responses are highly sensitive to eruption latitude, which points to the utility of isotopic records in constraining aerosol distribution patterns associated with past eruptions. This is most effective using precipitationδ¹⁸O; all Southern eruptions and the majority (66%) of Northern eruptions can be correctly identified. This work thus serves as a starting point for new, quantitative uses of isotopic records for understanding volcanic impacts on climate.

Because of the pervasive role of water in the Earth system, the relative abundances of stable isotopologues of water are valuable for understanding atmospheric, oceanic, and biospheric processes, and for interpreting paleoclimate proxy reconstructions. Isotopologues are transported by both large‐scale and turbulent flows, and the ratio of heavy to light isotopologues changes due to fractionation that can accompany condensation and evaporation processes. Correctly predicting the isotopic distributions requires resolving the relationships between large‐scale ocean and atmospheric circulation and smaller‐scale hydrological processes, which can be accomplished within a coupled climate modeling framework. Here we present the water isotope‐enabled version of the Community Earth System Model version 1 (iCESM1), which simulates global variations in water isotopic ratios in the atmosphere, land, ocean, and sea ice. In a transient Last Millennium simulation covering the 850–2005 period, iCESM1 correctly captures the late‐twentieth‐century structure of δ¹⁸O and δD over the global oceans, with more limited accuracy over land. The relationship between salinity and seawater δ¹⁸O is also well represented over the observational period, including interbasin variations. We illustrate the utility of coupled, isotope‐enabled simulations using both Last Millennium simulations and freshwater hosing experiments with iCESM1. Closing the isotopic mass balance between all components of the coupled model provides new confidence in the underlying depiction of the water cycle in CESM, while also highlighting areas where the underlying hydrologic balance can be improved. The iCESM1 is poised to be a vital community resource for ongoing model development with both modern and paleoclimate applications.