31 In order to complement the picture of the atmospheric water cycle in the Southern Ocean, we 32 have continuously monitored water vapor isotopes since January 2020 in Amsterdam Island 33 (37.7983 °S, 77.5378 °E) in the Indian Ocean. We present here the first 2-year-long water 34 vapor isotopic record monitored on this site. We show that the vapor isotopic composition 35 largely follows the vapor mixing ratio, as expected in marine boundary layers. However, we 36 evidence 11 cold front periods of a few days where there is a strong loss of correlation 37 between water vapor δ 18 O and mixing ratio. These periods are associated with abrupt negative 38 excursions of water vapor δ 18 Ο, often occurring toward the end of precipitation events. Six of 39 these events show a decrease in gaseous elemental mercury suggesting subsidence of air from 40 higher altitude. 41 Accurately representing the water isotopic signal during these cold fronts is a real challenge 42 for the atmospheric components of Earth System models equipped with water isotopes. While 43 the ECHAM6-wiso model was able to reproduce most of the sharp negative water vapor  18 O 44 excursions, the LMDZ-iso model at 2° (3°) resolution was only able to reproduce 7 (1) of the 45 negative excursions. Based on a detail model-data comparison, we conclude that the most 46 plausible explanations for such isotopic excursions are rain-vapor interactions associated with 47 subsidence at the rear of a precipitation event. 48

21 Stable water isotopes are natural tracers in the hydrological cycle and have been applied 22 in hydrology, atmospheric science, ecology, and paleoclimatology. However, the factors 23 controlling the isotopic distribution, both at spatial and temporal scales, are debated in East 24 Asia. For the first time, we made large scale (order 10000 km) continuous observations of near- 25 surface vapor isotopes across China in both pre-monsoon and monsoon seasons, using a newly- 26 designed vehicle-based vapor isotope monitoring system. For both seasons, the observed 27 variations along the sampling route are mainly due to spatial variations, and marginally 28 influenced by synoptic-scale variations. The data thus documents the spatial and seasonal 29 variability of vapor isotopes. The spatial variations of vapor δ 18 O are mainly controlled by 30 Rayleigh distillation during the pre-monsoon period, but significantly influenced by different 31 moisture sources, continental recycling processes and convection during moisture transport 32 during the monsoon period. The seasonal variation of vapor δ 18 O reflects the influence of the 33 summer monsoon convective precipitation in southern China, and a dependence on temperature 34 in the North. The spatial and seasonal variations in d-excess reflect the different moisture 35 sources and the influence of continental recycling. The isotope-incorporated global spectral 36 model (Iso-GSM) successfully captures the spatial variation of vapor δ 18 O during the pre- 37 monsoon period owing to the large latitudinal contrast in humidity and temperature, the overall 38 performance is weaker during the monsoon period. These results provides an overview of the 39 spatial distribution and seasonal variability of water isotopic composition in East Asia and their 40 controlling factors, and emphasize the need to interpret proxy records in the context of the 41 regional system and moisture sources. unique and novel isotope dataset consisting of vehicle-based spatially continuous near-surface vapor isotopes across a large spatial scale in China during the pre-monsoon and monsoon periods. The data provide a detailed description of the spatial and seasonal variability of vapor isotopes and their controlling mechanisms in the middle and low 84 latitudes. Our results reveal two types of isotopic patterns: (1) spatial variations at region. Our results are consistent 630 with previous studies showing that many models underestimate the heavy isotope depletion in 631 pre-monsoon seasons in subtropical and mid-latitudes, especially in very dry regions (Risi et 632

This study interprets 11 years (2006 to 2016) and 6 months (March to August in 2017) of respectively monthly and daily isotopic (delta D and delta O-18) monitoring of rain at Douala (Cameroon), a humid tropical station in Western Africa. The main scope is to analyze the climate controls on precipitation isotopes at different timescales. Firstly, we examine the annual cycles of delta O-18. Over the 11 years of survey, the annual cycle exhibits a W shape that is quite reproducible from year to year, with two minima in spring and autumn periods. Based on back trajectory calculations and remote sensing observations of water vapor isotopic composition, we show that the observed depletion in spring and autumn is due to strong convective activity along air mass trajectories. The same effect of convective activity can be observed at the daily timescale. At seasonal and daily time scales, the isotopic composition is also strongly tied to the convective organization and cloud types. More depleted precipitation is associated with larger areas of high clouds. Very low to low clouds are observed in July-August, mid-level to high clouds are dominant in June and high to very high clouds characterize March-April-May, thus explaining the enriched (depleted) values in summer (spring). Finally, this paper highlights the importance of large scale meteorological conditions controls on precipitation stable isotope composition in the Gulf of Guinea.

. Palaeoelevation reconstructions of mountain belts have become a focus of modern science since surface elevation provides crucial information for understanding both geodynamic mechanisms of Earth’s interior and the influence of mountain growth on climate. Stable oxygen isotopes palaeoaltimetry is one of the most popular techniques nowa-days, and relies on the difference between δ 18 O of palaeoprecipitation reconstructed using the natural archives, and modern measured values for the point of interest. Our goal is to understand where and how complex climatic changes linked with the growth of mountains affect δ 18 O in precipitation. For this purpose, we develop a theoretical expression for the precipitation composition based on the Rayleigh distillation and the isotope-equipped atmospheric general circulation model LMDZ-iso outputs. Experiments with reduced height over the Tibetan Plateau and the Himalayas have been designed. Our results show that the isotopic composition of precipitation is very sensitive to climate changes related to the growth of the Himalayas and Tibetan Plateau. Specif-ically our simulations suggest that only 40 palaeoaltimetry

We propose a framework using water vapor isotopes to study mixing processes in the marine boundary layer (MBL) during quiescent conditions, where we expect evaporation to contribute to the moisture budget. This framework complements the existing models, by taking into account the changing isotopic composition of the evaporation flux (δe), both directly in response to the mixing and indirectly in response to mixing and surface conditions through variations in MBL humidity. The robustness of the model is demonstrated using measurements from the North Atlantic Ocean. This shows the importance of considering the δe variability simultaneous to the mixing of the lower free troposphere to the MBL, to simulate the MBL water vapor, whereas a mixing model using a constant δe fails to reproduce the data. The sensitivity of isotope observations to evaporation and shallow mixing further demonstrates how these observations can constrain uncertainties associated with these key processes for climate feedback predictions.

The skills of isotope‐enabled general circulation models are evaluated against atmospheric water vapor isotopes. We have combined in situ observations of surface water vapor isotopes spanning multiple field seasons (2010, 2011, and 2012) from the top of the Greenland Ice Sheet (NEEM site: 77.45°N, 51.05°W, 2484 m above sea level) with observations from the marine boundary layer of the North Atlantic and Arctic Ocean (Bermuda Islands 32.26°N, 64.88°W, year: 2012; south coast of Iceland 63.83°N, 21.47°W, year: 2012; South Greenland 61.21°N, 47.17°W, year: 2012; Svalbard 78.92°N, 11.92°E, year: 2014). This allows us to benchmark the ability to simulate the daily water vapor isotope variations from five different simulations using isotope‐enabled general circulation models. Our model‐data comparison documents clear isotope biases both on top of the Greenland Ice Sheet (1–11‰ for δ18O and 4–19‰ for d‐excess depending on model and season) and in the marine boundary layer (maximum differences for the following: Bermuda δ18O = ~1‰, d‐excess = ~3‰; South coast of Iceland δ18O = ~2‰, d‐excess = ~ 5‰; South Greenland δ18O = ~4‰, d‐excess = ~7‰; Svalbard δ18O = ~2‰, d‐excess = ~7‰). We find that the simulated isotope biases are not just explained by simulated biases in temperature and humidity. Instead, we argue that these isotope biases are related to a poor simulation of the spatial structure of the marine boundary layer water vapor isotopic composition. Furthermore, we specifically show that the marine boundary layer water vapor isotopes of the Baffin Bay region show strong influence on the water vapor isotopes at the NEEM deep ice core‐drilling site in northwest Greenland. Our evaluation of the simulations using isotope‐enabled general circulation models also documents wide intermodel spatial variability in the Arctic. This stresses the importance of a coordinated water vapor isotope‐monitoring network in order to discriminate amongst these model behaviors.

29 Oxygen isotopes in tree-rings (δOTR) are widely used to reconstruct past climates. However, the 30 complexity of climatic and biological processes controlling isotopic fractionation is not yet fully 31 understood. Here, we use the MAIDENiso model to decipher the variability of δOTR of two 32 temperature-sensitive species of relevant paleoclimatological interest (Picea mariana and 33 Nothofagus pumilio) and growing at cold high-latitudes in North and South America. In this first 34 modelling study on δOTR values in both northeastern Canada (53.86°N) and western Argentina 35 (41.10°S), we specifically aim at: 1) evaluating the predictive skill of MAIDENiso to simulate 36 δOTR values, 2) identifying the physical processes controlling δOTR by mechanistic modelling 37 and, 3) defining the origin of the temperature signal recorded in the two species. Although the 38 linear regression models used here to predict daily δO of precipitation (δOP) may need to be 39 improved in the future, the resulting daily δOP values adequately reproduce observed (from 40 weather stations) and simulated (by global circulation model) δOP series. The δOTR values of 41 the two species are correctly simulated using the δOP estimation as MAIDENiso input, although 42 some offset in mean δOTR levels is observed for the South American site. For both species, the 43 variability of δOTR series is more likely linked to the effect of temperature on isotopic 44 enrichment of the leaf water rather than on the isotopic composition of the source water. We 45 show that MAIDENiso is a powerful tool for investigating isotopic fractionation processes but 46 that the lack of a denser isotope-enabled monitoring network recording oxygen fractionation in 47 the soil-vegetation-atmosphere compartments limits our capacity to decipher the processes at 48 play. This study proves that the eco-physiological modelling of δOTR values is necessary to 49 interpret the recorded climate signal more reliably. 50 51

Abstract. Past climate is an important benchmark to assess the ability of climate models to simulate key processes and feedbacks. Numerous proxy records exist for stable isotopes of water and/or carbon, which are also implemented inside the components of a growing number of Earth system model. Model-data comparisons can help to constrain the uncertainties associated with transfer functions. This motivates the need of producing a comprehensive compilation of different proxy sources. We have put together a global database of proxy records of oxygen (δ18O), hydrogen (δD) and carbon (δ13C) stable isotopes from different archives: ocean and lake sediments, corals, ice cores, speleothems and tree-ring cellulose. Source records were obtained from the georeferenced open access PANGAEA and NOAA libraries, complemented by additional data obtained from a literature survey. About 3000 source records were screened for chronological information and temporal resolution of proxy records. Altogether, this database consists of hundreds of dated δ18O, δ13C and δD records in a standardized simple text format, complemented with a metadata Excel catalog. For coherency with the Paleoclimate Modelling Intercomparison Project (PMIP), we focus on records spanning the past 200 years, the mid-Holocene (MH, 5.5-6.5 ka; calendar kilo years before 1950), and the Last Glacial Maximum (LGM, 19-23 ka), and those spanning the last interglacial period (LIG, 115-130 ka). For the LGM, a quality control flag was implemented to describe age markers and inform on chronological uncertainty. Basic statistics have been applied to characterize anomalies between these different periods. Most changes from the MH to present day, and LIG to MH appear statistically insignificant. Significant global differences are reported from LGM to MH with regional discrepancies in signals from different archives and complex patterns. This compilation effort highlights the need to homogenize the format of datasets and chronological information, and enhance the distribution of published datasets that are currently highly-fragmented and scattered. We also provide an online portal with an intuitive and interactive platform (http://climateproxiesfinder.ipsl.fr/), allowing one to easily select, visualize and download subsets of the homogeneously-formatted records that conform this database, following a choice of search criteria, and to upload new datasets.

Improving the representation of the hydrological cycle in atmospheric general circulation models (AGCMs) is one of the main challenges in modeling the Earth's climate system. One way to evaluate model performance is to simulate the transport of water isotopes. Among those available, tritium is an extremely valuable tracer, because its content in the different reservoirs involved in the water cycle (stratosphere, troposphere, and ocean) varies by order of magnitude. Previous work incorporated natural tritium into Laboratoire de Météorologie Dynamique Zoom (LMDZ)‐iso, a version of the LMDZ general circulation model enhanced by water isotope diagnostics. Here for the first time, the anthropogenic tritium injected by each of the atmospheric nuclear bomb tests between 1945 and 1980 has been first estimated and further implemented in the model; it creates an opportunity to evaluate certain aspects of LDMZ over several decades by following the bomb tritium transient signal through the hydrological cycle. Simulations of tritium in water vapor and precipitation for the period 1950–2008, with both natural and anthropogenic components, are presented in this study. LMDZ‐iso satisfactorily reproduces the general shape of the temporal evolution of tritium. However, LMDZ‐iso simulates too high a bomb tritium peak followed by too strong a decrease of tritium in precipitation. The too diffusive vertical advection in AGCMs crucially affects the residence time of tritium in the stratosphere. This insight into model performance demonstrates that the implementation of tritium in an AGCM provides a new and valuable test of the modeled atmospheric transport, complementing water stable isotope modeling.

Improvements in the evaluation of land surface models would translate into more reliable predictions of future climate changes, as significant uncertainties persist in the quantification and representation of the relative contributions of soil and vegetation to the water and energy cycles. In this paper, we investigate the usefulness of water stable isotopes in land surface models studying land surface processes. To achieve this, we implemented 18O and 2H and the computation of the oxygen (δ18O) and deuterium (δD) stable isotope composition of soil and leaf water pools in a~recent version of the land surface model ORCHIDEE. We performed point-wise simulations with this new model and evaluated its performance on vertical profiles of soil water isotope ratios measured in summer 2012 at four experimental sites located in a boreal region of the Artic zone of western Siberia. The model performed relatively well in simulating some features of the δ18O soil profiles, but poorly reproduced the d-excess profiles, at all four stations. The response of the simulated δ18O profiles to variations in key hydrological parameters revealed the importance of the choice of a correct infiltration pathway in ORCHIDEE. Our results show also that the strength of the evaporative enrichment signal plays a role in shaping the profiles, too and, therefore, the relevance of the vegetation and bare soil characterization. We investigated furthermore to which extent we are able to determine the relative contribution of the evaporation to the evapotranspiration. This study's results confirm that the use of water stable isotopes measurements helps constrain the representation of key land surface processes in land surface models.

Combined records of snow accumulation rate, δ18O and deuterium excess were produced from several shallow ice cores and snow pits at NEEM (North Greenland Eemian Ice Drilling), covering the period from 1724 to 2007. They are used to investigate recent climate variability and characterise the isotope–temperature relationship. We find that NEEM records are only weakly affected by inter-annual changes in the North Atlantic Oscillation. Decadal δ18O and accumulation variability is related to North Atlantic sea surface temperature and is enhanced at the beginning of the 19th century. No long-term trend is observed in the accumulation record. By contrast, NEEM δ18O shows multidecadal increasing trends in the late 19th century and since the 1980s. The strongest annual positive δ18O values are recorded at NEEM in 1928 and 2010, while maximum accumulation occurs in 1933. The last decade is the most enriched in δ18O (warmest), while the 11-year periods with the strongest depletion (coldest) are depicted at NEEM in 1815–1825 and 1836–1846, which are also the driest 11-year periods. The NEEM accumulation and δ18O records are strongly correlated with outputs from atmospheric models, nudged to atmospheric reanalyses. Best performance is observed for ERA reanalyses. Gridded temperature reconstructions, instrumental data and model outputs at NEEM are used to estimate the multidecadal accumulation–temperature and δ18O–temperature relationships for the strong warming period in 1979–2007. The accumulation sensitivity to temperature is estimated at 11 ± 2 % °C−1 and the δ18O–temperature slope at 1.1 ± 0.2 ‰ °C−1, about twice as large as previously used to estimate last interglacial temperature change from the bottom part of the NEEM deep ice core.

The Infrared Atmospheric Sounding Interferometer (IASI) flying onboard MetOpA and MetOpB is able to capture fine isotopic variations of the HDO to H2O ratio (δD) in the troposphere. Such observations at the high spatio-temporal resolution of the sounder are of great interest to improve our understanding of the mechanisms controlling humidity in the troposphere. In this study we aim to empirically assess the validity of our error estimation previously evaluated theoretically. To achieve this, we compare IASI δD retrieved profiles with other available profiles of δD, from the TES infrared sounder onboard AURA and from three ground-based FTIR stations produced within the MUSICA project: the NDACC (Network for the Detection of Atmospheric Composition Change) sites Kiruna and Izaña, and the TCCON site Karlsruhe, which in addition to near-infrared TCCON spectra also records mid-infrared spectra. We describe the achievable level of agreement between the different retrievals and show that these theoretical errors are in good agreement with empirical differences. The comparisons are made at different locations from tropical to Arctic latitudes, above sea and above land. Generally IASI and TES are similarly sensitive to δD in the free troposphere which allows one to compare their measurements directly. At tropical latitudes where IASI's sensitivity is lower than that of TES, we show that the agreement improves when taking into account the sensitivity of IASI in the TES retrieval. For the comparison IASI-FTIR only direct comparisons are performed because the sensitivity profiles of the two observing systems do not allow to take into account their differences of sensitivity. We identify a quasi negligible bias in the free troposphere (−3‰) between IASI retrieved δD with the TES, which are bias corrected, but important with the ground-based FTIR reaching −47‰. We also suggest that model-satellite observation comparisons could be optimized with IASI thanks to its high spatial and temporal sampling.

Supplementary tables S1--S5 describing correlation statistics between NEEM records and other records.• Figure S1 describing the characteristics of the first principal component of Greenland accumulation records.

Abstract. δ18O of atmospheric oxygen (δ18Oatm) undergoes millennial-scale variations during the last glacial period, and systematically increases during Heinrich stadials (HSs). Changes in δ18Oatm combine variations in biospheric and water cycle processes. The identification of the main driver of the millennial variability in δ18Oatm is thus not straightforward. Here, we quantify the response of δ18Oatm to such millennial events using a freshwater hosing simulation performed under glacial boundary conditions. Our global approach takes into account the latest estimates of isotope fractionation factor for respiratory and photosynthetic processes and make use of atmospheric water isotope and vegetation changes. Our modeling approach allows to reproduce the main observed features of a HS in terms of climatic conditions, vegetation distribution and δ18O of precipitation. We use it to decipher the relative importance of the different processes behind the observed changes in δ18Oatm. The results highlight the dominant role of hydrology on δ18Oatm and confirm that δ18Oatm can be seen as a global integrator of hydrological changes over vegetated areas.

The description of the hydrological cycle in Atmospheric General Circulation Models (GCMs) can be validated using water isotopes as tracers. Many GCMs now simulate the movement of the stable isotopes of water, but here we present the first GCM simulations modelling the content of natural tritium in water. These simulations were obtained using a version of the LMDZ General Circulation Model enhanced by water isotopes diagnostics, LMDZ-iso. To avoid tritium generated by nuclear bomb testing, the simulations have been evaluated against a compilation of published tritium datasets dating from before 1950, or measured recently. LMDZ-iso correctly captures the observed tritium enrichment in precipitation as oceanic air moves inland (the so-called continental effect) and the observed north–south variations due to the latitudinal dependency of the cosmogenic tritium production rate. The seasonal variability, linked to the stratospheric intrusions of air masses with higher tritium content into the troposphere, is correctly reproduced for Antarctica with a maximum in winter. LMDZ-iso reproduces the spring maximum of tritium over Europe, but underestimates it and produces a peak in winter that is not apparent in the data. This implementation of tritium in a GCM promises to provide a better constraint on: (1) the intrusions and transport of air masses from the stratosphere, and (2) the dynamics of the modelled water cycle. The method complements the existing approach of using stable water isotopes.

This study aims to investigate some characteristics of the moist processes of the Madden-Julian oscillation (MJO), by making use of joint HDO (or delta D) and H2O vapor measurements. The MJO is the main intraseasonal mode of the tropical climate but is hard to properly simulate in global atmospheric models. The joint use of delta D-H2O diagnostics yields additional information compared to sole humidity measurements. We use midtropospheric Infrared Atmospheric Sounding Interferometer (IASI) satellite delta D and H2O measurements to determine the mean MJO humidity and delta D evolution. Moreover, by making use of high temporal resolution data, we determine the variability in this evolution during about eight MJO events from 2010 to 2012 (including those monitored during the DYNAMO (the Dynamics of the MJO), CINDY (Cooperative Indian Ocean Experiment in Y2011) campaign). These data have a higher spatiotemporal coverage than previous delta D measurements, enabling the sampling of individual MJO events. IASI measurements over the Indian Ocean confirm earlier findings that the moistening before the precipitation peak of an MJO event is due to water vapor slightly enriched in HDO. There is then a HDO depletion around the precipitation peak that also corresponds to the moister environment. Most interevent variability determined in the current study occurs 5 to 10 days after the MJO event. In 75% of the events, humidity decreases while the atmosphere remains depleted. In a quarter of the events, humidity increases simultaneously with an increase in delta D. After this, the advection of relatively dry and enriched air brings back the state to the mean. Over the maritime continent, delta D-H2O cycles are more variable on time scales shorter than the MJO and the interevent variability is larger than over the Indian Ocean. The sequence of moistening and drying processes as revealed by the q-delta D cycles can be used as a benchmark to evaluate the representation of moist processes in models. This is done here by comparing observations to simulations of the isotope enabled LMDZ (Laboratoire de Meteorologie Dynamique Zoom) global climate model nudged with reanalysis wind fields. These simulations also give information to investigate possible physical origins of the observed q-delta D cycles.

Reconstructions of Quaternary climate are often based on the isotopic content of paleo-precipitation preserved in proxy records. While many paleo-precipitation isotope records are available, few studies have synthesized these dispersed records to explore spatial patterns of late-glacial precipitation δ18O. Here we present a synthesis of 86 globally distributed groundwater (n = 59), cave calcite (n = 15) and ice core (n = 12) isotope records spanning the late-glacial (defined as ~ 50 000 to ~ 20 000 years ago) to the late-Holocene (within the past ~ 5000 years). We show that precipitation δ18O changes from the late-glacial to the late-Holocene range from −7.1 ‰ (δ18Olate-Holocene > δ18Olate-glacial) to +1.7 ‰ (δ18Olate-glacial > δ18Olate-Holocene), with the majority (77 %) of records having lower late-glacial δ18O than late-Holocene δ18O values. High-magnitude, negative precipitation δ18O shifts are common at high latitudes, high altitudes and continental interiors (δ18Olate-Holocene > δ18Olate-glacial by more than 3 ‰). Conversely, low-magnitude, positive precipitation δ18O shifts are concentrated along tropical and subtropical coasts (δ18Olate-glacial > δ18Olate-Holocene by less than 2 ‰). Broad, global patterns of late-glacial to late-Holocene precipitation δ18O shifts suggest that stronger-than-modern isotopic distillation of air masses prevailed during the late-glacial, likely impacted by larger global temperature differences between the tropics and the poles. Further, to test how well general circulation models reproduce global precipitation δ18O shifts, we compiled simulated precipitation δ18O shifts from five isotope-enabled general circulation models simulated under recent and last glacial maximum climate states. Climate simulations generally show better inter-model and model-measurement agreement in temperate regions than in the tropics, highlighting a need for further research to better understand how inter-model spread in convective rainout, seawater δ18O and glacial topography parameterizations impact simulated precipitation δ18O. Future research on paleo-precipitation δ18O records can use the global maps of measured and simulated late-glacial precipitation isotope compositions to target and prioritize field sites.