Description: Variations in oxygen isotope compositions (δ18O) provide insight into modern climate and past changes in climate and topography. In addition, in regions such as Tibet, geologic archives of isotope ratios record climate change driven by plateau uplift and therefore also provide information about the surface uplift history. A good understanding of modern-day controls on δ18O is crucial for interpreting geologic δ18O in this context. We use the ECHAM5-wiso global atmospheric general circulation model to calculate δ18O in precipitation (δ18Op) for the present-day climate. In the region of the Tibetan Plateau, spatial variations of monthly means of δ18Op are statistically related to spatial variations of 2 m air temperature and precipitation rate, as well as to topography. The size and location of investigated regions are varied in our study to capture regional differences in these relationships and the processes governing the modern δ18Op. In addition to correlation analyses, a cross-validated stepwise multiple regression is carried out using δ18Op as the predictand, and topography and atmospheric variables (temperature and precipitation amount) as predictors. The 2 m air temperature and topography yield the highest spatial correlation coefficients of 〉0.9 and 〈 0.9, respectively, throughout most of the year. Particularly high correlation coefficients are calculated for the region along the Himalayan orogeny and parts of western China. The predictors explain 〉90% of the δ18Op spatial variance in the same regions. The 2 m air temperature is the dominant predictor and contributes 93.6% to the total explained spatial variance on average. The results demonstrate that most of the δ18Op pattern on and around the Tibetan Plateau can be explained by variation in 2 m air temperature and altitude. Correlation of the dependent predictors indicate that in low-altitude regions where topography does not determine temperature variability, the high correlation of temperature and δ18Op may partially be explained by variations in precipitation rates.

Description: 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

Description: The oxygen isotopes of water (H218O and H216O) are tracers widely used for the investigation of Earth science problems. The tracer applications are based on the premise that the 18O/16O ratio of open-water evaporation (δ18O ) can be calculated from environmental conditions. A long-standing issue concerns the role of kinetic fractionation, or diffusion transport, in the evaporation process. Here we deployed an optical instrument at a large lake (area 2,400 km2) to make in situ measurement of δ18O and δD of atmospheric vapor, then determined δ18O and δD of open-water evaporation using the gradient-diffusion method. Our results show a much weaker kinetic effect than suggested by the kinetic factor εk adopted in some previous studies of lake hydrology (14.2‰). By incorporating into the H218O isotopic mass balance of the lake a lower εk value (about 6.2‰) used for ocean evaporation in global climate models, we obtain an annual lake evaporation rate that agrees with an independent eddy-covariance observation, but the rate is 72% higher than if the commonly used lake εk value of 14.2‰ is applied. The applicability of this results to small lakes is uncertain and in need of field-based assessment.

Description: The seasonality of hydroclimate during past periods of warmer than modern global temperatures is a critical component for understanding future climate change scenarios. Although only partially analogous to these scenarios, the last interglacial (LIG, Marine Isotope Stage 5e, ~127–117 ka) is a popular test bed. We present coral δ18O monthly resolved records from multiple Bonaire (southern Caribbean) fossil corals (Diploria strigosa) that date to between 130 and 118 ka. These records represent up to 37 years and cover a total of 105 years, offering insights into the seasonality and characteristics of LIG tropical Atlantic hydroclimate. Our coral δ18O records and available coral Sr/Ca-sea surface temperature (SST) records reveal new insights into the variable relationship between the seasonality of tropical Atlantic seawater δ18O (δ18Oseawater) and SST. Coral δ18O seasonality is found to covary with SST and insolation seasonality throughout the LIG, culminating in significantly higher than modern values at 124 and 126 ka. At 124 ka, we reconstruct a 2 month lead of the coral δ18O versus the Sr/Ca-SST annual cycle and increased δ18Oseawater seasonality. A fully coupled climate model simulates a concomitant increase of southern Caribbean Sea summer precipitation and depletion of summer δ18Oseawater. LIG hydroclimate at Bonaire differed from today's semiarid climate with a minor rainy season during winter. Cumulatively, our coral δ18O, δ18Oseawater, and model findings indicate a mid-LIG northward expansion of the South American Intertropical Convergence Zone into the southern Caribbean Sea, highlighting the importance of regional aspects within model and proxy reconstructions of LIG hydroclimate seasonality.

Description: All types of applications of stable water isotopes, e.g. for the reconstruction of paleotemperatures or for climate model validation, rely on a proper understanding of the mechanisms determining the isotopic composition of water vapor and precipitation. In this study, we use the isotope-enabled limited-area model COSMOiso to characterize the impacts of continental evapotranspiration, rainout, and subcloud processes on δD of European water vapor and precipitation. To this end, we first confirm a reliable implementation of the most important isotope fractionation processes in COSMOiso by comparing 5 years of modeled δD values with multi-platform δD observations from Europe (remote sensing observations of the δD of water vapor around 2.6 km a.g.l., in situ δD measurements in near-surface water vapor, and δD precipitation data from the Global Network of Isotopes in Precipitation). Based on six 15 year sensitivity simulations, we then quantify the climatological impacts of the different fractionation processes on the δD values. We find δD of European water vapor and precipitation to be most strongly controlled by rainout. Superimposed to this are the effect of subcloud processes, which especially affects δD in precipitation under warm conditions, and the effect of continental evapotranspiration, which exerts an important control over the δD of near-surface water vapor. In future studies, the validated COSMOiso model can be employed in a similar way for a comprehensive interpretation of European isotope records from climatologically different time periods.

Description: We present here results of new isotope-enabled simulations with an enhanced ECHAM6-wiso model version nudged to the ERA5 reanalyses, at two different spatial resolutions, for the period 1979–2018. The isotopic content of snow on sea ice is considered, yielding surface water vapor with lower isotope ratios over sea ice covered areas, and the kinetic fractionation factors for oceanic evaporation are assumed as independent of wind speed. Also, the supersaturation equation was slightly re-tuned for a better agreement with the Antarctic isotope observations. In addition to the spatial resolution, the impacts of the improved ECHAM6 model physics and the chosen updated ERA5 reanalyses data set on our simulation results are investigated. For this purpose, detailed comparisons to simulation results obtained from the predecessor ECHAM5-wiso model nudged to ERA5 reanalyses and from ECHAM6-wiso nudged to ERA-Interim are performed. Compared to the ERA-Interim reanalyses, the nudging to ERA5 does not result in substantial changes of modeled surface isotope values on a global scale but water transport over the tropics clearly changed, with increased precipitation amounts over the Amazonian area and related changes of isotopic contents in water vapor in the high troposphere. An ECHAM6-wiso simulation at high spatial resolution (0.9° horizontal resolution and 95 vertical levels) generally further improves the modeling of annual mean isotope values. The modeling of temporal isotopic variations at seasonal and (sub-)daily time scales is also slightly improved for the high spatial resolution configuration. The latter will be very useful for an improved interpretation of various water isotope records.