Water Isotopes In Precipitation Research Articles

Influence of orbital forcing and solar activity on water isotopes in precipitation during the mid- and late Holocene

Abstract. In this study we investigate the impact of mid- and late Holocene orbital forcing and solar activity on variations of the oxygen isotopic composition in precipitation. The investigation is motivated by a recently published speleothem δ18O record from the well-monitored Bunker Cave in Germany. The record reveals some high variability on multi-centennial to millennial scales that does not linearly correspond to orbital forcing. Our model study is based on a set of novel climate simulations performed with the atmosphere general circulation model ECHAM5-wiso enhanced by explicit water isotope diagnostics. From the performed model experiments, we derive the following major results: (1) the response of both orbital and solar forcing lead to changes in surface temperatures and δ18O in precipitation with similar magnitudes during the mid- and late Holocene. (2) Past δ18O anomalies correspond to changing temperatures in the orbital driven simulations. This does not hold true if an additional solar forcing is added. (3) Two orbital driven mid-Holocene experiments, simulating the mean climate state approximately 5000 and 6000 yr ago, yield very similar results. However, if an identical additional solar activity-induced forcing is added, the simulated changes of surface temperatures as well as δ18O between both periods differ. We conclude from our simulation results that non-linear effects and feedbacks of the orbital and solar activity forcing substantially alter the δ18O in precipitation pattern and its relation to temperature change.

High‐frequency storm event isotope sampling reveals time‐variant transit time distributions and influence of diurnal cycles

AbstractHigh‐frequency sampling of stable water isotopes in precipitation and stream water during winter and summer storm events was carried out in a 2·3 km2 lowland agricultural catchment. During peak flows of monitored events, the responses of δ2H and δ18O were comparable and inferred the dominance (ca 70%) of ‘old’ pre‐event water. Transit Time Distribution (TTD) inferred by a gamma function were fitted (Nash–Sutcliffe = 0·8) and were also similar for δ2H and δ18O. However, the shape (α) and scaling (β) parameters were markedly different for summer and winter events. Consequently, when antecedent wetness was high, mean transit times were in the order of days; when drier, they increased to months. Moreover, while the responses of δ2H and δ18O exhibited similar gradual recovery to pre‐event conditions during winter hydrograph recessions, they differed dramatically on summer recessions. Time series analysis showed that δ2H isotope content was correlated with the diurnal cycle of air temperature, suggesting an evaporative fractionation pattern which could be reproduced by a temperature‐based first‐order autoregressive model. The heavier δ18O isotope showed no evidence for such diurnal variability. The study highlights the utility of high‐frequency stable isotope sampling to explore the time‐variant nature of TTDs. Furthermore, it shows that the time of sampling in a diurnal cycle may have crucial significance for interpreting stream isotope signatures, particularly δ2H. Copyright © 2011 John Wiley & Sons, Ltd.

Influence of convective processes on the isotopic composition (δ18O and δD) of precipitation and water vapor in the tropics: 2. Physical interpretation of the amount effect

In the tropics, the proportion of heavier water isotopes in precipitation is anticorrelated with the precipitation amount. The physical processes underlying this so‐called amount effect are still poorly understood and quantified. In the present study, stable water isotopes (H218O and HDO) have been introduced in a single column model including the Emanuel convection parameterization. We investigate the physical processes underlying the amount effect and propose a methodology to quantify their relative contributions. We focus on convective processes, since the idealized framework of the single column models does not allow us to consider the effects of large‐scale horizontal advections of air masses of different isotopic signatures. We show that two kinds of processes predominantly explain the amount effect: first, the reevaporation of the falling rain and the diffusive exchanges with the surrounding vapor; and second, the recycling of the subcloud layer vapor feeding the convective system by convective fluxes. This highlights the importance of a detailed representation of rain evaporation processes to simulate accurately the isotopic composition of precipitation in the tropics. The variability of the isotopic composition on different timescales (from days to months) is also studied using a unidimensional simulation of the Tropical Ocean–Global Atmosphere–Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE) campaign. The amount effect is best observable at intraseasonal or longer timescales. The period of time over which convective activity significantly affects the isotopic composition of precipitation is related to the residence time of water within atmospheric reservoirs.

Analysis of the global distribution of water isotopes using the NCAR atmospheric general circulation model

We have incorporated the cycling of water isotopes into the NCAR atmospheric general circulation model, CAM2. Isotope dynamics mostly follow those of previous isotope GCMs, with fractionation being produced by evaporation at the surface and by cloud processes. A new feature that we have added is the direct estimation of the degree of isotopic equilibration between vapor and raindrops as a function of temperature and rain rate. The model yields a reasonable global pattern of water isotopes in precipitation, but detailed comparison with observations is limited by known inaccuracies in precipitation and temperatures yielded by CAM2. We use the results to evaluate the fundamental controls on water isotopic composition in precipitation. We emphasize that, over much of the surface of the Earth, the concept of Rayleigh distillation is inadequate to understand the large‐scale geographic distribution of water isotopes in precipitation, because the effects of surface fluxes are more important than those of distillation, in particular at low and midlatitudes and over the oceans. In oceanic regions the balance between precipitation and evaporation (P − E), which reflects the large‐scale atmospheric circulation, is the primary determinant of the isotopic composition of precipitation and vapor. Variations of P − E at low latitudes over the oceans produce about 7‰ variation of precipitation δ18O that is independent of temperature variation. Where P > E, the convergence of atmospheric vapor derived from various sources leads to low values and a particularly wide range in δppt. In the tropical and subtropical troposphere the vertical decrease of δ18O in vapor is different from the values expected from Rayleigh distillation because of entrainment, convective mixing, detrainment, evaporation of detrained water, and subsidence of low‐δ18O high‐altitude air. The low δ18O of atmospheric vapor over the oceans at high (ca. 55°) latitudes produces, as a result of kinetic effects associated with evaporation, a zone of heavy (high‐δ18O) evaporation from the oceans. This effect may account for the low δ18O of some high‐latitude ocean surface waters and also helps attenuate the effects of global temperature changes on the isotopic composition of polar precipitation.

Simulation of the Stable Water Isotopes in Precipitation over South America: Comparing Regional to Global Circulation Models

Abstract A simulation of the stable water isotope cycle over South America by the regional circulation model REMOiso is discussed. The performance of the regional model, with a resolution of 0.5° (∼55 km), is compared to simulations by the global circulation model ECHAMiso at two coarser resolutions and evaluated against observations of precipitation and δ18O. Here REMOiso is demonstrated to reproduce reasonably well climatic and isotopic features across South America. This paper explores further insights of δ18O as a climate proxy, based on REMOiso’s improvements as compared to ECHAMiso. In particular, the authors focus on the seasonal variation of the amount effect (δ18O decrease with precipitation amounts) and the anomalous δ18O continental gradient across the Amazon basin, as inferred from the REMOiso, ECHAMiso, and GNIP datasets. The finer resolution of topography in REMOiso enables a detailed analysis of the altitude effect: not only the first, but also the second derivative of δ18O with altitude is considered. It appears that high-altitude grid cells show an isotopic signature similar to Rayleigh distillation, in accordance with experimental studies. Finally, a Lagrangian reference frame is adopted to describe the evolution of δ18O in precipitation along its trajectory, in order to relate the simulation analysis to the fractionation mechanisms. This confirms that the amount effect, via Rayleigh distillation processes, is dominant during the wet season. During the dry season, the δ18O in precipitation is controlled by isotopic reequilibration of rain droplets with surrounding vapor, reflecting the impact of nonfractionating transpiration by the vegetation.

Water isotopes in precipitation:: data/model comparison for present-day and past climates

Variations of HDO and H 2 18 O concentrations are observed in precipitation both on a geographical and on a temporal basis. These variations, resulting from successive isotopic fractionation processes at each phase change of water during its atmospheric cycle, are well documented through the IAEA/WMO network. Isotope concentrations are, in middle and high latitudes, linearly related to the annual mean temperature at the precipitation site. Paleoclimatologists have used this relationship to infer paleotemperatures from isotope paleodata extractable from ice cores, deep groundwater and other such sources. For this application to be valid, however, the spatial relationship must also hold in time at a given location as the location undergoes a series of climatic changes. Progress in water isotope modeling aimed at examining and evaluating this assumption has been recently reviewed (Jouzel et al., 1997) with a focus on polar regions and, more specifically, on Greenland. We extend this review in comparing the results of two different isotopic AGCMs (NASA/GISS and ECHAM) and in examining, with a more global perspective, the validity of the above assumption, i.e. the equivalence of the spatial and temporal isotope–temperature relationships. These results confirm the dominating role of local temperature changes on the paleo isotope signal in most regions. However, the exact calibration of this valuable paleothermometer is biased by, for example, the seasonality of precipitation and other factors. We forced the two models by the climatic boundary conditions of the mild-holocene at 6 kyr BP which only slightly differs from today's climate. The isotope response on this weak forcing is quite heterogeneous. The only robust common response is the intensification of the hydrological cycle in low latitudes and, therefore, isotopically more depleted precipitation in the tropics and subtropics. We also examine recent progress made in modeling: the relationship between the conditions prevailing in moisture source regions for precipitation and the deuterium excess of that precipitation.

Water isotope modeling in the Asian monsoon region

Abstract The isotopic composition of paleoprecipitation is widely used as proxy data for temperature and precipitation. In order to investigate the validity of these transfer functions for water isotopes, oxygen 18 and deuterium, we installed a water isotope model into the ECHAM Atmospheric General Circulation Model (AGCM). Such an isotope model calculates all fractionations between the stable water isotopes in the hydrological cycle of the AGCM. We ran the model under present day conditions and under the boundary conditions of the Last Glacial Maximum (LGM) for four years each. On a global scale the model satisfactorily reproduces the main features of the water isotopes in the hydrological cycle (temperature effect, amount effect). The analysis focusses on the Asian monsoon area where atmospheric circulation strongly affects the seasonal cycle of water isotopes in precipitation. The model successfully simulates the influence of circulation, temperature and precipitation amount in this region under present day conditions. Under LGM conditions the model shows a weakened summer monsoon causing a strong positive isotope signal over East Asia although the continent is about 2–4°C cooler than today. This result underlines the difficulties to use the isotopic composition of paleoprecipitation simply as a measure of temperature. Furthermore, it demonstrates the importance of circulation changes and of changing moisture sources for the interpretation of a given isotope signal.