Analysis of the global distribution of water isotopes using the NCAR atmospheric general circulation model (original) (raw)
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Patterns of Evaporation and Precipitation Drive Global Isotopic Changes in Atmospheric Moisture
Geophysical Research Letters, 2018
Because water isotope ratios respond to phase changes during evaporation (E) and precipitation (P), they are candidate fingerprints of changing atmospheric hydrology. Moreover, through preservation in ice cores and other paleoproxies, they provide important insight into the past. Still, there is disagreement over what specific attributes of hydroclimate variability isotopes reveal. Here we argue that variations in zonal mean isotope ratios of water vapor and precipitation are largely a response to geographically shifting patterns of E and P. Differences in the relative importance of local versus remote changes in these moisture variables explain the apparent distinct isotopic sensitivities to temperature and precipitation amount in high and low latitudes, respectively. Not only does our work provide a unified framework for interpreting water isotopic measurements globally, but it also presents a novel approach for diagnosing water cycle changes in a warmer world. Plain Language Summary Observations that track changes in the water cycle are critical for improving our understanding of the climate system. Particularly important are measurements that can verify whether imbalances in evaporation and precipitation increase in response to global warming. Because the isotope ratios of hydrogen and oxygen in water vapor and precipitation vary with rates of evaporation and precipitation, they are candidate fingerprints of water cycle changes. Here we use a simple mass balance model to evaluate the isotopic response of water vapor in the atmosphere to spatial variations in evaporation and precipitation. We find that these spatial patterns shape the isotope ratios by changing two critical factors: the efficiency with which precipitation dries the atmosphere and the probability that moisture is transported downwind (rather than rained out). These two factors suggest that isotope ratios are influenced both by local and by remote changes in evaporation and precipitation. Low-latitude isotope ratios respond largely to local imbalances in evaporation and precipitation, while high-latitude isotope ratios depend more on what happens "upstream." These findings provide key guidance for interpreting climate changes of the past and offer a novel approach for diagnosing water cycle changes in a warmer future.
Seasonality of isotopes in precipitation: A global perspective
Journal of Geophysical Research, 2009
1] We use data from Global Network of Isotopes in Precipitation (GNIP) database to explore how the atmosphere's meridional circulation cells control the latitudinal and seasonal distribution of d 18 O and d-excess in precipitation. We demonstrate that the atmospheric general circulation (AGC) cells determine variations of zonally averaged isotopic composition of meteoric water; the local isotopic minimum near the equator coincides with the intertropical convergence (ITC), and two maxima on either side of the ITC coincide with the subtropical highs (STHs). Both the ITC and STHs migrate cum sole, as part of the systematic annual migration of the meridional cells. This migratory circulation pattern controls the phase of the annual oscillation of the precipitation d 18 O. At latitudes equatorward of the STHs, d 18 O reaches its maximum in the winter of the respective hemisphere and at higher latitudes in the summer. From the monthly latitudinal distribution of the vertical velocity at the 500-hPa level, we obtain the seasonal variations of the latitudinal positions of the subtropical moisture source regions and their climates. The sea surface temperature and relative humidity at the moisture source regions are used to predict seasonal changes of the d-excess of water vapor evaporated from the source regions. The GNIP data is consistent with the predicted phase of the d-excess. However, the observed magnitude of the seasonal oscillation is greater than the predicted values. This work provides a baseline for understanding the influence of subtropical moisture source regions and other climatological factors on the d-excess.
Stable water isotopologues, mainly 1 H 2 O, 1 H 2 HO (HDO), and 1 H 2 18 O, are useful tracers for processes in the global hydrological cycle. The incorporation of water isotopes into Atmospheric General Circulation Models (AGCMs) since 1984 has helped scientists gain substantial new insights into our present and past climate. In recent years, there have been several significant advances in water isotopes modeling in AGCMs. This paper reviews and synthesizes key advances accomplished in modeling (1) surface evaporation, (2) condensation, (3) supersaturation, (4) postcondensation processes, (5) vertical distribution of water isotopes, and (6) spatial 18 O-temperature slope and utilizing (1) spectral nudging technique, (2) higher model resolutions, and (3) coupled atmosphereocean models. It also reviews model validation through comparisons of model outputs and ground-based and spaceborne measurements. In the end, it identifies knowledge gaps and discusses future prospects of modeling and model validation.
Water isotopes in precipitation
Quaternary Science Reviews, 2000
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 with a focus on polar regions and, more speci"cally, on Greenland. We extend this review in comparing the results of two di!erent 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 con"rm 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 di!ers from today's climate. The isotope response on this weak forcing is quite heterogeneous. The only robust common response is the intensi"cation 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.
The Variability of Stable Isotopes and Water Origin of Precipitation over the Maritime Continent
SOLA, 2013
The seasonal water cycle features over the maritime continent were determined using water sources from seven regions produced by global Rayleigh-type circulation model. The model output was validated statistically to reproduce stable isotopes by the observed δ 18 O and δ D content of precipitation at eight stations. The model explains the Asian-Australian monsoon circulation well and demonstrates the seasonal changes of the water origin on the basis of three climatic patterns as a signature of rainy and dry season:
Stable isotopic composition of water vapor in the tropics
Journal of Geophysical Research, 2004
Water vapor samples collected during tropical field experiments at Puerto Escondido, Mexico, near Kwajalein (KWAJEX), and near Key West, Florida (CAMEX 4), were analyzed for their stable isotope contents, 1 H 2 18 O: 1 H 2 16 O and 2 H 1 H 16 O: 1 H 2 16 O. Highest d 18 O values approached isotopic equilibrium with seawater during quiescent weather or in regions of isolated or disorganized convection. Lowest d 18 O values occurred in or downwind from regions of organized mesoscale weather disturbances and ranged as low as 15% below isotopic equilibrium with seawater. The mean d 18 O value of vapor over the sea surface therefore decreases as storm activity and organization increases.
Stable water isotopes in the coupled atmosphere–land surface model ECHAM5-JSBACH
Geoscientific Model Development, 2013
In this study we present first results of a new model development, ECHAM5-JSBACHwiso, where we have incorporated the stable water isotopes H 2 18 O and HDO as tracers in the hydrological cycle of the coupled atmosphere-land surface model ECHAM5-JSBACH. The ECHAM5-JSBACH-wiso model was run under present-day climate con-5 ditions at two different resolutions (T31L19, T63L31). A comparison between ECHAM5-JSBACH-wiso and ECHAM5-wiso shows that the coupling has a strong impact on the simulated temperature and soil wetness. Caused by these changes of temperature and the hydrological cycle, the δ 18 O in precipitation also shows variations from −4.5 ‰ up to 4.5 ‰. One of the clearest anomalies is shown over North-East Asia where, depend-10 ing on an increase of temperature, the δ 18 O in precipitation increases as well. In order to analyze the sensitivity of the fractionation processes over land, we compare a set of simulations with various implementations of water isotope fractionation processes over the land surface. The simulations allow us to distinguish between no fractionation, fractionation included in the evaporation flux (from bare soil) and also fractionation included 15 in both evaporation and transpiration (from water transport through plants) fluxes. The simulated δ 18 O and δD in precipitation of these setups generally fit well with the observations and the best agreement between observation and simulation is given in the case where no fractionation over land surface is assumed.
Journal of Geophysical Research, 1998
Results are presented of a global simulation of the stable water isotopes H•80 and HD160 as implemented in the hydrological cycle of the ECHAM atmospheric general circulation model. The ECHAM model was run under present-day climate conditions at two spatial resolutions (T42,T21), and the simulation results are compared with observations. The high-resolution model (T42) more realistically reproduced the observations, thus demonstrating that an improved representation of advection and orography is critical when modeling the global isotopic water cycle.
2019
The stable isotopic composition of water vapor over the ocean is governed by the isotopic composition of surface water, ambient vapor isotopic composition, exchange and mixing processes at the water-air interface as well as the local meteorological conditions. In this study we present water vapor and surface water isotope ratios in samples collected across the latitudinal transect from Mauritius to Prydz Bay in the Antarctic coast. The samples were collected on-board the ocean research vessel SA Agulhas during the 9 th (Jan-2017) and 10 th (Dec-2017 to Jan-2018) Southern Ocean expeditions. The inter annual variability of the meteorological factors governing water vapor isotopic composition is explained. The parameters governing the isotopic composition of evaporation flux from the oceans can be considered separately or simultaneously in the Craig-Gordon(CG) models. The Traditional Craig-Gordon (TCG) (Craig and Gordon, 1965) and the Unified Craig-Gordon (UCG) (Gonfiantini et al., 2018) models were used to evaluate the isotopic composition of evaporation flux for the molecular diffusivity ratios suggested by (Merlivat, 1978)(MJ), (Cappa et al., 2003)(CD) and (Pfahl and Wernli, 2009)(PW) and for different ocean surface conditions. We found that the UCG model with CD molecular diffusivity ratios where equal contribution from molecular and turbulent diffusion is the best match for our observations. By assigning the representative end member isotopic compositions and solving the two-component mixing model, a relative contribution from locally generated and advected moisture was calculated along the transect. Our results suggest varying contribution of advected westerly component with an increasing trend up to 65°S. Beyond 65°S, the proportion of Antarctic moisture was found to be increasing linearly towards the coast.
Journal of Geophysical Research, 2003
Stable water isotopes (D and 18 O) in precipitation have large spatial and temporal variability and are used widely to trace the global hydrologic cycle. The two models that have been used in the past to examine the variability of precipitation isotopes are Rayleigh-type models and isotope-atmospheric general circulation models. The causes of short-term (1-10 day) variability in precipitation isotopes, however, remain unclear. This study seeks to explain isotope variability quantitatively at such scale. A new water isotope circulation model on a global scale that includes a Rayleigh equation and the use of external meteorological forcings is developed. Transport and mixing processes of water masses and isotopes that have been neglected in earlier Rayleigh models are included in the new model. A simulation of 18 O for 1998 is forced with data from the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiments (GAME) reanalysis. The results are validated by Global Network of Isotopes in Precipitation (GNIP) monthly observations with correlation R = 0.76 and a significance level >99% and by daily observations at three sites in Thailand with similar correlation and significance. A quantitative analysis of the results shows that among three factors that cause isotopic variability, the contribution of moisture flux is the largest, accounting for 37% at Chiangmai, and 46% globally. This highlights the importance of transport and mixing of air masses with different isotopic concentrations. A sensitivity analysis of the temporal and spatial resolution required for each variable is also made, and the model is applied to two additional data sets. The more accurate Global Precipitation Climatology Project (GPCP) precipitation data set yields improved model results at all three observation sites in Thailand. The National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis allows the simulation to cover 2 years, reproducing reasonable interannual isotopic variability.