C43D-01 INVITED 13:40h
Sources of variability in the multi-isotopic composition of Antarctic snow from climate model simulations
The isotopic composition of Antarctic snow is known to reflect climate variability on all time scales that have been sampled. These robust signals have been of great use in understanding the broad features of past climate variability, however, the desire to understand the changes in the mechanisms responsible for climate variability leads to the need to understand the partitioning of processes contributing to the isotopic signals. Specifically, any number of individual processes can modify the isotopic composition of polar snow, yet different combinations of processes can signify different climate conditions. Information on the set of processes active can be obtained by examining isotope-isotope associations where more than one species exists (such as the "deuterium exces" and the "O17 anomaly"), although there is as yet no consensus on the best application of multi-isotope analyses in climate reconstruction. As different environmental conditions impart differing signals on the set of isotopic species, a mapping can be constructed between different climatic processes and the isotopic state. While such a mapping can not provide an unambiguous method for reconstructing all aspects of climate variability, it does provide quantitative guidance to the confidence one may place on the interpretation of the isotope records. In this manner, we attempt to identify which processes dominate the isotopic signal using numerical simulations. A state-of-the-art atmospheric general circulation model has been fitted with a new isotopic tracer scheme which computes the spatiotemporal distribution of water nuclides, including the four most abundant stable isotopes of glaciological interest (H$_2$O, HDO, H$_2^{18}$O and H$_2$^{17}$O). The isotopic state of atmospheric water vapor, cloud condensate, and precipitation is of principal interest. With the model, a theoretical assessment of the isotopic response to changes in atmospheric processes is determined. Building on previous work for a single isotope, we establish the sensitivity of isotope-isotope associations to changes in the condensation history of moist parcels by considering 1) the conditions affecting cloud microphysics (ice/liquid partitioning), and 2) changes in dilution of the isotopic signal by both large-scale and turbulent mixing.
C43D-02 13:55h
The Deuterium Excess record from Siple Dome, Antarctica in the Context of Global Climate
Comparing the deuterium excess record from the Siple Dome ice core in West Antarctica with other Antarctic deuterium excess records reveals that Antarctica receives moisture from various regions whose signatures of climatic conditions are preserved despite strong circum-Antarctic atmospheric circulation. Deuterium excess is a proxy for conditions in the regions providing the moisture to ice core sites, and is derived from the combination of deuterium and oxygen isotope records. Correcting for moisture source conditions is also fundamental to interpreting temperature from the raw isotopes. Using the Siple Dome deuterium excess record as a proxy for conditions over the subtropical to polar Pacific Ocean provides us with a long, continuous, highly-resolved and well-dated indicator of sea surface conditions. We compare this record with other estimates of sea surface temperature, such as oxygen isotopes in corals and Mg/Ca analyses in foraminifera. The Siple excess record is also compared with more distant climate proxies, such as the GISP2 ice isotope record and the ice core methane record, ultimately providing insight into extra-Antarctic climate, including ENSO and millennial-scale climate change.
C43D-03 14:10h
ISOMAP-UK: A Combined Data - Modelling Investigation of Water Isotopes and Their Interpretation During Rapid Climate-Change Events
Stable isotopes of oxygen and hydrogen within the water molecule provide excellent tracers of climate and climate-related processes within the hydrological cycle. Archives of past variations in water isotopes provide invaluable insights into the nature and causes of abrupt climate change. Long-term changes in oxygen- and hydrogen-isotope ratios in precipitation reflect variations in vapour source, airmass trajectory, rainout history and air temperature, all of which may be influenced strongly by changes in the strength of the thermohaline circulation. Quantitative information about the past isotopic composition of precipitation is transferred to natural archives such as ice sheets, lake sediments, peat and speleothems. Isotopic analyses of such archives can be used to reconstruct long-term histories of the precipitation isotope composition. The aim of this project is to compare carefully-chosen, high-resolution, late-glacial and Holocene isotope records from a range of terrestrial archives in Northwest Europe with model simulations of isotopes in precipitation in order to investigate the role of different forcing factors in rapid climate change. Model simulations will be undertaken using the UK Hadley Centre model HadCM3: incorporation of water-isotope diagnostics into this model, coupled with appropriate validation, is an ongoing component of the project. The mapping and modelling of the past distribution of isotopes in precipitation form the focus of the IGBP PAGES initiative ISOMAP (ISOtope calibration and MAPping study): this project is a UK contribution to ISOMAP, with specific reference to the investigation of rapid climatic events in Britain and NW Europe.
C43D-04 14:25h
Stable Isotopes In Precipitation: An Analysis Of GNIP Data And The Roles Of Temperature And Moisture Source In Isotope Distribution
Arithmetic (unweighted) means of isotope ratios in 1960-2000 precipitation from nearly 410 stations of the IAEA/WMO's global network of isotopes in precipitation (GNIP) are described by the equation: d2H = 8.07(±0.02) d18O + 9.9 (±0.1) (n=3426; R2=0.98). Isotope data from 31 stations with a continuous record for 1960-2000 were evaluated for the periods 1960-1978, 1979-1987, and 1987-2000. Although the isotope relationships at these stations are slightly different than the global relationship based on all GNIP data, a discernible trend of isotope variations from 1961 to 2000 is absent. The temperature coefficient of d18O values is 0.82(±0.01) per oC for stations with a mean air temperature (MAT) range -20°C to -2°C, and 0.53(±0.01) per °C for stations with a MAT range of -20 to 0°C. These coefficients are slightly lower than those reported in previous reports based on a smaller data set. Tropical stations (MAT higher than about 18oC) do not show a temperature dependency or a consistent dependence on the amount of precipitation. An analysis of isotope data in the Asian Monsoon region (70 E - 160 W longitude) with air circulation and moisture transport patterns indicates that isotope distributions are controlled primarily by the moisture source in precipitation. Monthly isotope data from Vienna, Bangkok and Hong Kong were analyzed with respect to upper air parameters and moisture transfer patterns in the planetary boundary layer. These analyses provide a basis for understanding the primary control of temperature on isotopes in Vienna precipitation and of moisture source in Bangkok and Hongkong precipitations. A statistical analysis of horizontal variability based on the entire GNIP dataset indicates that a terrestrial network with about 980 stations distributed more or less uniformly over the globe would provide a strong basis for using the precipitation isotope compositions to study variations and changes in the hydrological cycle on an inter-annual or decadal scale.
C43D-05 14:40h
Water Isotopes in a Warmer World : Lessons from General Circulation Model Based Simulations.
Isotopic versions of atmospheric general circulation models (IGCMs) have been developed by several modeling groups over the last two decades. Their initial applications have focused on past climates (e.g. the Last Glacial Maximum: 20,000 years ago) with the aim of improving the interpretation of paleodata gathered from various archives, particularly polar ice cores. IGCMs are also ideal tools for predicting how the distributions of stable water isotopes (HDO and H218O) in atmospheric water vapor and precipitation will evolve in a warmer world. With this in mind, a 2*CO2 isotopic experiment was performed more than 10 years ago using the NASA/GISS low-resolution IGCM. As expected, this simulation produced isotopically heavier precipitation at high latitudes. Somewhat surprisingly, though, the simulation also produced slightly lighter precipitation in some mid-latitude areas and consistently lighter precipitation in tropical and equatorial regions. In these latter areas, d18O in precipitation is decreased by up to 3 permil, leading to the suggestion that some type of "compensation effect" (decreases in tropical d18O making up for increases in high latitude d18O) may be operating. Warmer climate experiments have now been conducted with the ECHAM IGCM and with new isotopic versions of the NASA/GISS model. We will review these more recent results and examine them in view of a possible isotopic compensation effect between high latitude and tropical regions.
C43D-06 14:55h
Stable Isotopic Variations in Precipitation and Moisture Transport on the Tibetan Plateau
Stable isotopic ratios in precipitation from the Network of Isotope in Precipitation on the Tibetan Plateau are discussed. A decade of continuous observation shows that stable isotopes in precipitation on the northern Tibetan Plateau (TP) are controlled by air temperature. A regression of the annual datasets for the Delingha location (northeast Tibetan Plateau) shows a close relationship ($\delta$$^{18}$O=0.638T-13.31, R$^{2}$=0.58) between $\delta$$^{18}$O in precipitation and air temperature. Along the southern Tibetan Plateau, the monsoon dominates the temporal and spatial variation of $\delta$$^{18}$O in precipitation such that summer precipitation is characterized by very low (depleted) $\delta$$^{18}$O values. A comparison of the temporal variation of $\delta$$^{18}$O at different stations on the TP also reveals the strong monsoonal influence. The results from our observations of present precipitation agree quite well with the results from shallow ice core records. At two northern sites, the Dunde and Muztagata ice core $\delta$$^{18}$O records show comparable variation with air temperatures from a nearby meteorological station. The $\delta$$^{18}$O record from Dasuopu, along the southern margin of the TP, shows a negative relation with the precipitation record from a nearby meteorological station. The spatial and seasonal variations of stable isotopes in precipitation on the TP show the apparent impact of different air masses. The spatial variation patterns of the stable isotopes in precipitation on the Tibetan Plateau show the northern extent of the southwest monsoon. The seasonal variation of moisture sources results in the large seasonal fluctuation of isotopic ratios and high d-excess in precipitation on the southern slope of the Himalayas. The spatial variations of stable isotopes also reveal the impact of the southwest monsoon and westerly transport.
C43D-07 15:10h
Variability and evolution of $\delta^{18}$O$_{precip}$ in a Coupled Atmosphere Ocean GCM
Variability and evolution of $\delta^{18}$O$_{precip}$ in a Coupled Atmosphere Ocean GCM The newest version of the GISS coupled GCM (modelE) tracks water isotopes through every step of the hydrologic cycle. As such, we have a unique ability to monitor the affect of oceanic and atmospheric conditions on $\delta^{18}$O$_{water}$ and $\delta$D. We assess the intrinsic spatial and temporal variability of coupled water isotopes in the ocean and atmosphere under modern conditions. We examine the isotopic response to forced climate changes, including results from an abrupt climate change scenario where we simulate the 8.2 ka event by forcing the coupled model with an isotopically depleted freshwater input at high latitude. In addition, we include coupled model water isotope results given glacial boundary conditions. We assess natural and forced climate influences on $\delta^{18}$O$_{precip}$ on seasonal, annual, decadal, and multi-decadal timescales. We infer the interplay between the various factors that control $\delta^{18}$O$_{precip}$ given different rates of climate change and different boundary conditions. Preliminary results from the 8.2 ka event show the same magnitude of isotopic depletion as observed in Greenland ice cores and the Ammersee record. Interestingly, these results suggest that temporal $\delta^{18}$O$_{precip}$ - temperature relationships are influenced by the nature of the climate change.
C43D-08 INVITED 15:25h
Stable Isotopes in Precipitation and the South American Summer Monsoon - Observations and Model Results
The South American Summer Monsoon (SASM) is a major component of the South American climate system during the austral wet season. A number of proxy records throughout the continent seem to have captured past variations in its strength and/or location on various time scales. Stable isotopic records (e.g. from ice cores or speleothems) are amongst the most prominent such proxies. Unfortunately little is known about the climatic controls on stable isotopes in precipitation at low latitudes and the available observational data are scarce and of very short record lengths. Here we employ a high-resolution Atmospheric General Circulation Model (AGCM), the ECHAM-4 T106 model, fitted with stable isotopic tracers, to investigate the relationship between interannual variations in monsoon strength and the stable isotopic composition of precipitation over tropical South America. Special emphasis will be given to the relative importance of the SASM versus other potential forcing mechanisms such as ENSO. We will show that the relative influence of SASM and ENSO on the stable isotopic composition of precipitation varies regionally. In addition our results suggest that the effects of the two climate modes on stable isotopes counterbalance each other in some regions, while they reinforce each other in other parts of South America.