An introduction to stable water isotopes in climate models: benefits of forward proxy modelling for paleoclimatology

Abstract

Abstract. Stable water isotopes have been measured in a wide range of climate archives, with the purpose of reconstructing regional climate variations. Yet the common assumption that the isotopic signal is a direct indicator of temperature proves to be misleading under certain circumstances, since its relationship with temperature also depends on e.g. atmospheric circulation and precipitation seasonality. Here we introduce the principles, benefits and caveats of using climate models with embedded water isotopes as a support for the interpretation of isotopic climate archives. A short overview of the limitations of empirical calibrations of isotopic proxy records is presented. In some cases, the underlying hypotheses are not fulfilled and the calibration contradicts the physical interpretation of isotopic fractionation. The simulation of climate and its associated isotopic signal, despite difficulties related to downscaling and intrinsic atmospheric variability, can provide a "transfer function" between the isotopic signal and the considered climate variable. The relationship between modelled temperature and isotopic signal is analysed under present-day, pre-industrial and midHolocene conditions. The linear regression relationship is statistically more significant for precipitation-weighted annual temperature than mean annual temperature, yet the regression slope varies greatly between the time-slice experiments. Temperature reconstructions that do not account for the slope variations will in this case underestimate the lowfrequency variability and overestimate high-frequency variability from the isotopic proxy record. The spatial variability of the simulated δ 18 O-temperature slope further indicates that the isotopic signal is primarily controlled by synoptic atmospheric circulation rather than local temperature.

Determining water sources in the boundary layer from tall tower profiles of water vapor and surface water isotope ratios after a snowstorm in Colorado

“…Because the stable hydrogen and oxygen isotope ratio of water vapor, liquid and ice reflects the balance of processes influencing regional hydrology, measurements of the oxygen and hydrogen isotope ratios of water can provide constraints for water balance studies and expose model shortcomings (Jouzel and Merlivat, 1984;Henderson-Sellers et al, 2004;Henderson-Sellers, 2006;Sturm et al, 2010). The utility of isotope ratio information stems from the fractionation that accompanies phase changes in which the heavier isotopologues preferentially remain in liquid or solid form during evaporation and condensation (Bigeleisen, 1961;Dansgaard, 1964).…”

How does sea ice influence <i>δ</i><sup>18</sup>O of Arctic precipitation?

“…The model is based on the Community Atmosphere Model version 3 (CAM3) (Collins et al, 2006), and the isotope module was developed by David Noone, University of Colorado. More details of isoCAM3 can be found in Noone and Sturm (2010) The isoCAM3 model has been applied in several studies that investigated the isotopic response to past climate changes (Tharammal et al, 2013;Speelman et al, 2010;Sturm et al, 2010;Pausata et al, 2011;Liu et al, 2012;Sewall and Fricke, 2013;Liu et al, 2014).…”

Variations of oxygen-18 in West Siberian precipitation during the last 50 years

“…Such isotope-enabled AGCMs provide a mechanistic understanding of the atmospheric processes influencing the isotopic composition of meteoric water. Since the pioneering work of Joussaume et al (1984), Jouzel et al (1987), Hoffmann et al (1998) and others, about a dozen stateof-the-art GCMs have been equipped with explicit isotope diagnostics (see Sturm et al, 2010, for a detailed model overview). A number of studies have clearly demonstrated their usefulness for an improved climatic interpretation of present and past water isotope variability (e.g.…”