Abstract
Abstract. In this study we present the first results of a new isotope-enabled general circulation model set-up. The model consists of the fully coupled ECHAM5/MPI-OM atmosphere–ocean model, enhanced by the JSBACH interactive land surface scheme and an explicit hydrological discharge scheme to close the global water budget. Stable water isotopes H218O and HDO have been incorporated into all relevant model components. Results of two equilibrium simulations under pre-industrial and Last Glacial Maximum conditions are analysed and compared to observational data and paleoclimate records for evaluating the model's performance in simulating spatial and temporal variations in the isotopic composition of the Earth's water cycle. For the pre-industrial climate, many aspects of the simulation results of meteoric waters are in good to very good agreement with both observations and earlier atmosphere-only simulations. The model is capable of adequately simulating the large spread in the isotopic composition of precipitation between low and high latitudes. A comparison to available ocean data also shows a good model–data agreement; however, a strong bias of overly depleted ocean surface waters is detected for the Arctic region. Simulation results under Last Glacial Maximum boundary conditions also fit to the wealth of available isotope records from polar ice cores, speleothems, as well as marine calcite data. Data–model evaluation of the isotopic composition in precipitation reveals a good match of the model results and indicates that the temporal glacial–interglacial isotope–temperature relation was substantially lower than the present spatial gradient for most mid- to high-latitudinal regions. As compared to older atmosphere-only simulations, a remarkable improvement is achieved for the modelling of the deuterium excess signal in Antarctic ice cores. Our simulation results indicate that cool sub-tropical and mid-latitudinal sea surface temperatures are key for this progress. A recently discussed revised interpretation of the deuterium excess record of Antarctic ice cores in terms of marine relative humidity changes on glacial–interglacial timescales is not supported by our model results.
Highlights
The water cycle is a key component of the Earth’s climate system
For an evaluation of the modelled temperature effect (Fig. 1c), we focus on the 71 data sets in mid- to high-latitudinal regions with an annual mean temperature value below 20 ◦C
The physical state of the glacial ocean of our Last Glacial Maximum (LGM) simulation has already been analysed and described in detail by Zhang et al (2013). In agreement with this previous study, we find a rather uniform sea surface temperatures (SSTs) cooling in the range of 2–4 ◦C during the LGM in our simulation, comparable to the results of several atmosphere–ocean general circulation models (GCMs) participating in PMIP2 and CMIP5/PMIP3 (Zhuang and Giardino, 2012)
Summary
The water cycle is a key component of the Earth’s climate system. Documenting and understanding its past evolution is essential to test our ability to model its future changes. The LGM climate is very different from the present and/or pre-industrial climate, but this latest glacial epoch offers a wealth of terrestrial, marine, and ice core proxy data for an in-depth model–data comparison As many of these data sets are based on water stable isotopes (e.g. speleothem data, marine calcite data, ice core records), several studies with isotope-enabled GCM have chosen the LGM as a key period for an evaluation of modelled δ18O and δD values with different proxy data (Jouzel et al, 2000; Lee et al, 2008; Lewis et al, 2013; Risi et al, 2010a). Our following analysis and presentation of simulation results focus on the following questions. (a) How well does this fully coupled Earth system model simulate first-order isotopic variations (δ18O, δD) within different parts of the Earth’s water cycle under pre-industrial and LGM boundary conditions? (b) Do the model results indicate substantial changes in the temperature–isotope relation of meteoric water? (c) Are simulated spatial and temporal variations of the deuterium excess in precipitation, a second-order isotope effect, in agreement with available observations and paleoproxy data? (d) If so, how are these variations of deuterium excess related to past changes in evaporation processes?
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