Water isotopes – climate relationships for the mid-Holocene and preindustrial period simulated with an isotope-enabled version of MPI-ESM

Abstract

Abstract. We present here the first results, for the preindustrial and mid-Holocene climatological periods, of the newly developed isotope-enhanced version of the fully coupled Earth system model MPI-ESM, called hereafter MPI-ESM-wiso. The water stable isotopes H216O, H218O and HDO have been implemented into all components of the coupled model setup. The mid-Holocene provides the opportunity to evaluate the model response to changes in the seasonal and latitudinal distribution of insolation induced by different orbital forcing conditions. The results of our equilibrium simulations allow us to evaluate the performance of the isotopic model in simulating the spatial and temporal variations of water isotopes in the different compartments of the hydrological system for warm climates. For the preindustrial climate, MPI-ESM-wiso reproduces very well the observed spatial distribution of the isotopic content in precipitation linked to the spatial variations in temperature and precipitation rate. We also find a good model–data agreement with the observed distribution of isotopic composition in surface seawater but a bias with the presence of surface seawater that is too 18O-depleted in the Arctic Ocean. All these results are improved compared to the previous model version ECHAM5/MPIOM. The spatial relationships of water isotopic composition with temperature, precipitation rate and salinity are consistent with observational data. For the preindustrial climate, the interannual relationships of water isotopes with temperature and salinity are globally lower than the spatial ones, consistent with previous studies. Simulated results under mid-Holocene conditions are in fair agreement with the isotopic measurements from ice cores and continental speleothems. MPI-ESM-wiso simulates a decrease in the isotopic composition of precipitation from North Africa to the Tibetan Plateau via India due to the enhanced monsoons during the mid-Holocene. Over Greenland, our simulation indicates a higher isotopic composition of precipitation linked to higher summer temperature and a reduction in sea ice, shown by positive isotope–temperature gradient. For the Antarctic continent, the model simulates lower isotopic values over the East Antarctic plateau, linked to the lower temperatures during the mid-Holocene period, while similar or higher isotopic values are modeled over the rest of the continent. While variations of isotopic contents in precipitation over West Antarctica between mid-Holocene and preindustrial periods are partly controlled by changes in temperature, the transport of relatively 18O-rich water vapor near the coast to the western ice core sites could play a role in the final isotopic composition. So, more caution has to be taken about the reconstruction of past temperature variations during warm periods over this area. The coupling of such a model with an ice sheet model or the use of a zoomed grid centered on this region could help to better describe the role of the water vapor transport and sea ice around West Antarctica. The reconstruction of past salinity through isotopic content in sea surface waters can be complicated for regions with strong ocean dynamics, variations in sea ice regimes or significant changes in freshwater budget, giving an extremely variable relationship between the isotopic content and salinity of ocean surface waters over small spatial scales. These complicating factors demonstrate the complexity of interpreting water isotopes as past climate signals of warm periods like the mid-Holocene. A systematic isotope model intercomparison study for further insights on the model dependency of these results would be beneficial.

Greenland temperature and precipitation over the last 20 000 years using data assimilation

“…Particularly valuable would be transient simulations that include water isotopes as prognostic variables, which allow for direct assimilation of water isotope ratios (Steiger et al, 2017;Okazaki and Yoshimura, 2017), rather than the use of an explicit proxy system model between temperature and δ 18 O. Recent work shows significant improvements to the realism of water-isotope enabled models in the polar regions (Nusbaumer et al, 2017;Dütsch et al, 2019;Cauquoin et al, 2019;Okazaki and Yoshimura, 2019), and longer simulations, once available, should allow us to further improve upon the results we have presented here. In principle, our…”

“…Borehole thermometry at the GISP2 and GRIP sites shows that for the low-frequency changes associated with the last glacialinterglacial transition, the temporal slope is less than half the modern, spatial slope (Cuffey and Clow, 1997;Jouzel et al, 1997). Numerous studies have suggested that precipitation seasonality is the largest source of nonlinearity in the δ 18 O-T site relationship (e.g., Steig et al, 1994;Pausata and Löfverström, 2015); changes in precipitation seasonality are thought to be the primary reason that the effective δ 18 O-T site relationship for the glacial-interglacial transition has such a low slope (Werner et al, 2000;Gierz et al, 2017;Cauquoin et al, 2019).…”

Abrupt Climate and Weather Changes Across Time Scales

“…Alternatively, their effects must be parameterized as in current state-of-the-art general circulation models, where the typical horizontal resolutions of the ocean component is about 1 • . Practically, given the present high-performance computer capacities, efficient and parallelized model codes, it is now possible to conduct isotope-enabled (e.g., Cauquoin et al, 2019;Werner et al, 2018) simulations for 50-100 model years per day even with a multiscale ansatz (Shi et al, 2020).…”

Speleothems of South American and Asian Monsoons Influenced by a Green Sahara

“…Further, many speleothem records come from these regions, and historically, model simulations have had difficulty capturing the speleothem‐suggested δ 18 O responses (Battisti et al, 2014; Comas‐Bru et al, 2019; Liu, Wen, et al, 2014; Liu & Battisti, 2015; Tabor et al, 2018). Although recent water isotopologue modeling work appears to have somewhat better model‐proxy agreement in East Asia, possibly due to the inclusion of dynamic vegetation (Cauquoin et al, 2019). We define monsoon regions as gridcells where >50% of total precipitation falls during the wet season (NDJFM for the SAM and MJJAS for the AM) and wet season minus dry season precipitation is >2 mm/day, similar to the definition presented in Wang et al (2011).…”

“…Climate models with water isotopologue tracking have been developed to allow for more direct comparison between models and proxies. Despite providing some refined interpretations, water isotopologue tracking‐enabled models also have difficulty capturing the amplitude of the δ 18 O spel response with changes in summer insolation, suggesting that the simulated hydrologic response bias is not entirely a result of incorrect proxy interpretation (Battisti et al, 2014; Cauquoin et al, 2019; Comas‐Bru et al, 2019; Liu, Wen, et al, 2014; Tabor et al, 2018).…”