Modeling of water stable isotopes in the fully coupled Earth system model MPI-ESM: current status and perspectives

The hydrological cycle is a fundamental component of the Earth’s climate system. Modeling the time response of this cycle and the implied physical processes challenges the general circulation models (GCM) used to study the climate system and to project future climate. Water stable isotopes (H216O, H218O and HD16O) are integrated tracers of climate processes occurring in various branches of the hydrological cycle. Changes of the isotopic com- position, which can be measured in various natural climate archives, have been used, for example, to reconstruct past temperatures changes at high resolution or to study the past dynamics of the monsoon. The explicit modeling of these isotopes in GCMs allows to evaluate their performance and to study the past and present-day hydrological cycle evolutions. We present here the first results, under present-day and Last Glacial Maximum (LGM) conditions, of the on- going implementation of water stable isotopes in the fully coupled Earth system model MPI-ESM, called here- after MPI-ESM-wiso. It includes the atmospheric model ECHAM6, the dynamic vegetation module JSBACH and the ocean/sea-ice module MPIOM. In addition to classical variables (temperatures, precipitation amount...), we evaluate the isotopic composition of precipitation, water vapor, ocean, etc. simulated by MPI-ESM-wiso against available observations. Our analyses concentrate also on a detailed comparison to the previous model release, COSMOS-wiso [1], and potential improvements in simulating the water stable isotopes signal (spatial variability, link with the local temperature...) due to overall model enhancements. This work will be an important contribution to the Paleoclimate Modelling Intercomparison Project. Indeed, the models with an explicit water stable isotope diagnostics make it possible to perform direct comparisons, at different time periods, with environmental records and to reduce the uncertainties resulting from the interpretation of these records in terms of climate signals in model-data comparisons. The project is part of the PalMod initiative (“Paleo Modelling: A national paleo climate modelling initiative”), funded by the German Federal Ministry of Education and Science (BMBF). [1] Werner et al., 2016, Geosci. Model Dev., 9, 647-670

Modeling of water stable isotopes in the ECHAM6 atmospheric general circulation model: current status and perspectives

We present here the first results for present-day conditions of the ongoing implementation of water stables isotopes in the latest version of the ECHAM atmospheric general circulation model, ECHAM6, enhanced by the JSBACH interactive land surface scheme (ECHAM6-wiso). Major changes with respect to its predecessor ECHAM5 have to do with the treatment of shortwave radiative transfer, the development of a new surface albedo representation, a new aerosol climatology, the height of the model top, and a more complex representation of the land surface [1]. Besides, a new five-layer soil hydrology scheme can be used instead of the single soil moisture reservoir in ECHAM5/JSBACH [2]. Our first analyses of the ECHAM6-wiso results concentrate on a detailed comparison to the previous model release, ECHAM5-wiso, and potential improvements in simulating the water stable isotopes signal due to overall model enhancements. This study represents the first step of the incorporation of water stable isotope tracers in all components of the fully coupled Earth system model MPI-ESM. The project is part of the PalMod initiative ("Paleo Modelling: A national paleo climate modelling initiative"), funded by the German Federal Ministry of Education and Science (BMBF). [1] Stevens et al., 2013, JAMES, 5, 146–172. [2] Hagemann and Stacke, 2015, Clim. Dyn., 44, 1731–1750

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

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 H162O, H182O 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

Links between central Greenland stable isotopes, blocking and extreme climate variability over Europe at decadal to multidecadal time scales

The link between central Greenland stable oxygen isotopes, atmospheric blocking frequency and cold temperature extremes at decadal to multidecadal time scales is investigated using observed and proxy data as well as model experiments. A composite analysis reveals that positive stable isotope anomalies in central Greenland are associated with enhanced blocking activity in the Atlantic European region. Several indices of blocking activity in the Atlantic European region are higher correlated with central Greenland stable isotope time series than with the North Atlantic Oscillation indices both in observations and model simulation. Furthermore, the blocking frequency anomaly pattern associated with central Greenland stable isotope variability is similar to the blocking anomaly pattern associated with the Atlantic Multidecadal Oscillation. A composite analysis reveals that stable isotope variations in central Greenland are related to a large-scale pattern in the frequency of extreme low temperature events with significant positive anomalies over Europe and a southwest to northeast dipolar pattern over Asia. During observational period central Greenland isotope records, blocking and extreme temperature indices over Europe show enhanced variability 10–30 and 50–70 years. Similar quasi-periodicities dominate the spectrum of central Greenland isotope variability during the last millennium. We argue that long-term variations of climate extreme indices over Europe and Asia, as derived from observational data, can be put into a long-term perspective using central Greenland stable isotope ice core records

Wave Kinematics at High Sea States

Measurements of currents close to the ocean surface and within the crests of large, steep waves have been acquired with an incoherent bistatic sonar mounted on the seafloor. The sonar uses a single narrow-beam transmitter/receiver and three fan-beam receivers set in a triangular configuration around the source. Acoustic pulses transmitted from the seafloor are scattered by bubble clouds and the sea surface to the four receivers and may be transformed into velocity components as a function of elevation. Individual estimates of the currents at, and close to, the surface are made with sufficient temporal resolution to identify kinematics in the crests of large waves. Observations acquired in the Danish sector of the North Sea are examined to evaluate both the potential merits and limitations of the measurement approach. At lower wind speeds, sidelobe scatter from the surface reaches the receiver simultaneously with the volume scattered signal arriving from a few meters beneath, contaminating the velocity measurement at this depth. At higher wind speeds, bubble clouds and increased roughness of the surface combine to suppress this effect, permitting reliable near-surface measurement. A numerical simulation has been implemented to explore some aspects of sonar performance including turbulent velocity fluctuations and bubble density gradients. Additional analysis is carried out to examine bubble suppression of sidelobe scatter. The observations lead to some conclusions regarding wave kinematics during a storm in which the wind speed reached ∼17 m s−1. At the ocean surface, the downwind velocity in the crests of large waves substantially exceeds that predicted by the second-order Stokes model, but in the wave troughs the current is close to the nonlinear prediction