ModE-Sim (short for Modern Era Simulations) is a medium-size ensemble of model simulations using the ECHAM6 atmosphere general circulation model (model version 6.3.5p2, doi:10.17617/2.1810480). Its setup is based on the PMIP4 experiments, but uses a forced AGCM rather than a fully coupled model.

ModE-Sim was originally designed to form the a-priori state for a climate reconstruction (Modern Era Reanalysis, ModE-RA, to be found as separate experiment within this WDC project) that uses an offline data assimilation technique to combine the output of ModE-Sim with historical climate information. However, beyond its original purpose ModE-Sim on its own can be used as a tool to study climate variability, providing a high number of posible climate states that are physically plausible under the given forcings and boundary conditions. This might include, e.g. the separation of internal variability from the response to externally forced signals, understanding of teleconnection patterns, or the study of extreme events.

The ensemble uses observed/reconstructed forcings and boundary conditions, while accounting in uncertainties in these. For 1850 to 2009 ModE-Sim offers 36 members grouped in two subsets, all using PMIP4 radiative forcings. The subset 1850-1 provided in this dataset group has 20 members and uses 10 different realizations of HadISST2 (two different ModE-Sim members share one realization of HadISST while they differ in their initialization) and according sea ice as ocean boundary conditions.

In contrast to the other ModE-Sim sets, for set 1850-1 no forcings are provided within this dataset group. This is because for set 1850-1 only standard forcings and boundary conditions were used, i.e. PMIP4 volcanoes and HadISST sea surface temperatures and sea ice concentrations https://www.metoffice.gov.uk/hadobs/hadisst2/.