Coupled models and initialization procedures. The DEMETER system comprises seven global coupled ocean-atmosphere models. A brief summary (detailed information on the models and the initialization procedures can be found on the DEMETER Web site www.ecmwf.int/research/ demeter/general/docmodel/index.html) of the different coupled models used in DEMETER is given in Table 1. From each model, except that of the Max- Planck Institute (MPI), uncertainties in the initial state are represented through an ensemble of nine different ocean initial conditions (Fig. 1). This is achieved by creating three different ocean analyses; a control ocean analysis is forced with momentum, heat, and mass flux data from the ECMWF 40-yr Re- analysis (ERA-40 henceforth; ERA-40 intends to pro- duce a global analysis of variables for the atmosphere, land, and ocean surface for the period 1958-2001; more information is available online at www.ecmwf. int/research/era), and two perturbed ocean analyses are created by adding daily wind stress perturbations to the ERA-40 momentum fluxes. The wind stress perturbations are randomly taken from a set of monthly differences between two quasi-independent analyses. In addition, in order to represent the uncertainty in SSTs, four SST perturbations are added and subtracted at the start of the hindcasts. As in the case of the wind perturbations, the SST perturbations are based on differences between two quasi-independent SST analyses. Atmospheric and land surface initial conditions are taken directly from ERA-40. A separate ensemble initialization procedure is used for the MPI model. Ocean data assimilation has been used in the Met Office experiment after 1987 as well as in a sensitivity experiment carried out with the ECMWF model.
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