The monthly forecasting system is run 51 times from slightly different initial conditions. One forecast, called the control forecast, is run from the operational ocean and atmosphere ECMWF analyses. 50 additional integrations, the perturbed members, are made from slightly different initial atmospheric and oceanic conditions, which are designed to represent the uncertainties inherent in the operational analyses.
Atmospheric perturbations: Same as for ENS
a) 50 perturbations of initial conditions are produced using the singular vector method. These include perturbations in the extratropics + perturbations in some tropical areas by targeting tropical cyclones.
b) Stochastic physics: in order to take into account the effect of uncertainties in the model formulation, the tendencies in the atmospheric physics are randomly perturbed during the model integrations.
c) Ensemble data assimilation (EDA): perturbations computed from an ensemble of atmospheric data assimilations are applied to the atmospheric initial conditions in addition to the singular vector perturbations.
A set of wind stress perturbations is calculated by taking the difference between different wind stress analyses. 5 ocean assimilations (1 control and 4 perturbed) are produced by randomly picking 2 perturbations from the set of wind stress perturbations for each month of data assimilation and adding them
with a + or - sign to the analysed wind stress. The same perturbations cannot be chosen for 2 consecutive months.