Bias correction is applied to forecasts launched between 1 November and 31 March inclusive. The bias is calculated as the difference between the 1999-2010 hindcasts (as stored on the S2S database) and ERA-Interim reanalysis. The CFSv2 has a significant bias toward a weak vortex (shown here as a function of launch date), especially for forecasts launched in early winter, so accounting for this drift is necessary.
Please note that when bias correction is not active (i.e. April through October), comparisons between the ensemble plume and the ERA reanalysis are not encouraged.