Historical reanalyses combine observations of past weather with simulations from modern numerical weather prediction models to provide a consistent, global history of the weather. CERA-20C is the first coupled reanalysis over the 20th century produced by ECMWF. Atmospheric and ocean observations are assimilated simultaneously, thereby providing more balanced estimates of the atmosphere-ocean system and allowing for a broader range of studies of the entire Earth system. The question of whether the North Atlantic Oscillation (NAO) and associated climate anomalies over the North Atlantic-European area during winter can be predicted with any confidence is still a matter of on-going scientific debate. Long retrospective seasonal forecast experiments for all boreal winter seasons during the period 1901 to 2009 have been produced using CERA-20C for the initialisation of the coupled model. There is significant skill for early and most recent decades while there is a lack of skill mid-century. This shows that short hindcast periods are not sufficiently representative for longer-term behaviour due to decadal climate variability.