Please see this paper:
Molteni, F, Stockdale, T, M. Balmaseda, A, Basalmo, G, Buizza, R, Ferranti, L, Magnusson, L, Mogensen, K, Palmer, TN, Vitart, F (2011): The new ECMWF seasonal forecast system (System 4). ECMWF Technical Memorandum number 656. (http://www.ecmwf.int/en/elibrary/11209-new-ecmwf-seasonal-forecast-system-system-4)
If you want to acknowledge C3S Seasonal Forecast data in a publication we also recommend to reference this paper.