Predictability of Semiannual Atmospheric Circulation Type Frequencies in the Extratropics Using the EC‐Earth3 Decadal Forecasting System

Abstract We investigate the skill of the fully initialized decadal forecasting system EC‐Earth3 in re‐forecasting the semiannual occurrence frequencies of synoptic‐scale atmospheric circulation types in the extratropics. Six‐hourly sea‐level pressure data from 10 model integrations of the dcppA‐hindcast experiment are transformed into discrete circulation‐type time series using the Jenkinson‐Collison approach. The frequencies of these types are then compared to those derived from a reference reanalysis. The resulting skill is assessed and compared to that of a 10‐member ensemble run with historical forcing to evaluate the added value of initialization. In the Pacific‐North American sector, both the skill and added value are significant for the December‐to‐May season of the first forecast year. In the Southern Hemisphere, significant results are found for the same season through the second forecast year and also for the June‐to‐November season of the first forecast year. Significant signal‐to‐noise paradox situations are virtually absent in the analyses conducted here.