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 reforecasting 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.