Process Evaluation of Subseasonal North Atlantic Oscillation Prediction in the ECMWF Ensemble Forecast System

This study evaluates the prediction skill of the North Atlantic Oscillation (NAO) pattern and its associated energy budget as simulated by the European Center for Medium-Range Weather Forecasts ensemble forecast system. By classifying NAO events into high- and low-skill cases, we analyzed the stationarity of NAO patterns and the role of baroclinic energy conversion in NAO prediction. In both positive and negative NAO phases, high-skill cases exhibited more stationary NAO patterns than low-skill cases. The analysis of processes indicates that high-skill NAO cases are due to stronger baroclinic maintenance of NAO, with its initial position at the climatological thermal trough, whereas low-skill NAO cases result from forecast biases in wave propagation from the North Pacific. Specifically, biases in baroclinic energy conversion in the meridional direction from week 2 lead to weak advection of the eddy available potential energy (EAPE), resulting in lower prediction skill.