State-dependent predictability in subseasonal to seasonal forecasts over North America

Forecast skill on subseasonal-to-seasonal (S2S) timescales varies strongly with the large-scale atmospheric state, creating intermittent “windows of opportunity’ ’ for skillful prediction. Here we evaluate state-dependent predictability of 2m temperature over North America using CESM2 and ECMWF subseasonal forecasts. Skill is quantified as a function of the Pacific-North American (PNA) pattern, the phase of the El Nino Southern Oscillation, the Madden-Julian Oscillation (MJO) and the North Atlantic Oscillation (NAO). Both models exhibit regionally significantly enhanced subseasonal skill during dynamically organized flow regimes like the PNA, El Niño or La Nina, and certain MJO phases, when tropical forcing projects onto an amplified winter jet and supports coherent Rossby wave propagation. The resulting predictability is modulated by the seasonality of the background flow. Our findings highlight the importance of the initial circulation state and demonstrate that regional S2S forecast skill may be higher than suggested by spatial averages.