Directional Asymmetry of Forecast-to-Power Error Propagation in Multi-Horizon Wave-State Forecasting for a Pontoon WEC

Forecast errors in significant wave height and dominant wave period can propagate differently to wave energy converter (WEC) power, depending on error direction and the device response. Hourly records from NDBC Station 46014 (2019–2020) were used for direct 1, 3, and 6 h forecasts from 24 h histories. Valid wind-direction observations recorded as 360° (north) were retained during preprocessing. Persistence, raw-input GRU, and forecast-origin CEEMDAN-GRU were compared. Paired forecasts were converted to power using a published Pontoon WEC power matrix on shared valid timestamps. Finite-change analysis separated height, period, and joint power-error contributions. A sign-reversal index compared equal-magnitude positive and negative forecast errors at each observed sea state. CEEMDAN-GRU produced mapped-power RMSEs of 83.22, 98.15, and 115.65 kW; its differences from raw-input GRU were small, with calendar-block confidence intervals including zero. The period–direction mean absolute asymmetry index exceeded the height–direction index by 0.194, 0.180, and 0.164 on jointly valid samples, with all 95% confidence intervals above zero. This ordering persisted under grid-normalized perturbations and two alternative interpolation methods. The results concern forecast-error propagation through the published matrix at one buoy station, without measured electrical-output validation.