Diagnosing Spatial Distribution of Week 3&4 Ocean Forecast Bias in NCEP/EMC's UFS Coupled Model Prototype 8

Abstract

NOAA's Environmental Modeling Center led a community‐wide effort to build a robust global coupled configuration within the Unified Forecast System (UFS) framework to upgrade operational systems. To reach a fully coupled configuration, the components and their interactions were iteratively refined, resulting in a more connected and comprehensive coupled modeling system. A significant milestone was delivering a mature coupled prototype (P8) as the baseline configuration for subsequent development toward operational systems. This is the first study to evaluate the spatial pattern of P8 week 3&4 ocean forecasts, with NOAA's current coupled operational system, CFSv2, serving as a comparison benchmark. Most notably, P8 demonstrates improved sea surface height (SSH) forecast skill, reduced upper ocean biases, and improved El Niño anomaly and its eastward propagation, albeit with room for further improvements. Given the numerous changes between CFSv2 and P8, isolating specific reasons for forecast improvements is difficult. Instead, a comparison to P8's predecessor, P7, suggests that iterative revisions to atmospheric physics have reduced the overly warm bias in general, and improved the wind‐driven upwelling near the equator and along the US West coast. Further improvements may be sought by refining atmospheric physics parameterization, including model uncertainty, and initializing from cycled coupled data assimilation. This study establishes a critical benchmark necessary to aid in assessing the efficacy of future upgrades, and in evaluating the relative roles of model physics versus initialization in resolving prominent ocean forecast biases.