An EnOI‐based data assimilation system with DART for a high‐resolution version of the CESM2 ocean component

An ensemble optimal interpolation (EnOI) data assimilation system for a high-resolution (0.1 degrees horizontal) version of the Community Earth System Model Version 2 (CESM2) ocean component is presented. For this purpose, a new version of the Data Assimilation Research Testbed (DART Manhattan) that enables large-state data assimilation by distributing state vector information across multiple processors at high resolution is used. The EnOI scheme uses a static (but seasonally varying) 84-member ensemble of precomputed perturbations to approximate samples from the forecast error covariance and utilizes a single model integration to estimate the forecast mean. Satellite altimetry and sea surface temperature observations along with in situ temperature and salinity observations are assimilated. This new data assimilation framework is then used to produce a global high-resolution retrospective analysis for the 2005-2016 period. Not surprisingly, the assimilation is shown to generally improve the time-mean ocean state estimate relative to an identically forced ocean model simulation where no observations are ingested. However, diminished improvements are found in undersampled regions. Lack of adequate salinity observations in the upper ocean actually results in deterioration of salinity there. The EnOI scheme is found to provide a practical and cost-effective alternative to the use of an ensemble of forecasts.