Analysis of four‐dimensional variational state estimation of the Hawaiian waters

Abstract

In this study, we evaluate results from an incremental strong constraint four‐dimensional variational data assimilation (IS4D‐Var) experiment applied to the circulation around the Hawaiian Islands using the Regional Ocean Modeling System (ROMS). Assimilated observations include (1) satellite‐derived high‐resolution swath radiometer sea surface temperatures (SST) and along‐track altimetric sea surface heights (SSH) and (2) in situ temperature and salinity profiles from Argo floats, autonomous Seagliders, and shipboard conductivity‐temperature‐depth. Two assimilation configurations are compared: adjusting initial conditions versus adjusting both initial conditions along with atmospheric forcing. In the latter case, we compare two separate forcing products. For all experiments, we investigate how the assimilation alters the tidal, inertial, and mesoscale variability. Significant improvements in the observation‐model fit are found for SST and salinity regardless of assimilation configuration or atmospheric forcing; however, significant change to the subsurface temperature is made when adjusting only initial conditions. Baroclinic tides are altered during the assimilation because of changes in the density field in regions of strong internal tide generation. Spurious inertial oscillations are found in assimilation circulations that are associated with the IS4D‐Var increment when using either SST or SSH observations; however, this increase in the inertial energy had minimal effect on the mesoscale variability.