Ocean State and Surface Forcing Correction using the ROMS- IS4DVAR Data Assimilation System

The Incremental Strong constraint 4D-Variational (IS4DVAR) system of the Regional Ocean Model System (ROMS) for data assimilation is presented. It has been applied to the California Current System (CCS) to assimilate remotely-sensed and in situ oceanic observations. Results from both twin and realistic experiments are presented. The IS4DVAR control vector is comprised of the model initial conditions and surface forcing, and a focus is given on the ability of ROMS-IS4DVAR to correct the latter. ROMS-IS4DVAR always reduces misfits between the model and the observations that are assimilated. However, without corrections to the surface forcing, the assimilation of surface data can degrade the temperature structure at depth. This behavior is prevented by using surface forcing adjustment in ROMS-IS4DVAR which can reduce errors between the model and surface observations through corrections to surface forcing rather than to temperature at depth. Wind stress corrections generate abnormal spatial and temporal variability in its divergence and curl. However, twin experiments indicate that corrections to wind stress and surface heat flux tend to reduce errors in these forcing fields.