The Regional Ocean Modeling System (ROMS) 4-dimensional variational data assimilation systems

► The impact of observations on California Current circulation estimates is assessed. ► All observations are found to be important in controlling coastal transports. ► In situ observations have significant impact despite their relatively small number. ► The adjoint of 4D-Var can be used reliably for observing system experiments. ► The impact of uncertainties in the observations on coastal transports is assessed.