Impacts of a 4-dimensional variational data assimilation in a coastal ocean model of southern Tyrrhenian Sea

Abstract The impact of the assimilation of satellite sea surface height, sea surface temperature and surface velocity fields observed by a set of high-frequency (HF) radars is studied using a three-dimensional ocean circulation model configured for the southern Tyrrhenian Sea. The study period is October–December 2010 covered by a large number of data. The nonlinear model is based on the Regional Ocean Modeling System (ROMS) and the data assimilation component on the four-dimensional variational (4D-Var) algorithm. Assimilation proceeds in a series of 7-day windows, providing an analysis solution in each window. The assimilation of surface velocity combined with other observations has more utility in recovering the density fields based on the theory of geostrophic adjustment and a strong impact both on near-surface horizontal volume fluxes and subsurface flows, constraining surface geostrophic currents in the area not covered by the HF radar data. The adjoint of the 4D-Var gain matrix was used to quantify the impact of individual observations and observation platforms on different aspects of the 4D-Var circulation estimates during both the analysis and subsequent forecast cycles. In this study, we focus on the alongshore transport of the surface and intermediate waters in the eastern zone of southern Tyrrhenian Sea. The majority of the observations available during any given analysis cycle are from HF radar, and on average these data, together with SSH data, exert the largest controlling influence on the analysis increments of coastal transport. Also, observations from satellite platforms in the form of SST have a considerable impact on analyses and forecasts of coastal transport, even though these observations represent a relatively small fraction of the available data at any particular time. During 4D-Var, the observations are used to correct for uncertainties in the model control variables, namely, the initial conditions, surface forcing, and open boundary conditions. It is found that correcting for uncertainties in the initial conditions and only secondarily in the boundary conditions has the largest impact on the analysis increments in alongshore transport. Finally, we note that both the control vector and the observation impact calculations are a useful way for monitoring the performance of the data assimilation system, as well as quantifying the impact of the observations on the circulation estimates.