A 4D-Var Data Assimilation System for the North-Western Mediterranean Sea and its Impact on the Corsica Channel Transport

Abstract. We present a 4D-Var data assimilation (DA) system covering the North-Western Mediterranean Sea implemented with the Regional Ocean Modeling System (ROMS). We study, throughout the year 2022, its ability to improve the description of the overall circulation and the capability to constrain the transport across the Corsica Channel (CC), the dynamics of which are crucial in determining the circulation throughout the region. The system assimilates Sea Surface Temperature (SST) and Sea Level Anomaly (SLA) observations from satellites, surface velocity data from High-Frequency Radars (HFRs), and in situ temperature, salinity and velocity observations, the latter from a mooring located in the CC. For all the observed state variables, DA is able to improve the forecast and the analysis compared to a free run without DA, with root mean squared error reduction up to 60 % and correlation increase up to 0.4. The general circulation after DA is characterized by a reduction of the Eastern Corsica Current (ECC) and an increase of the Western Corsica Current (WCC). An adjoint sensitivity-based method was used to evaluate the impact of observations on the CC transport state estimates. The net reduction in transport induced by DA, changed the annual average value of 0.49 Sv for the free run, to 0.31 Sv and 0.28 Sv for the forecast and analysis, respectively. The observations that contribute most to the transport changes are the in situ velocity data and those from HFRs. The observation impacts were found to vary seasonally, and sometimes act in competition to shape the circulation pathways across the CC. The sensitivity of the transport to SST and in situ temperature and salinity observations indicates that remote measurements (e.g. those from the Gulf of Lion) can potentially play a significant role in constraining the CC transport. Transport variations are largely affected by free surface gradients contained in the increment, and promoted by modifications to the open boundary conditions. This indicates that the CC dynamic is controlled by mechanisms operating at basin scales.