Deliverable 6.4 Ice-ocean statistics and state estimation V1

Based on a coupled ocean-sea ice model and its adjoint model, we evaluate the capacity of the existing Arctic Ocean observing system on capturing different aspects of the Arctic Ocean changes, using Observing System Simulation Experiments (OSSEs). Subsequently, we produce a 10-year ocean-sea ice synthesis by assimilating all available observations into the system. The main findings are: 1) Moorings deployed in the Fram Strait and along the pathway of Atlantic inflow to the Arctic Ocean are critical to capture the remote-propagating signals. The existing mooring system needs to be enhanced. 2) Sea ice processes, especially sea ice concentration changes, are the dominant mode that is improved by data assimilation. The improvement benefits from the spatiotemporal coverage of sea ice concentration observations. 3) Changes in the temperature and salinity are tiny in the ice-covered region and are caused by enhanced air-sea interaction due to sea ice reduction. Although the availability of hydrographic profiles increased significantly over the past decades, its effects are not apparent in the data assimilation experiments.