Model-Based Observing System Evaluation in a Western Boundary Current: Observation Impact from the Coherent Jet to the Eddy Field

Ocean forecast models rely on observations to provide regular updates in order to correctly represent dynamic ocean circulation. This synthesis of observations and models is referred to as data assimilation. Since initial conditions dominate the quality of short-term ocean forecasts, accurate ocean state estimates, achieved through data assimilation, are key to improving prediction. Western boundary current (WBC) regions are particularly challenging to model and predict because they are highly variable. Understanding how specific observation types, platforms, locations, and observing frequencies impact model estimates is key to effective observing system design. The East Australian Current (EAC), the South Pacific’s WBC, is a relatively well-observed current system that allows us to study the impact of observations on prediction across different dynamical regimes, from where the current flows as a mostly coherent jet to the downstream eddy field. Here we present a review of the impact of observations on model estimates of the EAC using three different methods. Consistent results across the three approaches provide a comprehensive understanding of observation impact in this dynamic WBC. Observations made in regions of greater natural variability contribute most to constraining the model estimates, and subsurface observations have a high impact relative to the number of observations. Significantly, sampling the downstream eddy-rich region constrains the upstream circulation, whereas observing the upstream coherent jet provides less improvement to downstream eddy field estimates. Studies such as these provide powerful insights into both observing system design and modeling approaches that are vital for optimizing observation and prediction efforts.