Spurious internal wave generation during data assimilation in eddy resolving ocean model simulations

Data assimilation (DA) combines observational data and the dynamical ocean model to forecast the ocean state in a matter that is not possible from either observations or models by themselves. However, the incorporation of data-derived corrections into the model introduces the potential to disrupt the dynamical balance of the model state, leading to initialization shocks. These shocks arise as the model undergoes a process of adjustment to restore the perturbed dynamic balance, involving the generation of spurious near-inertial motions. Notably, the US Navy’s global ocean forecast system strives to mitigate these issues through the implementation of the Incremental Analysis Update (IAU) method, distributing the DA corrections to the model state across a specified time window (3 h in the operational system). Our study shows that, despite the implementation of this 3 h IAU period, the initialization shocks still persist in the model analysis fields. We find that during DA updates, spurious internal waves are generated that are in a broad near-inertial frequency range and propagate long distances from their generation sites in the form of low-mode near-inertial waves. The depth-integrated, time-mean near-inertial kinetic energy in a simulation with DA is 68% higher than in a corresponding forward simulation (free-run, without DA) of the simulation with the same surface wind forcing. The presence of these spurious near-inertial waves disrupts the ocean model energetics, and minimizing them is crucial for using the assimilative model simulations to study small scale/high-frequency ocean dynamics. We also examine a possible way to minimize the spurious internal waves by extending the IAU period in numerical experiments using regional model simulations. We demonstrate that the generation of spurious near-inertial waves can be minimized if we insert increments of smaller magnitude at each time step during the update, which can be achieved by extending the IAU period. Our findings indicate that in simulations with a 24 h IAU period, the variance of near-inertial kinetic energy between the assimilative and forward simulations is reduced to approximately 1%.