Wave-Current Interactions at Meso and Submesoscales: Insights from Idealized Numerical Simulations

Surface gravity waves play a major role in the exchange of momentum, heat, energy, and gases between the ocean and the atmosphere. Waves are modulated by ocean currents via wave–current interactions, which lead to variations in their direction, frequency, and amplitude. Recent studies of wave-current interactions suggest that the spatial variability of the significant wave height at submesoscales (10-100 km) is dominated by the spatial variability of the current field. At these scales, divergent motions associated with tides and inertia-gravity waves may contribute to a large portion of the surface kinetic energy (EKE). In the California Current region, for example, the EKE at submesoscales is mostly dominated by balanced (rotational) motions in late winter/spring, while divergence is stronger in late summer/fall. If surface waves respond differently to divergence and vorticity, seasonal changes in the dominant regime of surface currents may lead to significant changes in the surface wave field. In the present work, we use an ensemble of synthetic flow fields to force WAVEWATCH III and assess the relative impact of current divergence and vorticity in modifying several properties of the waves. We find that the wavenumber spectrum of the significant wave height (Hs) is highly sensitive to the nature of the underlying current and that at wavelengths from 10-100 km the spectral slope of Hs nearly follows EKE spectral slope.

In addition, the response behavior diagnosed with the idealized ensembles are used to interpret the results of simulations using surface currents from the 2 km resolution LLC4320 MITgcm output in the California Current region. The results suggest that wave parameters could be used to detect and characterize strong gradients in the velocity field which is particularly relevant for SWOT, as well as CFOSAT and several proposed satellite missions.