The impact of internal waves on small-scale geostrophic motions: Implications for submesoscale lateral dispersion

Marie-Pascale Lelong, NorthWest Research Associates, Boulder, CO, United States, Gerardo Hernandez-Duenas, UNAM, Queretaro, QA, Mexico and Leslie M Smith, University of Wisconsin Madison, Madison, United States
A number of questions pertaining to the role of internal waves in driving lateral dispersion on scales of 100m-10km remain unanswered. Flows in this regime, known as the submesoscale, are dominated by internal waves and small-scale geostrophic (vortical) motions.
To better understand the impact of internal waves on lateral dispersion, we have performed a series of numerical simulations with a set of four intermediate nonlinear models which include/exclude subsets of wave/vortical nonlinear interactions. These models range in complexity from ones with no waves (QG) or no vortical motions (GGG), to a model that excludes wave-triad interactions, and finally to the full Boussinesq (FB) that includes all possible nonlinear interactions. The models rely on a linear eigenmode decomposition which splits the flow into two propagating wave eigenvectors and the zero-frequency eigenmode representing vortical motion.

The four models are spun up to statistical equilibrium with identical forcing and the dispersion characteristics of a passive tracer and Lagrangian particles are examined to identify dynamical differences. The parameter regime of interest is Bu=O(1).

In this regime, we find that lateral dispersion is primarily governed by vortical motions, even in cases where the flow is strongly dominated by waves. Somewhat surprisingly, the presence of a strong non-breaking wave field significantly enhances the dispersing capabilities of weaker vortical motions through a direct energy transfer via wave/wave interactions. The scale dependence of the dispersion in each model is also discussed and compared to existing theories.