Impact of Horizontal Model Resolution on Mixing and Dispersion in the Northeastern Gulf of Mexico

In this paper, the importance of model horizontal resolution in mixing and dispersion is investigated by comparing two data-assimilative high-resolution simulations (4 and 1 km), one of which is submesoscale-permitting. By employing both Eulerian and Lagrangian metrics, upper-ocean differences between the mesoscale-resolving and submesoscale-permitting simulations are examined in the northeastern Gulf of Mexico, a region of high mesoscale and submesoscale activity. Mixing in both simulations is explored by conducting Lagrangian experiments to track the generation of Lagrangian coherent structures (LCSs) and their associated transport barriers. Finite-time Lyapunov exponent (FTLE) fields show higher separation rates of fluid particles in the submesoscale-permitting case, which indicate more vigorous mixing with differences being more pronounced in the shelf regions (depths ≤ 500 m). The extent of the mixing homogeneity is examined using probability density functions (PDFs) of FTLEs with results suggesting that mixing is heterogeneous in both simulations, but some homogeneity is exhibited in the submesoscale-permitting case. The FTLE fields also indicate that chaotic advection dominates turbulent mixing in both simulations regardless of the horizontal resolution. In the submesoscale-permitting experiment, however, smaller scale LCSs emerge as noise-like filaments that suggest a larger turbulent mixing component than in the mesoscale-resolving experiment. The impact of resolution is then explored by investigating the spread of oil particles at the location of the Deepwater Horizon oil spill.