Numerical simulation of upper ocean responses to the passage of a submesoscale eddy

In this study, a large eddy simulation model (LES) is used to investigate the effect of an idealized westward propagating submesoscale eddy on ocean surface boundary layer turbulence. Large scale forcing (LSF) terms are introduced to the model to represent the effects of submesoscale eddies using the scale separation approach. Although re-stratification is observed at the arrival of the eddy center, consistent with previous studies, strong mixing and deepening is detected before/after the arrival of the eddy center. LES experiments are conducted to investigate the mechanism behind these dynamical responses, and explore the relative importance of the LSF terms. The interaction among these forcing terms is shown to be highly nonlinear, and the combined effects of buoyancy and momentum eddy forcing dominate the boundary layer response to the submesoscale eddy. While buoyancy flux is responsible for the enhanced mixing in the boundary layer, our analysis suggests that the momentum eddy forcing can significantly alter the timing of the enhanced mixing and boost its strength through generating strong mean current in the water column that accelerates westward density advection generated by the buoyancy eddy forcing. The lighter/heavier fluid intrusion brought by the mean flow leads to strong upwelling/downwelling, enhanced mixing and deepening of the mixed layer before the arrival of the eddy. Simulations using a general circulation model do not show similar response in the upper ocean because the K profile parameterization used in the model is a function of vertical shear only, and cannot represent the enhanced turbulence due to lateral mixing brought by the submesoscale eddy. Eddy distortion due to its asymmetric decay with time and northward Ekman transport generated by the wind stress also leads to substantial differences in boundary layer responses at different locations relative to the eddy center.