Numerical modeling of transport by materially coherent vortices in the ocean

Abstract

Transport of material in the ocean is complex and operates on multiple length and time scales. Material coherence, however, is ubiquitous, and ocean flows can often be partitioned into regions dynamically distinct from their surroundings with regards to material transport. This partitioning can assist search-and-rescue operations by reducing the search domain, identify material being transported by breaking waves, or explain the entrainment of material from seagrass beds. The trajectory clustering method partitions fluid elements in a flow according to the similarity between their trajectories. This method systematically decomposes Lagrangian trajectories into coherent and incoherent families, providing a conceptual simplification of the underlying dynamics. The partitioning validity depends on the accuracy of the ocean model used, which is subject to several sources of uncertainty. Ocean forecast realizations with varying model parameters can span a range of potential outcomes. We evaluate the robustness of the trajectory clustering partitions by quantifying their sensitivity to both method free-parameters and model parameters. Then, we analyze an operational coastal ocean ensemble forecast and compare the clustering results to drifter trajectories south of Martha's Vineyard. Next, we develop a model for breaking internal waves and analyze the resulting transport using trajectory clustering. Internal waves shoaling on the continental slope can break and form materially coherent vortices called boluses. These boluses are able to trap and transport material up the continental slope. We model the density stratification in our simulations with a finite pycnocline and demonstrate its impact on the bolus. Trajectory clustering is used to identify the bolus as a coherent structure that contains the material advected upslope. The bolus size and displacement upslope are examined as a function of the pycnocline thickness, incoming wave energy, density change across the pycnocline and topographic slope. Last, we analyze transport of material resulting from the interaction between fluid flow and submerged, deformable seagrass beds. Shear-driven instabilities in the flow evolve into vortices, induce grass blade oscillations, and affect the transport of sediment and nutrients in aquatic systems. We develop a numerical model for the two-way coupled system, using buoyant blades that deform to adjust instantaneously to the fluid drag. The instability onset with respect to a steady state is studied as a function of seagrass buoyancy and Reynolds number. We then visualize how the vortices induce a collective waving motion in the seagrass bed, known as monami, and the vortex-driven material exchange resulting from this interaction.--Author's abstract