Horizontal transport of buoyant material by turbulent jets in the upper ocean

Abstract

Currents in the ocean surface boundary layer (OSBL) determine the horizontal transport of submerged buoyant material, such as pollutants, plankton, and bubbles. Commonly, the mean horizontal transport, that is, the transport that changes the horizontal position of the material’s center of mass, is assumed to be accomplished by horizontal mean currents. However, surface convergence zones due to OSBL turbulence organize both wind-driven horizontal currents and near-surface concentrated buoyant material. In such surface convergence zones, concentrations of buoyant material are enhanced (e.g., apparent as windrows) and collocate with increased horizontal turbulent currents, here referred to as turbulent jets. In turn, the correlation of turbulent jet flow and material concentrations leads to a net mean horizontal transport due to turbulent motion. To examine this turbulent jet transport, an idealized model is devised for a wind-driven flow that is perturbed by prescribed cellular flow structures with crosswind surface convergence zones. Model solutions of the jet flow and material concentrations reveal that turbulent jet transport is comparable to the transport by horizontal mean currents for sufficiently strong cellular flow and material buoyancy. To test this model, we also perform more realistic turbulence-resolving large-eddy simulations (LESs) of wind and wave-driven OSBL turbulence. LES results are consistent with many features of the idealized model and suggest that the commonly overlooked turbulent jet transport is about 20%–50% of the traditional transport by horizontal mean currents. Thus, turbulent jet transport should be taken into account for accurate transport models of buoyant material in the OSBL.