Characteristics and Evolution of Reactive Tracers in the Oceanic Mixed Layer

Reactive tracers such as plankton and carbonate chemical species play important roles in in the oceanic carbon cycle,allowing the ocean to hold 60 times more carbon than the atmosphere. These tracers react primarily in the mixed layer, where air-sea gas exchange occurs and light is plentiful for photosynthesis. It is well known that there can be substantial heterogeneity, or “patchiness,” in the spatial distribution of ocean tracers, but the contribution of sub-kilometer scale turbulent processes to these distributions remains poorly understood. In this scale range, both submesoscale (100m - 10 km), largely geostrophic, mixed layer eddies that act to restratify the mixed layer and small-scale, three-dimensional turbulence that promotes mixing are active, resulting in substantial scale complexity from which tracer heterogeneity can arise. In this presentation, we take a foundational approach to understanding tracer characteristics and evolution in the presence of realistic mixed layer ocean turbulence using results from two sets of simulations. In the first set, we perform large eddy simulations (LES) of the spin-down of a large-scale temperature front to understand the characteristics and evolution of tracers released at different locations and tracers fluxing across the air-sea interface on horizontal scales from 20km down to 5m. The simulations include the effects of wave-driven Langmuir turbulence by solving the wave-averaged Boussinesq equations with an imposed Stokes drift velocity. In the second set of simulations, we perform LES of idealized biogeochemical tracers in the oceanic mixed layer from scales of hundreds of meters down to sub-meter scales. By examining different tracer sources, flux rates, reaction mechanisms, and reaction time scales in these simulations, we connect the coupled turbulence-tracer dynamics with spatial, spectral, and statistical properties of the resulting tracer fields. We also highlight the dependence of tracer properties on mixed layer turbulent processes occurring at different scales, including vertical mixing by meter-scale Langmuir turbulence and kilometer-scale stirring by submesoscale eddies, fronts, and filaments. Eddy diffusivities are calculated for each of the tracers and the implications of the simulation results for parameterizations of submesoscale reactive tracer dynamics in large-scale climate simulations are discussed.