The interactions between Langmuir turbulence and submesoscale processes in the oceanic mixed layer are described using large-eddy simulations of the spin-down of a temperature front in the presence of submesoscale eddies, winds, and waves. The simulations solve the Craik-Leibovich equations with Stokes drift wave forcing at a resolution that is sufficiently fine to capture small-scale Langmuir turbulence. A simulation without Stokes drift forcing is also performed for comparison. The spatial and spectral properties of temperature, velocity, and vorticity fields are described, and these fields are scale-decomposed in order to examine multiscale fluxes of momentum and temperature (or buoyancy). Buoyancy flux results indicate that Langmuir turbulence counters the restratifying effects of submesoscale eddies, leading to greater vertical transport and mixing than in the simulations without Stokes drift forcing. The observed fluxes are also shown to be in good agreement with results from an asymptotic analysis of the Craik-Leibovich equations. Regions of potential instability in the flow are identified using Richardson and Rossby numbers, and it is found that Langmuir turbulence is associated with increased prevalence of gravitational instabilities, in contrast to simulations without Stokes drift forcing which are dominated by symmetric instabilities. The mixed layer is deepened in the presence of Stokes forcing, and the distinction between the depths of uniform potential vorticity and uniform temperature gradient – a signature of symmetric instability – is less pronounced.
Reactive tracers such as carbonate chemical species and plankton play key roles in determining the biogeochemistry of the ocean, which is the largest reservoir of carbon in the Earth system active on short timescales. These tracers react primarily in the mixed layer, where air-sea exchange occurs and light is plentiful for photosynthesis. The mixing of these tracers is parameterized in large- scale, mesoscale eddy-resolving simulations of the global carbon cycle and climate, but so far the coupling between their reactions and flow physics is not represented. Understanding this coupling in the upper ocean is complicated by the presence of turbulent processes spanning a wide range of scales, including vertical mixing by meter-scale Langmuir turbulence and kilometer-scale stirring by submesoscale eddies, fronts, and filaments. As such, reactive tracers are not fully mixed to the point of having zero gradients at all scales, even in the mixed layer. This leads to heterogeneity, or “patchiness,” in the spatial distribution of tracers; the degree and spectral properties of this heterogeneity are determined by the interactions between turbulent mixing and biological or chemical reactions. Characterizing the effects of realistic mixed layer turbulence on reactive tracers with different reaction dynamics and rates is a fundamental challenge in understanding the interplay between physical processes and biological and chemical species in the ocean. In this project, we use large eddy simulations (LES) to study the effects of multiscale turbulent processes on reactive tracers in the oceanic mixed layer, spanning scales from meters (Langmuir) to tens of kilometers (submesoscale fronts and instabilities). Relatively little work on turbulence and oceanic tracer reactions over this range of scales is extant, yet small-scale turbulent mixing has time scales similar to those of chemical processes (such as CO2 hydration), and submesoscale eddies evolve on time scales similar to those of plankton blooms. Many prior studies of reactive tracers have relied on simple flow fields (such as two-dimensional or quasi-geostrophic turbulence); this project will address the full three dimensional complexity of upper ocean turbulence. At first, relatively simple tracer reactions will be used, building toward more realistic biological and chemical models. This approach will allow fundamental turbulence-tracer interactions to be understood before introducing overly complex reaction models. This project will bring together concepts from fundamental turbulence physics, chemical and biological reacting flows, and physical oceanography.