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.
Intellectual Merit: This project will 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). The investigators will examine mixed layer tracer dynamics by drawing on their prior experience studying Langmuir turbulence, submesoscale features, reacting flows, and the global carbon cycle. 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.
Broader Impacts: Three graduate students will be trained at the nexus of oceanic physics, biology, and chemistry. Insights obtained from this project will be directly relevant to gas and carbon budgets at the interface between the atmosphere and ocean, as well as biological dynamics in the upper ocean. Both of these processes play a critical role in the organic and inorganic global carbon cycles, and thus have a direct impact on understanding and predicting atmospheric CO2 levels and climate change. Although this study will be focused on fundamental processes, with an emphasis on physical oceanography and idealized reaction models, there are clear applications to problems in chemical and biological oceanography. In particular, with the numerical framework, diagnostics, and fundamental understanding developed in the current study, the study of reactive tracers can be extended in the future to more complicated models of biological and chemical reactions in the upper ocean. The team of investigators is also experienced in more applied aspects of modeling and parameterization development, and so will ensure the practical utility of the fundamental work. The data obtained from these simulations will be made publicly available to support further research on reactive tracers in the upper ocean.
