Effects of Langmuir Turbulence on Upper Ocean Carbonate Chemistry

Reactive tracers such as carbonate chemical species play important roles in the oceanic carbon cycle, allowing the ocean to hold 60 times more carbon than the atmosphere. However, the uncertainties in regional ocean sinks for anthropogenic CO₂ are still relatively high. Many carbonate species are non-conserved, flux across the air-sea interface, and react on time scales similar to those of turbulent processes in the ocean, such as small-scale Langmuir turbulence. All of this complexity can give rise to heterogeneous tracer distributions that are not fully understood and can greatly affect the rate at which CO₂ fluxes across the air-sea interface. In order to more accurately model the biogeochemistry of the ocean in Earth system models (ESMs), a better understanding of the fundamental interactions between these reactive tracers and relevant turbulent processes is required.

The most significant tracer-flow couplings occur when coherent structures in the flow have timescales that rival reaction time scales. Langmuir turbulence, a three-dimensional, small-scale process that results from surface waves tilting vertical vorticity anomalies into the horizontal, has length and time scale on the order of Ο(1-100m) and Ο(1-10min), respectively. Once CO₂ transfers across the air-sea interface it reacts with seawater to produce bicarbonate (HCO₃^-) and carbonate (CO₃^2-) in a series of reactions whose rate limiting steps have time scales of approximately 10-25s. Due to the similarity in scales between these small-scale physical and chemical processes, further examination is warranted.

In this talk, we present results from large eddy simulations of carbonate chemical species in the presence of realistic mixed layer ocean turbulence. The simulations explore the effects of wave-driven Langmuir turbulence by solving the wave-averaged Boussinesq equations with an imposed Stokes drift velocity. Comparisons are made between simulations with time dependent chemistry, equilibrium chemistry, and no chemistry. Additionally, we vary the strength of the Langmuir turbulence in order to determine a relationship between the degree of enhancement (or reduction) of carbon that is fluxed across the air-sea interface due to the presence of Langmuir turbulence. By examining different reaction chemistry and surface forcing scenarios in these simulations, we connect the coupled turbulence-reactive tracer dynamics with spatial and statistical properties of the resulting tracer fields. These results along with implications for development of reduced order reactive tracer models will be discussed.