Parameterizing Eddy Transport of Biogeochemical Tracers

The distribution of oceanic biogeochemical tracers is fundamentally tied to physical dynamics at and below the mesoscale. Since global climate models rarely resolve those scales, turbulent transport is parameterized in terms of the large-scale gradients in the mean tracer distribution and the physical fields. Here, we demonstrate that this form of the eddy flux is not necessarily appropriate for reactive tracers, such as nutrients and phytoplankton. In an idealized nutrient-phytoplankton system, we show that the eddy flux of one tracer should depend on the gradients of itself and the other. For certain parameter regimes, incorporating cross-diffusion can significantly improve the representation of both phytoplankton and nutrient eddy fluxes. We also show that the efficacy of eddy diffusion parameterizations requires timescale separation between the flow and reactions. This result has ramifications for parameterizing subgrid scale biogeochemistry in more complex ocean models since many biological processes have comparable timescales to submesoscale motions.

Automated summary

The distribution of oceanic biogeochemical tracers is closely related to physical dynamics at and below the mesoscale. Current parameterizations for turbulent transport in global climate models may not be appropriate for reactive tracers like nutrients and phytoplankton. Incorporating cross-diffusion can significantly improve the representation of phytoplankton and nutrient eddy fluxes in an idealized system. Eddy diffusion parameterizations require timescale separation between flow and reactions, which has implications for parameterizing subgrid scale biogeochemistry in more complex ocean models.