Stirring, Mixing, Growing: Modeling Microscale Processes that Change Larger Scale Phytoplankton Dynamics.

The quantitative description of marine systems is constrained by a major issue of scale separation: most marine biochemical processes occur at sub-centimeter scales, while the contribution to the Earth’s biogeochemical cycles is expressed at much larger scales, up to the planetary one. In spite of vastly improved computing power and observational capabilities, the modeling approach has remained anchored to an old view that sees the microscales as unable to substantially affect larger ones. The lack of a theoretical appreciation of the interactions between vastly different scales has led to the proliferation of numerical models with uncertain predictive capabilities.

We discuss a new Lagrangian modeling framework (doi:10.1016/j.jcp.2018.01.031, https://arxiv.org/abs/1909.04334) that allows biogeochemical processes to be modeled as they occur at the submicroscale; that includes irreversible mixing processes whose intensity is not constrained by the model resolution; that does not use diffusion for modeling unresolved transport processes.

We compare Lagrangian water column simulations relative to ocean station PAPA and to the sub-Antarctic zone of the Southern Ocean with their Eulerian, eddy-diffusive counterpart.

We show that, when irreversible mixing processes are reduced to reasonably realistic levels, rather than being used to parameterize unresolved stirring, plankton patchiness spontaneously emerges in the mixed layer, coherently with recent high-resolution chlorophyll observations.

We give theoretical reasons and numerical evidence that patchiness, in turn, strongly affects the bulk growth rate, shifting the onset of the spring bloom by as much as 15 days, and the primary productivity during the bloom by a factor 6 at PAPA, and a factor 2 at the more vigorously stirred SAZ.