Data from: Fluxes and mixing of reacting biogeochemical scalars in a stratified shear layer

Biogeochemical scalars, representing many different species of
phytoplankton, zooplankton, nutrients, etc., play an important role in
marine ecosystems. Here we examine a simplified two-species system of
generic phytoplankton and nutrient populations, modeled here as reacting
scalars governed by advection-diffusion equations with added terms
governing the biological reaction (nutrient uptake, phytoplankton
death/remineralization). We perform direct numerical simulations of an
idealized scenario in which initial layers of phytoplankton and nutrients
are separated by an unstable stratified shear layer susceptible to either
Kelvin-Helmholtz or Holmboe instability (canonical examples of overturning
and scouring mixing events, respectively). We further vary the ratio of
the reaction and flow timescales, considering cases of fast, slow, and
comparable reaction rates. We find that turbulent advective fluxes show
large oscillations during the evolution of the shear instabilities,
associated with reversible fluid motions. To isolate the irreversible
turbulent scalar fluxes, we instead calculate diascalar fluxes, that is,
diffusive fluxes across isoscalar surfaces. This concept has been
previously used to quantify irreversible mixing of buoyancy in stratified
mixing events; here, we extend this approach to also account for the
effects of biogeochemical reactions. With this analysis, we show that both
instabilities accelerate phytoplankton growth, that the KH-driven mixing
tends to have a larger impact on the phytoplankton growth than the Holmboe
flow, and that the largest impact of the flow on the scalar evolution
occurs when reaction timescales are comparable to the buoyancy period.