Submesoscales enhance storm-driven vertical mixing of nutrients: insights from a biogeochemical large eddy simulation

Storms deepen the mixed layer, entrain nutrients from the pycnocline, and fuel phytoplankton blooms in mid-latitude oceans. However, the effects of oceanic submesoscale (0.1-10 km horizontal scale) physical heterogeneity on the physical-biogeochemical response to a storm are not well understood. Here, we explore these effects numerically in a Biogeochemical Large Eddy Simulation (BLES), where a four-component biogeochemical model is coupled with a physical model that resolves some submesoscales and some smaller turbulent scales (2 km to 2 m) in an idealized storm forcing scenario. Results are obtained via comparisons to BLES in smaller domains that do not resolve submesoscales and to one-dimensional column simulations with the same biogeochemical model, initial conditions, and boundary conditions but parameterized turbulence and submesoscales. These comparisons show different behaviors during and shortly after the storm. During the storm, resolved submesoscales double the vertical nutrient flux. The vertical diffusivity is increased by a factor of 10 near the mixed layer base, and the mixing-induced increase in potential energy is double. Resolved submesoscales also enhance horizontal nutrient and phytoplankton variance by a factor of 10. After the storm, resolved submesoscales maintain higher nutrient and phytoplankton variance within the mixed layer. However, submesoscales reduce net vertical nutrient fluxes by 50% and nearly shut off the turbulent diffusivity. Over the whole scenario, resolved submesoscales double storm-driven biological production. Current parameterizations of submesoscales and turbulence fail to capture both the enhanced nutrient flux during the storm and the enhanced biological production.