Energetic Submesoscales Maintain Strong Mixed Layer Stratification during an Autumn Storm

Abstract

Atmospheric storms are an important driver of changes in upper-ocean stratification and small-scale (1–100 m) turbulence. Yet, the modifying effects of submesoscale (0.1–10 km) motions in the ocean mixed layer on stratification and small-scale turbulence during a storm are not well understood. Here, large-eddy simulations are used to study the coupled response of submesoscale and small-scale turbulence to the passage of an idealized autumn storm, with a wind stress representative of a storm observed in the North Atlantic above the Porcupine Abyssal Plain. Because of a relatively shallow mixed layer and a strong downfront wind, existing scaling theory predicts that submesoscales should be unable to restratify the mixed layer during the storm. In contrast, the simulations reveal a persistent and strong mean stratification in the mixed layer both during and after the storm. In addition, the mean dissipation rate remains elevated throughout the mixed layer during the storm, despite the strong mean stratification. These results are attributed to strong spatial variability in stratification and small-scale turbulence at the submesoscale and have important implications for sampling and modeling submesoscales and their effects on stratification and turbulence in the upper ocean.

Submesoscales Enhance Storm‐Driven Vertical Mixing of Nutrients: Insights From a Biogeochemical Large Eddy Simulation

“…The rotating stratified Boussinesq equations are solved on an f ‐plane without surface wave effects over horizontal wavelengths ranging from about 2 km to about 4 m, which include both submesoscales and smaller‐scale turbulence. The numerical model, spatial domain, and physical initial condition (of density, velocity, and pressure) are as described in Whitt and Taylor (; see also Taylor, ). Briefly, the model solves the Boussinesq equations together with the evolution equations for the four reactive biogeochemical tracers using a low‐storage third‐order Runge‐Kutta time marching method, a pseudospectral method to represent horizontal derivatives, and second‐order centered differences to represent vertical derivatives.…”

“…The turbulent physical initial condition at the onset of the storm with finite amplitude submesoscale eddies (Figure ) is obtained from a 3‐day spin‐up simulation that is initialized with low‐amplitude red noise in the frontal zone with a vertical density profile based on the observed density profile above the Porcupine Abyssal Plain during September 2012. The spin‐up simulation is forced by a constant air‐sea buoyancy flux B A =3×10 −9 m 2 /s 3 (equivalent to about 10 W/m 2 cooling) that is chosen to balance the restratifying effects of submesoscale mixed layer baroclinic instabilities, as discussed in Whitt and Taylor (; see also Mahadevan et al, , ). At the start of the spin‐up simulation, the fastest‐growing mixed layer baroclinic instability has a horizontal length scale of about 985 m and a growth timescale of about 13 hr, as discussed in Whitt and Taylor (; see also Stone, ).…”

“…The spin‐up simulation is forced by a constant air‐sea buoyancy flux B A =3×10 −9 m 2 /s 3 (equivalent to about 10 W/m 2 cooling) that is chosen to balance the restratifying effects of submesoscale mixed layer baroclinic instabilities, as discussed in Whitt and Taylor (; see also Mahadevan et al, , ). At the start of the spin‐up simulation, the fastest‐growing mixed layer baroclinic instability has a horizontal length scale of about 985 m and a growth timescale of about 13 hr, as discussed in Whitt and Taylor (; see also Stone, ). The domain size is chosen so that it contains two wavelengths of the fastest growing baroclinic mixed layer instability mode in each horizontal dimension.…”

“…If turbulent scale motions are more energetic as a result, this may enhance vertical fluxes. In a recent paper, Whitt and Taylor () used LES to explore the physical response of submesoscales coupled with small‐scale turbulence to a storm. Their simulations showed that energetic wind‐driven submesoscale motions nonuniformly restratified the mixed layer while deepening the reach of boundary‐layer turbulence during the storm.…”

Subduction by Submesoscales

“…Submesoscales (SM) are near‐surface oceanic features with horizontal scales (0.1–10 km), vertical scales (0.01–1 km) and lifetimes (hours–days). Since these features make their measurement difficult, their importance has been demonstrated mostly by high‐resolution numerical simulations (Bachman, Fox‐Kemper, et al, ; Bachman, Taylor, et al, ; Capet et al, ; Klein et al, ; Lapeyre et al, ; Lévy et al, ; Mahadevan & Tandon, ; Rosso et al, , ; Skyllingstad & Samelson, ; Thomas ; Whitt & Taylor, ). In a recent review article, Mahadevan () pointed out that the relevance of SM processes to phytoplankton productivity is because the time scales on which they operate are similar to those of phytoplankton growth.…”