Submesoscale vortical wakes in the lee of topography

An idealized framework of steady barotropic flow past an isolated seamount in a background of constant stratification (with frequency N) and rotation (with Coriolis parameter f) is used to examine the formation, separation, instability of the turbulent bottom boundary layers (BBLs), and ultimately, the genesis of submesoscale coherent vortices (SCVs) in the ocean interior. The BBLs generate vertical vorticity ζ and potential vorticity q on slopes; the flow separates and spawns shear layers; barotropic and centrifugal shear instabilities form submesoscale vortical filaments and induce a high rate of local energy dissipation; the filaments organize into vortices that then horizontally merge and vertically align to form SCVs. These SCVs have O(1) Rossby numbers (Ro_ζ = ζ/f) and horizontal and vertical scales that are much larger than those of the separated shear layers and associated vortical filaments. Although the upstream flow is barotropic, downstream baroclinicity manifests in the wake, depending on the value of the nondimensional heightĥ, which is the ratio of the seamount height h_s to that of the Taylor height h_T = fL/N, where L is the seamount half-width. Whenĥ < 1, SCVs span the vertical extent of the seamount itself. However, forĥ > 1, there is greater range of variation in the sizes of the SCVs in the wake, reflecting the wake baroclinicity caused by the topographic interaction. The aspect ratio of the wake SCVs has the scaling L_z /L_h ~ √ f /N, instead of the quasigeostrophic scaling L_z /L_h ~ f /N.