Jet Formation and Evolution in Baroclinic Turbulence with Simple Topography

Satellite altimetry and high-resolution ocean models indicate that the Southern Ocean is comprised of an intricate web of narrow, meandering jets that undergo spontaneous formation, merger and splitting events, and rapid latitude shifts over periods of weeks to months. The role of topography in controlling jet variability is explored using over 100 simulations from a doubly-periodic, forced-dissipative, two-layer quasigeostrophic model. The system is forced by a baroclinically-unstable, vertically-sheared mean flow in a domain that is large enough to accommodate multiple jets. The dependence of (i) meridional jet spacing, (ii) jet variability and (iii) large-scale, domainaveraged transport properties on changes in the length scale and steepness of simple sinusoidal topographical features is analyzed. The Rhines scale lβ = 2π √ Ve/β, where Ve is an eddy velocity scale and β is the barotropic potential vorticity gradient, measures the meridional extent of eddy mixing by a single jet. The ratio lβ/lT , where lT is the topographic length scale, governs jet behavior. Multiple, steady jets with fixed meridional spacing are observed when lβ ≫ lT or when lβ ≈ lT . When lβ < lT , a pattern of perpetual jet formation and jet merger dominates the time evolution of the system. This unsteady structure reduces transport by a factor of two if the topography consists of zonally-invariant ridges, and increases transport by an order of magnitude or more if the topography consists of two-dimensional sinusoidal bumps. For certain parameters, bumpy topography gives rise to periodic oscillations in jet structure between purely-zonal and topographicallysteered states. In these cases, transport is dominated by bursts of mixing associated with the shifts between the two regimes. Unsteady jet behavior depends crucially on the feedback between changes in mean flow orientation, caused by topographic steering, and the conversion of potential energy to kinetic energy through baroclinic instability, as well as on asymmetric Reynolds stresses created by topographical modifications to the large-scale potential vorticity gradient. It is likely that these processes play a role in the dynamic nature of Southern Ocean jets.