Séminaire
Oceanic submesoscale currents near the surface primarily have patterns of lines or streaks with widths of 10-1000 m. The lines are made visible by high concentrations of buoyant surface materials (much of which is biogenic scum) that are pushed together where horizontal currents converge and feed into downward current sheets, leaving the surfactants behind. These lines are created by the process of frontogenesis where horizontal differences in density are compacted into smaller cross-front transition zones, with the associated current patterns following along. Three primary points are made in this talk: (1) the currents that cause the frontogenesis are created by the combined effects of the density differences, Coriolis force, surface gravity wave mass transport, and vertical momentum mixing associated with turbulence in the surface boundary layer; (2) the rate of frontogenesis is equivalent to the rate of convergence once the front is strong enough; and (3) there is typical life-cycle of frontogenesis, instability, arrest. and decay that strongly influences the local boundary layer turbulence. Statements (1)-(2) are not the common view of meteorological weather fronts where the frontogenesis rate is usually attributed to the strain rate of the winds, which can be true for weak and moderate fronts, but not for strong ones; statement (3) is, to my knowledge, only lightly explored in the meteorological context.
sabrina.speich@lmd.ens.fr