WHOI PO

Abstract:

Finescale turbulence and tracer transport are strongly modulated by submesoscale flows through both eddy-driven restratification and changes in turbulent kinetic energy production. While the impacts of restratification can be represented in traditional boundary layer turbulence theory via changes in mixed layer depth, the effects of energetics cannot. In the first part of this talk, I will show that the modification of turbulence energetics at fronts can be understood using a generic constraint, regardless of wind-front alignment or the presence of symmetric instability. This constraint arises from the coupled momentum and buoyancy budgets through cross-front advection, which dictates that the sum of vertical buoyancy flux and geostrophic shear production is set by the total buoyancy forcing from surface flux and cross-frontal Ekman transport. Therefore, a generic bidirectional transfer of kinetic energy exists between the finescale and submesoscale motions, enhancing (diminishing) turbulence energetics according to the relative strength of downfront (upfront) wind and frontal strength. In the second part, I will show that finescale tracer transport across the mixed layer depth is significantly enhanced at sharp fronts created by the mixed layer instability. Analysis of the finescale kinetic energy budget reveals that the enhanced finescale tracer flux is linked to horizontal shear production associated with frontogenesis, rather than the surface forcing. Together, these findings offer new insights for developing energetically consistent parameterizations of both finescale and submesoscale processes in climate models.