Turbulent Dissipation in the Mid-Latitude Mixed Layer/Thermocline Transition Layer

The oceanic mixed layer is a region of intense mixing, where turbulence homogenizes vertical temperature and salinity distributions down to depths of O(100) m. Below the mixed layer, in the upper layers of the stratified thermocline, turbulent energy levels are greatly reduced. The transition between these two regions is the focus of this investigation. Traditionally, this transition is assumed to take place abruptly at the mixed-layer base. However, observations suggest that enhanced turbulence penetrates significantly into the stratified water below the mixed layer. Here, I present an examination of existing turbulence data documenting open ocean conditions with steady wind forcing in both the sub-tropical Atlantic and Pacific. These data will allow for direct estimates of diffusivity and diapycnal flux occurring in the mixed layer/thermocline transition layer. This analysis establishes statistics for turbulence dissipation levels occurring just below the well-mixed layer, which have not been previously documented. My investigation suggests that while the transition layer thickness can vary considerably (from O(10) to O(100) m for my data), the diabatic fluxes in this region, as measured by the turbulent dissipation rate, are 4-8 times greater than in the thermocline under typical surface forcing conditions.