Ocean Mixed Layer Depth From Dissipation

Abstract

AbstractThe mixed layer depth (MLD) is a widely used parameter for physical, chemical, and biological oceanography. The MLD delimits that region of the ocean, which is directly influenced by the atmosphere. There is a similar length scale referred to as the mixing layer depth, which is determined from profiles of dissipation rate of turbulent kinetic energy, but is less prevalent due to the requirement for specialized instrumentation. Here, we suggest that the MLD can be estimated using density/temperature (henceforth hD) or using dissipation (henceforth hϵ). Utilizing two data sets in the North Atlantic collected with the autonomous Air‐Sea Interaction Profiler, hD and hϵ in the upper 100 m are compared. A new method based on the shape of the dissipation profile for estimating hϵ is presented. The main sources of turbulence in the upper ocean (i.e., wind, waves, and buoyancy fluxes), which tend to deepen the MLD, are more strongly correlated with hϵ variability for both data sets, confirming that the MLD derived from dissipation measurements represents more accurately the variability in the mixed layer. Given that dissipation measurements have become more operational due to the development of ocean microstructure technology, then it is appropriate that hϵ be adopted in the future for estimation of the MLD. This is additionally supported by the fact that hD typically overestimates the MLD when compared to hϵ, thus having consequences for studies where hD is used to represent the extent of mixing in the upper ocean.