Ocean Mixed Layer Depth From Dissipation. Issue 4 (4th April 2022) - Giunta, Valentina
Abstract: The 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 h D ) or using dissipation (henceforth h ϵ ). Utilizing two data sets in the North Atlantic collected with the autonomous Air‐Sea Interaction Profiler, h D and h ϵ in the upper 100 m are compared. A new method based on the shape of the dissipation profile for estimating h ϵ 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 h ϵ 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 h ϵ be adopted in the future for estimation of the MLD. This is additionally supported by the fact that h D typically overestimates the MLD when compared to h ϵ, thus having consequences for studies where h D is used to represent the extent of mixing in the upper ocean. Plain Language Summary: The mixed layer is the region of the ocean, which is directly influenced by atmospheric processes. The mixed layer plays a large role in regulating air‐sea exchange and climate. The mixed layer depth (MLD) is a critical parameter for physical, chemical, and biological oceanography and is typically estimated using vertical profiles of temperature and density. However, since this layer is being actively mixed by different sources of turbulence (e.g., wind, waves, and buoyancy fluxes), a criterion based on turbulence measurements is more desirable. In this article, we explore different methods to estimate the MLD based on temperature, density, and turbulence data using two data sets in the North Atlantic collected with an autonomous vertical profiler that records these data simultaneously. Significant differences were found among the methods used in this work, showing that a method based on the shape of the turbulent profile proves to be more robust for estimating the MLD. Moreover, by looking at the sources of turbulence during these cruises, it is possible to observe that the MLD based on turbulence measurements represents more accurately the variability in the upper ocean, having consequences for studies where the MLD is used. Key Points: A new method to estimate the mixed layer depth (MLD) based on the shape of the vertical profile of dissipation is presented The MLD derived from dissipation offers a more realistic description of turbulent mixing than density, which tends to overestimate the MLD It is appropriate that dissipation‐based estimates of the MLD be adopted, given that microstructure measurements are becoming increasingly available operationally