A Potential Energy Analysis of Ocean Surface Mixed Layers

Turbulent mixing in the ocean surface boundary layer leads to the presence of a surface mixed layer. This mixed layer is important for many phenomena including large-scale ocean dynamics, ocean-atmosphere coupling, and biological and biogeochemical processes. Analysis of the ocean mixed layer requires one to estimate its vertical extent, for which there are various definitions. Correspondingly, there are uncertainties on how to best identify an ocean surface mixed layer for a given application. We propose defining the mixed layer depth (MLD) from energetic principles through the potential energy (PE). The PE based MLD is based on the concept of PE anomaly, which measures the stratification of a layer of seawater by estimating its energetic distance from a well-mixed state. We apply the PE anomaly to diagnose the MLD as the depth to which a given energy could homogenize a layer of seawater. We evaluate the MLD defined by common existing methods and demonstrate that they contain a wide range of PE anomalies for the same MLD, particularly evident for deep winter mixed layers. The MLD defined from the PE anomaly ensures a more consistent MLD identified for a large range of stratifications. Furthermore, the PE method relates to the turbulent kinetic energy budget of the ocean surface boundary layer, which is fundamental to upper ocean mixing processes and parameterizations. The resulting MLD is more representative of active boundary layer turbulence, and is more robust to small anomalies in seawater properties.