Source: Journal of Geophysical Research: Oceans
The effects of winds, waves, and atmospheric cooling create a near-surface region in the ocean that is often well-mixed vertically. This so-called “surface-mixed layer” (SML) is essential to upper ocean dynamics and atmosphere-ocean interactions: it mediates the exchange of energy, momentum, and gases between the ocean and the atmosphere and hosts a wealth of crucial biological and biogeochemical transformation processes. The thickness of the SML is one of the most important properties of this layer. It has traditionally been defined based on fixed thresholds for maximum density and temperature differences (or gradients) across the SML, but these were largely arbitrary and not generally applicable.
Reichl et al.  propose to define the SML based solely on energetic considerations. Their starting point is the observation that the SML, despite its name, is almost always weakly stratified. Therefore, to perfectly mix this weakly stratified near-surface region to a given depth (the SML depth), turbulence in the upper ocean must perform work against gravity that can be accurately quantified using available density profiles. Comparing this “potential energy anomaly” to the estimated energy available for mixing (e.g., from winds), the SML depth can be determined (see figure).
This method of estimating SML depth is physically attractive because it relates to fundamental physical principles and promises to be more robust, more flexible, and more generally applicable compared to conventional approaches.
Citation: Reichl, B. G., Adcroft, A., Griffies, S. M., & Hallberg, R. (2022). A potential energy analysis of ocean surface mixed layers. Journal of Geophysical Research: Oceans, 127, e2021JC018140. https://doi.org/10.1029/2021JC018140
—Lars Umlauf, Editor, Journal of Geophysical Research: Oceans