A Potential Energy Analysis of Ocean Surface Mixed Layers. Issue 7 (9th July 2022)

• Record Type: Journal Article
• Authors: Reichl, Brandon G.; Adcroft, Alistair; Griffies, Stephen M.; Hallberg, Robert
• Is Part Of: Journal of geophysical research. Volume 127:Issue 7(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-07-09
• Subjects: ocean mixed layer -- ocean boundary layer -- upper ocean mixing; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2021JC018140
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: 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. Plain Language Summary: Properties such as temperature, salinity, nutrients, and chemical and biogeochemical tracers are usually well‐mixed vertically near the surface of the ocean. This layer is strongly mixed because winds, waves, and convective plumes are actively churning and stirring the near‐surface fluid. Oceanographers are interested in this surface mixed layer because it plays a significant role in how the ocean works from physical, biological, and chemical perspectives. However, the boundary between the mixed layer and the interior is difficult to identify, and therefore scientists have proposed numerous methods to identify this depth. In this paper we suggest to identify the mixed layer depth (MLD) based on potential energy (PE). A well mixed column has more PE than a stratified column because it has more mass located further from the center of the Earth. The PE difference between a column and its well‐mixed state is often called the PE anomaly, and the closer a column is to well‐mixed the closer the PE anomaly is to zero. In this paper we show that using the PE anomaly to identify the MLD is practical and it offers conceptual benefits by directly linking to the physics of upper ocean vertical mixing. Key Points: Diagnosing the mixed layer depth (MLD) from potential energy (PE) anomaly yields a MLD constrained by energy The PE‐based MLD considers the integrated stratification within the mixed layer The PE‐based MLD is strongly correlated with the active boundary layer depth