The global ocean mixed layer depth derived from an energy approach

Abstract. The mixed layer depth (MLD) is critical for understanding ocean-atmosphere interactions and internal ocean dynamics. Traditional methods for determining the MLD commonly rely on hydrographic thresholds that vary spatially and temporally with local oceanographic conditions, limiting their global consistency and applicability. To address this, we propose an energy-based methodology that defines the MLD as the depth at which the work done by buoyancy (WB) reaches 20 J m^-3. Based on the structural change in WB, the MLD criterion identifies the upper ocean's well-mixed layer in energetic terms. This approach provides a robust criterion based on physical principles, which is globally and temporally consistent and easy to implement. Our methodology aligns with turbulent boundary layer dynamics while maintaining quasi-homogeneity in density and temperature for most of the global ocean throughout the year. A global monthly MLD climatology derived from this method demonstrates its reliability across diverse oceanic conditions and its accuracy in regions and seasons where conventional methods struggle. Our study advances the development of MLD energy-based methodologies by providing a single energy value to define the MLD globally during all months. This energy-based approach could offer significant potential for advancing the study of dynamic, and thermodynamic processes, including heat content and vertical exchanges. It could also serve as a robust tool for validating ocean circulation models and to support intercomparison studies in initiatives such as the Ocean Model Intercomparison Project (OMIP) and the International Coupled Model Intercomparison Project (CMIP). Future research will explore its applicability to high-frequency processes and regional variability, further enhancing its utility for understanding and modeling oceanic phenomena.