This study by Guillaume Serazin and co-authors documents the first climatology of the stratification below the mixed layer: the upper ocean pycnocline. The paper was published in march 2023 in Frontiers in Marine Science : https://doi.org/10.3389/fmars.2023.1120112
The dataset itself is published on SEANOE : https://www.seanoe.org/data/00798/91020/ (Sérazin et al., 2022).