Ocean stratification plays a crucial role in regulating climate and marine ecosystems. This study investigates global ocean stratification changes over a 26-year period (1992–2017) from the ECCOv4r4 ocean state estimate. We quantify stratification trends using the depth anomaly of the ocean’s center of mass (COM), defined as the excess depth of the COM relative to a fully mixed reference state. The global depth anomaly has increased at a steady rate of 0.66 cm per decade, corresponding to a significant 1% stratification increase per decade. The global mean squared buoyancy frequency has increased at a similar rate, albeit with larger interannual variability, making it a less robust metric. The primary driver of stratification increase is the upper ocean warming, with a nearly perfect correlation between the global depth anomaly and the ocean heat content. Depth anomalies are mapped spatially for the upper 2000 m water column, showing large spatial heterogeneity. Most of the stratification increase occurs in Indo-Pacific tropical regions and along western boundary currents, with local rates more than five times higher. Salinity changes provide a secondary contribution, dominant only in polar regions. In contrast, stratification has weakened in most of the North Atlantic sector during the same period.
Roquet et al. (Thu,) studied this question.