Representation of the upper ocean stratification changes in different observational datasets

Ocean stratification changes impact Earth’s energy and water cycles and ocean ecosystems, but the robustness of its changes has not been fully evaluated. This study compares seven widely-used observational gridded analyzed products to investigate the consistency and differences of past stratification changes. The most pronounced increase in ocean stratification since 1960 is found in the near-surface Arctic Ocean (0–200 m), with a trend of 1.1 × 10−7 s−2 per year. However, estimates of this trend show the greatest inconsistency among datasets, varying from − 0.6 to 3.3 × 10−7 s−2 per year, largely due to limited salinity data. In low- and mid-latitude oceans, the long-term strengthening of stratification is more pronounced in the upper 200 m than in deeper layers, primarily driven by thermal changes. Moreover, the estimated trends show particularly large spreads in regions with steep vertical density gradients or sparse observations, such as the Eastern Pacific and the Southern Tropical Indian Ocean. The inter-data uncertainties in the basin and regional means of ocean stratification have decreased significantly during the Argo Period, except in the Arctic Ocean and some coastal regions like the Northwestern Tropical Atlantic Ocean. These findings highlight the need for improved ocean observation systems and more inter-comparison analyses to estimate the ocean stratification change.