Stratifying of the Global Ocean due to temperature and salinity changes

Ocean stratification is associated with the vertical gradient of density, and can inhibit the mixing processes and mass exchanges between different layers. It therefore has important consequences to the ocean circulation and oceanic transports of heat, carbon, oxygen, and nutrients, with implication for the Ecosystem and Climate system. The increase in near-surface stratification between 0 and 200m has been observed over the past decades, which was ascribed to the surface intensification of the warming signal. However, the estimates in previous studies are mostly restricted to the depths of 0-200 meters and remain some controversies due to the limitation of historical data and analysis approaches. Here we attempt to quantify this change with more complete water column based on our observations, combined with other observational data. We show that the global average of upper-2000m stratification has increased by 5.56±1.04% since 1960, with a stratified rate of 3.14±0.70 10^-6 s^-2 per 6 decades. Almost all ocean basins examined have experienced the continued strengthening of stratification. We further demonstrate that temperature impact amounts to more than 90% of the observed increase in upper-2000m stratification, and salinity effect is also important in high latitudes, tropical oceans, and subtropical gyre regions. This updated estimate with higher confidence provides a more stable state at upper ocean than previously possible, indicating a stronger influence on the ocean system.