The global ocean has become more layered and stable over the past few decades with global warming, which affects life in the ocean by reducing nutrients and oxygen.

Researchers from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences and their collaborators from the U.S. found that the global ocean has become more layered and resistant to vertical mixing as warming from the surface creates increasing stratification. 

The study was published in Nature Climate Change on Sept. 28.

Sea water generally forms layers with lighter water near the surface and denser water at greater depth, i.e. warmer fresher water atop colder water with more saline. This stable stratified configuration acts as a barrier to water mixing, influencing the efficiency of vertical exchanges of heat, carbon, oxygen and other constituents.

As human-caused greenhouse warming has fundamentally altered oceanic temperature and salinity fields, impacts to stratification are expected but the details have been difficult to discern until now.

The main basis for estimating the stratification change is the sparse distribution of ocean observations both horizontally and vertically. Previous quantification of stratification change has been limited to a simple index and has neglected the spatial complexity of ocean density change.

Ocean color difference when two water mass meets (Image by ZHU Jiang) 

The new study provided an estimate on ocean stratification for the upper 2000m and its spatial structures as well. It used carefully evaluated ocean temperature and salinity data (IAP products), which overcomes previous systematic biases associated with sampling.

Moreover, the study adopted an improved metric of stratification (related to the density gradient over depth), and then provided a true estimate of ocean stratification and its changes.

New data showed that ocean has become more layered by 5.3% since 1960 for the upper 2000m. An even stronger ocean stratification increase, as much as 18%, has been observed for the upper 150m.

This observed long-term increasing trend of stratification is mainly caused by stronger ocean warming for upper layers versus the deep oceans (~97%), but salinity changes play an important role locally. This stratification increase reveals a robust human-driven change in the ocean due the long-term temperature and salinity change structures. Therefore, the observed ocean stratification increase is another irrefutable piece of evidence of human-driven global warming.

In the tropics, there is a very strong stratification increase at upper 200m. This indicates a significant change in tropical thermocline depth, which is important to the El Nino phenomenon. In the middle and high latitudes, significant increases of ocean stratification appear below 500m, implying an impact on deep ocean stability by climate change.

With increased stratification, heat from climate warming cannot penetrate into the deep ocean as readily, which helps to raise the surface temperature. It also reduces the capability of ocean carbon storage, exacerbating the global warming. It prevents the vertical exchanges of nutrients and oxygen, and impacts the food supply of the whole marine ecosystems.

This study was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, National Key R&D Program of China and Key Deployment Project of Center for Ocean Mega-Research of Science, CAS.