Trend and Variability in Global Upper-Ocean Stratification Since the 1960s

Many studies on future climate projection point out that with progressing of global warming, upper-ocean density stratification will strengthen over this century, and consequently, global-averaged ocean primary productivity will decrease. Observed long-term changes in the stratification to date, however, still show large uncertainties of the change itself and its driver. Focusing on the vertical difference in the emergence of the global warming signals, we used only observational profiles to describe the spatiotemporal characteristic of long-term trend and variability in the upper-ocean stratification (defined as the density difference between the surface and 200-m depth). Statistically significant strengthening of the stratification since the 1960s was detected in similar to 40% of the global ocean area. The global average increase in the stratification corresponds to 3.3-6.1% of the mean stratification. The strengthening trends considerably change depending on the regions and show dominant contribution from the tropical region. In addition to the well-documented explanation of strengthening stratification, namely the surface intensification of global warming signal, we found that changes in subsurface temperature and salinity stratification associated with changes in atmospheric/ocean circulations significantly contribute to the long-term change in the stratification and setting its regional difference. In midlatitude and high-latitude ocean of the Northern Hemisphere, the long-term trend exhibits noteworthy seasonality, which shows faster increase trend in the summer than in the winter. From the detrended time series, interannual variabilities correlated with a particular climate mode are detected in several ocean regions, suggesting that these variabilities are mainly driven by associated sea surface temperature variation. Plain Language Summary In general, upper-ocean stratification (i.e., vertical changes in sea water density) serves as a barrier between relatively warm and nutrient-depleted surface water and relatively cool and nutrient-rich subsurface water. As a consequence of global warming, globally averaged upper-ocean thermal stratification is thought to be enhanced due to the surface intensification of the warming signal. Strengthened stratification will have a large impact on oceanic biological activity through changes in light availability and vertical nutrient supply to the upper-ocean phytoplankton community. Because investigations of the change in stratification are limited in ocean regions and have employed relatively short-term observational data, large uncertainties remain. In this study we investigated the changes in stratification with as great a spatial and temporal coverage as possible, using only historically collected profile data. We found strengthened stratification in the large part of the global ocean with large spatial heterogeneity. Changes in the vertical structure of salinity and subsurface temperature contribute to the changes in density stratification, in addition to the surface intensification of the global warming signal.