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 ~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.
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