Observed long-term trend and variability in global upper-ocean stratification
Many studies on future projections by climate models of the Coupled Model Intercomparison Project (CMIP) point out the long-term strengthening of the upper-ocean stratification due to the surface intensification of the ocean warming associated with global warming. The strengthened stratification will reduce vertical nutrient supply from the subsurface by more stabilizing the upper-ocean water column, and then the primary production of the whole ocean is concerned to decrease. Observed long-term changes in the stratification at this present time, however, show large uncertainties of the change itself and its driver. The strengthening trend of the upper-ocean thermal stratification has been reported from an estimate using globally-averaged sea surface and subsurface temperature time series in the IPCC AR5. On the other hand, some recent observational studies demonstrate that the upper-ocean stratifications are weakening over the last decades in some ocean regions, although the sea surface temperature is warming. These suggest that the simplest relation, namely that ocean warming is intensified near the surface and will result in increases in the local stratification, does not always hold in any ocean region and there are some other drivers of the change in stratification.
Here, we examined globally the long-term change and variability in the upper-ocean stratification, using only unprocessed historical observational profiles with as spatial and temporal coverage as wide as possible. In the seminar, we would like to talk about the detected long-term changes and its spatiotemporal characteristics and causes. We also present the interannual/decadal variability in each ocean region and its relationship to particular climate modes.