Warming of the Global Ocean: Spatial Structure and Water-mass Trends

This study investigates the multidecadal warming and interannual and decadal heat content changes in the upper ocean focusing on aspects of vertical and horizontal variability. Our results are based on objectively analyzed gridded observational data sets and from a modeled state estimate. We find that warming climate has a strong signal of sinking in the mid-thermocline isopycnals, which can be diagnosed directly from hydrographic data. Due to sinking basin-average multidecadal warming is dominated by a contribution from deepening of isopycnals, which mainly expand the subtropical mode water volume, rather than shifts of the temperature/salinity relationship, known as ‘spice’ variability. Multidecadal isopycnal sinking has been the strongest over the southern basins. Theory and circulation models suggest that observed strengthening of winds over the Southern Ocean can also contribute to global sinking of isopycnals, hence to increasing heat content. On interannual to decadal scales, sinking and shoaling of density surfaces still dominates ocean heat content changes, while the contribution from temperature changes along density surfaces tend to decrease as time scales shorten.