Decadal ocean water mass changes : global observations and interpretation

Using a combination of three quality controlled oceanographic datasets, changes in temperature, salinity, and oxygen were interpolated to neutral density surfaces and investigated on a global scale. The analysis was broken into two comparisons, where 759,713 profiles (1940-1988 with a mean year of 1970) and then 242,087 Argo profiles (mean year of 2005), were objectively mapped to the locations of 38,463 full-depth WOCE profiles (mean year of 1992). The largest water property changes along neutral density surfaces (1970-1992) occurred in the upper 1500 m, with a near-global cooling and freshening on density surfaces around the salinity minimum in both hemispheres (AAIW, NPIW), warming and salinity increases at the shallow salinity-maximum, freshening of NADW in the North Atlantic and salinity increases further south, and coherent salinity increases in CDWin the Southern Ocean for this period. Mid and high-latitude density surfaces all deepened, while there was shoaling in the equatorial low-latitudes. The pattern of water mass changes from 1992 to 2005 was generally consistent with the 1970-1992 observations on a global scale. However some regional decadal variability in the South Indian and the North Atlantic Oceans occurred, with the cooling and freshening pattern switching to warming and salinity increases on density surfaces from 1992 to 2005. Oxygen concentration levels in the upper-1500 m have reduced across all ocean basins at an average 1.82 μmol l−1 (1970-1992), with changes being strongest in mode, intermediate, and deep water ventilation regions. Less than 15% of the oxygen decreases can be attributed to temperature increases. The pattern of oxygen change poleward of 25± is correlated with upper-ocean stratification increases in both hemispheres. Integrating changes along density layers from the equator to the surface outcrop of the layer highlights the importance of high-latitude regions to global budgets, with 117% of apparent heat flux increases, 116% of apparent freshwater flux increases, and 94% of oxygen flux decreases occurring in density surfaces that outcrop poleward of 40±. Statistically significant heat flux increases were evident at all latitudes, but the largest apparent heat and freshwater flux increases occurred around 50±N and 55±S (1970-1992). Precipitation-minus-evaporation decreases of up to 10 mm yr−1 have occurred in the low latitudes. Using the pure warming, freshening, and heave modes of subduction (Bindoff and McDougall, 1994), changes in salinity appear to have had relatively little contribution to density and hence to steric sea-level change. Instead pure warming and pure heave appear as the dominant subduction scenarios with both explaining the observed density changes with equal skill. Both of these subduction mechanisms are the basis for a proposed physical model of a shift in the poleward boundaries of the subtropical gyres. The observed water mass changes imply an enhanced global hydrological cycle, and increased surface warming. Together these changes have lead to increased upper-ocean stratification, consistent with a reduced renewal rate of all water masses. Our observations compare well with the patterns of ocean change produced by model simulations of increasing carbon dioxide.