Water mass changes in the North and South Pacific Oceans between the 1960s and 1985-94

Comparisons have been made along five modern hydrographic sections against historical hydro graphic data to investigate water mass changes in the North and South Pacific oceans. The five modern hydrographic sections were sampled in the decade 1985-94, while the historical data were mostly from the late 1960s. Below the seasonal mixed layer, statistically significant temporal differences in temperature and salinity have been detected in the water masses that occur in the top 2000 dbar of the water column. These differences in water mass properties are assumed to result from sea surface changes at the formation regions. Of all the water mass changes, the most spatially coherent ones come from the shallow salinity maxima of North Pacific Subtropical Water (NPSTW) and South Pacific Subtropical Water (SPSTW), and the intermediate salinity minima of North Pacific Intermediate Water (NPIW) and Antarctic Intermediate Water (AAIW). The two shallow salinity maxima, NPSTW and SPSTW, have shown signs of salinity increase, while the two intermediate salinity minima, NPIW and AAIW, have become fresher and, except along l 7¬∞S, have become warmer. Since NPSTW and SPSTW originate under the high evaporative cells of the subtropical North and South Pacific, and NPIW and AAIW acquire their properties near the polar gyres, these changes in the ocean interior imply an increase in net evaporation over the mid-latitudes, and an increase in net precipitation over the high-latitudes in both hemispheres. Together these results imply a strengthening of the hydrological cycle over the North and South Pacific oceans. Outputs from a coupled climate model show that under increasing atmospheric C02 , the model ocean responds with a warming of the water column in the top 300 dbar. Superimposed on this background warming trend is a decrease in salinity in the two intermediate salinity minima of the Pacific (NPIW and AAIW), and corresponds to near-surface freshening where their respective isopycnals outcrop. Hence the freshening signature that has been detected in NPIW and AAIW from observational data is qualitatively consistent with this climate model's response to increasing C02 ‚ However, natural variability cannot be discarded as a possible cause for the observed changes. The steric sea level change for the area in the Pacific between 60¬∞N and 31.5¬∞S over the roughly 20-year study period is estimated to be a rise of 0.85 mm/yr. This is larger than that estimated by numerical models, and so demonstrates the usefulness of observational studies such as this in testing the effectiveness of numerical models in simulating the natural variability of the climate system.