Large-scale freshening of intermediate waters in the Pacific and Indian oceans

Despite the central role of the oceans in the global hydrological cycle, direct observations of precipitation over the oceans are too sparse to infer global patterns of variability. For the regions of water-mass formation (the high latitudes), however, it is possible to obtain indirect information on changes in the surface salinity budget from salinity measurements elsewhere, as water masses in the ocean carry distinct signatures in temperature and salinity over long distances. Here we present a comparison of historical hydrographic data collected between 1930 and 1980 with six more-recent trans- oceanic hydrographic sections (1985-94) from the intermediate waters of the Pacific and Indian oceans. North Pacific Intermediate Water and Antarctic Intermediate Water both show coherent basin-wide salinity decreases with time. The simplest explanation for these changes is a freshening of surface waters, over approximately 22 years, in the high-latitude North Pacific and Southern oceans, suggesting that precipitation (minus evaporation) has increased over the polar gyres. We estimate an increase by about 31 mm yr -1 for the Southern Ocean (between 55°S and 65°S), which is about three times larger than the values suggested by coupled atmosphere-ocean models with increasing atmospheric greenhouse-gas concentrations for the same period. The patterns of change are, however, qualitatively consistent between models and observations, and our results provide evidence for an intensification of the global hydrological cycle over the past decades.