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

Despite the central role of the oceans in the global hydrological cycle, di rect observations of precipitation over the oceans are too sparse to infer global patterns of variability. For the regions of water-mass formation (th e high latitudes), however, it is possible to obtain indirect information o n changes in the surface salinity budget from salinity measurements elsewhe re, as water masses in the ocean carry distinct signatures in temperature a nd salinity over long distances. Here we present a comparison of historical hydrographic data collected between 1930 and 1980(1,2) with six more-recen t trans-oceanic hydrographic sections (1985-94) from the intermediate water s of the Pacific and Indian oceans(3,4). North Pacific Intermediate Water a nd Antarctic Intermediate Water both show coherent basin-wide salinity decr eases 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 evaporat ion) has increased over the polar gyres. We estimate an increase by about 3 1 mm yr(-1) far the Southern Ocean (between 55 degrees S and 65 degrees S), which is about three times larger than the values suggested by coupled atm osphere-ocean models with increasing atmospheric greenhouse-gas concentrati ons for the same period(5-8). The patterns of change are, however, qualitat ively consistent between models and observations, and our results provide e vidence for an intensification of the global hydrological cycle over the pa st decades.