Salinity along 150W in the North Pacific during the Marathon-II expedition (May-June 1984). The subsurface low salinity water mass (green colors) characteristic of NPIW occurs between ~26.6 and 27.0 kg/m3 potential density horizons. Note the low salinity surface waters of the high-latitude North Pacific and the resulting sharp density contrast. It is this sharp density contrast which inhibits direct ventilation of interior waters in the Gulf of Alaska. Image courtesy of Exploring Alaska's Seamounts 2002, NOAA/OER.