Shallow Salinity Minima in the North Pacific

Abstract

CTD/STD data from 24 cruises in the North Pacific are studied for their vertical salinity structure and compared to bottle observations. A triple-salinity minimum is found in two separated regions in the eastern North Pacific. In the first region, bounded by the northern edge of the subarctic frontal zone and the 34°N front between 160° and 150°W, a middle salinity minimum is found below the permanent pycnocline in the density range of 26.0 and 26.5 σ?. This middle minimum underlies Reid's shallow salinity minimum and overlies the North Pacific Intermediate Water (NPIW). In the second region, southeast of the first, a seasonal salinity minimum appears above the shallow salinity minimum at densities lower than 25.1 σ?. The shallow salinity minimum and the NPIW can be found throughout year, while the seasonal minimum only appears in summer and fall. The middle and shallow salinity minima, as well as the seasonal minimum, originate at the sea surface in the northeast Pacific. The properties at the minima depend on the surface conditions in their source areas. The source of the middle minimum is the winter surface water in a narrow band between the gyre boundary and the subarctic front west of 170°W. The shallow salinity minimum is generated in winter and is present throughout the year. The seasonal salinity minimum has the same source area as the shallow salinity minimum but is formed in summer and fall at lower density and is not present in winter. A tropical shallow salinity minimum found south of 18°N does not appear to be connected with the shallow salinity minimum in the eastern North Pacific. South of 20°N, the shallow salinity minimum and the NPIW appear to merge into a thick, low salinity water mass. When an intrusion of high salinity water breaks through this low salinity water mass south of 18°N, this tropical salinity minimum appears at the same density as the shallow salinity minimum. Though the water mass of the tropical minimum is deprived from the water in the shallow salinity minimum, the formation of the vertical minimum is different.