| The World Ocean Atlas 1994 is used to study the formation and transformation of North Pacific Intermediate Water (NPIW), paying attention to the broad subarctic-tropical frontal zone (SATFZ). Four neutral density surfaces sigma_N=26.5, 26.9, 27.2 and 27.4 are mapped to span the NPIW layer with the sigma_N=26.9 surface identified with the salinity minimum of NPIW. Various dianeutral motion processes, cabbeling, thermobaricity, vertical turbulent mixing and double-diffusion, and water-mass property conversion are quantified; cabbeling appears to be the most important. Spatially, dianeutral transport is strongest in the SATFZ. Within the SATFZ subdomain 36-44N, 142E-140W, with assumed epineutral and dianeutral diffusivities of K=10(3) m(2)/s and D=10(-5) m(2)/s cabbeling contributes to a maximum dianeutral downwelling transport of 0.9 Sv on the sigma_N=26.5 neutral density surface (the upper boundary of NPIW) and 0.7 Sv on the sigma_N=26.9 surface (at the NPIW core). In meridional sections, cabbeling shows epineutral extension from the SATFZ towards the subtropical gyre, most apparent along the sigma_N=26.9 surface. Dianeutral transport is found to be sensitive to the epineutral diffusivity K on the upper two surfaces sigma_N=26.5 and 26.9 where the transport is dominated by cabbeling. With only half an order of magnitude increase in diffusivity K=10(3.5) m(2)/s, the total NPIW (about 3 Sv) can be accomplished by cabbeling alone. These results suggest that cabbeling plays a crucial role in the NPIW transformation. Since cabbeling describes physically the dianeutral downwelling resulted from epineutral mixing, strong cabbeling in the SATFZ implies that NPIW sources, and subsequently, NPIW is carried by epineutral advection and gyre circulation into the subtropical interior. Thus cabbeling is revealed as the principle mechanism responsible for transforming subpolar source waters into subtropical NPIW water via the SATFZ. |
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