The North Pacific Intermediate Water, characterized by a salinity minimum confined to density surfaces of σθ = 26.7–26.9, exists throughout the subtropical gyre and has been observed to originate in the subarctic North Pacific. The physical processes that determine the density range on which the NPIW resides are not yet well understood. This study attempts to clarify these processes by combining observational data and a simple advection–diffusion isopycnal model. Due to the regional excessive precipitation over evaporation, the salinity in the upper-layer subarctic North Pacific generally decreases with decreasing water depth. Both alongisopycnal advection and diffusion work to carry this salinity/depth characteristic into the subtropical circulation. For the isopycnal surfaces overlying the NPIW, however, this transport mechanism is hindered by the seasonal outcropping. The outcropping not only blocks the fresh subarctic water from advecting and diffusing along these isopycnals into the subtropi... Abstract The North Pacific Intermediate Water, characterized by a salinity minimum confined to density surfaces of σθ = 26.7–26.9, exists throughout the subtropical gyre and has been observed to originate in the subarctic North Pacific. The physical processes that determine the density range on which the NPIW resides are not yet well understood. This study attempts to clarify these processes by combining observational data and a simple advection–diffusion isopycnal model. Due to the regional excessive precipitation over evaporation, the salinity in the upper-layer subarctic North Pacific generally decreases with decreasing water depth. Both alongisopycnal advection and diffusion work to carry this salinity/depth characteristic into the subtropical circulation. For the isopycnal surfaces overlying the NPIW, however, this transport mechanism is hindered by the seasonal outcropping. The outcropping not only blocks the fresh subarctic water from advecting and diffusing along these isopycnals into the subtropi...