Why Is the Spreading of the North Pacific Intermediate Water Confined on Density Surfaces around σθ = 26.8?

Abstract

The North Pacific Intermediate Water, characterized by a salinity minimum confined to density surfaces of σ? = 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 subtropical gyre, but also results in shoaling of the isopycnals in the Kuroshio?Oyashio mixed water region, where the turbulent mixing in the deep winter mixed layer is able to conduit the surface salt flux into these outcropping isopycnal surfaces. This seasonal forcing creates a high-salinity overlying layer, leaving σ? = 26.7?26.9 the lightest density surfaces that are free to transport the uppermost (i.e., the freshest) subarctic water into the subtropical North Pacific. This model result is consistent with high-resolution CTD observations that showed that σ?a = 26.7?26.9 are the least dense isopycnal surfaces on which the alongisopycnal potential vorticity is homogenized. The NPIW surfaces contrast with the shallower isopycnal surfaces where strong potential vorticity gradients exist.