WHY IS THE SPREADING OF THE NORTH PACIFIC INTERMEDIATE WATER CONFINEDON DENSITY SURFACES AROUND SIGMA(THETA)=26.8

The North Pacific Intermediate Water, characterized by a salinity mini mum confined to density surfaces of sigma(t)heta = 26.7-26.9, exists t hroughout the subtropical gyre and has been observed to originate in t he subarctic North Pacific. The physical processes that determine the density range on which the NPIW resides are not yet well understood. T his study attempts to clarify these processes by combining observation al data and a simple advection-diffusion isopycnal model. Due to the r egional excessive precipitation over evaporation, the salinity in the upper-layer subarctic North Pacific generally decreases with decreasin g water depth. Both alongisopycnal advection and diffusion work to car ry this salinity/depth characteristic into the subtropical circulation . For the isopycnal surfaces overlying the NPIW, however, this transpo rt 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 surfac es. This seasonal forcing creates a high-salinity overlying layer, lea ving sigma(t)heta = 26.7-26.9 the lightest density surfaces that are f ree to transport the uppermost (i.e., the freshest) subarctic water in to the subtropical North Pacific. This model result is consistent with high-resolution CTD observations that showed that sigma(t)heta = 26.7 -26.9 are the least dense isopycnal surfaces on which the alongisopycn al potential vorticity is homogenized. The NPIW surfaces contrast with the shallower isopycnal surfaces where strong potential vorticity gra dients exist.