B. Qiu, WHY IS THE SPREADING OF THE NORTH PACIFIC INTERMEDIATE WATER CONFINEDON DENSITY SURFACES AROUND SIGMA(THETA)=26.8, Journal of physical oceanography, 25(1), 1995, pp. 168-180
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.