Structure of intermediate water layer in the North Western Pacific

Data of long time Japanese surveys of the NW Pacific are used to investigate the spatial structure and seasonal variability of the layer of the North Pacific Intermediate Water (NPIW). The authors found that depth of salinity minimum, salinity, potential density and dissolved oxygen content at that depth express significant interannual disturbances and regular spatial trend. NPIW layer outcrops around 40-42°N where salinity, potential density and oxygen content equal 33.3 psu, 26.5 and 6.9 ml/l, respectively. No significant zonal variability has been found along 41°N section as well as no important differences between 144°E, 148°E and 150°E meridional sections. No important seasonal NPIW variability was found below 200 m depth. In meridional direction the NPIW layer deepens southward to 900 m depth at 30°N. At the same time, its salinity rises to 34.3 psu, potential density to 26.95 and dissolved oxygen content drops to 2.0 ml/l. Typical standard deviations of that value reached then 100 m, 0.2 psu, 0.15 and 1.0 ml/l, respectively. That means that NPIW layer cannot be traced in the northwestern Pacific by use of any fixed isohaline or isopycnal. The authors also analyzed data of six “Megapolygon” hydrographic surveys conducted in Summer-Fall of 1987 at 500×500 km area centered at 40°N, 155°E. CTD-profiling was carried out from seasurface through 1500 m depth with 0.1-1 m vertical resolution at 20 mile horizontal separation. Regular feature revealed is a well-developed finestructure at some profiles located around the layer of the North Pacific Intermediate Water (NPIW) traced with a salinity minimum