How were the sources of the subtropical North Pacific Intermediate Water (NPIW), which has a core density of 26.8σθ formed in the subpolar region, given that the maximum surface density in the late winter (density reaches a maximum in March due to successive cooling through the winter) of the entire North Pacific is only 26.5σθ? How, then, did these sources cross the broad subarctic-tropical frontal zone (SATFZ) (which is about 800 km in width and has predominantly eastward flowing currents) and enter the subtropical gyre southward, forming the NPIW? These questions have been addressed recently. This article provides an update.
The previously conjectured pathway (a path of NPIW's source water en route to the subtropical gyre from the formation region, the Okhotsk Sea) was a shortcut directly from the western subpolar gyre into the subtropical gyre. Such a pathway required an isopycnal (along a constant density surface) salinity minimum that is not found immediately south of the SATFZ in the northwestern subtropical gyre.