Study of the formation of North Pacific Intermediate Water by a general circulation model and the particle‐tracking method: 2. Formation mechanism of salinity minimum from the view of the “critical gradient” of the Oyashio mixing ratio

Abstract

In order to avoid the “false North Pacific Intermediate Water (NPIW)” problem a general circulation model is driven by the winter‐like North Pacific wind that can separate the modeled Kuroshio from the coast around 36°N. In the model a salinity minimum appears around 26.8σθ in the North Pacific subtropical region, which does not outcrop in the North Pacific. Formation processes of the salinity minimum are examined. Comparing them with observations and discussing them sufficiently, this study clarified that NPIW salinity minimum is formed just above the less saline water influenced largely by the Oyashio water. The formation of NPIW is basically controlled by the rapid increase of the Oyashio layer thickness around 26.5–26.8σθ, which layer structure is determined by Okhotsk Sea Mode Water pycnostad at 26.8–26.9σθ and the winter sea surface density of ∼26.5σθ. For NPIW salinity minimum formation it is most important that the gradient of the Oyashio mixing ratio exceeds the “critical gradient” around 26.4–26.7σθ. NPIW salinity minimum density is almost determined by the heaviest density of this range (26.7σθ), which is ∼0.1σθ less dense than the bottom of the rapid increase of the Oyashio layer thickness. Then cabbeling leads to an increase in its density to 26.8σθ. The Sea of Okhotsk is very important for the formation of NPIW through the formation of the Oyashio thick layers, and it practically determines NPIW salinity minimum density.