Formation of the salinity minimum in the Mixed Water Region between the Oyashio and Kuroshio Fronts

Abstract

The salinity minimum frequently occurring in the Mixed Water Region between the Oyashio and Kuroshio Fronts seems to originate from the salinity minimum at the density of 26.8 σ θ called the North Pacific Intermediate Water. We examined water exchange of this region with the Oyashio and the Kuroshio Extension using mixing ratio R K defined as ( θ− θ OY)/( θ K− θ OY)×100, where θ OY , θ K , and θ represent potential temperature of the Oyashio and Kuroshio Waters and their mixture on the isopycnal surfaces, respectively. CTD data were obtained by repeated observation from January 1990 to May 1991. R K increases southward from the Oyashio Front to the Kuroshio Front with the range of −20 to 120%. The gradient of R K on the isopycnal surfaces is large around the Oyashio Front above the 26.8 σ θ surface, while it is large around the Kuroshio Front below it. This agrees with the average R K in the Mixed Water Region decreasing greatly with the increase of density at densities less dense than 26.8 σ θ . We calculated thickness and volume transport of the Oyashio between the isopycnal surfaces near the coast of Hokkaido. They increase largely with density at densities less dense than 26.8 σ θ . It is supposed that the salinity minimum in the Mixed Water Region is the upper limit of the water largely influenced by the Oyashio Water. Its density could depend only on the density structure of the Oyashio.