Differential Formation and Circulation of North Pacific Central Mode Water

Abstract

Abstract A repeat hydrographic section along 165°E was analyzed to verify a westward extension of the formation region of the North Pacific Ocean Central Mode Water (CMW) suggested by previous synoptic observations, and to investigate the relation between the formation region and thermohaline fronts. The CMW formation region extends at least as far west as 155°E, much farther than recognized in a previous study based on climatology. It is located in two interfrontal regions between the Kuroshio Extension front and the Kuroshio Bifurcation front (KBF), and between the KBF and the subarctic front, where two types of CMW—namely, the lighter variety with potential density of 25.8–26.2 kg m−3 and the denser one of 26.3–26.4 kg m−3—are formed. How this differential formation of CMW is reflected in its gyrewide distribution was examined using one-time sections of the World Ocean Circulation Experiment (WOCE) Hydrographic Program in the North Pacific. The main circulation paths of the two types of CMW diverge east of the date line; the lighter variety is located in the inner part of the eastern subtropical gyre, and the denser variety is located in the outer part. These results demonstrate that the frontal structure around the northern boundary of the subtropical gyre, particularly the existence of KBF, is essential in determining the properties and the gyrewide distribution of CMW.