Abstract
The Miami Isopycnic Coordinate Ocean Model configured with 1° horizontal resolution and 23 layers is used to examine processes that maintain the mesothermal structure, a subsurface temperature inversion, in the subarctic North Pacific. The model successfully reproduces the mesothermal structure consisting of a shallow temperature minimum and an underlying temperature maximum that are called the dichothermal and mesothermal waters, respectively. The mesothermal water is formed through cross-gyre exchange between the subtropical and subarctic gyres, whereas the dichothermal water originates from cold and low-salinity waters formed in the winter mixed layer. The horizontal distribution of the passive tracer injected into the subsurface layers south of Japan shows that warm and saline water of the Kuroshio in the density range of 26.8–27.0 σθ is the source of the mesothermal water. There are three pathways through which the Kuroshio waters enter the subarctic region. First, the Kuroshio waters that cross the gyre boundary in the western boundary region are carried to the Alaskan gyre by the northern part of the North Pacific Current. Second, the Kuroshio waters carried by the southern part of the North Pacific Current enter the Alaskan gyre through a cross-gyre window in the eastern basin. Third, the Kuroshio waters that diffuse along the isopycnal in the Kuroshio–Oyashio Extension enter the western subarctic gyre. The mesothermal water thus formed in the subarctic region is entrained into the winter mixed layer and returns to the subtropics as surface water by the southward Ekman drift, forming the subpolar cell.
Corresponding author address: Dr. Takahiro Endoh, International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, 1680 East West Road, Honolulu, HI 96822. Email: tendo@hawaii.edu
