| Using the Miami Isopycnic Coordinate Ocean Model (MICOM) configured with 1-degree horizontal resolution and 23 layers, we examine processes that maintain the mesothermal structure, a subsurface temperature inversion, in the subarctic region of the North Pacific. The model successfully reproduces the mesothermal structure consisting of the shallow temperature minimum and the underlying temperature maximum that are so 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 wintertime mixed layer. The horizontal distribution of the passive tracer injected into the subsurface layers south of Japan clearly shows that warm and saline waters of the Kuroshio in the density range of 26.8-27.0 σθ are the sources of the mesothermal water. There are three pathways of the Kuroshio waters from the subtropics to 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 the cross-gyre window in the eastern basin. Third, the Kuroshio waters diffuse along the isopycnal into the western subarctic gyre, while they are carried by the northern part of the North Pacific Current. The mesothermal water thus formed in the subarctic region is entrained into the mixed layer in winter, and flows out to the subtropics by southward Ekman drift, forming the subpolar cell (SPC). |
|
|