Numerical study on the formation of the mesothermal structure in the North Pacific subarctic gyres

\hspace*{7}We examine processes that maintain the mesothermal structure, a subsurface temperature inversion, in the subarctic region of the North Pacific. The study proceeds numerically by using the Miami Isopycnic Coordinate Ocean Model (MICOM) configured with 1-degree horizontal resolution and 23 layers. 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 26.8-27.2 σ \theta are the sources of the mesothermal water. The mesothermal water thus formed in the subarctic region upwells into the mixed layer during winter due to Ekman suction, and flows out to the subtropics by southward Ekman drift, forming a shallow overturning cell. There are two cross-gyre windows that permit flow from the subtropical gyre to the subarctic gyre. One is the eastern window near 140°W, through which the Kuroshio waters carried by the North Pacific Current enter the Alaskan gyre. It takes about 10 years for the Kuroshio waters to reach the eastern window, consistent with the observed results. The other cross-gyre window, the central window, is found near 170°$E. The Kuroshio waters carried along the Oyashio front enter the western subarctic gyre through the central window only within 3 years. Considering that the mesothermal structure is mainly distributed in the western North Pacific, the central window, whose importance has been overlooked in the previous studies, plays an essential role in maintaining the mesothermal structure.