Pathway of the Kuroshio water traveling to the Bering Sea in a western North Pacific eddy-resolving model analyzed with the tangent linear and adjoint models

Transport of the Kuroshio water to the Bering Sea can play an important role in the North Pacific overturn. In this study, we analyze the pathway of the Kuroshio water that travels to the Bering Sea simulated by a western North Pacific eddy-resolving Ocean General Circulation Model (OGCM) using the product between concentration of a passive tracer and its adjoint variable. We trace the water that is transported eastward across 140ºE by the Kuroshio and travels to the Bering Sea without going out of the model domain, which spans from 117ºE to 160ºW and 15ºN to 65ºN. The water initially flows eastward in the Kuroshio and the Kuroshio Extension. But, most of it deviates northward from the Kuroshio Extension before reaching 160ºE, and moves back to the west through eddy activities. A large fraction of it, then, enters the quasi-stationary jet that flows at 43ºN,-155ºE, or the Isoguchi Jet, and is transported northeastward. The water flows along the southern edge of the subarctic gyre after that. It is, then, transported northward through the gaps between the western subarctic gyre and the Alaskan gyre, and enters the Bering Sea. The number and positions of the gaps has interannual variability and it affects the pathway of the Kuroshio water in the model. It should, however, be noted that the water traveling to the Bering Sea via the Gulf of Alaska is not traced in this analysis due to the limitation of the model domain. In the presentation, we also show that the analyzed pathway is basically consistent with the variability of the Isoguchi Jet and the subarctic gyre in the 30-year 4-Dimensional variational (4DVAR) Ocean Reanalysis for the western North Pacific (FORA-WNP30), in which the realistic ocean variation is reconstructed through the eddy-resolving 4DVAR ocean data assimilation system in the Japan Meteorological Agency (JMA).