A peculiar water mass forms as a result of stirring and mixing between water originating from the Kuroshio and Oyashio currents in the Transition Domain, which is a boundary region between the subtropical and subarctic gyres extending zonally at approximately 40°N in the western North Pacific west of the dateline. The domain is important not only for oceanographic processes such as the inter-gyre exchange of water masses but also for mid-latitude climate and biological processes. We investigated how water from the Kuroshio and Oyashio currents is transported to the Transition Domain and then mixed, especially in relation to the geostrophic velocity fields, bottom topographic features, and the local eddy activity in the upstream region of the domain. We have clarified the pathways of the water from the Kuroshio and Oyashio currents from a Lagrangian point of view by conducting drifting buoy observations in 2015 and 2017, as well as by particle tracking using geostrophic surface flow fields. Herein, we visualize how the water pathways within and around the Transition Domain are closely tied to low-rise bottom topographic features. Time-variation of the geostrophic field in the upstream region of the Transition Domain also likely plays an important role in supplying water from the Kuroshio region to the Transition Domain; if a steady climatological flow field is used, the Transition Domain will be occupied solely by water from the Oyashio current. Once the water from the Kuroshio region enters a quasi-stationary jet in the western North Pacific (the so-called western Isoguchi Jet; J1) due to the eddies at the Oyashio Second Branch in addition to eddies at another region of the J1′s meander, it is further transported northeastward in the subarctic gyre via the Transition Domain. The subtropical–subarctic exchange through the Transition Domain could essentially be driven by the eddies in these local regions.

来自北太平洋西部西部约 40°N 纬向延伸的亚热带和亚北极环流之间的边界区域的过渡域是源自黑潮和亲潮流的水之间的搅拌和混合的结果形成的特殊水团日期变更线的。该域不仅对海洋学过程（如水团间环流交换）很重要，而且对中纬度气候和生物过程也很重要。我们研究了来自黑潮和亲潮流的水如何输送到过渡域然后混合，特别是与地转速度场、底部地形特征和域上游区域的局部涡流活动有关。我们通过 2015 年和 2017 年的漂流浮标观测以及使用地转表面流场的粒子跟踪，从拉格朗日的角度阐明了黑潮和亲潮流中水的路径。在此，我们形象化了过渡域内部和周围的水路如何与低层底部地形特征密切相关。过渡域上游地区地转场的时间变化也可能在黑潮地区向过渡域供水方面发挥重要作用；如果使用稳定的气候流场，过渡域将完全被亲潮流的水占据。一旦来自黑潮地区的水进入北太平洋西部的准静止急流（所谓的西部矶口急流；J1) 由于亲潮二支的涡流和 J1 曲流的另一个区域的涡流，它通过过渡域在亚北极环流中进一步向东北输送。通过过渡域的亚热带-亚北极交换基本上可以由这些局部区域的涡流驱动。