Surface water pathways in the subtropical-subarctic frontal zone of the western North Pacific

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 degrees 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 midlatitude 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.