Fig. 1

a Schematic diagram of the flows around Japan. b Locations of stations along the repeated observation lines in the Tsugaru Strait. c Observation site of the present study and distribution of the temporal mean velocity at each grid point for 2017–2019. The contours in b denote bathymetry. The rectangle in c indicates the subregions (R1 to R20) used to calculate the zonal average of the zonal velocity