Figure 1

(a) Bottom topography and schematic ocean currents around Japan based on the study by Yabe et al.¹³. (b) Schematic diagram depicting the path of jets around the study region (green arrows; the coastal/offshore branches of the Tsushima Warm Current and the subpolar front) based on Naganuma²⁵ and Wagawa et al.²⁶ and the upper-layer low-salinity water (ULSW) (blue ellipse) observed using an underwater glider in Wagawa et al.^26,27 (black line). Contours of 200 m and 1000 m depths are denoted respectively by black and gray contours. The red rectangles denote the mouths of the major rivers, in order from north to south: the Omono, Mogami, Agano, and Shinano Rivers. Red open circles and blue circles represent the locations of CTD and AMeDAS stations, respectively. The abbreviation for the topography name is SR, Sado Ridge. (c) Vertical cross-section of the upper-layer low-salinity water (ULSW, blue) from the glider survey in Wagawa et al.²⁶ with salinity and potential density definitions of Wagawa et al.²⁷. Contours represent the potential density (kg m⁻³). (d) Springtime (March–May) mean sea-surface chlorophyll-a (mg m⁻³) averaged during 2010–2017 from satellite color measurement. Black lines and circles represent the glider track and CTD stations, as in (b). IDL 9.0 (https://www.nv5geospatialsoftware.com/Products/IDL) is used in generating the bathymetry based on the 1-km JTOPO30 data set provided by the Marine Information Research Center of the Japan Hydrographic Association (www.mirc.jha.or.jp/products/JTOPO30v2/).