Fig. 5: Correlation maps between the surface chlorophyll-a concentration in the Tsugaru Gyre development season and the divergence of the surface velocity.

The area for divergence estimation is delineated by the cyan rectangle, which is situated on the eastern (western) flank of the Shiriya Spur, ranging from 41.65°N–41.70°N, 141.60°E–141.65°E (41.65°N–41.70°N, 141.50°E–141.55°E) in (a) and (b) (c and d). a, c Cases with a 1-day lag, whereas b and d represent estimations with a 7-day lag. The grey dots indicate the grids in which the p value is greater than 0.05. The grey contours denote bathymetry. The black contours and vectors are the standardized mean relative vorticity by the inertial frequency at 41°N and the velocity at the surface estimated from OSCAR data between August and September for the years 2014–2021, respectively. Source data are provided as a Source Data file.