Fig. 4

Seasonal mean of the surface velocity obtained from the high-frequency radar data in a winter, b spring, c summer, and d autumn. The color denotes the relative vorticity estimated from the radar data as ∂v/∂x − ∂u/∂y, where u and v are the zonal and meridional velocities, respectively. The yellow dashed (solid) line shows the temporal mean (standard deviation) of the Tsugaru Warm Current axis. The bathymetry is along 141.3°E