Fig. 1

Quasi-stationary jets in the Subarctic Frontal Zone. a Annual mean SST (shade), surface current (arrows) and SST front position (white solid lines) derived from the JCOPE2 ocean reanalysis (see Methods). Surface currents faster than 0.1 ms⁻¹ are denoted by black arrows, while slower than 0.1 ms⁻¹ by gray arrows. As for the SST front position, it is represented by the latitude Φ(x, t) where the meridional gradient of SST is maximum in the region 145°E–170°E, 35°N–47°N, derived from JCOPE2. Here, Φ(x, t) is decomposed to \({\Phi} \left( {x,t} \right)\sim \overline { \Phi (x)} + {\rm{EOF}}1(x) \cdot {\rm{OEI}}(t)\), where \(\overline {\Phi (x)}\) is the temporal mean position, and EOF1(x) and OEI(t) denote the 1st mode empirical orthogonal function (EOF) and its time coefficient, respectively. OEI(t) is normalized so that its standard deviation σ to be unity. White solid lines denote \(\overline {\Phi (x)} +\rm EOF1( {\it x} ) \cdot \sigma\), where σ = 0 (thin line) and σ = 1 (thick line). b Barotropic streamfunction (contours, ×10⁶ m³ s⁻¹) and bottom topography (shades). Barotropic streamfunction is obtained from depth-integrated transport using horizontal velocity field of JCOPE2 reanalysis. Black (light blue) contours denote the positive (negative) value in terms of the streamfunction. The bottom topography is based on ETOPO5, in which bathymetry between 5500 m and 5000 m is emphasized. A, A′, B, and C denote the ocean-floor rises that are paid attention in this paper. J1 and J2 are indicated by dashed lines with arrows