Poleward Shift of the Kuroshio Extension Front and Its Impact on the North Pacific Subtropical Mode Water in the Recent Decades

The meridional shift of the Kuroshio Extension (KE) front and changes in the formation of the North Pacific Subtropical Mode Water (STMW) during 1979-2018 are reported. The surface-to-subsurface structure of the KE front averaged over 142 degrees-165 degrees E has shifted poleward at a rate of similar to 0.23 degrees +/- 0.16 degrees decade(-1). The shift was caused mainly by the poleward shift of the downstream KE front (153 degrees-165 degrees E, similar to 0.41 degrees +/- 0.29 degrees decade(-1)) and barely by the upstream KE front (142 degrees-153 degrees E). The long-term shift trend of the KE front showed two distinct behaviors before and after 2002. Before 2002, the surface KE front moved northward with a faster rate than the subsurface. After 2002, the surface KE front showed no obvious trend, but the subsurface KE front continued to move northward. The ventilation zone of the STMW, defined by the area between the 16 degrees and 18 degrees C isotherms or between the 25 and 25.5 kg m(-3) isopycnals, contracted and displaced northward with a shoaling of the mixed layer depth h(m) before 2002 when the KE front moved northward. The STMW subduction rate was reduced by 0.76 Sv (63%; 1 Sv equivalent to = 10(6) m(3) s(-1)) during 1979-2018, most of which occurred before 2002. Of the three components affecting the total subduction rate, the temporal induction (-partial derivative h(m)/partial derivative t) was dominant accounting for 91% of the rate reduction, while the vertical pumping (-w(mb)) amounted to 8% and the lateral induction (-u(mb) . del h(m)) was insignificant. The reduced temporal induction was attributed to both the contracted ventilation zone and the shallowed h(m) that were incurred by the poleward shift of KE front.