Subtropical Mode Water in a recent persisting Kuroshio large-meander period: part I—formation and advection over the entire distribution region

Since August 2017, the Kuroshio has taken a large-meander (LM) path, which has forced the Kuroshio extension (KE) to be in its stable state against its wind-forced decadal variability. How such current conditions have impacted the formation and advection of North Pacific subtropical mode water (STMW) over its distribution region was examined using Argo float data during 2005–2020. Out of the whole STMW defined as a low-potential vorticity layer of 16–19.5 ºC, a relatively cold variety of 16–18 ºC, which was formed south of the KE and advected westward and southward, occupied more than 80% of the total volume. The formation rate of the 16–18 ºC variety was low during 2006–2009 in an unstable-KE period and high during 2010–2015 in a stable-KE period, and then dropped drastically in 2016 despite the KE still being in the stable state. After a short unstable-KE period in 2016–2017, the LM-forced, stable-KE period began, but the formation rate of the 16–18 ºC variety has not restored, possibly due to stronger background stratification propagated from the central North Pacific. In addition, the 16–18 ºC variety has had to make a southern detour around the LM, and its westward advection from the formation region south of the KE to the region south of Japan has been significantly decreased, possibly because it is dissipated more strongly over a southern part of the Izu–Ogasawara Ridge. Due to such decline in the formation and advection, the volume of the 16–18 ºC variety and hence that of the whole STMW have gradually decreased since 2016.

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

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°E-165°E has shifted poleward at a rate of ~ 0.23±0.16° per decade. The shift was caused mainly by the poleward shift of the downstream KE front (153°E-165°E, ~ 0.41±0.29° per decade), barely by the upstream KE front (142°E-153°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 16°C and 18°C isotherms or between 25 kg m-3 and 25.5 kg m-3 isopycnals, contracted and displaced northward with a shoaling of the mixed layer depth (hm) before 2002 when the KE front moved northward. The STMW subduction rate was reduced by 0.76 Sv (63%) during 1979-2018, most of which occurred before 2002. Of the three components affecting the total subduction rate, the temporal induction ( −∂hm/∂t ) was dominant accounting for 91% of the rate reduction, while the vertical pumping (−wmb) amounted to 8% and the lateral induction (−umb · ∇hm) was insignificant. The reduced temporal induction was attributed to both the contracted ventilation zone and the shallowed hm that were incurred by the poleward shift of KE front.

Decadal-scale variability of the North Pacific subtropical mode water and its influence on the pycnocline observed along 137°E

Decadal-scale variability of the North Pacific subtropical mode water (STMW) and its influence on the pycnocline are examined by analyzing Japan Meteorological Agency (JMA) repeat hydrographic observations along the 137°E meridian from 1972 to 2019, with a particular focus on the summer season when the seasonal upper pycnocline develops above the STMW. The STMW appears between 20° and 32°N at 137°E, with the thickness varying on decadal timescales of approximately 9–15 years. Argo float observations suggest that the observed change in the STMW thickness originates in the wintertime mixed layer south of the Kuroshio Extension in the preceding year. The STMW has a substantial impact on the pycnocline. The presence of thick STMW shoals the upper pycnocline, occasionally concurrent with the deepening of the lower main pycnocline. The change is robust in the upper pycnocline, where the heaving of isopycnal surfaces occurs with density anomalies up near the surface. The subtropical front (STF) at subsurface depths, which is associated with a northward shoaling of the upper pycnocline and is maintained by the STMW in the climatology, also changes on decadal timescales. A thick STMW increases the northward shoaling of the upper pycnocline and intensifies the STF. On decadal timescales, the STF variations are accounted for by the STMW-induced change in the upper pycnocline slope. The change in the STF due to mode waters is consistent with previous findings from numerical models.