Decadal Variations of North Pacific Subtropical Mode Water and Their Influence on the Downstream Region Observed at the 137°E Repeat Hydrographic Section

North Pacific Subtropical Mode Water (STMW) forms in a wintertime deep surface mixed layer south of the Kuroshio Extension, subducted and advected southward by the gyre circulation, producing a thick layer of vertically homogeneous water in a wide region of the northwestern subtropical gyre. STMW shoals the upper pycnocline above STMW and makes the pycnocline tilted to the south, maintaining subtropical front at subsurface depths along the southern flank of STMW. STMW exhibits significant decadal variability in the formation site, while its influence on the downstream region has not been fully understood yet. The present study focuses on decadal variability of the STMW thickness and addresses the following questions: how does STMW change in the downstream region? How does it affect the upper pycnocline above STMW and the main pycnocline below STMW? Does it affect subtropical front? We analyze the Japan Meteorological Agency repeat hydrographic observations along the 137°E meridian for the period of 1972 to 2018 and also analyze recent Argo float observations. STMW shows obvious decadal variability at 137°E. It occurs almost coherently in the cross section over the entire STMW. STMW is likely spread to 137°E very quickly within one year from the formation site. The change in the STMW thickness is well correlated with that in the isopycnal depths. The thicker STMW deepens the main pycnocline while shoaling the upper pycnocline. The shoaling is very obvious above STMW all the depths up to the near surface, suggesting possible influence on the surface temperature and density. Simulation of a linear two-layer model of the ocean driven by wind stress curl from an atmospheric reanalysis indicates a minor contribution of wind stress curl forcing and Rossby wave propagation to the change in the isopycnal depths in the STMW region. The thick STMW tends to enhance the northward shoaling of the upper pycnocline and thus intensify subtropical front on decadal timescales.