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

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ORIGINAL ARTICLE

Decadal‑scale variability of the North Pacific subtropical mode water and its influence on the pycnocline observed along 137°E Fumiaki Kobashi1   · Toshiya Nakano2 · Naoto Iwasaka1 · Tomomichi Ogata3 Received: 16 June 2020 / Revised: 7 November 2020 / Accepted: 7 November 2020 © The Author(s) 2020

Abstract 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. Keywords  137°E section · North Pacific subtropical mode water · Subtropical front · Pycnocline · Decadal-scale variations

1 Introduction The North Pacific subtropical mode water (STMW) is a distinct water mass that exists between the upper pycnocline and the lower main pycnocline in the western subtropical North Pacific (e.g., Masuzawa 1969; Suga et al. 1989; Hanawa and Talley 2001; Oka and Qiu 2012; Feucher et al. 2019). It appears as a layer of nearly vertically homogeneous properties and is dynamically characterized by a vertical minimum in potential vorticity (PV; Fig. 1a). The STMW forms in the deep mixed layer south of the Kuroshio and the Kuroshio Extension in winter, which arises from strong * Fumiaki Kobashi [email protected] 1



Tokyo University of Marine Science and Technology, Tokyo, Japan

2



Nagasaki Meteorological Office, Japan Meteorological Agency, Nagasaki, Japan

3

Application Laboratory, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, Japan



ocean surface cooling and wind stirring in conjunction with weak vertical stratification below the mixed layer (Tozuka et al. 2017). The STMW is then subducted and transported to a wide region of the subtropical gyre by the mean g