Decadal-scale variability of the North Pacific subtropical mode water and its influence on the pycnocline observed along 137\xb0E

Fumiaki Kobashi,Naoto Iwasaka,Toshiya Nakano,Tomomichi Ogata

doi:10.1007/s10872-020-00579-x

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.

Highlights

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)
The subtropical front (STF) is manifested as the northward shoaling of the upper pycnocline at subsurface depths of 50–200 m in the latitudinal range of 17°–27°N (Fig. 1b; Uda and Hasunuma 1969)
The present study addresses the following questions: how does the change in the STMW thickness alter the upper and lower main pycnoclines at 137°E on a decadal timescale? Does the STMW affect the variability of the STF? We focus on the summer season when the upper pycnocline develops well above the STMW

Summary

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). Eddy-resolving ocean general circulation models (Yamanaka et al 2008; Nonaka et al 2012; Sugimoto et al 2012) and coupled climate models (Xie et al 2011; Xu et al 2012) revealed that the STF is anchored by mode waters in the climatology, in agreement with observations (Aoki et al 2002; Kobashi et al 2006) They showed that the advection of thick, low-PV mode waters causes the upper pycnocline to shoal more than normal, intensifying the STF and STCC on decadal timescales. This mode water-induced change in Decadal‐scale variability of the North Pacific subtropical mode water and its influence on.

Observation data and method

Linear baroclinic Rossby wave model

Variations in the STMW thickness

Variations in the pycnocline

Variations in the STF and their cause

Summary and discussion