Subtropical Mode Water in a recent persisting Kuroshio large-meander period: part I\u2014formation and advection over the entire distribution region

Eitarou Oka,Hatsumi Nishikawa,Bo Qiu,Shusaku Sugimoto,Niklas Schneider

doi:10.1007/s10872-021-00608-3

Abstract

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.

Highlights

The Kuroshio has taken a large-meander (LM) path south of Japan since August 2017 (Usui 2019)
The LM occurrence impacts currents and water masses in the surrounding area. It reset the decadal variability of the Kuroshio extension (KE) system (Qiu et al 2020), which had been controlled by the wind stress curl variability in the central North Pacific associated with the Pacific decadal oscillation (PDO; Mantua et al 1997) and the subsequent westward propagation of sea surface height anomalies as baroclinic Rossby waves (Qiu and Chen 2005; Qiu et al 2007)
This paper examines the formation and advection of subtropical mode water (STMW) over its entire distribution region in relation to a persisting Kuroshio LM and the resultant stable state of the KE

Summary

Introduction

The Kuroshio has taken a large-meander (LM) path south of Japan since August 2017 (Usui 2019). The LM occurrence impacts currents and water masses in the surrounding area. It reset the decadal variability of the Kuroshio extension (KE) system (Qiu et al 2020), which had been controlled by the wind stress curl variability in the central North Pacific associated with the Pacific decadal oscillation (PDO; Mantua et al 1997) and the subsequent westward propagation of sea surface height (and main thermocline depth) anomalies as baroclinic Rossby waves (Qiu and Chen 2005; Qiu et al 2007).

Results

Discussion

Conclusion