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

Published in Journal of Oceanography, June 2021

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.

Fig1. Time series of cross-sectional area of STMW in the 137ºE section in winter (blue) and summer (red). Thin curve with dots and thick curve denote the yearly values and values smoothed with a 1-3-4-3-1 filter, respectively. Solid (dotted) bars indicate stable (unstable) periods of the KE during 1977–1992 based on a numerical hindcast (Qiu et al. 2014) and after 1992 based on satellite altimeter data (Qiu et al. 2020). Shade denotes LM periods of the Kuroshio

Oka, E., Nishikawa, H., Sugimoto, S. et al. Subtropical Mode Water in a recent persisting Kuroshio large-meander period: part I—formation and advection over the entire distribution region. J Oceanogr (2021). https://doi.org/10.1007/s10872-021-00608-3