Formation of Central Mode Water based on two zonal hydrographic sections in spring 2013 and 2016
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ORIGINAL ARTICLE
Formation of Central Mode Water based on two zonal hydrographic sections in spring 2013 and 2016 Eitarou Oka1 · Shinya Kouketsu2 · Daigo Yanagimoto1 · Daiki Ito3 · Yoshimi Kawai2 · Shusaku Sugimoto4 · Bo Qiu5 Received: 7 March 2020 / Revised: 23 April 2020 / Accepted: 20 May 2020 © The Author(s) 2020
Abstract Two zonal high-density hydrographic sections along 41° N and 37.5° N east of Japan were occupied in April 2013 and June 2016 to examine the formation of Central Mode Water (CMW) and Transition Region Mode Water (TRMW) in relation to fronts and eddies. In the 41° N section traversing the meandering subarctic front, the denser variety of CMW (D-CMW) and TRMW was formed continuously on both sides of the front, except for the part of the section located south of the Kuroshio bifurcation front where the lighter variety of CMW (L-CMW) and D-CMW was formed instead. L-CMW and D-CMW were also formed in the eastern part of the 37.5° N section between the Kuroshio Extension front and the Kuroshio bifurcation front, but were hardly formed in the western part of the section west of the bifurcation point of the two fronts. D-CMW and TRMW pycnostads in the western part of the 41° N section observed in April 2013 tended to exhibit more than one core (vertical minimum of potential vorticity), which might be formed by destruction of deep winter mixed layers. Such multiplecore structure was also observed in L-CMW and D-CMW pycnostads in the eastern part of both the sections south of the Kuroshio bifurcation front in June 2016, being particularly abundant in three anticyclonic eddies. It was likely to be formed by the exchange of low-potential vorticity water among the eddies and the ambient region in association with eddy-to-eddy interaction, suggesting a new mechanism of mode water subduction. Keywords Central Mode Water · Transition Region Mode Water · Formation · Fronts · Mesoscale eddies
1 Introduction Central Mode Water (CMW; Nakamura 1996; Suga et al. 1997) is a low-potential vorticity (Q) water found in the lower permanent pycnocline of the North Pacific subtropical gyre. It is formed as deep winter mixed layers north of * Eitarou Oka [email protected]‑tokyo.ac.jp 1
Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa 277‑8564, Japan
2
Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237‑0061, Japan
3
National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, Yokohama 236‑8648, Japan
4
Department of Geophysics, Graduate School of Science, Tohoku University, Aoba‑ku, Sendai 980‑8578, Japan
5
Department of Oceanography, University of Hawaii at Manoa, Honolulu, HI 96822, USA
the Kuroshio Extension, subducted in the central northern part of the subtropical gyre, and transported anticyclonically toward the western boundary (Suga et al. 2004; Oka et al. 2011a). The formation, subduction, and circulation of CMW and their temporal variability are believed to be important for various
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