Formation of Central Mode Water based on two zonal hydrographic sections in spring 2013 and 2016

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.

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

“…This definition differs from that used in our previous studies using Argo profiles, i.e., a layer where Q is lower than 2.0 × 10 -10 m −1 s −1 and θ at the Q minimum (called core θ hereafter) is between 16 and 19.5 °C (Oka 2009;Oka et al 2011Oka et al , 2015. In the new definition, we permit the existence of more than one STMW layer in each profile, which has been frequently observed in mode waters (Taneda et al 2000;Oka et al 2011Oka et al , 2020Gao et al 2016;Liu et al 2017Liu et al , 2019. It should also be noted that the θ range for the STMW definition is different between the 137ºE and Argo float data; the narrower and relatively warm θ range for the more contemporary Argo data reflects long-term warming of STMW (Sugimoto et al 2017;Oka et al 2019).…”

Spiciness anomalies in the upper North Pacific based on Argo observations

“…The inconsistency in interannual variations corresponded to the different formation regions and circulation pathways in the light part (L-CMW) and the dense part (D-CMW) of the CMW. The L-CMW is formed between the KE front and the Kuroshio bifurcation front (KBF) and distributed along the inner part of the eastern subtropical gyre, while the D-CMW is formed between the KBF and the subarctic frontal zone (SAFZ) and distributed in the outer part (Oka and Suga, 2005;Oka et al, 2011;Oka et al, 2020).…”

Heat and salinity transport between the permanent pycnocline and the mixed layer due to the obduction process evaluated from a gridded Argo dataset

“…2a), these mode waters spread to around 300 m depth or deeper (e.g. Qiu et al 2006;Oka et al 2020), which is captured in the Eulerian definition. The vertical shift of water column is considered solely by the vertical pumping term in the Lagrangian definition, but the vertical pumping rate is a few tens of meters per year in the North Pacific (Qiu and Huang 1995;Suga et al 2008) and is not large enough to explain the vertical displacement of the mode waters.…”