4:00 PM - 4:15 PM
[AOS18-03] Subtropical Mode Water in a recent persisting Kuroshio large meander period. Part II: Formation and temporal evolution in the recirculation gyre off Shikoku.
Keywords:Subtropical Mode Water, Kuroshio large meander, recirculation gyre off Shikoku
The Kuroshio large meander (LM) since in August 2017 has continued for more the 3 years for the first time in 40 years. When the Kuroshio took a meandering path (i.e., a typical large meander path and an offshore non-large-meander path), an anomalously warm Subtropical Mode Water (STMW) exceeding 19℃ was formed in the recirculation gyre off Shikoku, forming a two-layer structure with a colder STMW formed in the previous years (Sugimoto and Hanawa, 2014), although each meandering period examined in the preceding study spanned only one year. This study aims to examine how an LM event longer than 2 years influences the formation and temporal evolution of STMW in the recirculation gyre off Shikoku, based on the data from Argo floats, satellite altimeter, and shipboard observation.
In spring 2017 before the Kuroshio large meander began, a single STMW layer with thickness of 400m and temperature of 18℃, which was thought to be formed in the late winter 2017, existed at depths of 100-500 m in the recirculation gyre off Shikoku. In the late winter 2018 after the current LM began, warm STMW with temperature exceeding 19℃ was formed to 300m depth, while the STMW layer formed in the previous year shifted downward to the depths of 350-650m, forming a two-layer structure. The shallower warmer STMW layer was not renewed in the late winter of 2019 due to insufficient mixed layer development, persisting for nearly 2 years, and then renewed in the late winter of 2020. The deeper STMW layer formed in the late winter of 2017 survived until 2020 while its thickness gradually decreased to 200 m. The two-layer structure of STMW was thus maintained.
A shipboard CTDO2 profile obtained by the Japan Meteorological Agency near the center of the recirculation gyre in June 2018 clearly captured the two-layer structure. Apparent oxygen utilization at the potential vorticity minima in the shallower and deeper STMW layers was 0.2560 ml/l (236 m) and 0.4229 ml/l (545 m), respectively. The oxygen utilization rate estimated from their difference was 0.1669 ml/l/yr, which was much smaller than values presented by previous studies. This is presumably because the STMW layer formed in 2017 shifted to the greater depths where oxygen consumption was slow, and because the dissipation of the STMW trapped and isolated in the recirculation gyre was slow.
In spring 2017 before the Kuroshio large meander began, a single STMW layer with thickness of 400m and temperature of 18℃, which was thought to be formed in the late winter 2017, existed at depths of 100-500 m in the recirculation gyre off Shikoku. In the late winter 2018 after the current LM began, warm STMW with temperature exceeding 19℃ was formed to 300m depth, while the STMW layer formed in the previous year shifted downward to the depths of 350-650m, forming a two-layer structure. The shallower warmer STMW layer was not renewed in the late winter of 2019 due to insufficient mixed layer development, persisting for nearly 2 years, and then renewed in the late winter of 2020. The deeper STMW layer formed in the late winter of 2017 survived until 2020 while its thickness gradually decreased to 200 m. The two-layer structure of STMW was thus maintained.
A shipboard CTDO2 profile obtained by the Japan Meteorological Agency near the center of the recirculation gyre in June 2018 clearly captured the two-layer structure. Apparent oxygen utilization at the potential vorticity minima in the shallower and deeper STMW layers was 0.2560 ml/l (236 m) and 0.4229 ml/l (545 m), respectively. The oxygen utilization rate estimated from their difference was 0.1669 ml/l/yr, which was much smaller than values presented by previous studies. This is presumably because the STMW layer formed in 2017 shifted to the greater depths where oxygen consumption was slow, and because the dissipation of the STMW trapped and isolated in the recirculation gyre was slow.