[A-OS19] Coastal physical processes associated with mixing, eddies, internal waves

[AOS19-01] Seasonal cycle of the confluence of the Tsugaru Warm, Oyashio, and Kuroshio currents east of Japan

★Invited Papers

*Sachihiko Itoh¹, Eisuke Tsutsumi¹, Eiji Masunaga², Takashi T Sakamoto³, Kazuo Ishikawa¹, Daigo Yanagimoto¹, Yasuhiro Hoshiba¹, Hitoshi Kaneko³, Daisuke Hasegawa⁴, Kiyoshi Tanaka¹, Hideki Fukuda¹, Toshi Nagata¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Ibaraki University, 3.Japan Agency for Marine-Earth Science and Technology, 4.Japan Fisheries Research and Education Agency)

Keywords:Submesoscale, Fronts, Underway CTD, Oyashio, Tsugaru Warm Current

The Sanriku Confluence east of Japan is the region of confluence of the Tsugaru Warm Current from the Sea of Japan, the Oyashio Current from the western subarctic gyre and the Sea of Okhotsk, and Kuroshio water that has detached from the subtropical gyre. It is a field of vigorous stirring driven by variability in current systems, but transition processes between water masses have yet to be clarified. High-resolution underwater conductivity–temperature–depth observations were undertaken during each of the four seasons to test a hypothesis that the water-mass gradient in the Sanriku Confluence is set without full stirring. Analyses in isopycnal coordinates indicate the seasonal occurrence of prominent fronts of both salinity and displacement. After fitting error functions to frontal features, the seasonally emerging isopycnal salinity fronts often became extremely sharp with widths typically narrower than 3 km, supporting the hypothesis. Disturbances distinguished from frontal structures indicate variance peaks with ranges similar to or slightly greater than the baroclinic Rossby radius, likely stirring the large-scale tracer gradient at this scale. Elevated variances in isopycnal salinity were often observed around the fronts, and variances caused by isolated patches with salinity anomalies were also large. By determining the horizontal scales at which the effects of internal wave heaving becomes dominant, we were able to calculate the potential vorticity (PV) of the balanced flow field. As the PV becomes negative or near zero around the fronts, symmetric instability might have developed.