Fine-scale structure and mixing across the front between the Tsugaru Warm and Oyashio Currents in summer along the Sanriku Coast, east of Japan
High-resolution shipboard observations were made across the front between the Tsugaru Warm Current (TWC) and the Oyashio Current in July 2013. Fine structure in the frontal zones was successfully captured with an underway conductivity–temperature–depth profiler deployed with a typical horizontal interval of 2–3 nautical miles. The front characterized by marked horizontal gradients in temperature and salinity extended from the subsurface onto the shelf. Along this frontal layer, the minimum frequency for internal waves became substantially lower than the local inertial frequency, mainly due to the strong vertical shear of the geostrophic velocity. Turbulent energy dissipation rates ε (vertical diffusivity K ρ ) were frequently elevated along the front and its offshore side up to 3 × 10−8 W kg−1 (10−4 m2 s−1), which may have been caused by an “internal tide chimney”, trapping low-frequency internal waves within the band of strong shear. At the onshore side of the TWC on the shelf, strong mixing with ε (K ρ ) exceeding 10−6 W kg−1 (10−3 m2 s−1) was also observed. A large portion of the water columns in the frontal area provided suitable conditions for double diffusion; in some layers with moderate turbulence, temperature microstructures indicative of double diffusion were observed. The vigorous mixing processes around the front are likely to modify the properties of the TWC downstream, which could then produce a latitudinal gradient in environments along the coast.
KeywordsTsugaru Warm Current Oyashio (Oyashio Current) Front Vertical mixing Internal tide chimney 2011 off the Pacific coast of Tohoku earthquake The Great East Japan earthquake
This research was supported by the Tohoku Ecosystem-Associated Marine Sciences (TEAMS) by the Ministry of Education, Culture, Sports, Science and Technology in Japan. S. Itoh, H. Kaneko and T. Okuhishi were also supported by The New Ocean Paradigm on its Biogeochemistry, Ecosystem, and Sustainable Use (NEOPS). The authors thank S. Kouketsu, H. Kawahara and I. Yasuda for preparation and deployments of the UCTD profiler. The data obtained by R/V Daisan Kaiyo Maru are available on the Research Information and Data Access Site of TEAMS (RIAS). The data from the Iwate Fisheries Technology Center are available on its website (http://www2.pref.iwate.jp/~hp5507/).
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