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Fine-scale structure and mixing across the front between the Tsugaru Warm and Oyashio Currents in summer along the Sanriku Coast, east of Japan

  • Special Section: Original Article
  • Oceanographic observations after the 2011 earthquake off the Pacific coast of Tohoku
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Abstract

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.

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Acknowledgments

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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Authors and Affiliations

  1. Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8564, Japan

    Sachihiko Itoh, Hitoshi Kaneko, Miho Ishizu, Daigo Yanagimoto & Kiyoshi Tanaka

  2. Tohoku National Fisheries Research Institute, Fisheries Research Agency, Shiogama, Japan

    Takeshi Okunishi

  3. Faculty of Education and Welfare Science, Oita University, Oita, Japan

    Hajime Nishigaki

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  1. Sachihiko Itoh
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  2. Hitoshi Kaneko
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  3. Miho Ishizu
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  4. Daigo Yanagimoto
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  5. Takeshi Okunishi
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  6. Hajime Nishigaki
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  7. Kiyoshi Tanaka
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Correspondence to Sachihiko Itoh.

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Itoh, S., Kaneko, H., Ishizu, M. et al. Fine-scale structure and mixing across the front between the Tsugaru Warm and Oyashio Currents in summer along the Sanriku Coast, east of Japan. J Oceanogr 72, 23–37 (2016). https://doi.org/10.1007/s10872-015-0320-6

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  • DOI: https://doi.org/10.1007/s10872-015-0320-6

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