Advertisement

Journal of Oceanography

, Volume 72, Issue 1, pp 23–37 | Cite as

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

  • Sachihiko Itoh
  • Hitoshi Kaneko
  • Miho Ishizu
  • Daigo Yanagimoto
  • Takeshi Okunishi
  • Hajime Nishigaki
  • Kiyoshi Tanaka
Special Section: Original Article Oceanographic observations after the 2011 earthquake off the Pacific coast of Tohoku

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.

Keywords

Tsugaru Warm Current Oyashio (Oyashio Current) Front Vertical mixing Internal tide chimney 2011 off the Pacific coast of Tohoku earthquake The Great East Japan earthquake 

Notes

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/).

References

  1. Conlon DM (1982) On the outflow modes of the Tsugaru Warm Current. La mer 20:60–64Google Scholar
  2. D’Asaro E, Lee C, Rainville L, Harcourt R, Thomas L (2011) Enhanced turbulence and energy dissipation at ocean fronts. Science 332(6027):318–322CrossRefGoogle Scholar
  3. Hanawa K, Mitsudera H (1987) Variation of water system distribution in the Sanriku coastal area. J Oceanogr Soc Jpn 42:435–446CrossRefGoogle Scholar
  4. Inoue R, Gregg MC, Harcourt RR (2010) Mixing rates across the Gulf Stream, part 1: on the formation of Eighteen Degree Water. J Mar Res 68(5):643–671CrossRefGoogle Scholar
  5. Inoue R, Yamazaki H, Wolk F, Kono T, Yoshida J (2007) An estimation of buoyancy flux for a mixture of turbulence and double diffusion. J Phys Oceanogr 37(3):611–624. doi: 10.1175/jpo2996.1 CrossRefGoogle Scholar
  6. Ishizu M, Itoh S, Tanaka K, Komatsu K (2015) Influences of the Tsushima and Oyashio Currents on the circulation and water properties of Otsuchi Bay. J Oceanogr (submitted)Google Scholar
  7. Ishizu M, Kitade Y, Matsuyama M (2008) Characteristics of the cold-water belt formed off Soya Warm Current, J Geophys Res 113(C12). doi: 10.1029/2008jc004786
  8. Itoh S, Sugimoto T (2002) Direct current measurements off Sanriku, east of Japan. J Oceanogr 58(6):877–882. doi: 10.1023/A:1022883415672 CrossRefGoogle Scholar
  9. Itoh S, Yasuda I, Nakatsuka T, Nishioka J, Volkov YN (2010) Fine- and microstructure observations in the Urup Strait, Kuril Islands, during August 2006. J Geophys Res 115. doi: 10.1029/2009JC005629
  10. Itoh S, Tanaka Y, Osafune S, Yasuda I, Yagi M, Kaneko H, Konda S, Nishioka J, Volkov YN (2014) Direct breaking of large-amplitude internal waves in the Urup Strait. Prog Oceanogr 126:109–120. doi: 10.1016/J.Pocean.2014.04.014 CrossRefGoogle Scholar
  11. Johnston TMS, Rudnick DL, Pallas-Sanz E (2011) Elevated mixing at a front. J Geophys Res 116(C11). doi: 10.1029/2011jc007192
  12. Joyce TM (1989) On in situ “calibration” of shipboard ADCPs. J Atmos Ocean Technol 6(1):169–172. doi: 10.1175/1520-0426(1989)006<0169:oisosa>2.0.co;2 CrossRefGoogle Scholar
  13. Kaneko H, Yasuda I, Komatsu K, Itoh S (2012) Observations of the structure of turbulent mixing across the Kuroshio. Geophys Res Lett 39(15). doi: 10.1029/2012gl052419
  14. Kaneko H, Yasuda I, Komatsu K, Itoh S (2013) Observations of vertical turbulent nitrate flux across the Kuroshio. Geophys Res Lett 40(12):3123–3127. doi: 10.1002/Grl.50613 CrossRefGoogle Scholar
  15. Klymak JM, Legg SM, Pinkel R (2010) High-mode stationary waves in stratified flow over large obstacles. J Fluid Mech 644:321. doi: 10.1017/s0022112009992503 CrossRefGoogle Scholar
  16. Koslow JA, Pesant S, Feng M, Pearce A, Fearns P, Moore T, Matear R, Waite A (2008) The effect of the Leeuwin Current on phytoplankton biomass and production off Southwestern Australia. J Geophys Res 113(C7). doi: 10.1029/2007jc004102
  17. Kunze E (1985) Near-inertial wave-propagation in geostrophic shear. J Phys Oceanogr 15(5):544–565CrossRefGoogle Scholar
  18. Lee DK, Niiler PP (1998) The inertial chimney: the near-inertial energy drainage from the ocean surface to the deep layer. J Geophys Res 103(C4):7579–7591. doi: 10.1029/97jc03200 CrossRefGoogle Scholar
  19. Masunaga E, Fringer O, Yamazaki H (2015) An observational and numerical study of river plume dynamics in Otsuchi Bay, Japan. J Oceanogr. doi: 10.1007/s10872-015-0324-2 Google Scholar
  20. Nagai T, Tandon A, Yamazaki H, Doubell MJ (2009) Evidence of enhanced turbulent dissipation in the frontogenetic Kuroshio Front thermocline. Geophys Res Lett 36. doi: 10.1029/2009GL038832
  21. Nagai T, Tandon A, Yamazaki H, Doubell MJ, Gallager S (2012) Direct observations of microscale turbulence and thermohaline structure in the Kuroshio Front. J Geophys Res 117(C8):C08013. doi: 10.1029/2011jc007228 Google Scholar
  22. Osborn TR (1980) Estimates of the local-rate of vertical diffusion from dissipation measurements. J Phys Oceanogr 10(1):83–89CrossRefGoogle Scholar
  23. Rainville L, Pinkel R (2004) Observations of energetic high-wavenumber internal waves in the Kuroshio. J Phys Oceanogr 34(7):1495–1505CrossRefGoogle Scholar
  24. Ruddick B (1983) A practical indicator of the stability of the water column to double-diffusive activity. Deep Sea Res Part A Oceanogr Res Pap 30(10):1105–1107. doi: 10.1016/0198-0149(83)90063-8 CrossRefGoogle Scholar
  25. Rudnick DL, Klinke J (2007) The underway conductivity–temperature–depth instrument. J Atmos Ocean Technol 24(11):1910–1923. doi: 10.1175/jtech2100.1 CrossRefGoogle Scholar
  26. Sakamoto T, Urakawa LS, Hasumi H, Ishizu M, Itoh S, Tanaka K (2015) Numerical simulation of Pacific water intrusions into Otsuchi Bay, northeast of Japan, with a nested-grid OGCM. J Oceanogr (submitted)Google Scholar
  27. Shimizu Y, Yasuda I, Ito S (2001) Distribution and circulation of the coastal Oyashio intrusion. J Phys Oceanogr 31(6):1561–1578CrossRefGoogle Scholar
  28. Skagseth Ø, Drinkwater KF, Terrile E (2011) Wind- and buoyancy-induced transport of the Norwegian Coastal Current in the Barents Sea. J Geophys Res 116(C8). doi: 10.1029/2011jc006996
  29. Stabeno PJ, Ladd C, Reed RK (2009) Observations of the Aleutian North Slope Current, Bering Sea, 1996–2001. J Geophys Res 114(C5). doi: 10.1029/2007jc004705
  30. Takikawa T, Yoon JH, Cho KD (2005) The Tsushima warm current through Tsushima Straits estimated from ferryboat ADCP data. J Phys Oceanogr 35(6):1154–1168. doi: 10.1175/jpo2742.1 CrossRefGoogle Scholar
  31. Wagawa T, Kuroda H, Ito S-I, Kakehi S, Yamanome T, Tanaka K, Endoh Y, Kaga S (2015) Variability in water properties and predictability of sea surface temperature along Sanriku coast, Japan. Cont Shelf Res 103:12–22. doi: 10.1016/j.csr.2015.04.016 CrossRefGoogle Scholar
  32. Whitt DB, Thomas LN (2013) Near-inertial waves in strongly baroclinic currents. J Phys Oceanogr 43(4):706–725. doi: 10.1175/jpo-d-12-0132.1 CrossRefGoogle Scholar
  33. Yasuda I (2003) Hydrographic structure and variability in the Kuroshio–Oyashio Transition Area. J Oceanogr 59(4):389–402CrossRefGoogle Scholar
  34. Yasuda I, Okuda K, Hirai M, Ogawa Y, Kudoh H, Fukushima S, Mizuno K (1988) Short-term variations of the Tsugaru Warm Current in autumn (in Japanese with English abstract). Bull Tohoku Reg Fish Res Inst 50:153–191Google Scholar
  35. Zhai X, Johnson HL, Marshall DP (2010) Significant sink of ocean-eddy energy near western boundaries. Nat Geosci 3(9):608–612. doi: 10.1038/ngeo943 CrossRefGoogle Scholar

Copyright information

© The Oceanographic Society of Japan and Springer Japan 2015

Authors and Affiliations

  • Sachihiko Itoh
    • 1
  • Hitoshi Kaneko
    • 1
  • Miho Ishizu
    • 1
  • Daigo Yanagimoto
    • 1
  • Takeshi Okunishi
    • 2
  • Hajime Nishigaki
    • 3
  • Kiyoshi Tanaka
    • 1
  1. 1.Atmosphere and Ocean Research InstituteThe University of TokyoKashiwaJapan
  2. 2.Tohoku National Fisheries Research InstituteFisheries Research AgencyShiogamaJapan
  3. 3.Faculty of Education and Welfare ScienceOita UniversityOitaJapan

Personalised recommendations