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An observational and numerical study of river plume dynamics in Otsuchi Bay, Japan

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

A newly developed tow-yo profiler (YODA Profiler) and a fully nonhydrostatic numerical model, SUNTANS, are used in this study to investigate river plume mixing in Otsuchi Bay, a ria estuary located in Iwate, Japan. Several field campaigns were conducted in the bay during early summer, late summer, and late winter. The YODA Profiler reveals fine features related to a shallow river plume that experiences rapid mixing events during the summer campaigns with a time scale of O(1) hour. These events coincide with strong baroclinic currents and large fluctuations in thermocline and pycnocline depths related to shoaling internal tides. The combined effects of wind stress and baroclinic flow appear to generate a strong shear near the surface layer and enhance mixing of the river plume, with typical eddy diffusivity values of O(10−4) m2 s−1. To investigate the physical mechanisms involved, numerical simulations are conducted with tidal and wind forcing. Analysis of tidal forcing mechanisms reveals that mixing near the river mouth (and upstream) is dominated by the barotropic tide, while turbulent mixing in the middle of the bay is significantly enhanced by the baroclinic internal tide. Wind forcing is also important for river plume dynamics; along-channel wind forcing mixes the river plume and transports it horizontally. Overall, this study suggests that all three forcing mechanisms (barotropic tide, baroclinic tide, and wind) are important for mixing processes of the river plume in Otsuchi Bay.

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Acknowledgments

We thank the crew of the R/V Grand-maillet (the University of Tokyo), the fishing boat Senshu-maru (Shin-Otsuchi Fishery), and the fishing boat Daimutsu-maru (Shin-Otsuchi Fishery) for their time and help. The authors gratefully acknowledge Kiyoshi Tanaka, Hideki Fukuda, Yuichiro Nishibe, and Yuka Saito for coordinating our observation plans. We are indebted to the members of Laboratory of Ocean Ecosystem Dynamics (Tokyo University of Marine Science and Technology) for their assistance with the deployment of the instruments. This study was supported by funding from Tohoku Ecosystem-Associated Marine Sciences (TEAMS), a research program launched by the Ministry of Education, Culture, Sports, Science and Technology (MEXT).

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Authors and Affiliations

  1. Department of Ocean Sciences, Tokyo University of Marine Science and Technology, 4-5-7 Konan, Minato-ku, Tokyo, Japan

    Eiji Masunaga & Hidekatsu Yamazaki

  2. Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, Office 187, Stanford, CA, USA

    Oliver B. Fringer

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  1. Eiji Masunaga
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  2. Oliver B. Fringer
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  3. Hidekatsu Yamazaki
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Correspondence to Hidekatsu Yamazaki.

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Masunaga, E., Fringer, O.B. & Yamazaki, H. An observational and numerical study of river plume dynamics in Otsuchi Bay, Japan. J Oceanogr 72, 3–21 (2016). https://doi.org/10.1007/s10872-015-0324-2

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  • DOI: https://doi.org/10.1007/s10872-015-0324-2

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