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

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.