The strength of the vertical eddy diffusivity determines the structure of oceanʼs general circulation, which affects the climate change. Therefore, global estimation of vertical eddy diffusivity is required; however, focusing on the direct observation of turbulence (dissipation rates) for estimation within limited ship time is difficult. Even if observations are made, understanding factors such as wind, tide, internal wave breaking, and sea surface cooling, which are energy sources of turbulence, and parameters characterizing the turbulence is crucial. In this review, the length scales ranging from a few millimeters to several tens of meters and dimensionless numbers used in ocean turbulence analysis are summarized to promote the understanding of the turbulence; particularly, we focus on practical estimation methods for dissipation rates to accelerate research using observed turbulence data.

Seasonal variations in water system distributions were examined in Onagawa Bay of the Sanriku area using long-term hydrographic data obtained from April 2000 to February 2017. In this study, we classified the waters in the bay into six water systems defined by Hanawa and Mitsudera (1987) and coastal water (CW), which appears in the nearshore region but not in the offshore region. The mode water system analysis revealed clear seasonal variations in the water system distributions in the bay, characterized by the predominance of the Tsugaru Warm Current water system in the lower layer in fall, the entire water column in winter, the Oyashio water system in the lower layer in spring, and CW in the surface layer from spring to summer. CW in the bay was less saline and/or less dense than the offshore water and mostly distributed in the surface layer from April to October, indicating that CW was formed by mixing freshwater discharged from the river with water in the bay. The relationship between surface salinity in the mouth part of Onagawa Bay and freshwater supply from the Kitakami River flowing into Oppa Bay indicated that a part of the CW in Onagawa Bay was derived from outside the bay.