This study investigates sea-level variability along the south coast of Japan during the 2017 Kuroshio large meander (LM), focusing specifically on the Kuroshio path. To estimate sea-level changes associated with variations in the Kuroshio path, we utilize Observation Minus Hindcast (OMH) data obtained by subtracting meteorological sea-level changes from observed sea-level anomalies. Sea levels rose by 3-5 cm at eight stations along the Tokai district during the LM period of 2018-2023 as compared to the 2011-2016 non-LM period. During the LM period, high sea-level regions shifted depending on the Kuroshio path off the Tokai district. This variability can be categorized into three patterns based on composite and empirical orthogonal function (EOF) analyses: (1) when the Kuroshio flows close to the Izu and Boso Peninsulas, sea levels tend to rise along the coast west of the Tokai district; (2) when the westward flow bifurcates from the Kuroshio off the Tokai district, sea levels tend to rise throughout the Tokai district; (3) when the Kuroshio adopts an S-shaped path and flows near the eastern side of the Kii Peninsula, sea levels tend to rise west of Owase. Consequently, sea-level variability differs between the eastern and western areas of the Kii Peninsula during the LM period.

Internal gravity waves generated by tidal flows passing over steep seafloor topography, known as internal tides, promote the vertical mixing of seawater by generating microscale turbulence. This process significantly influences global ocean circulation, climate change, and marine ecosystems. However, where and how internal tides induce turbulent mixing in the ocean remain poorly understood. To tackle this issue, it is essential to clarify the mechanism by which the energy of internal tides, with horizontal wavelengths exceeding 100 km, cascades down to smaller scales. This paper reviews the historical background and the author's theoretical contributions to the study of parametric subharmonic instability (PSI)-a resonant interaction between internal waves that facilitates the energy cascade from internal tides in mid-latitude regions. Beginning with an elementary explanation of the PSI mechanism, the paper introduces techniques such as the wave kinetic equation, Floquet theory, and direct numerical simulations in transformed coordinate systems. These methods are utilized to address and overcome the challenges faced in previous studies attempting to quantify PSI. In conclusion, the paper discusses recent advances in turbulent mixing and deep ocean circulation and suggests future directions. Additionally, it provides an overview of the author's latest research endeavors, which were inspired by concepts from condensed matter physics.