Near-inertial internal waves and multiple-inertial oscillations trapped by negative vorticity anomaly in the central Sea of Japan

Abstract

This study aims to reveal the interaction between wind-induced internal waves and mesoscale features in a frontal region of the central Sea of Japan. A mooring system with an upward-looking acoustic Doppler current profiler was deployed over a meandering pathway of the Tsushima Warm Current. The annual record of horizontal current showed an extraordinary event of near-inertial internal waves (NIWs) that were trapped in a mesoscale warm-core eddy, characterized by negative vorticity anomaly. Near the critical depth inside the eddy, where background vorticity vanishes, the amplitude of downward-traveling NIW reached 50–70 cm s−1 in root-mean-square current magnitude. The amplified NIWs were concurrent with the emergence of superinertial oscillations, peaking at multiple inertial (MI) frequencies, i.e., double, triple, and quadruple inertial. The MI oscillations are regarded as arising through nonlinear resonance of NIW. Examining local surface winds suggest that a fast-moving cyclone passing over the site initially generated the NIW event. The trapping and amplification processes of wind-generated NIW are examined by a series of ray-tracing simulations, where background vorticity is reconstructed from in situ hydrographic observations. Based on the results above, a specific picture model of kinetic energy circulation around the Sea of Japan can be presented: the atmospheric event provides momentum energy into the ocean surface, resulting in the generation of NIWs and trapping within anticyclonic vortices. A part of the near-inertial kinetic energy is then transferred into the MI oscillations if NIW is vigorously amplified near the critical depth; then it likely dissipates into turbulence.