Typhoon-Induced Near-Inertial Waves Around Miyakojima Island in 2015

In 2015, four typhoons traversed the regions surrounding the Ryukyu Island Chain, resulting in different near-inertial waves (NIWs), the characteristics of which were investigated through in situ observations around Miyakojima Island and numerical simulations covering the East Asian marginal seas. The spatial distribution of typhoon-induced near-inertial motions was significantly correlated with the typhoon tracks and background currents. Typhoons Chan-hom and Goni traversed the observation transect, resulting in different spatial patterns of NIWs owing to their different tracks. Observations on both the western and eastern sides of the Ryukyu Island Chain failed to capture NIWs after typhoon Chan-hom, because they were positioned to the left of the typhoon track where near-inertial motions were enhanced only in the upper 50 m. In contrast, NIWs were negligible on the left side and energetic on the right side of typhoon Goni. Despite being hundreds of kilometers from the observation transect, typhoons Soudelor and Dujuan induced NIWs with higher energy levels than those induced by typhoons Chan-hom and Goni. The energy propagation of NIWs after typhoons Soudelor and Dujuan was significantly influenced by background currents. The western boundary currents, including the Kuroshio and Ryukyu Currents, create negative relative vorticity on their right, which works like a waveguide for the poleward advection of NIWs. However, the Ryukyu Current can be impeded by westward-propagating cyclonic eddies from the North Pacific, which further disrupts the waveguide.