Abstract

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.

Plain Language Summary

Near‐inertial waves (NIWs) transfer energy from the surface to the deep ocean and play a crucial role in energy cascades. Based on observations and numerical simulations conducted around Miyakojima Island in the northwestern Pacific, the different spatial patterns of NIWs after four typhoons in 2015 were influenced by variations in typhoon tracks and background currents. The NIWs were negligible and energetic on the left and right sides of the typhoon tracks, respectively. Energetic NIWs can be advected poleward along the northward western boundary currents, including the Kuroshio Current and Ryukyu Current. However, the poleward energy propagation of NIWs can be hindered by cyclonic eddies originating from the North Pacific, which impede the Ryukyu Current and disrupt the favorable conditions for their energy propagation. This study enhances our understanding of the propagation dynamics of wind‐induced NIWs and provides valuable insights into the multiscale interactions in regions with abundant dynamic processes.

Key Points

Near‐inertial waves (NIWs) with different features were observed around the Ryukyu Islands following four typhoons in 2015

NIWs induced by remote typhoons were observed, but at times, the observations failed to capture those induced by regional typhoons

The energy propagation of NIWs is influenced by the Kuroshio Current, the Ryukyu Current, and eddies