Poleward-propagating near-inertial waves enabled by the western boundary current

Chanhyung Jeon1, Jae-Hun Park2, Hirohiko Nakamura3, Ayako Nishina3, Xiao Hua Zhu4, Dong Guk Kim5, Hong Sik Min5, Sok Kuh Kang6, Hanna Na7 and Naoki Hirose8, (1)Pusan National University, Department of Oceanography, Busan, South Korea, (2)Inha University, Ocean Sciences, Incheon, South Korea, (3)Fac. Fisheries, Kagoshima University, Kagoshima, Japan, (4)Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, China, (5)KIOST, Ocean Circulation and Climate Research Center, Busan, South Korea, (6)KIOST, Ocean Circulation and Climate Research Center, Ansan, South Korea, (7)Seoul National University, School of Earth and Environmental Sciences, Seoul, South Korea, (8)Kyushu University, Research Institute for Applied Mechanics, Fukuoka, Japan
Near-inertial waves (NIWs) exist everywhere in the ocean except at the equator; their frequencies are largely determined by the local inertial frequency, f. It is thought that they supply about 25% of the energy for global ocean mixing through turbulence resulting from their strong current shear and breaking; this contributes mainly to upper-ocean mixing which is related to air-sea interaction, typhoon genesis, marine ecosystem, carbon cycle, and climate change. Observations and numerical simulations have shown that the low-mode NIWs can travel many hundreds of kilometres from a source region toward the equator because the lower inertial frequency at lower latitudes allows their free propagation. Here, using observations and a numerical simulation, we demonstrate poleward propagation of typhoon-induced NIWs by a western boundary current, the Kuroshio. Negative relative vorticity, meaning anticyclonic rotational tendency opposite to the Earth’s spin, existing along the right-hand side of the Kuroshio path, makes the local inertial frequency shift to a lower value, thereby trapping the waves. This negative vorticity region works like a waveguide for NIW propagation, and the strong Kuroshio current advects the waves poleward with a speed ~85% of the local current. This finding emphasizes that background currents such as the Kuroshio and the Gulf Stream play a significant role in redistribution of the NIW energy available for global ocean mixing.